1010302S Cross Support Beams under table top
 

Gerald,

I have some surplus Galv. steel left over from some idustrial projects in the yard and I wonder if some of it would make sense to use until I get to the: main beams, XY-gantry, and YZ-car.

For example I have 7 ea. of 4x4- L -1/2" thick 78" long and I wonder if they could be used for cross supports or would they be to heavy for the main beams to support.

I have only briefly looked at some of your drawings as I am just starting look deeper into buiding another machine. When I see steel lying around that will likely end up hauled as scrap it makes me think of a friend who like to say "free goes along ways."

I admire your work on this forum and forward to learning more.

Thanks, Robert

Hi Robert. The idea of L-section as a cross-support puts me off a little. C-section is just so much better. My book on "sections" is at the office and I'll have a look at the numbers there tomorrow. The weight doesn't worry me because it is static. When the main x-beams sag a little under that weight, the x-rails are shimmed to get them straight.

Hi again Robert. Had a look at the data tables and seen that those angles would be great as cross-supports. Have fun drilling them! http://www.mechmate.com/Forum/clipart/happy.gif

Note added July 15, 2009:

Robert's L-sections are particularly "heavy". Here are the minimum L-sections that will be about as stiff as the C-section in the drawings:
80x10 [ L3x3x3/8 ]
90x6 [ L3.5x3.5x1/4 ]

Further note July 23rd, 2009

Minimum square tube is:
80 x 80 x 7 mm
90 x 90 x 4.5 mm

Minimum rectangular tube is:
100 x 50 x 6 mm

Gerald,

Thanks for your response and I agree in the first person drilling will not be the fun part of possibly using them. I am going to take stock of what is lying around before it all gets hauled off. And just because it is free does not mean it has to be used at all costs. The fun part is finding workable pieces... then the work begins $500 worth a drill bits for $200 work of steel.

Gerald,
I have in my mind a system that has a work area of bout 2500 x 500 x 200 (xyz) for joinery and small solid wood part work - 99% of my work would fit into 1000 of the x of such a table. How well would your system adapt to such a size, and what could the minimum footprint be (seriously teeny workshop).

Just fishing at present, regards, r./

Ralph, the motor/gantry arrangement of a ShopBot or MechMate must make them of the worst systems for small footprints. Inboard motors would help a lot. A single-beam gantry would also help.

However, I don't see a serious obstacle in adapting the MechMate down to that size. A lot of the x-rails would probably be covered by a board to serve as a general purpose worktop most of the time. Therefore, you might build the whole thing lower so that this worktop is at a convenient height.

I have a design brewing in the background for a wall-mounted system..... The x-rails fixed to the wall (one high, one low). The "car" running on them to have an arm (ala radial arm saw) extending horizontally over a standard workbench, with the y-rails. Then a y-car and z-slide running on this y-arm...

(:
Will ponder a while...

Hi Gerald

There was a school teacher in the UK who built a wall mounted router as you described, the guy posted a build history on cnczone.com.

CamTech makes a router that generally fits the description... http://www.camtech.ca/products/route...ceOverview.cfm

Is that what you were thinking?

I've always liked this idea due to the improved space utilization of this design but it seems to me that the idea is reliant on having a vacuum table. Or maybe a t-slot table at a minimum. I've also wondered about how beefy of a Y motor you'd need for such a design either that or a hefty gas spring or counter-weight.

See this thread

Lee, we are talking of something quite different....

Imagine a standard workbench against a wall in a small workshop....and then a CNC machine that is attached to the wall above the bench, cutting a workpiece that is lying horizontally on the workbench. Naturally this is for smaller stuff, but these are mostly the type of guys that need "spacesavers". One-man sign companies, for example. Or guys using high-end natural timber planks. The really big machines are for the MDF brigade. http://www.mechmate.com/Forum/clipart/happy.gif

(Attaching to the wall implies a solid brick_&_mortar wall, which is common around here.)

Gerald.. I'm thinking of making my own crossmember bearers. I'm going to cut them in 5mm mild steel, and then bend them 2 places to make a C-channel like piece. it will be 40mm high and 80mm wide, the main board is going to lay on the 80mm side of the beam. is this possible.??? If you want a picture pls. let me know...

I have a difficulty to understand your description (main board?) - a rough picture will be appreciated.

Gerald, offcourse I meant the Support board... Sorry, my mistake....

Hi Kim, do mean with the 40mm legs pointing down and the 80mm horizontal? If so, that will unfortunately be quite weak.

Rotate that channel to the vertical and then it is about 5 times stronger to carry a vertical load. 80x40x5 Channel will be okay when vertical.

I'm not sure why this would be weak. I can't bend them, twist them or anything, If I stand on them they don't even bend. But now they have been bent, and pre drilled and such, so I'm going to use this setup for now, then I'm going to see how weak it really would be. And make changes later if needed.

