Table: Adjustable/Lift Design

[Temuba]

Structural Question:

I'm in the process of designing an adjustable table that would raise the spoil board portion of the table up and down 12". This would incorporate building a free floating table surface inside the outer base. It would have vertical movement via 3/4" Acme thread using chain & sprockets via an electric motor to rotate the threads. This would then cause the table to rise/lower from the attached welded 3/4" hex nuts.

Below are some preliminary images with some sections hidden for clarity. Please let me know if more details are needed to explain my idea better.

In the design, there are four corner points with the acme threads. The table/cutting dimensions are 100"x53". The perimeter of the steel assembly will be C4x5.4 c-channel and the interior 4 cross members will be C3x4.1 c-channel. The thickness of the spoil board will be 1.25" (3/4"+1/2") MDF.

My question (this is where I loose it), should I put an additional riser support pair in the center of the longer C4x5.4 (see red question mark in image)? My concern is the weight issue of the spoil board and any addtional material on top and whether the longer c-channel will deflect somewhat to affect the quality of the work above.

Adding another support pair in the center will somewhat complicate the chain and sproket design at those points. I'm wondering if I can get away with just the four outer point and having the chain wrap around all four at 90°.

Thank you for any comments, suggestions or ideas - David

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT01.jpg
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[bradm]

I don't see an issue with your floating table per se, but I think you've greatly compromised the rigidity of the base frame which will destroy the performance of the machine. Remember that the cross supports and the table itself are structural elements of the MM design, and you'll need to compensate for their removal.

[Temuba]

Brad- The images don't show it and I haven't yet drawn it out, but the front and back base sections will be include into the design. These two elements will be bolted directly to the side bases. The top member will be 1.5"x3" steel tube, the bottom and diagnals will be 1.5" square steel tubes. Similar in design and construction as the two longer side bases.

Basically the base will be 4 sided with the appropriate diagnals. All sides will be welded and then bolted to each other at the corners. The adjustable table will then float insided this box to freely move up and down. Thanks-David

[Regnar]

Dave I was wondering why you would need such a deep cutting area? Do you plan on extending the Z axis? Would it be feasible to machine what ever you are making in layers?

As mentioned I think you will loose a good amount of rigidity. It also looks like a lot of critical elements to make this work. Do you have the tools to machine and measure. I think 6 screws would be a starting point. You might want to consider running the screws all the way up to the top of the x rails and install 2 nuts on each screw. Install one towards the surface and one where you have it located now. This should help out with any swaying on the table. You could even mount a brace in the center to take out any bowing of the screw under pressure.

I also notice that you didnt abandon the ideal of X rails from the other topic. Should also mention that the racks get installed under the V rails and you will not need the extra pieces of angle iron. As drawn the x motors will not reach. Good luck with you adventure.

[Temuba]

Here's a better image showing the base only without the adjustable table within.
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT05.jpg

[Regnar]

Right now you have nothing keeping your table square. A small hit from loading your work can knock it out of aligment. With the orignal design all the cross bracing and spoil board keep the table square. I can't remember the number off the top of my head but I think it's around 100 bolts that hold it square. Your ideal will keep the X rials at equal distance apart but will not keep it square. Needs a lot of cross bracing.

[Temuba]

Russel- First, yes I have a 22" z travel and there are two purposes for the adjustable table:
a) By bringing the table up closer to the gantry, at that point it will limit the z travel to 12" max. This will help with a better finish in the project for smaller 3d objects. Basically less play in the z direction.
b) I plan to add an indexer for turning. By lowering the table, I'll get 22-24" of clearance below the bottom of the gantry for larger diameter projects.

Second,my original idea was to extend the threads near the top closer to the x beam. However with the table near the bottom and the bolts exposed above the z-plate and router would hit the threads, unless I shrink the width of the table. I need to further study this idea to get better stability in the base. Post#5 actually show the base with all the proper components and diagnals.

Please disregard the details on the x beam, those are other ideas I had but are not concrete.

Thanks for the input.-David

[Temuba]

Russel- I can't believe I overlooked the horizontal cross/diagnal bracing. After a while you're worried about the small details that sometimes you overlook the obvious.:o

Also, there are dead spaces (non cutting areas) at the front and rear of the x axis. I can install bracings/gussets at each top corner. Also as you can see in red I can add futher longer diagnal bracing at the bottom horizontal cross members. Hopefully this should further stabilize any movement diagnally.

Need to still think about your previous suggestion to stiffen the table in relation to the all-threads.

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT06.jpg

[JLFIN]

I think I would consider a sub base that you could remove for the extra Z space that would give you the best of both worlds
or just extend out the front travel and put index on adjustable z

[Alan_c]

To stiffen the table you could move the vertical adjustment screws further toward the centre of the table and iclude angle bracing from the nut up to the bottom of the C4x5.4, i.e. the four jacking points dont have to be at the corners, by having a bit of cantilever you reduce the chance of the centre section deflecting (move in no more than 1/4 of the total from each end) That may also give you more room to cross brace the main outer frame.

[Temuba]

Alan-Great point, thanks. I can move those assemblies to the next interior C3x4.1 cross member, which are 20" on center. This would be them within the 25% range you recommended to only 20% from each corner.

There would then be only a span of 60" between points. Do you think I can still get away with four points instead of six? Like I said before having four instead of six points would greatly simplify the chain and sproket design to drive the threads. Thanks-David

[KenC]

60" is a long span for your material choice. since there are unused void under the board, Should you consider the bed a lattice space frame made out of think wall square hollow section or pipes & you can span a lot longer then that.

[Gerald D]

A CNC table must be be able to carry rapidy changing horizontal cutter forces. It is the x-rails that impart these forces, and thus a stiff connection between the x-rails and table surface is desired.

[Temuba]

Here's some more work done to better improve rigidity and strength, not to mention additional weight.

Russel- To answer one of your original questions regarding if I have the know-how and machines to take this task: the answer is yes. Twenty years in carpentry and building, both using wood and metal. I have most of the tools necassary to cut, weld and measure.

