New Machine Build- Desktop CNC Design Ideas

[bmsgaffer86]

Hi all. I have been a lurker on these forums for a while (great information but theres so much i get lost here for hours!), but this will be my first tech post. I have been interested in building a large CNC router, but I figured I would start small with my first machine like you guys tend to recommend. I am building this for a senior project. [shameless plug] If you know anyone looking to get rid of any building supplies I will happy accept them to help me stay under budget. [/shameless plug ]

The plan for this machine is somewhere around a 12" x 12" cutting area, and fairly accurate. I will be looking to build this machine out of whatever cheap (but good) parts I can get a hold of. I have access to some machine shop equipment and a Cincinnati CNC machine at my college. So that will be helping me get stuff cheap.

This is a very preliminary design and I dont have the drive system worked out 100%, but I plan on ball-screws for the drives. I am looking for tips, experiences, and suggestions based on what you see here. I am more of an electrical engineer, but dabble in the mechanical side of things every once in a while. Theres a good chance there are some serious design flaws, and Ive looked at it so much I cant see them So let me know what you think! Thanks!

[edit]
frame = extruded 8020.net aluminum
green arms = milled aluminum
table = tslot table milled from aluminum plate

[bmsgaffer86]

I know it looks alot like many other machines, but that is because i like the ideas you all have and I'm just combining them into my own machine.

Any thoughts at all?

[Riceburner98]

What will be driving the Y axis (green arms)? (2 ball screws, 1 screw...) Are the connected under the table? Might get some "racking" there if not careful. Why the curve in the green arms, and the holes? Looks like that might take away some stability... Looks cool though. Milling T slots in a thick aluminum plate for the bed might cause the aluminum to bow / warp quite a bit as well.. (as it releases internal stress) I think they're typically extruded with T slots to keep that from happening... Taking a "skim" pass over the top after it's built may fix that though. Just a few observations from a complete non-expert. Still haven't scratch-built my own yet...

[bmsgaffer86]

Originally Posted by Riceburner98 What will be driving the Y axis (green arms)? (2 ball screws, 1 screw...) Are the connected under the table? Might get some "racking" there if not careful. Why the curve in the green arms, and the holes? Looks like that might take away some stability... Looks cool though. Milling T slots in a thick aluminum plate for the bed might cause the aluminum to bow / warp quite a bit as well.. (as it releases internal stress) I think they're typically extruded with T slots to keep that from happening... Taking a "skim" pass over the top after it's built may fix that though. Just a few observations from a complete non-expert. Still haven't scratch-built my own yet...

I was hoping to get away with one ball screw on one side, but I wasnt sure what kind of torqing the linear slides would allow or if they would help keep it in check. Thats the reason i put the extruded aluminum AND the slides on the gantry to help prevent that twisting.

So say i did two ball screws, would it be better to try and attempt two motors and hope they dont loose different steps during the cutting, or could i use belts to ensure that the motors dont cause twisting and save some money? Or could i just strengthen the gantry and not need two ball screws even?

Since the arms are made of aluminum, i basically lightened them as much as I could (w/stress analysis) to reduce the weight of the gantry without loosing strength. and then adjusted the shape to put the center of gravity right dead center of the linear bearing pillow block once everything is attached (thats the theory anyways... you know how that goes )

I never even thought about stressing (and thus bowing) the aluminum plate. I may just drill and tap a square pattern in it to accomplish the same thing... that should solve my problem, right?

Thanks for the input! I hope to get this thing started before the end of the year if i can find the cashflow around here somewhere.

[bmsgaffer86]

Oh and another question... what is the general stance on syncronous-belt driven ball screw? It would be 1:1 ratio just to get the motor into a better mounting location. Is this a good idea or does it just introduce too many problems (belt stretching, vibration, etc)?

[Riceburner98]

I had thought of using a single motor with a belt to drive the 2 ballscrews to save $$. I think for the fairly low forces involved here, that would be fine. I've also seen people have good success (on much larger machines) using a drive on one side, then a cable / pulley setup (like a drafting table) to keep the other side square. In the end I decided to just try the 2 ballscrews / 2 steppers because I ended up with an extra set. I've never lost a step on my current machine that wasn't caused by me crashing it, so I don't think losing steps on one motor would be a problem with a proper electronics setup..

As for the plate, many great machines have a drilled / tapped grid of holes that works great. Take a look at Hoss's machine for example. You know that thing gets a workout! Another issue with an aluminum T-slot table can be buggering up the bottom of the slots with the steel T nuts if they're overtightened.. The Taig seems to be OK with them, though they are fairly thick aluminum. Then again, it's always possible to strip out a 1/4" thread if you go too crazy.. I was personally hoping to find some (cheap! lol) T-slot cast iron stock to use, but those words just don't seem to get along when you get any decent size going..

As far as the arm strength goes, it sounds like you're on top of it! Now if you can anodize them green to match the drawing.....