Gerald,

my local steel shop have 5 Kg/m (76mm X 38mm) channel (instead of 7 Kg/m); would that be stiff enough for my cross bearer. I intended to have 7 of them instead of 8 as per MechMate drawing... fit better on my already built frame... upgrading here...

Hi Paco, what cross-bearers do you have now? I can tell you the difference in stiffness between your existing cross-bearers and the 5kg/m channel. That should give you a feeling of how much you are "upgrading"

Right now my frame has 2" X 2" X 3/16" angles over 64"; that's 51mm X 51mm X 4.76mm over 1626mm.

We already discuss this and you suggested me to go with channels and I look on the MechMate drawings and that's where I picked up the 7Kg/m (76mm X 38mm channel). I could do the upgrade this weekend with 5Kg/m 76mm X 38mm channel instead but I wonder if I would be as stiff as what the MechMate drawings suggest which I use as a reference.http://www.mechmate.com/Forum/clipart/proud.gif

Right now I have 7 of those angles as cross bearer so I would upgrade with the same number of 76mm X 38mm @ 5Kg/m if that's stiff enough.

I have problem with the vacuum that cup my machine bed up to 0.03" in the center over hours and mess up my pockets and sign carvings; that's why I need an upgrade.

Paco, I do not have my reference tables here at home, but 5kg/m is probably about 85% the strength of 7kg/m and about 300% the strength of the 2x2x3/16". (I am guessing from my (big) stomach). Let us just say it is a HUGE improvement over the angle iron.

I end up ordering 6.12Kg/m for a fast delivery (no 7Kg/m in stock)... I can't wait to have this part of my life resolved.

Thanks Gerald!

This morning at the office I see my tables only list one weight of 76x38 channel, 6.72kg/m

If I compare that channel to 50x50x4 angle (my handy table is metric only), the channel is 825% "stiffer". Compared to 50x50x5 it is 675% stiffer.

Even if we compare that 6.72kg/m 76x38 channel to the same mass of angle iron 70x70x6, then that channel is still 200% stiffer. The point is that channels are inherently a much stronger beam than an angle iron because of their shape.

When I talk of "stiffer" I am comparing the amount of deflection caused by a vertical load when the beam is horizontal and supported at the two ends. It is not true that channels are always better than angles, because the force directions and supports can be different.

Thanks a bunch for the details! It should help keep my bed straight even under this deformation force from the vacuum... I hope... it's a must...

We had a 40mm thick MDF bed on our old ShopBot (96x48"), built up over the years of glueing on and skimming off "spoilboards" all of MDF, and we removed it after about 5 years because it had too many holes through it. Well, when we loosened the screws, that board curled up by about 30mm! Imagine if you try to pull a 40mm thick MDF, curled by 30mm, flat onto some 50x50x4 angle irons......you can be sure that angle iron will bend by about 10 to 15mm as it fights with the heavy MDF. We want to be sure that the cross-bearer always wins the fight with the table.

My problem happen with the vacuum usage; say I'm carving a sign (no matter what material) with intricate pockets (raising letters and logos from the background), as the vacuum hold flat the material, the bed start to cup upward. This happen over hours; I generally don't have problem with 15 minutes milling but when I pocket or 3D raster carve where the tool machine from one section to another the get back later over long period of time, I can see that the material slightly raised.

It doesn't help me look like a PRO in front of customers...

Gerald, engineering question
 

Hi Gerald,

Could you tell if a 2"X3" rectangular tube with 1/4" wall thickness is as strong/stiff as a 1 1/2"x3" channel? I know that channels have different weights (lb/ft), is there one that has equal strength/stiffness as the tubing.

Paul

I don't have tables to hand for the inch sizes, but I can safely say that tube will be much stiffer than the channel. I don't think that you will get a channel as thick/heavy to compare with the 1/4" thick tube. Snag with the tube is to bolt through it without collapsing the sides, but 1/4" thick sides should be okay for spoilboard bolts.

Hi Gerald

Thanks for the reply. I was concentrating, too much, on the attachment of the cross members to the main beams that I had overlooked the attachment of the support board. You're right, other than longer bolts being required, there would not be enough torque to crush the 1/4" tube walls before pulling the carriage bolts completely through the support board material.
That brings up another question. Is MDF board,more commonly, used for the support board or is another material (eg plywood) a better choice?

Paul

Paul,

I seem to recall an earlier post from Gerald where he said either one will work. For what it's worth, I'm currently using mdf.

Plywood might be a slightly better choice, but we havn't tried it because here it isn't available in big sizes. The reason I say this is because we did have some problems with the bolt heads tearing out the bottom of the MDF. Decent plywood might be better for this. The typical plywood we have here would be a disaster because it splits with the slightest provocation. The drawing mentions pouring epoxy over the bolt head to soak into the MDF and reinforce the area - this has cured our problem.

Gerald,
Have you ever tried thin Cyanoacrylate adhesive for strengthening the board holes? It is very thin and would soak right into the pores of the mdf. Lots of people use it when they want to thread a hole in mdf to strengthen the threads. Just a thought.:)