My first Longmill project(belt grinder) with a bit of a oops

I got the Longmill CNC late last year and just now got the time to really start playing around with it. To learn it and fusion 360 I decided to go ahead and make a new belt grinder that could tilt 90 degrees with the goal of using the Longmill to do all of the cuts. Here are a few pictures of it:

I learned a lot while doing this(obviously). Especially a lot of basic things. Like the speed and feeds and depth of cuts I would need to run. And how key finishing passes are to clean up deflection and get super accurate parts(within 2-3 thou). As well as squaring up the machine and calibrating the machine.

The only thing that I still kind of don’t know how to do well(in terms of things I have tried so far) are small-ish holes for bolts. Using a 1/4 inch end mill and even doing a slower repeat finishing pass they would still end up with some deflection on a 1/2 inch hole and be ~10-12thou under. I assuming going slower and doing it in more passes/lighter cuts would help dramatically.

And now the problem. So this was meant to be a 2x72 belt grinder. I took the measurements off my old one and used those in cad to start designing it. Did not even really question anything at that point(I did think about modeling the belt in and everything else but decided not to in the end…). The issue is that with my old one, the motor sat in the middle of it on the base with a hole cut out for it through the grinder(so everything was much tighter/closer together). And when I moved the motor to the back of this one so it could rotate 90degrees, I did not take into account that I was obviously moving the motor ~10-12inches further away from the machine…
That is a 2x72 belt in the last image. As you can see it is a bit small for the grinder. And oh boy did I feel kind of dumb when I finally got everything together and brought a belt over to test it out. I ended up needing to order some custom belts(2x88). Not the end of the world by any means. Would have been nice if it was smaller but not that big of a deal. Just a small oops…and a very valuable lesson learned.

As a whole I am super happy with how it turned out, though. It’s super solid/rigid(more so than I thought it would be) and the tilting actually works really well(it turned out to be basically perfectly squared in both positions). And I also really enjoyed the first steps in learning/using the Longmill and the programs associated with it(fusion, gsender, ect).

Incase anyone was wondering, I was actually able to make the whole thing out of just a 4x8 sheet of plywood(just the grinder, not the base/cart for it). Cut it up into 8 2x2 squares then just did part sheets. Would’ve been able to save a bit of that wood if I had actually made it the right size, though…


Awesome challenging first project for putting everything to the test Deonto! As we all do in life and woodworking, we learn the hard way then make it better the next time. I’m a newbie also waiting on the 48x30 LongMill and can’t wait to put it through tests and building some custom modifications to it.

How did you like using Fusion 360? I’ve been playing with it and really like it. I’m an old (by young in heart) AutoDesk 2D/3D AutoCAD/Revit/3DStudio guru, so that seemed to help the transition but look forward to sending code to the LongMill with gsender and making some sawdust.

I actually really enjoyed using fusion 360. I was kind of like a kid in a candy/toy store when learning the basics of it. I can’t really compare it to other programs as this is the first one I have used, though. But, again, I am really enjoying it. Currently in the process of designing a few other things for the grinder(such as a tool rest and a surface grinding attachment for it).

Grats on your 48x30! While I don’t have any other experience with different router cnc machines, I am pretty impressed with the Longmill so far.

Hi Deonto! Thanks for this great post. I’m struggling with what I think is a deflection issue, and hearing that you could only get dimensional cuts under control with finishing passes gives me some hope that I can resolve this too.

For context: I notice that a straight cut (say a jog of 10mm) will be deflected perpendicular from the cut line by 0.15 - 0.3mm. So you see sort of round bumps at the beginning and end of the line, perpendicular to the jog line. It seems like the bit plunges, then as soon as the jog starts it deflects to one side, then settles again at the end of the jog. Curiously the deflection is always away from the “climb” direction. So if the line is a jog in +Y, then the bit deflects -X; similarly if the jog is in -Y, then the big deflects +X. This might be an unrelated issue, but for my own troubleshooting, would you have some pointers on how you “cleaned up deflection”?