[Pairair]

Hi group!
This is my first post on the forum, been a member for a while. I designed and built a small cnc router last year, appr. 50 x 30 cm table. Most of it is mdf, and it's using steel rods and ball bearings for linear guides.

Now I've purchased some more powerful steppers and started drawing on a bigger machine, something like 150 x 110 cm (60 x 43 inches). I'm thinking about using 80 x 40 x 2 mm rectangular tubing (steel) for the main frame, and some 40 x 40 x 2 mm.

I don't know if you can make anything out of the pics attached, but if you can I'd be happy for any comments on the design and choice of material.

Regards,
Pär

[Chris D]

The images look great and the design is pretty good too.

There are two things I would change instantly....

1) Add an angled brace from the vertical riser of the carriage to the horizontal component. This brace, or gussett, will add a lot of strength to the "L" shape you have now.

2) Forget the end supported rails as you have them. Those things will flex and bend like crazy, even under light to moderate loads. You need to have the rails (rods or tubes), attached to something along their entire length. It makes it much more difficult to align and make, but the benefit of added rigidity is paramount.

Chris

[ViperTX]

Check out the CNC tab (Igis88 mill) at http://www.buildyouridea.com for thoughts and inspiration.

[jeffs555]

I would agree with Chris. It doesn't make much sense to build a good steel frame and then use small guide rods like that. You either need guide rods many times larger than what you show, or else they should be continuously supported as Chris suggested.

[Pairair]

Thanks for your input!
These rails are pretty sturdy, 25 mm (1 inch) hardened steel rod, and I planned to have them supported on the middle. But I agree, best would be to come up with a way to support them all the way. I'll try to do some load testing. I'll have to re-design anyway, work area is too much limited with this kind of ...gantry... is that the word for it?

/Pär

[andy_ck87028]

For a while, I also had a similar design however I concluded that one small design change would create huge advantage.

What I did was to put the X axis stepper on the gantry and the y axis stepper on the head assembly. Then, if I ever need to increase the cutting footprint, all I need to do is to extend the frame and the 'x' for one dimension and widen the gantry and 'y' rail for the other dimension.

My reasoning was based on that fact that all the complexity / expense will have been worked through by completion so why limit the design and restrict the cutting footprint unnecessarily.

I may never need to increase the cutting footprint but if I do, it will be easy and not very costly.

Andy

[jeffs555]

Hardend 1" steel rods may seem sturdy, but they are not very rigid. A 60 inch rod will deflect over 0.03 inches with just a 10 pound load.

Also, I took a second look at your pictures and saw something else that is not good at all. You have no roller bearings on the outside of your X axis guides, or the top and bottom of your Y axis guides. You seem to be counting on the carriages being wedged between the two rails. Because of mechanical leverage, the force trying to spread the two guides will be much higher than the actual cutting forces. You will need at least 12 bearings per axis, not the eight that you show.

[DieGuy]

take a look at the end of this thread, you could adapt the mdf concepts there to a metal design. I may provide a more ridge inexpensive guideway assemble.

http://www.cnczone.com/forums/showthread.php?t=12681