Inaccurate sized cuts

So, I’m sure there is someone else out there with this same issue and hopefully it’s been resolved. I set up a 2” square to cut (externally) from 1/2” acrylic. so after I cut it with my 1/8” endmill, I measured the square and it’s 2” on the bottom but it’s 1.89” on the top of the cut. Is there any valuable resources out there that can help me get this machine to cut a 2” square accurately?

Which machine? And how are you setting up the stock? Are you cutting all 4 sides of the square? or using a fence for the bottom & left, and cutting the top & right?
To check the machine, make sure it cuts all 4 sides.
Can you verify that the corners are square? A master square, tri-square, measure corner to corner…

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I’m using the shapeoko pro xxl. I’m cutting all four sides. I’ll check the squareness of the cuts and get back with you.

Or cut 2 pieces, put them together side to side, and see if the resulting edge is straight


They should be if they are positioned as they came off the machine, but if you rotate one of them & check all 4 sides, I expect there will be a discrepancy. Then it’s a matter of figuring out how to tram that machine in so the Y axis & gantry are square. (currently above my paygrade, outside my wheelhouse) :frowning:

Start by verifying that the machine is mechanically sound and everything is square/trammed — if everything checks out mechanically, calibrate for belt stretch:

I had kind of a similar problem with my XXL Pro, only I was cutting finger joints. The top of the joint would be about the right width, but the bottom would be narrow. Like what you have, only I was making square holes. My holes were always under sized leaving my remaining part too big. They wouldn’t fit together like intended. I did a roughing pass, then used a contour pass with negative stock to leave to get the correct dimension.

I was still cutting angled parts until I slowed down the contour pass. I was using a 1/4" bit, and initially cutting the last 0.015" (1/2" depth of cut) of the side wall to square things up at 60IPM in wood. It came out angled like you have. I slowed it down to 20IPM, and things finally came out square.

Long story short, I think bit deflection and maybe bearing play (I could be totally wrong here) can be big if you are moving with any speed. For good final dimensions the last pass needs to be really thin and rather slow to reduce deflection as much as possible. I am no expert, but I have been able to get a lot better joinery using this technique. Good luck!

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It shouldn’t deflect that much — what spindle are you using?

What’s the runout like and how true do tools turn?

If things are off mechanically, check in w/ the spindle manufacturer — for a Carbide Compact Router.

Thank you so much for that! I’m going to try this. I think Tramming the machine is going to be my last resort. I think it’s pretty square. Just seems kinda crazy to have to go thru all this.

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It’s the standard carbide trim router that came with the machine. I actually got a new spindle too because the bit wasn’t spinning as it should but I had thought this was resolved.

Even multi-million dollar machines have to be leveled and aligned and trammed when they are setup — should be a once and done thing (unless there’s a mechanical change).

In that case, it’s probably as good as one can get w/ a trim router (which is pretty good for most folks, but these tools you are using are tiny) — some folks have replaced the bearings and gotten good, possibly better results, but that’s not something we expect folks to do.

Is a VFD Spindle kit an option you could consider? I think it would help w/ the very, very small tooling.

Carbide Create router, and I measure 1/2 thou runout at the root of my bit. I used a 1/4 bit with two flutes in part to have it be stiffer. It is a 1" long cutter and I had it sticking out a little far. I might try cutting the same file with a shorter bit and make sure I choke up all the way. I got a bunch of good drawers out of it once I slowed my feed on the contour. Post to follow sometime soon on my lessons learned.