CNC foam cutter, part IX?

I haven’t posted anything here yet so I figure I ought to fix that.  One project I’m working on is a 1.5M discus launched radio controlled sailplane.  “What in the heck is that?” you say.  If you go to Google and search for ‘dlg sailplane’ you’re bound to find better explanations than I can give and some wicked cool videos.  You see, at my core I’m really cheap.  I can’t see spending several hundred dollars on a 60″ sailplane that I’m going to beat the living tar out of when I can build one myself.  Thus the CNC foam cutter project was born. I have built several wooden winged DLGs and now I’m getting to the point where I need more consistency, lighter weight and more strength.  This is accomplished by using insulation foam as the substrate and laminating that with epoxy saturated fiberglass, carbon fiber and kevlar.

The CNC project has been under way for a few weeks but mostly in the collection of parts and first steps at testing.  I’m using a HobbyCNC 4-axis driver board that is available as a kit.  There is support but only if you’re an engineer and have read all the engineer-written documentation and haven’t made a mistake.  Otherwise you’re at the mercy of other users found in the Yahoo Group for said CNC controller.  The board itself works brilliantly, the support not so much.  If you can solder and follow the sparse instructions you’ll be fine.

The following account, reproduced here, is my experience with using locally source (and cheap) home improvement store bits as critical mechanical components of this computerized hot wire foam cutting machine.


ACME screw-lite, or, How to make threaded rod suck less

Acme screws are dang expensive. Really expensive. Even in the tiny little 1/4-x sizes they’re $100 for 6 feet of screw + $20-$40 Acme nuts for each axis. Very quickly two axes would be more than the balance of my CNC investment. Solutions I’ve seen frequently for ultra-light duty are timing belts on linear slides and threaded rod with blind nuts. I opted for the later since there’s a Lowe’s Home Improvement almost within walking distance and a Home Depot across the street and down half a block. That’s the good news.

The bad news is that threaded rod sucks as a lead screw. Particularly the inconsistent and frequently bent stuff often found in bulk barrels at said stores. I figured this out after I’d assembled my X and U axes. My $5 stepper motors kept stalling regardless of how well I lubricated them and aligned and realigned the axis. Finally I disassembled it and figured out quickly why. The stupid thing was binding horribly thanks to the awesomeness of the bulk threaded rod. I couldn’t run even a normal nut up and down the thread without some serious effort. I read on some site that I can’t find at the moment that you can hone the threads with a coupler and some sort of mild abrasive that escapes my mind. I got a coupler and ran it down the thread a few times with one end of the threaded rod firmly attached to the chuck of my drill and the coupler in a nice gigantic wrench. It definitely loosened up so I reassembled the axis and gave it another shot. Much, much better. And then it bound. And then it stalled. And then I cursed. A lot.

Fast forward to today. After waking at the crack of 7:30 and getting some much needed C8-H10-N4-O2 (caffeine) I wandered across the street to Lowe’s. There I acquired a tap and die set and set about truing the thread on my threaded rods. While waiting for my laptop to boot, VPN to connect, etc. I cleaned up one axis and reassembled it. Thence over the course of the next several hours the next axis was cleaned up and reassembled it just before lunch. There were shavings all over the table so it must be perfect by now, right? Boot up ye olde CNC machine and start running tests. Absolutely, positively . . . . worse. Much, much more cursing.

It was at this time that I remembered seeing somewhere that a local store had some nice shiny threaded rod instead of this bronzish looking stuff I found at Lowe’s. I figure it must have been Home Depot so off I went at lunch to search for it. Sure enough, right in front of all the other threaded rod were some shiny threaded rod whose tags said ‘Zinc’ on them. I assume these are some kind of mild steel that are zinc plated. I also got some push-in threaded furniture inserts made of what looks and feels like pig iron. I cleaned the threads up quickly with the tap. These furniture inserts would make several rotations under their own inertia if I gave them a good spin. These had to have been 10x better than the blind nuts but the longer barrel length with demand greater precision in alignment, or so I thought.

After wrapping up work for the day I cut some more bearing plates, measured and drilled them for the inserts and pressed them in with my handy, dandy desktop vise. A careful alignment at the far end of the threaded rod, threaded in an inch or so for good measure. I was surprised at how smooth and consistent this assembly was. I ran it for two solid hours in a loop, forward and back across the length of the lead screw. No issues. This is my final answer. I found that this new solution was able to traverse literally twice as fast as the previous configuration. Conclusion: a crap ton less drag equals a boat load more speed. It should also aid in accuracy and repeatability but I suppose time will tell.


  • Most threaded rods suck, some suck less. Use a long coupler to field test thread quality.
  • Blind nuts suck as lead screw nuts.
  • Furniture inserts suck less.
  • Cut your threads yourself. Invest in a tap and die if you don’t have one.
  • Measure four times, adjust infinitely.