Some of the changes and improvements are as follow:
1-Added the missing horizontal diagnal bracing at the bottom and top of the base, image 3 &5.
2-On the floating table, moved the corner thread assemblies 20" closer to the center. This reduces the longer C4 channel (X axis) span from 100" to 60", image 4 & 5. As per Alan_c recomendation.
3-Added addional diagnal bracing to the thread assemblies for better rigidity, image 4 & 5.
4-At the thread assembly where the two nuts are in contact with the thread, I increase the contact distance from 1.75" to 5.5", image 5. I was never happy with the original contact distance and think that this will reduce any flexing of the thread as per Russel's recomendation.

The last image shows the inner table in the top most raised position. I know that there may still be room for improvement, so please comment. Thanks-David.

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT07.jpg
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[Temuba]

I think I would consider a sub base that you could remove for the extra Z space that would give you the best of both worlds
or just extend out the front travel and put index on adjustable z

My first original idea was to do such a thing. However, I'm having an issue trying to fit this machine in my small shop without an additional 100"x53"x12" sub base taking up more space.I'm trying to utilize the space under the table to serve this purpose. Thanks-David.

[Alan_c]

Thats looking better, I would add more bracing as indicated below, and some method of locking the table to the X-rails while working as Gerald indicated.

9145

[KenC]

9146

[Temuba]

Ken- I see, I know get where you were going with your original post. I like it. Basically you are refering to a triangular truss structure using lighter components similar to the image below.
http://i585.photobucket.com/albums/ss292/temuba/CNC/FloatingTable_StructuralTrussDesign.png
I understand this concept very well, as I use it many times in the construction field. Correct me if I'm wrong, but bydoing it this way I can change to steel component in the floating table:

a) The current four pairs of 1.5"sq tube diagnals can be replaced with a smaller .75" or 1" sq. tube, since a broader design with more triangular points will be attached to the c-channel platform.
b) The perimeter C4 c-channel can be reduced to the same as the interior C3 c-channel cross members. Since there are more tringular points welded to it, it no longer has to be that deep.

After absorbing this idea, I think this may further help stiffen the rigidity in relationship to the side base-sides. Not noly is the floating table rigid at the horizontal area where the top c-channel structure is, but will also be as rigid at the bottom section of this assembly where the nuts are that ride on the thread.

Alan- In additon, with this new concept I still agree with your point and trying to "lock/clamp" the top of the floating table to the xbeams and base-sides. I've been thinking of a linear guide that would always be in contact between the top of the floating table and the base-sides. Once the desired height has been achive then a clamping or bolting system can then lock both the floating table and base to itself.

Any ideas? Thank-David.

There's a saying I've been using for decades: "I see said the blind man to the deaf man as he picked up his hammer and saw":)

[bradm]

I believe the idea of a lower table with riser blocks was rejected due to storage space. Here's another take on that:

If it fits your usage, why not build 1/2 the table at normal height, and 1/2 at the lower height. Build your riser blocks as storage containers for various shop items, and design the system so that the riser blocks can go underneath the high side of the table when they aren't in use.

[Gerald D]

Alan- In additon, with this new concept I still agree with your point and trying to "lock/clamp" the top of the floating table to the xbeams and base-sides. I've been thinking of a linear guide that would always be in contact between the top of the floating table and the base-sides. Once the desired height has been achive then a clamping or bolting system can then lock both the floating table and base to itself.

2 Comments:
- if you can achieve a locks/clamps, then you can discard 50% of the steel you have shown above.
- whatever clamp system you employ, it must not disturb the alignment of the x-rails on tightening the clamps.

[JLFIN]

linear guide system could be a little pricey... cam followers up and down a channel would be a nice guide or a simple set of rails made out of U.H.M.W. plastic would also work.

[Temuba]

linear guide system could be a little pricey... cam followers up and down a channel would be a nice guide or a simple set of rails made out of U.H.M.W. plastic would also work.

Jim- I was only using the words 'linear guide' as a term to describe the idea and function. I realize they are pricey and I cannot afford such a concept. One idea I do have is to create and make a linear guide using a 3/4" dia. rod that would go up and down inside a 3/4" ID piece of pipe. There would be a hole on the side of the pipe with a welded nut, then a screw that would tighten onto the rod and press against the opposite wall of the pipe. Very simple, effective and strong wothout any complicated parts. Thanks-David.

[JLFIN]

any guide might be a problem if you are trying to run those acme screws independently,
is it possible to lift in the center and just bolt in the correct position

[Tokamak]

4931T116
Steel, 2" Square, 6'Length, Perforated Tubing for Heavy Duty Telescoping-Tube Framing $34.80 Each.

This tubing looks like a good linear slide. Perforations provide safety locking pins. Still need a bolt like you mentioned to tighten the gap between the tubes.

Ever thought about using small air pistons to do the lifting. You could lift evenly without binding on low cost linear slides.

[Regnar]

Dave, I am still wondering why you need the table to rise and lower.

With as much work that would go into a rising and lowering the one table you might want to consider building 2 purposeful tables.

[Temuba]

Russel- In post#7 I answer that question you posted originally.Post #15 explains why I cannot have any additional components outside the envelope of this machine. I have a TINY shop and will have to consider moving some of the larger woodworking machines into the adjacent utility room. Thanks-David

[Temuba]

Ok guys, here another sketchup based on some of the recomendations here. First, thanks for the advice and help.

1)The Base has remaind the same, however the adjustable table has changed somewhat. I decided to go with a truss system for the structure that supports the c-channel sub-structure which eventually holds the spoil board. I lighten this lower truss structure to 1" sq. tube, since the truss design will allow for better rigidity and strength. This also allows to reduce the original C4 c-channel permiter to a C3 c-channel like the rest of the interior cross members.

2) As per Gerald's recommendation, a verical linear guide system was added to the four outside corners of the table. A guide/clamp block will help in two ways: prevent any sway or flex from the 3/4" riser threads and most importantly lock the table to the side base in the X axis. This will hopefully stiffen the base for any horizontal and diagnal forces applied.

The following images show some details of the changes. Some sections were hidden for clarity.

Gerald-in a previous post you mentioned "if you can achieve a locks/clamps, then you can discard 50% of the steel you have shown above." I added the locks/clamps but I'm a little unsure as to what components can be discarded. Please explain or draw on one of the images, and thank you.

Again as always, comments are welcomed and appreciated. Thanks-David

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT13.jpg
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[domino11]

Do you really think you will need the infinite range of the threaded rod? I would think you would set the table to a reasonable working height and would only very rarely change it for the odd large piece and then put it back for a long time again at the regular table height. Why not just have a few pre-drilled locations to which you could set the table to. Already leveled, etc.... Just a thought.

