Keeping the cutting bed from bowing?

[matt888]

OK, it's been a slow build due to so many other projects, but progress has been made. I have the support and cutting bed built, but am not satisfied with the bow in the middle due to the weight and length. It's an extended length JGRO at approx 60" long. I put angle iron on each side and the bed is very stiff for cutting, but I don't like the 1/8"-1/4" bow in the middle.

The only solution I can think of is that I'm stuck with this problem due to the design of the JGRO (and the fact that I have a longer cutting bed) and try to work around it later by:

- placing a brace under the middle part of the bed and dividing the project up into multiple "chunks" so that I can make sure the gantry doesn't run into the brace.

I'm really struggling here. I'm contemplating removing the rails and re-designing the gantry to slide on linear rails so that the bed can lay flat. This "suspended" cutting bed seems to really introduce alot of cutting error in the z-axis along the length of the bed.

[joecnc2006]

if the torsion box is stable, then plane the bed when the machine is finished, with the router and a large bit, you can do this prior to installation of the lead screws in the Z and Y, by moving the gantry in those directions back and forth manually, and use the lead screw to adjust the height of the Z-Axis and lock into place.

[matt888]

Originally Posted by joecnc2006 if the torsion box is stable, then plane the bed when the machine is finished, with the router and a large bit, you can do this prior to installation of the lead screws in the Z and Y, by moving the gantry in those directions back and forth manually, and use the lead screw to adjust the height of the Z-Axis and lock into place.

Brilliant!!!

So, shouldn't I NOT install the X and Y leadscrews, install the Z leadscrew, lock the Z down at the lowest (middle) portion of the bed, and manually move the router out from the middle of the table?

I've got the M12 Hitachi router that most builders seem to be using. Any suggestions on the bit I should use to do this particular job? I'm thinking that it may have to be done a few times over the life of the machine if the cutting bed gets too beat up from cutting down the road.

Thanks again!

Matt

[joecnc2006]

any large flat bit will work, most larger ones use a 1/2" shank.

Joe

[Glidergider]

Matt,
After you surface the board, get another large piece of MDF and screw it down with deep countersinks on top. This will become your sacrificial surface. You may have to shave this surface too. Later you can discard this surface when it gets abused by overcuts.
Dave

[jhowelb]

Is the bed bowing or are the rails sagging? Milling the bed won't help with sagging rails be cause the downward thrust of the Z axis will lift every thing as you attempt to cut into it.
\
IMHO you need supported rail of some kind.

[matt888]

Not sure what you mean by rails sagging? Are you asking about the bed support rails or the galvanized rails the bearings slide on?

The bed seems fairly firm (I can apply alot of downward force without more bowing).

I probably shouldn't stress too much about it and just finish building the machine. Maybe I can learn the basics on this machine and then use it to build a JoeCNC for a more firm milling platform.

I'm about to cut the aluminum angle for assembling the bearings. What is the best way to cut the notches in the aluminum that you run the mounting bolts through (these bolts attach the aluminum angle to the gantry) and ensure they're flat, straight, and the correct depth?

Matt

[jhowelb]

I have the support and cutting bed built, but am not satisfied with the bow in the middle due to the weight and length. It's an extended length JGRO at approx 60" long. I put angle iron on each side and the bed is very stiff for cutting, but I don't like the 1/8"-1/4" bow in the middle.

Those 60" pipes, unsupported will sag under the weight of the gantry. Guaranteed! Saw it happen on a friends machine and he used 1" Thompson Shaft. When the Z pushes the tool into the work they will then bow upward rather that making the total plunge inti the material. Just a matter of physics, if you don't support those rails you will be disappointed in the accuracy in performance.

[Glidergider]

Matt
I put a dial indicator on my machine just 4 days ago. I was surfacing the spoil board and was interested in how much I was going to take off. So while I was zeroing the dial indicator, I noticed that it was jumping +/- .005 inches. Then I realized that I was resting my other hand on either the table or the X rail. Just the weight of my hand and forearm would change the dial indicator .005 inches.

As bad as that is, I've learned that most of my wood working is not affected by that little sag. Lots of fun can be had with the JGRO and it's saggy rails.

If you want a stiffer structure, you got to build either a Joe2006 or a LionClaw.

I like that you lengthened your JGRO table. The limited x-y size is the biggest draw back for me. I wished I had lengthened my table.

dave

[Glidergider]

Just to show you how much fun I have with my JGRO, here's a link to photos from last night's cutting effort. A Pine Wood Derby chassis for a friend. Solid modeled using Unigraphics and cut using Vectrics Cut3D.
http://flickr.com/photos/davidcousin...7603509563559/

By the way, don't forget to add the Rockler Tee rails for hold down support. You won't regret it. $60 well spent.
http://www.rockler.com/product.cfm?page=5325

[matt888]

Originally Posted by Glidergider Just to show you how much fun I have with my JGRO, here's a link to photos from last night's cutting effort. A Pine Wood Derby chassis for a friend. Solid modeled using Unigraphics and cut using Vectrics Cut3D. http://flickr.com/photos/davidcousin...7603509563559/ By the way, don't forget to add the Rockler Tee rails for hold down support. You won't regret it. $60 well spent. http://www.rockler.com/product.cfm?page=5325

Very cool. I'm looking to add some style to wooden spearguns. They basically start off as a block piece of teak that is 2"x2"x55" long. Not a very hard wood, but not as soft as pine either. Between building the machine and learning how to use solid edge... I've got my hands full!

[Glidergider]

ah-oh, Matt you got problems. You said your bed was 60 inches long, and later you write that you want to cut 55 inch objects. The sad news is that your cutting limits will be as small as 47 inches. I've got a 44 inch bed and I can only cut 31 inches. So, your 60 inch be will yield 13 inches less volume too. Your Spear guns will likely be too long for your CNC. Machining them in two sections is going to be your solution.
There are a couple of threads about how to do that in the DIY forum.