Chasing Holes that are undersized then oversized

Well the last great mystery of setting up a new machine is chasing bore sizing issues. I’m having flashbacks to setting up my SO3. I’ve calibrated everything so square, tram, measured steps etc. I’m trying to run a boring operation on a 16mm hole, and I run it with spring and finishing passes. Dimensionally the holes are coming out nice and round but now I’m chasing sizing problems. First attempt it was .5mm undersized using a 1/4" 278z single flute. I rechecked everything. I know these tools are not actually .25" in diameter and without any cutter comp I’ve seen similar issues when the toolpaths are posted from Fusion. So I monkeyed with trying to measure the tool diameter and dropped the tool size to .225" in Fusion and reran it. Now I’m getting 16.2mm holes. I know some of that is runout. But I’m wondering if I’m forgetting something or if I need to relook at the machine calibration before I go chasing tool diameter settings. I’d definitely prefer things to be undersized vs oversized.

  1. Runout will be present, but it shouldn’t be that significant (if we are talking about a router in good mechanical shape, not an ultra-cheap drill press).
  2. I’d believe you if you said your tool might be 0.001" - 0.002" undersized, I’d really doubt it is so far off unless you chipped the tip of your 1-flute.
  3. If all you have are calipers, measuring inner diameter is not super easy and there can easily be variation in your measurement.
  4. Depending on your material and cutting aggressiveness, there can be some tool deflection. This can be reduced with better CAM strategies.
  5. I would think the biggest contributor would be the belts; are your belts in good shape and in good tension?
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Yeah I don’t think its that much run out, I didn’t see that on a similar setup on my SO3. This a brand new SO4 Pro. with the C3d Er-11 router. The tooling is brand new in this case. None of the C3d single flute tooling I have is actual diameter so that is not all that surprising, the shaft diameter might be .25" but the cutting diameter is always smaller. In this case I’m measuring with calipers the hole but also have a precision shaft that fits through this particular hole so its not hard to tell when its under or oversized. Its 6061 aluminum which I’ve cut a bunch of on my SO3, I’m just running a boring option and leaving .001-.002" of stock to clean up on finishing to minimize deflection. Belts are brand new and equally tensioned as best as you can on a machine like this. I can rerun my calibration routine again to check squares and triangles but my holes are coming out cylindrical just sized wrong. It’s possible I need to look at my step calibration again.

I’d try cutting two square pillars, one 10mm, one 16mm, using the same feeds and speeds and finishing pass as your boring operation, then measure them with a micrometer:

  • If they both show the same discrepancy, you just need to adjust your tool diameter compensation to match.
  • If they show different discrepancies, it means your step calibration is off.
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I was already headed that direction when I saw this post. Thanks. I should have done this sooner.