[Temuba]

any guide might be a problem if you are trying to run those acme screws independently,
is it possible to lift in the center and just bolt in the correct position

Jim- Post #1 explains how the table will raise and lower. I plan to use sprokets attached at the bottom ends of the 3/4" riser threads and then employ a continuos chain to drive all four threads at the same time. I haven't yet figured out the details for this, I just want to get the structure above finallize before I tackle that new design issue.

I have seen this method used in woodworking workbenches, where the operator cranks a handle and raises and lowers the surface top of the workbench. This allows for various work heights for different projects.

At this point I may just use the same principle and use a hand-wheel to operate the table. The table will maybe be only moved once per project then locked in place while the router does its job.

[KenC]

Dave,
Now that you have a rigid bed, the screw won't work, I'll go with jim's idea on using jack for the job, even easier construction. I would suggest a scissor lift for the job.
http://www.lift-equipment-parts.com/parts/images/scissor-lift-parts.jpg
Your lock & latch don't kook good to my eye. Why don't you look into scaffolding joints for inspiration?
http://www.alps-scaffolding.com/images/scaffolding%20home.jpg

[swatkins]

Do you really think you will need the infinite range of the threaded rod? I would think you would set the table to a reasonable working height and would only very rarely change it for the odd large piece and then put it back for a long time again at the regular table height. Why not just have a few pre-drilled locations to which you could set the table to. Already leveled, etc.... Just a thought.

Then put a cheap bottle jack under each corner and raise and lower the table with those..

[Temuba]

Do you really think you will need the infinite range of the threaded rod? I would think you would set the table to a reasonable working height and would only very rarely change it for the odd large piece and then put it back for a long time again at the regular table height. Why not just have a few pre-drilled locations to which you could set the table to. Already leveled, etc.... Just a thought.

Yes you're correct, I won't be using the threads for precise lifting. I can always Zero the router bit to the general height of the table. The threads just make it easy to raise and lower the table and weight.

The previous post and #1 explain how the system will work. This is another reason I'm just using "over-the-counter" standard thread or all-thread as we call it around here, I'm not looking for precision, just a comfortable working height. Thanks-David

[KenC]

No matter which choice of lifting mechanism, you still need fix holes + pin/bolt to locate the bed position before lock/latching the bed.
Dave,
The design is getting more complicated then necessary, you need to consolidate...

I have another approach, build a honeycomb-ish sandwiched bed that is very rigid out of MDF, place a scissor lift in the middle fot the lifting, attach 3 or 4 pipe poles to the corners of the bed with position holes & use the scaffolding joints to lock the position on to the fixed table.

[Temuba]

Ken- How won't it work? All four points will be moving at the same time. If you can image a machinisht vise employing the intergrated dovetail ways and using the linear screw to move the table left to right, now just take this concept and rotate it 90°. The only difference is that my guide/clamp block will not be so tight to the linear vertical rail.

I also thought about the scissor lift, as in the one I use for my ATV. However, cost and rigidity was a factor in not using this idea.

In Post#22 I explain the same concept that you mention using scaffoling joints. But the way I saw it was that the rod riding inside the joint would only be attached to the structure at the top and bottom. So if the table were half way lifted the rod may (or may not) have some flex. By using the C-channel as a guide or rail the channel is technically attached to the structure from top to bottom. Thanks-David

[Gerald D]

Your x-rails beams are weak in the y-direction when the gantry sits in the middle of a board . . . . .those beams will wobble a lot.

[KenC]

Dave,
On paper, running 4 screws to move thing in unison is perfect, but my ex-day jobs told me it isn't as simple as they seem. A simple illustration is 2 man 3 leg contest. A single point push or pull works with less hassle both in building & in the long run.
My design philosophy is let each mechanism handle their purpose well. This had helped me to come up with things that are so fundamental that many think its out-of-the-box. On that, the lifting mechanism should only do lifting & the lock & latch should just do the lock & latch & do it extremely well. once you get into position, you lock the bed & the lifting mechanism can go take a cuppa in the pantry ;)

Having a few more lock & latching point securely fasten to the bed onto the table also make the two as one and hence improve the rigidity of the table when its most needed during production. In this aspect, the space frame is ideal as it can span a lot further & still has very good rigidity.

BTW, you can make a scisor lift cheaply, if cost is a constrain, replace the hydraulic ramp with a screw & nut, that will also allow your hand crank adjustment.

[Temuba]

Gerald- That thought crossed my mind. These are just prelimanary conceptual ideas. I will take it into consideration and adjust the design accordingly. I'm still curious to your previous comment regarding "discarding 50% of the abpve material". Thank you

[Gerald D]

Once you clamp/tie the table top to the x-rails, then you suddenly get a lot of strength, as per basic plans, which need very much less steel.

[Temuba]

Ken- I unerstand your opinions and thoughts. Like I say to many around me, "I smell you".:D

However after over 23 in carpentry and building, I've had my share of erecting and using scafoldings and I don't beleive that these types of joints will be sufficient to reduce flex or movement like the more rigid and continiuos connection of the c-channel or for that matter any type of linear rail system.

I also agree with you and Gerald that the xbeam will be weak in the center with no locking point to the table. Like I said before that thought crossed my mind, I just haven't gotten around to all the details, just the general drawings of how the different components of the structure work with each other.

Before I conpleted the last revision, my first thought was to punch holes on the c-channel rail and just bolt the table to the fixed based at the four points(now should be at least six points). I wasn't too thrilled with the clamp system. It could lead to slipage. I agree, it should be a pin or bolt like system once the general height has been established.

As far as your first comment, the four threads working on paper, I'm not to sure if I agree with this. I have personally seen this concept work on a workbench. Also having worked on some high-end mult-million dollar homes, I have had my share of taking an idea from a piece of paper and making it work in the end. That was my role, job superviser, making sure what was on paper and spoken about got built.

Again thank you-David

[Temuba]

Ken- Cuppa? Had to look that one up in the dictionary.:)

If we simplify to using a bolt system to lock between the table and base, what would be your opinion on the following? As you saw in the last revison, I added four vertical linear rails at the outside corners of the table. Originally, I wanted to put them right next to the riser thread assembly but didn't have the room. Now that we have established that we don't need a guide rail system, but just have two plates riding by each other with a series of holes that would align and bolts placed through them. This can be as simple as two smaller width L steel sliding by each other.

a) Should I keep the four outside corner points at their current location and add another one in the center. Three points on each side.
b) Still keep the points at the outside corners and add another two in between for a total of four contact points on each side.
c) Move them next to the riser thread assembly (approximatly 20" from ends) and still add one in between.

