Hi, newbie to cnc here...

I'm making a very very basic CNC foam cutter to apply towards my metalcasting hobby. In fact, it will be less than two feet long or wide (Reason being my metalcasting equipment can only cast about 1lb or so at once).

For cutting foam I guess the recommendation is slow and cool, so it doesn't have to move super fast. For some reason I'm having a hard time getting my head thru this part of the project to come up with something. It almost seems like overkill to use rollerblade bearings and rods since the machine won't be very big. I'd really like to use belt drive if I could (less backlash hassle). So far the only thing I can think of is taking apart some old printers and using their mechanical guts?

Just wondering if you guys might have any ideas.

The basic design I'm shooting for is one X slide, and one Z slide (NOT TWO like most foam cutters) The Z Slide holds the hotwire in a copying saw type bracket about 8" deep or so. This way I only need two motors and two slides. For example, the foam is mounted on its edge in the machine. X moves back and forth horizontally, and Z moves up and down vertically off of the X plane, and has a horizontally mounted wire protuding off it, and the other end of the wire is held by a bracket arm the comes over the top like a coping saw, with some throat depth. Granted it won't be able to cut very large pieces, but for now I Just want to get it up an running to see how well it will work.

I've also looked at drawer slides but they seem a bit heavy to use with belt drive. Actually, I'm thinking I'll probably end up with belt drive for the X way and screw drive for the Z no matter what I do, because I don't know if a belt will hold the hotwire arrangement in place or not (if the motor torque will be strong enough to hold the wire and the Z way both).

The thing is the hotwire really doesn't undergo any large mechanical forces, so the ways don't have to be heavy duty.

Perhaps there is also a better gantry design than what I had in mind? I'm just trying to avoid using more than 2 motors since I only have so much in the "recycle" area if you know what I mean...


Another thing I found interesting as I was searching the web last night was the theory behind how to draw a straight line - the linkages that they came up with long ago in the 1800's. I wondered if this could somehow be applied to a lightweight foam cutter machine. In other words, use a linkage for the X axis movement rather than a traditional linear bearing. Have a look at what I mean:

http://www.math.cornell.edu/~dwh/courses/M451-F02/peaucellier.html

The linkage forces a straight line. Wonder if anyone ever measured its precision?

Nice to be here!

-niko

Anyway, here is a render of what I'm talking about. This would be using drawer slides for linear motion...probably would be able to maybe use a pulley and cable for the Z axis :) ?

-niko

Hi Niko, Interesting concept. I use a foam cutting in my business and I can safely tell you that this would work. One thing I would change is to make your "bow" similar to the old style woodcutting bowsaws, with a turnbuckle to increase tension on the wire. Wire tension is very important in foamcutting. The thicker the cut the more important it is. For all practical purposes, Id go with leadscrew and nut on both axis. On a smallish machine its very easy to make a backlash free leadnut that pretty much eliminates any play. Plus it makes it easier to calculate steps per inch and such. Some software will only interpolate circles if steps per inch is identical on both axis.
Im not sure what motors you have or are planning to use, but Im pretty confident that all but the smallest stepper motors would have plenty of power to both move and hold your upright axis. They are very powerful for their size at slower RPMs, and you are right slow and cool is best for what you will be doing. I suppose drawer slides would work, but they to tend to get stiff or bind from time to time. Plus it would be very hard to align them to run freely in the opposed manner you have them set up since you would need to align both up and down as well as the exact distance between them, looks like a shimming and adjusting nightmare. For me, stuff like that never just "works". Feel free to email me if you have any questions.

Ed Rees
erase42@aol.com

Thanks Ed, good to hear from someone in the "biz" already.

I have some small motors (400 steps/ rev), but they have quite a bit of torque in two phase mode and shouldn't have any problem moving a lead screw even with the Z axis.

The only reason I didn't like the thought of leadscrews was even if I use 1/4-20" then I get 400 steps * 20 threads/inch which is 8000 steps / inch. Which means the motors would have to move very fast to go any quicker than 1 inch/min. (In fact I don't think they can handle it well using KCam software anyway..) This is offset by the fact however that I don't think I'll be able to go too fast anyway being that I'm cutting foam and it's supposed to be slow and cool ! So it might work out to be ok !


I'm thinking I'll probably try to go with some rollerblade bearings for now as well since I can make it just like the normal big machines (with adjustments on the end of the pipes).

I was actually thinking of some different bearing arrangements if using drawer slides, just using one for X and one for Z instead of two opposing. That would eliminate aligning problems. Z lead screw would be pre-loaded by gravity so backlash would be eliimated in Z, so I would only have to worry about X..

Thanks for your feedback :)

-niko

I dont think you are on the right track with your numbers. To go as slow as 1 inch per minute your motor would only be going 20 rpm. that is very very slow. I have what I concider to be slow motors. And I can rapid transit at one inch every few seconds. Keep in mind that those 400 steps happen at a speed of about 25k per second or so I guess, faster on some controllers unless im getting numbers confused.

Ed

Hi, I run a small mdf mill at around 6 IPM with printer motors rated at 3v running on 12v with balast resistors and 5/16 all-thread to drive the x and Y.

I get 200 steps OR 400 half-steps using my controler. Frequency I can reach on the controler is around 1000hertz which are steps or half steps. Of course, using half-steps enhance resolution but decrease a bit the torque. I probably would be able to drive it in steps at around 1200 hertz for about 8IPM but that would be limit.

Well, I say all this because I made the mill, then added a small bow to cut foam on the Z axis carriage and swaped Y and Z in my FOAM config file for TurboCNC. Now I can draw x-y stuff and cut it vertically.

Good luck, it's really a blast to make those forms.

Para