Dowel holes varying in diameter

Wanted to create a two sided project - a number of them with one holding jig - and used a 6mm end mill in a pocket toolpath to cut 8.33mm dia and 15mm deep.
At first the holes were all nice and tight but they have now seemed to vary in size - larger now so don’t get a nice tight fit with the dowels.
I have not changed anything regarding toolpaths etc and can’t work out what has happened - all other associated toolpaths are all accurate
Using Vcarve pro 11.5 on a Woodworker x50
Appreciate any advice on what might be happening
Kerry Cain

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What is the material?
I have seen this happen in aluminum. The chips will pack up in the flutes if not evacuated and actually push the hole walls out as the cutter goes around until they are oversize.
Maybe some types of wood will do this too if the chips pack up?

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Is the bit hot or the bearings getting loose causing more runout

Hi Chris thanks for your reply but I was only using pine.
Did some more tests today with a spiral profile cut and had a similar result
Cutting an 8.3mm diameter with a 6mm end mill and 10mm deep and resulting hole finished up 8.76mm.
Did a number slowly reducing dia till got to 8.18mm and finished up close to the 8.33mm I needed to fit a steel dowel.
Seems strange as the first holes in the holding jig were machined pretty accurate a number of weeks ago using a pocket toolpath - I have since had the same problem this way also so changed to the spiral profile cut.

Don’t think heat is a part as only cut two holes 8.33mm x 12mm deep.
As to bearings I don’t seem to have any other trouble with accuracy doing 3D carving.
This is why I need to cut accurate dowel holes to be able to turn the work piece over for double sided machining.
Thanks Kerry

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You may have better luck using drilling toolpath than a pocket tool path if you want that inf of accuracy

8.3mm Metric M35 Cobalt Steel Extremely Heat Resistant Twist Drill Bit of 1pcs with Straight Shank to Cut Through Hard Metals Such as Stainless Steel and Cast Iron, 5% Cobalt M35 Grade HSS-CO - -

Kerry, what troubleshooting steps have you already taken? Have you e.g. tried a different size of bit like a true 1/4" or a 1/8" and creating a new toolpath? That would check if its peculiar to the bit, toolpath or definition of the 6mm bit in the library.

(I’ll note that if you are, for example, actually using a 1/4" bit when you think you’re using a 6mm bit, you’ll wind up with a slightly larger hole than you expect…)


It could be how the machine is interpreting the arcs as it is a series of straight lines and not true arcs… There was something a long time ago about circular interpolation and skipped steps… I wonder if you are facing something similar where being such a small pocket it is doing something odd or as a previous response mentioned the bit geometry is working against you… Trying a larger hole and smaller bit may help prove something

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Hey Kerry,

such issues come up here from time to time, typically when milling 3/4" dog holes into the wasteboard. Causes can be

  • Deflection of the machine under load
  • Bit deflection, mostly because of selecting too long bit
  • Backlash of the machine
  • Dulling of the endmill (when holes get too small)
  • Increasing play in the bearings of the spindle/router
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I have no problem with this when doing holes. I would suspect either a bit, collet, or bearing problem.
Have you trammed it to see if the router and/or slider is running true?