I personally prefer choice 'a' because this would spread out better the contact points to the x-beam side base and not concentrate it so much near the center. However go with 'b' for more bolted points.

Also, by incorporating choice a or b, would the horizontal diagnals at the bottom of fixed based be required any more? I believe this is where Gerald is going with his last post.

[KenC]

David,
I'm impressed by your job!

As engineers, it is our obligation to eliminate as many "things" that won't or very difficult to implement from reaching the paper. ;)

You can always try to perfect the design. With enough time, effort & perseverance, they will definitely work eventually.

Its another philosophical thingy...

Normally, these multi-million dollar project can afford the time & money, yours salary & your crews'... NOT when build for own use especially to generate income, i.e. put on the business man cap.

I always keep the two very distinctive from each other.

I meant to say look into the scaffolding joints for inspiration, but if really tight with time, just use them untill a better solution arises.

[Alan_c]

Moving the four jacks independantly will be frought with problems and lifting it with hydraulics could be just as troublesome, the four jacks driven by one chain and motor WILL work - have a close look at any large wide belt sander - very solid table moved vertically with four jack screws driven by one motor and I have seen some very big ones move effortlesly (see Heeseman machines). All this proposed system needs is some small refinements to the locking/consolidating mechanism.

[KenC]

Philosophy again...

My idea for the lift is to move the bed to a close proximity of the latching position. Then latch & lock at precise location, using tapered shaft/ dovel pin or what ever that fits the purpose to mate the locating holes on the bed & the frame then secure them with bolting through or other methods.

By doing so, we eliminate headache of having a "precision" height adjustment. Only require precision height locking which is a lot easier & cost effective.

But really, if the height need to be adjust after every single job... You then need to surface the spoil board after every shift.... Unless you make sure the positioning holes are accurate within o.1mm i.e. +/-0.05mm.

Why not buy a knee mill or something else convert it? All will be precision. Milling machine can be a lot larger then 10' x 40', with Z-stroke of over 6' and used one can be found with a little digging.
Just my opinion, one has their own.

[Gerald D]

If I look at the sketch plans in this thread, the moving table has become heavier than the static x-rails. And that leads to the question; Why not make the table static and do the height adjustment on the x-rails?

[KenC]

AAAAAHHHHHH!
That a good approach too.
Or just make the gantry wheels move up/down?

[bradm]

Dave, I assume that once you've finished the fun part of design brainstorming, you're going to step back, sort out what design you're using for an extended Z if necessary, and do at least an armchair evaluation of what the forces will look like in the more challenging cutting positions. Then check this carefully against the types and shapes of materials you plan to cut, and see if your overall design will really work better than the original.

Brainstorming is great, and I encourage you to continue, but to my eye, you're already flirting past the boundary of K.I.S.S. if the actual requirements are just a table with an 8-10" cutting range that can be adjusted for materials from 0" to 20" or so, and not use more than it's own envelope for storage inside the shop.

[Temuba]

All I can say is WOW, the gears are smoking' now:).

Ken- One main thing I forgot to mention regarding my field of work and working on high-end multi million dollar homes or not high-end at all, the homes were not new, but remodels. Remodeling a home is a different animal. I wish they were new, you would have a clean slate to work from when ever the architect, engineer and/or designer adds, changes anything. But when you have an architect and engineer want to put an elaborate steel skeleton inside an 80 year old home and you're not allowed to knock it down because it's 'grandfathered' into the site, that's were I excelled the most. Figuring out how to put a square peg in a round hole. This has always been my mode of seeing things, always trying to figure out a good solution to a difficult or near impossible idea. When I walk into someones home, whether new or falling apart, I don't see paint, trim, furniture, etc., I see framing, wiring, piping, structure, etc. Strange, but that's the way I see things.

This is how I see this part of the project, as a challenge to an idea and criteria. The criteria? a)Trying to eliminate as much play in the extended z axis as possible for smaller more precise projects, b) Create a deep cavity for use of an indexer and c)no space in the shop to allow for other components to help alleviate a and b above.

The main reason for this particular design because I have seen it work. Like Alan pointed out, this concept is used many times in woodworking and other stationary tools. My own Dewalt Thickness Planer uses this exact engineering, much compact machine, but same principle. I agree that a hydraulic system would be too complicated for the infrequent use of the table, however a chain and sprocket system is very simple and effective, either by means of an electric motor or simple hand crank.

The discussion here is to see how to take a conceptual idea and take it through a working machine. From the beginning of this topic, the original sketch has progressed and evolved thanks to the many innovative members of this forum.

As far as Gerald latest opinion to keep the table static and move the gantry to the work, I'm not so sure. This would require more precise engineering to make the gantry move to a precise and exact location. At that point you may have to introduce linear guides and ball screw with anti-back lash nuts. Keeping the base and gantry static would allow for a better job if the gantry were already level and tuned to the base. Its as simple as a 20 ton shop press: the jack, gantry, legs and base are stationary and static, but the working table moves up and down and held at any location with pins. In this case the pins are replaced with bolts and require better precise hole locations.

This still leaves us with the original idea, move the table a general distance from the gantry. Still locking/bolting the table to the static base (which so far has been the most effective and simple solution for this). The thought of the spoil board being perfectly level at any position has crossed my mind and would require precise hole locations to achieve this. I believe the additional material under the table is to keep it as rigid and stable as possible and then transfer this rigidity to the base once it is locked in place.

I believe that some minor adjustments, leveling, shimming may be required every time the table is moved. But it would be infrequent and the time required to do this is a trade off I have to live with for both point a and b above. If in the future this becomes a problem due to productivity, then and only then I will have to make a decision: move from my current home to a place I can build a bigger shop and at that point build one or two more CNC machines to do a specific role.

