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  #1  
Old Tue 30 March 2010, 16:13
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Table: Adjustable/Lift Design

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




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  #2  
Old Tue 30 March 2010, 18:10
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
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.
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  #3  
Old Tue 30 March 2010, 18:30
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
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
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  #4  
Old Tue 30 March 2010, 19:46
Regnar
Just call me: Russell #69
 
Mobile, Alabama
United States of America
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.
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  #5  
Old Tue 30 March 2010, 19:54
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Here's a better image showing the base only without the adjustable table within.
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  #6  
Old Tue 30 March 2010, 20:18
Regnar
Just call me: Russell #69
 
Mobile, Alabama
United States of America
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.
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  #7  
Old Tue 30 March 2010, 20:20
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
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

Last edited by Temuba; Tue 30 March 2010 at 20:34.. Reason: More details added
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  #8  
Old Tue 30 March 2010, 20:54
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
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.

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.

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  #9  
Old Tue 30 March 2010, 23:04
JLFIN
Just call me: Jim
 
glenwood iowa
United States of America
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

Last edited by JLFIN; Tue 30 March 2010 at 23:22..
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  #10  
Old Wed 31 March 2010, 09:53
Alan_c
Just call me: Alan (#11)
 
Cape Town (Western Cape)
South Africa
Send a message via Skype™ to 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.
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  #11  
Old Wed 31 March 2010, 10:18
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
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
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  #12  
Old Wed 31 March 2010, 20:13
KenC
Just call me: Ken
 
Klang
Malaysia
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.
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  #13  
Old Wed 31 March 2010, 22:22
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
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.
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  #14  
Old Wed 31 March 2010, 23:13
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Table: Adjustable/Lift Design-Version 2

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.






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  #15  
Old Wed 31 March 2010, 23:18
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Quote:
Originally Posted by JLFIN View Post
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.
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  #16  
Old Thu 01 April 2010, 01:24
Alan_c
Just call me: Alan (#11)
 
Cape Town (Western Cape)
South Africa
Send a message via Skype™ to 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.

TABLELIFT10.jpg
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  #17  
Old Thu 01 April 2010, 01:43
KenC
Just call me: Ken
 
Klang
Malaysia
TABLELIFT.jpg
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  #18  
Old Thu 01 April 2010, 08:56
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
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.

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"
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  #19  
Old Thu 01 April 2010, 10:22
bradm
Just call me: Brad #10
 
Somerville(MA)
United States of America
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.
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  #20  
Old Thu 01 April 2010, 10:51
Gerald D
Just call me: Gerald (retired)
 
Cape Town
South Africa
Quote:
Originally Posted by Temuba View Post
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.
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  #21  
Old Thu 01 April 2010, 12:03
JLFIN
Just call me: Jim
 
glenwood iowa
United States of America
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.
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  #22  
Old Thu 01 April 2010, 12:16
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Quote:
Originally Posted by JLFIN View Post
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.
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  #23  
Old Thu 01 April 2010, 17:12
JLFIN
Just call me: Jim
 
glenwood iowa
United States of America
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
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  #24  
Old Thu 01 April 2010, 20:30
Tokamak
Just call me: John #121
 
Monrovia (ca)
United States of America
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.
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  #25  
Old Thu 01 April 2010, 20:40
Regnar
Just call me: Russell #69
 
Mobile, Alabama
United States of America
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.
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  #26  
Old Thu 01 April 2010, 20:57
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
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
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  #27  
Old Thu 01 April 2010, 21:49
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Table: Adjustable/Lift Design-Version 3

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





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  #28  
Old Thu 01 April 2010, 22:13
domino11
Just call me: Heath
 
Cornwall, Ontario
Canada
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.
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  #29  
Old Thu 01 April 2010, 22:15
Temuba
Just call me: Dave
 
Vineland,NJ
United States of America
Quote:
Originally Posted by JLFIN View Post
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.
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  #30  
Old Thu 01 April 2010, 22:24
KenC
Just call me: Ken
 
Klang
Malaysia
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.

Your lock & latch don't kook good to my eye. Why don't you look into scaffolding joints for inspiration?
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