Deep Z conversion mod for foam- feasible?

I want to start CNC cutting XPS foam to create huge 3D shapes.

The professional solution for this is a ~$50k 5-axis CNC, but I’m not made of money. Instead, I can just break up the objects into smaller sections without overhang, and cut them on a 3-axis.

The Longmill seems like a great machine, but the 4.9" of Z-plunge might be a bit too shallow for efficient cutting of foam. Since foam requires significantly less rigidity than wood, I was wondering about modding the machine with…

  • Longer XZ gantry plate cut on the Longmill out of aluminum
  • Smaller spindle mounted to reduce weight
  • Longer lead screw and rails purchased

These are just first-pass ideas, and I’d only attempt them after fully mastering the normal build. This configuration would also only be used for foam of course.

Would it be feasible to do this, and if yes, approximately how much extra Z motion might be obtained?

I was also considering a MPCNC for this purpose since they’re so modular, but I’d prefer to put my $ into a normal reliable machine that can also do weird deep foam jobs.

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@helix_3 Welcome to the group! How long of a bit are you able to obtain? I just searched real quick and I didn’t see anything over 6". Unless you have a source for long bits it sounds like a lot of work to gain ~1".

Hi _Michael, I found this 1/2" shank, 10" long foam ball end bit here- I assume it could be combined with an adapter like this to get it on a router with a 1/4" collet. There’s also an 8" version.

However, I don’t know if this would be safe in practice. Maybe a router with a 1/2" collet would be better.

Wow, that’s a monster end mill! A full 6 inches for the cutting part! I guess you would need to mount the Y rails with a couple 4x4’s or something under them to get the X rail to have something like 8" clearance. Interesting proposition to say the least.

EDIT: Was thinking some more and maybe it would be better to make a table that had two levels of table top. Then you remove the top level when you cut foam. Jacking up the machine like I mentioned above would require you to remount the whole machine to go from normal to foam mode.

Well I know the Makita’s 1/4" collet holds well and I had the ER style on my first machine and that style worked well too. I would definitely spin the router by hand with that long of a bit to make sure it was true. Don’t want it to be off center and vibrate like crazy.

That’s my 2 cents for know anyway, like I said it sounds like an interesting idea if you can pull it off.

EDIT2: One more thing you might find this stiffness and deflection testing for the MK2 useful.

I’d be reluctant to try an adapter for a bit that long and heavy. Run out is real and if there’s any wobble at all you have yourself a deadly projectile. You’re going to need a spindle made for such a bit. You’ll have a better grip and less chance of a hospital visit.

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Good point Jim, the adapter adds length, another place to come apart, and possibly more run out, I had forgot the term. Plus I think a spindle allows you to run at a slower RPM, at least the little one I had did, whereas the Makita is ~10,000RPM at a minimum.

A little quick digging reveals that Sienci does sell an 80mm mount for the LM for modest coin ($28). This is significant as the only spindles I see that accommodate the 1/2" bits are the larger 80mm spindles. The quality ones don’t come cheap but for foam, something like this [CNC Spindle CNC Motor 110V 1.5KW Air Cooled Spindle Motor 80mm 4 Bearings 24000RPM 400HZ for CNC Router Machine] might fit the bill for cheap money. At least at $160 it can prove the concept.

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Thanks everyone! I’ll mark this down as a possibility if there’s really a strong need, and proceed with a normal build for now.

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