As always thanks for the opinions, thoughts and ideas from Gerald, Alan, Ken and all you guys. It's an enjoyable topic going on here and look forward to more.-David

[JLFIN]

like the scissor lift concept,
as i'm pondering i thought of your bolting system and it came to me that in order for the table to bolt up tight it has to be a perfect fit all the way around or its going to pull on your x rails and the rigidity of your adj. table looks like it would win, so my question is would it be better to have your table come up and run into stop/bolting blocks same with going down however in the down position add adj(slotted) angle iron over to your side rails, that way they won't pull and you will be tight, drawing a concept and welding it up are two completely different animals (which great drawings by the way ) I think if your tie in were up and down it would give alot more tolerance on the table
side note Harbour Freight sells those scissor tables fairly cheap
I wrote this before I updated all the posts HOPE it still pertains i need to go back and read to catch up!!!! good luck

[Temuba]

Jim- Thanks for the comments. Yes below shows a pin setup using 3/4" bolts w/nuts onto holes vertically 2" OC. As you mentioned about the guide blocks having a perfect fit all the way around would probably be next to impossible to achieve during welding.

So I made the guide plates to have adjustment or play in the y-axis as the table is being raised or lowered. Once its to height the top two bolts are tighten and then the 3/4" bolt assembly is inserted through the side of the guide plate and into the guide rail. At this point after tightening, there will be eight points tightening the table to the base.

I have been giving some consideration into the scissor lift table idea, it's sounds like a cleaner and simpler way of vertical adjustment. I'll have to do some more homework on that. As mentioned before the chain and sprocket idea has not been finalized, just trying to work the structure and mechanics of the table first.

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT18.jpg
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT19.jpg
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT20.jpg

[Temuba]

Dave, I assume that once you've finished the fun part of design brainstorming, you're going to step back, sort out what design you're using for an extended Z if necessary, and do at least an armchair evaluation of what the forces will look like in the more challenging cutting positions. Then check this carefully against the types and shapes of materials you plan to cut, and see if your overall design will really work better than the original.

Brainstorming is great, and I encourage you to continue, but to my eye, you're already flirting past the boundary of K.I.S.S. if the actual requirements are just a table with an 8-10" cutting range that can be adjusted for materials from 0" to 20" or so, and not use more than it's own envelope for storage inside the shop.

Brad- the cutting range will be 22" not 8-10". As mentioned before this is the reason for the adjustable table and also to create a deep cavity for the indexer.

And yes, after all the brainstorming, I will step back and see what works and what needs to be changed. I pass the KISS boundary the day I decided I prefer a longer z-axis and an indexer. There's no turning back now.

I'm trying to build a machine that will allow me to perform a variety of projects. Before someone post a response to this comment, I know there's a price to pay for it in the design. This is something I want to do and need to do.

Thanks-David

[KenC]

Not disputing your choice but this is too complex to achieve a simple goal.

Why not just make the gantry bracket a feet taller & make the Z-slide long enough to drill for oil. that should meet your needs.

If you are worried about precision, don't. Anyway you already knew that a multi-purpose machine is a compromised machine....

BTW, The way you construct your location holes will be impossible to be near to achieve "precision". Why? the lef itself is not precise, the hole drilling also won't be precise & you can't find a reliable datum to work around...
You should have a precision machined plate which you will bolt on after the table is constructed.

[TheDave]

Have you seen Art's thread for his version of the MechMate?

http://www.mechmate.com/forums/showthread.php?t=174

It sounds to me like you want something similar to what he built.

[KenC]

Not exactly, but he focus on achieving his need & he wasn't greedy for a multi purpose machine. He did it real simple & I like his modification a lot.

My brain storming question for you is.
Must you move your spoil board around to achieve your 22" cavity?

[cab. guy]

Hi Dave,
Interesting design,using four screw jacks to lift the table with an adaptable impact wrench,would be easy but you could end up racking the table as well.Turning the
screws by ratchet,well that would be a long day.The scissor jack in the center of the table seems beneficial.The real challenge of your design,isn't so much the movement
of the table.The real technical challenge is how to move the table without disturbance of the rail alignment.If your design takes those rails out of parallel alignment as little as .004 ,the result is technically ,your vee rollers are trying to climb out of the track.
Heres on option that might work for you,using linear guides or even a crude guide system,having mechanical stops at both destinations,make the table a 64th of an inch undersized all the way around.The intended gap is for shim stock and through bolting at corners only. On the top position use the gantry as the tram (distance
between guides) shim and through bolt.Why the bolts? To help solidify the moving table to the to the outer frame.Dave, at faster speeds your operating no less than a jack hammer;).You can get those linear rails on eBay at 20-25 cents on the dollar.

[cab. guy]

Dave,
on the scissor jack,I have one on my late model GMC truck.It has plenty of stroke,payload not much issue. It also has 90 degree strait up throw(neutral).It can be used with an impact.At a junk yard,like brand new ---pocket change.How to implement?Rid yourself of jack screws,mount 4 linear guides,one per corner 20mm
no larger.One shoe per corner. Build the bed,find the exact balance point before
frame installation.This is where you mount the jack.You might be wondering if you
could get bound up in one of the corners?Only if you leave the table substantially out of balance.Its simple,cheap and should work very well.

[Temuba]

Not disputing your choice but this is too complex to achieve a simple goal.

Why not just make the gantry bracket a feet taller & make the Z-slide long enough to drill for oil. that should meet your needs.

If you are worried about precision, don't. Anyway you already knew that a multi-purpose machine is a compromised machine....

BTW, The way you construct your location holes will be impossible to be near to achieve "precision". Why? the lef itself is not precise, the hole drilling also won't be precise & you can't find a reliable datum to work around...
You should have a precision machined plate which you will bolt on after the table is constructed.

Ken- Gas around here just jumped 15 cents per gallon in the last week alone. You have a point, if I hit oil under my basement I can then take the money and buy several cnc'c already built to my specs. I'm still trying to source a well drilling bit to fit inside my 1/2" collect for my router.:rolleyes:

The taller gantry will not work also, I already covered that limitation due to the 7'4" ceiling height in my SMALL, did I mention SMALL, shop. As drawn with the 22" z the setup is at almost 7' from floor to top of up most z travel.

Many have here on this forum made multipurpose machines with great success, either with a tall z and/or indexer while still having the original machine some where in the build. The major compromise to this idea would be the setup every time the table is adjusted. The manufacturing and assembly of this idea is, to me at least, a challenge and relaxing part of it.

On a better note, I do agree with the precision drilling for those plates. I may have to at least get those sourced out at a local machine shop. I don't think that any amount of measuring I do, I'll be able to get the spacing tolerance to an acceptable level.-David

[Temuba]

Ron- I'm beginning to see the light here. Like stated before just ideas thrown out there for others like yourself to help with a better solution. I went this morning to my local steel supplier/surplus dealer and I saw this lift table below for $260, normally sells for $400 online. It's new, never used, and has a 600 lb capacity. I do agree this setup will make construction a lot quicker and the travel more efficient.

With the last two revisions I made, I always had the thought of using a type of linear guide or rail. However I couldn't find a cheap source for this idea, that's why I first had the clamping block riding behind an angle steel as a guide rail and the last idea was that the guide blocks would just ride loosely in front of the guide rail and then get bolted together.

If you can point me in the direction to examples of those linear rails you mention, I would greatly appreciated. Again thanks-David

http://i585.photobucket.com/albums/ss292/temuba/CNC/0403001151.jpg
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http://i585.photobucket.com/albums/ss292/temuba/CNC/0403001151b.jpg

[JLFIN]

Dave,
I like very much where your heading, great visual on drawings.
I would love to see a dual bolting in all positions on the vertical legs even if they are only 4 or 5 inches apart (i don't know how but you seem to be great at taking theory and putting it on paper) keep going even if you get more opinions ( like mine ) then you ask for
Good job
harbor freight has 1500lb atv lift for 80 dollars
500 lb table lift 139.00
1000 lb for 199.00

[Temuba]

Jim-thanks for the input and comment. There's a Harbor Freight right here in town, I'll have to stop by there a see what they have. I did a rough weight calculation of the table with 1-1/4" MDF and it comes to about 475#. The 500 lb would do, but the 1000 lb might be safer. I don't think that items at Harbor Freight hold up to their specified capacity, so the 1000 lb unit may have a safer tolerance.

Yeah, I can just add more length to the vertical side of the guide block in order to accommodate another bolt at 4"OC. The guide rail still has plenty of room at the bottom to allow for two more 3/4"D holes for this concept. Though, that's a lot of bolting, 16 total now, but like I stated before the table would be moved very little.-David

[cab. guy]

Dave,
Go to ebay site,search( thk linear) look for shs or hsr style rails and blocks. 25mm
to large 15mm to small 20mm just right according to the three little bears.The lift you have pictured ,quite nice.How much manual control at top would you have--hole alignment?Pistons can hang up in the seal,making it difficult to precisely control elevations.If the jack pressure isn't controlled at the top of the lift,you may risk damage to the table. Between pressure control and limit switch issues they would need to be sorted out.Personally I would consider it doable. Also make sure that its
travel is straight up and down.

[Temuba]

Ron- Jim gave me a good lead for a 1000lb capacity table lift at Harbor Freight for $199. There's one in town, so I can wait for a possible sale to go on to pick it up at that time.

I realize that the one I had pictured with the air hydraulics could pose a problem if it traveled too far. It would definitely damage the machine and getting into limit switches will be complicating things further. I had the same scenario when my first design was to use a motor to drive the sprockets. I saw the potential of the motor going to far without a sought of electronic control to stop it, that's why I later opted for a manual hand wheel.

I'll look into ebay with your suggestions. Starting to see a much simpler solution to this. Thanks-David
http://i585.photobucket.com/albums/ss292/temuba/CNC/Lifttable-HarborFreight.jpg

[JLFIN]

i think if you raise slightly past holes on this design you can pick holes off on the way back down ( one at a time )
and Dave I agree with double size as per harbor freight specs I just didn't know what the beastly table would weigh in at

[KenC]

The taller gantry will not work also, I already covered that limitation due to the 7'4" ceiling height in my SMALL, did I mention SMALL, shop. As drawn with the 22" z the setup is at almost 7' from floor to top of up most z travel.

David

You looked too far outside of the box ;)

Look closer at 1020456 & 1020251.
Extend 1020456 by 1' while keeping the mounting hole location relative to the rail as original.
Make 1020251 larger but keep the motor mounting holes relative to the V-wheels same as original
If you think the overall height of the machine is too much, you can increase the gantry height by 1' ft & lower the bed by 1' or which ever ratio that is convinient to you.

BTW, is your shop a container box? the dimension sure do look like one.

This is definately do-able, more economical, meets your needs & still keep the distinct MM identity. Most importantly, fail safe.

Wish I can model the drawings as fast as you could....

[Temuba]

Here's another crack at it based on some of the recommendations of some of you guys. The most effective, simplest and economical design is using a scissor lift table. The one so far chosen is the one on post #61. I didn't draw the scissor lift because I don't have it in front of me for detailed measurements. However it will be somehow attached directly underneath or within the table.

Also now changed to the previous design are four THK SR20 linear rails. These will replace the thread assembly to better guide and keep the table in alignment with the static base.

The details are not final, they will greatly depend on once I start purchasing parts and steel. At that point I will have to possibly reverse-engineer certain components and details to suite the parts at hand. At least the drawings will give me an idea of what I need to buy and how to go about the build.
Thanks-David

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT21.jpg
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT22.jpg
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT23.jpg

[cab. guy]

Dave,
Go back to your 1st design,look at the difference in weight on the moving table.The last design is not only overly designed, but puts all that weight in the worst possible place on the top---top heavy.Wheres the foundation to keep those rails properly
adjusted at both elevations?Think about keeping 60 -65% in the frame were it belongs.Consider a sub box frame that would cradle a table landed.A strong sub box frame is key to your design concept.The table itself can be rather simple,using rectangular tubing.Choosing the right size steel and wall thickness,It can be a flat
rectangle weldment. As for as center deflection goes,if its not to your own satisfaction add a couple of 5/8 tie rods along the beams for adjustments.
On a side note the sr are designed for vertical travel.The shs,hsr at the same price,
handle multi-directional loads.Excellent design work.

[Temuba]

Ron- I'm trying to absorb your opinions here, so please bear with me, thnaks. Are you referring to the first sketch in post#1?

As far as, "Wheres the foundation to keep those rails properly adjusted at both elevations?". Do you mean how they tie in to the side base? If so, the linear rail blocks are attached via a bracket to the horizontal L3x3x.5, which is then attached/connected to the side base. All these points are stationary while the table and linear guide rails move up and down.

The weight issue of the table you mention: Basically reduce the steel/weight of the c-channel sub-assembly to something similar to the 1" steel tube in the truss system below it?

I didn't mention it before but the reason for the c-channel is so I can have access to screwing the MDF from below. That way I wouldn't have any exposed screw head at the top of the MF board as I plane the board down over continuos use. Insted of the C3 c-channel, I can get a smaller profile like a C2. Basically 2" tall by 1" wide c-channel. You have a good point since the larger C3 will not improve the rigidity of the table any more than the C2. This change to the steel may reduce the weight at that point about 30-40%.

As far as the vertical C3 you see in the last design: First I need that size for the bolt plate guides to accept the bolts, washers and nuts. However I can reduce the ones that the linear guide rails are on to the smaller C2 c-channel size.

"if its not to your own satisfaction add a couple of 5/8 tie rods along the beams for adjustments." Can you please clarify or markup on one of my sketches?

So they SR I spec are good, right?

Thanks for everything Ron, great help.-David

[melissa]

I don't know if anyone has mentioned this in this thread yet... have you had a look at what Greg did for his mechmate #19?

http://www.mechmate.com/forums/showthread.php?p=21439&postcount=34

Michel

[Temuba]

Ron- Let me know if I followed you correctly based on Post #66 I wrote above. Still need some clarification on the last line. -David
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT24.jpg
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT25.jpg
http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT26.jpg

[Temuba]

I don't know if anyone has mentioned this in this thread yet... have you had a look at what Greg did for his mechmate #19?

http://www.mechmate.com/forums/showthread.php?p=21439&postcount=34

Michel

Yes, the idea is very similar to mine with the deep Z axis. However, he only built for a deep axis and not for a possible indexer that would require the table (or cavity) to be deeper. At least what I would like my specs to be.

Thanks, it's difficult for me to read every build done so far, I missed that one. Very interesting.-David

[cab. guy]

Dave,
Greg's pictures are exactly what I'm suggesting.The table itself,A simple flat platform,that goes up and down.The foundation that I'm referring to is also just like Greg's,note the stiff box welded frame.The box bottom helps to keep the outer posts
plumb and unified.Yes, this in principal is very similar to what I was suggesting.
I didn't know that this design existed,but cheers to Greg.I don't know that Greg ever
actually changed elevations (stage 2).Imagine in principle this same design with,
A.much larger rec steel for the table.
B.a centered mount scissor jack
c.adjustable linear guides.
D.mechanical stops top and bottom.
E. Incorporating a 4th axis that has yet to be discussed.

From you it would be helpful to know on a 100 x53 cut table, what is the distance between guides?How long are the guides?Is the distance between guides based on a prefab gantry or is this adjustable?
Tie rods if needed ,run length wise to the table platform.They can remove deflection in the table (sag).
If you contact Art in Texas,He should be able to give you valuable advise on that tall gantry.

[cab. guy]

Sr s should be fine .The rails can be cut .

[Temuba]

Ron- After carefully studying Greg's build, I understand your comments now. I've made some major changes from the previous sketch loosely based on the original one posted in the beginning.

1-You will notice the additional two cross members (1.5"x3" tube) in the center that tie both side bases together.
2-Linear guide rails and bolt guide rails are now attached to the side base, and the linear truck/block and bolt block are now attached to the table.
3-The table was changed back to the original C4 c-channel.
4-The hydraulic scissor table has been added with two C4 c-channels running alone the y-axis welded to the two longer x-axis C4 c-channel of the table. The scissor table is represented by the lower box (base unit) and the upper smaller box (moving part of the scissor table top).

As far as Greg's build, I didn't see mechanically how the table would be lifted or adjusted to the next higher level. No holes, steel, nothing. I'm thinking he might just place a large spacer between the cross members and the table to get the correct height and then either bolt or clamp in place. Great concept and design.

Indexer is another story, the empty or non-cutting area toward the rear will be dedicated for the indexer. The other business end will be bolted to the table and slide alone the x-axis depending on the length of the project.

Thanks for the inputs-David

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT27.jpg
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[KenC]

If you could run a finite element analysis on your structure to check for deflection under load.

[Temuba]

If you could run a finite element analysis on your structure to check for deflection under load.

Ken-you're going to have to step into this question above. I have no means of figuring this out. Most of my work/design here has been purely based on mechanical drawings using Sketchup and a lot of know-how. I do have access to Solidwork and understand it will analyse various criterias but I am not fluent in using the program.

If you wish to give advice or guidance, I am always willing to learn something new. If not for just the enjoyment of experiencing it once. Thanks-David.

[KenC]

I'm no guru, giving advice or guidence way over stated., discuss is more appropriate.
I mention the simulation because at the first glance, your latest structural rigidity bothers me & I could be wrong too. Also, just by saying "its not rigid enough" isn't going to help, so some numbers would be unbias.
I'm not familiar with Solidwork, but Acad, will see if I have time to run a simulation.
I too enjoy experiencing everything once ;)

[Temuba]

Ken- I have a friend who has Autocad 2010 in his office. I may be able to borrow a copy for personal use only (like this). Is it the same as yours by Autodesk. I am (or was) very fluent in Autocad. Is it possible to maybe get a few ideas on how to make this happen on my end so I can get an analysis? You now have my curiosity.

[cab. guy]

Dave good job,
Yes , this is very similar to what I was trying to describe.A modified thought was rather than leaving the tops of the vertical rails floating,bolt it on to a adjustable
bracket for fine tuning,at which time,with some confidence it can be pend top and bottom.Diagram #2 shows the table at the top floor,Here's where you can use horse shoe style shim and through bolt table to rails to help solidify the table to frame
in addition to stabilizing X axis rails.I think you will find the jack will weld into the base table just fine.If you decide to forgo the shim idea, then use thk 25shs on the vertical guides.I'm not sure that Greg ever changed elevations.It appeared that he was going to attempt to do so in a manual fashion.With regards to the jack, You might also consider a ball-bearing plate under the table,driven by the jack.
There's a lot of bolts shown here,how about the welding?
If you can get those measurements, I can help with a steel schedule.
Your looking good.

[Temuba]

Ron- In the previous post I omitted certain details off the sketches and description. As a new note I increased the previous C3x4.1 c-channel to C5x6.7 c-channel to have two purposes, see image 3.

1-The THK linear rail guides will be attached to the vertical C5x6.7 c-channel.

2-The THK linear guide blocks/trucks and bolt plate blocks will be attached to the table itself. The bolt plate block will have horizontal movement in the y-axis via two bolts. This will allow fine tuning to fit snug up against the bolt plate guides at the base.

I believe that no matter how careful I am with layout and measurements and decide to weld these, they will never be perfectly parallel to the opposite side, as the table moves up and down. By just loosening the top bolts the bolt plate blocks will have some play horizontally as the table is raised or lowered via the scissor lift and linear guide rails. Once the table position has been determined, two side bolts can be inserted and bolted to the side base and then the top two bolts can then re-tighten to remove the horizontal play from the table.

3-Also notice in the third image, that with the table at the top position the bolt plate blocks are directly below the TS5x2 tube steel (X rail beam). I believe that the table will be in this position the majority of the time. I hope I understood your comment (Diagram #2 shows the table at the top floor,Here's where you can use horse shoe style shim and through bolt table to rails to help solidify the table to frame), and this clarifies that. In your opinion, do you still think that the THK 20mm will work or should it be increased to 25mm?

My only concern is that in this latest sketch, I omitted the diagonals in the side bases. As you mentioned about welds, everything is welded except where you see bolts/bolt plates. I need to be able to disassemble this machine if I build it in my basement and decide later to move.

KenC is seeing into a deflection analysis or maybe guiding me somewhat to do it myself. As far as your comment regarding a steel schedule, do you mean a list of materials for this part? If so, I have that under control and familiar with the various steel sections and weights. If I'm wrong with your comment, please advise.

My decision to remove the diagonals is because the side base will be completely welded and there are plenty of heavy vertical steel members within. However I am not sure if this is somewhat true considering the heavy C5 channels I updated to the side bases. Some of the measurements in the table are not final and I cannot input them until I purchase the scissor table and the THK linear guides. At that point I will have to reverse-engineer some of the details and measurements based on detailed measurements of these two items.

Again thanks to you and the others for the great information and help. This part has not only been a design challenge for me but very enjoyable discussion here.

http://i585.photobucket.com/albums/ss292/temuba/CNC/TABLELIFT32.jpg
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[TheDave]

Your table is looking good! I just had a thought.

You are going to spend a lot of time coping both ends of those c-channels to fit into your table sides (red circle). Why not invert the channel so you can just butt them together?

Please excuse my amazingly crude drawing.:eek:

As I said, it's just a thought. I totally understand if you want to leave it as is for aesthetic reasons.

[KenC]

Its been a long while since I last did any FEA, there are many free software out in the net, have a look at here (http://www.freebyte.com/cad/fea.htm)
I am not recommanding any particular software as there are far more choices then when I need/have to do such analysis.

[cab. guy]

Dave,
Using brackets in lieu of shims will work fine.If you limit the amount of attachments
may work better for you, corners and maybe ctr. posts only,let the rest float.Caution on vertical holes,they should be dead on.Remember each elevation change requires
a resurfacing process that should be minimal skimming.You could drill those holes accurately by drilling them after the fact using the Z axis to define elevation.The Z axis is the reference. The linear
guides are as I pictured also.If you mount them on separate adjustable risers, you will be able to tune them.Their needs are not just parallel but on plane as well.If you raise the table leaving the rails loose the trucks will show you where to tightened
the riser brackets.The explanation is oversimplified but in principle is correct,the alternative to this is less than exciting.I believe that somewhere you commented on
using c-channel on the table ,because it was easy to through bolt to.On my machine
I have +/- 300 drilled and tapped holes,in1/4 -3/8 walled rec tubing.The average
time to tap a hole about 90 seconds with a cordless drill--kind of a non issue.Unlike the rest of the world ,here in the US rectangular tubing is bountiful.Here's where you might want to do some more research.
Dave, the welding is for any part of the machine that does not need adjustment,Think about the jack hammer effect.Provided that you tie the moving
table to frame base 20mm is no problem.In other words don't rely only on the linear
rails as a form of attachment.

Anything taken away from the steel will retard performance.

[KenC]

This build is going to be very complex & demanding with no promises. There are too many alignment for a structure IMHO...
1) hole location,
2) linear slide line & plane alignment. & there are so many of them... we are looking at 0.05" to overload the slide...
worst of all, I can not find a reliable datumn to work from...

My original suggestion was for a scissor lift as prime mover, some rod in pipe guide as very primative guide to ensure the bed doesn't run freely & using locating pins to locate hole accurate enough & secure with bolt through to secure & strengthten.

For the locating holes, you don't want/need to drill in situ, drill a set of precision holes on a separate plate, then mount them on while you are alignming the bed.

IMHO, it will be near impossible to achieve your kind of objectives with fabricated steel structure w/o precision jigs which will cost as much as your whole steel consignment. & very difficult to carryout in your 7' headroom workshop.

The more I look at it, I am more inclined toward JLFin's suggestion of a removable sub base on a fixed bed. Since you have space limitation, why not a small fold away sub base which you can stow away under the bed.? eg, a few boxes at for handling by one person build to precise height. Looks really crued but guarantee to work.

I'm considering this with the overll picture that you are building this for your business use, hence, money & time are important.

[Red_boards]

I don't know much, but from experience of my many "shortcuts" over the years it's pretty difficult to lift something horizontally and then line up bolt holes. I've found that lifting at an incline and locating holes at one end, then doing the same at the other and locating the holes, the "filling in" between can be easier.

[chunkychips]

Not sure if you all came up with the ultimate solution already but I have a suggestion that is relatively low tech which might work. This may be what Ken was talking about above but anyway here goes...

How about creating one or more torsion box platforms out of light mdf. These would be the same dimension as your base support board and could be bolted directly to it to provide a raised platform. You could store them under the table when you want to work on thicker stuff.

There are plenty of places online showing how to make a torsion box and if you use the MM to make the ribs it would be very accurate. If you then surface it it should remain flat and true (in theory).

You could make thinner ones and stack them to give you some flexible height options.

Of course I could be way off the mark :)

Here is a sketch of what I mean (in xray view). I have left the ribs exposed around the perimeter to facilitate bolting and as a bonus you can use the lip to clamp work or a spoiler board...

[smreish]

...in the Theatrical world we call those stress skin platforms - and they work very well.
A good affordable solution for adjustable table height.

Sean