I am trying to cut a 15/32" hole. In Sketchup I draw a circle with a 15/64" radius (15/32 diameter), and then generate the gcode for an inside cut. Here are 2 samples. The results are always too small. I end up having to ream the hole out with a 15/32" bit. The outside cuts are accurate, just not the inside. Wondering if there is something I'm missing.

This first sample is with an 1/8" bit cutting out a 15/64 radius (15/32" diameter hole). It was too small.

[pre]
(Generated by PhlatscripT {0.923})
(File: LargeStepperMountPlate.skp)
(Bit diameter: 0.125)
(Feed rate: 60)
(Material Thickness: 0.375)
(Material length: 22.0 X width: 22.0)
(www.PhlatBoyz.com)
G90 G20 G49
M3 S8000
G0 Z 0.050
(Pass: 1)
X 0.887838 Y 0.719354
G1 Z -0.100 F 60
G17 G3 X 0.923323 Y 0.692125 Z -0.100000 R 0.171875
X 0.964646 Y 0.675009 Z -0.100000 R 0.171875
X 1.008991 Y 0.669171 Z -0.100000 R 0.171875
X 1.053335 Y 0.675009 Z -0.100000 R 0.171875
X 1.094658 Y 0.692125 Z -0.100000 R 0.171875
X 1.130143 Y 0.719354 Z -0.100000 R 0.171875
X 1.157372 Y 0.754839 Z -0.100000 R 0.171875
X 1.174488 Y 0.796162 Z -0.100000 R 0.171875
X 1.180326 Y 0.840506 Z -0.100000 R 0.171875
X 1.174488 Y 0.884851 Z -0.100000 R 0.171875
X 1.157372 Y 0.926174 Z -0.100000 R 0.171875
X 1.130143 Y 0.961659 Z -0.100000 R 0.171875
X 1.094658 Y 0.988888 Z -0.100000 R 0.171875
X 1.053335 Y 1.006004 Z -0.100000 R 0.171875
X 1.008991 Y 1.011842 Z -0.100000 R 0.171875
X 0.964646 Y 1.006004 Z -0.100000 R 0.171875
X 0.923323 Y 0.988888 Z -0.100000 R 0.171875
X 0.887838 Y 0.961659 Z -0.100000 R 0.171875
X 0.860609 Y 0.926174 Z -0.100000 R 0.171875
X 0.843493 Y 0.884851 Z -0.100000 R 0.171875
X 0.837655 Y 0.840506 Z -0.100000 R 0.171875
X 0.843493 Y 0.796162 Z -0.100000 R 0.171875
X 0.860609 Y 0.754839 Z -0.100000 R 0.171875
X 0.887838 Y 0.719354 Z -0.100000 R 0.171875
...



This second example, I made the hole 1/4" radius (1/2" diameter) and used a 1/4" bit as my 1/8" was getting dull, it was still tool small.


(Generated by PhlatscripT {0.923})
(File: LargeStepperMountPlate.skp)
(Bit diameter: 0.25)
(Feed rate: 60)
(Material Thickness: 0.375)
(Material length: 22.0 X width: 22.0)
(www.PhlatBoyz.com)
G90 G20 G49
M3 S8000
G0 Z 0.050
(Pass: 1)
X 0.886012 Y 0.621269
G1 Z -0.100 F 60
G17 G3 X 0.911677 Y 0.601576 Z -0.100000 R 0.125000
X 0.941565 Y 0.589196 Z -0.100000 R 0.125000
X 0.973638 Y 0.584973 Z -0.100000 R 0.125000
X 1.005711 Y 0.589196 Z -0.100000 R 0.125000
X 1.035599 Y 0.601576 Z -0.100000 R 0.125000
X 1.061264 Y 0.621269 Z -0.100000 R 0.125000
X 1.080957 Y 0.646934 Z -0.100000 R 0.125000
X 1.093337 Y 0.676821 Z -0.100000 R 0.125000
X 1.097559 Y 0.708895 Z -0.100000 R 0.125000
X 1.093337 Y 0.740968 Z -0.100000 R 0.125000
X 1.080957 Y 0.770855 Z -0.100000 R 0.125000
X 1.061264 Y 0.796520 Z -0.100000 R 0.125000
X 1.035599 Y 0.816214 Z -0.100000 R 0.125000
X 1.005711 Y 0.828593 Z -0.100000 R 0.125000
X 0.973638 Y 0.832816 Z -0.100000 R 0.125000
X 0.941565 Y 0.828593 Z -0.100000 R 0.125000
X 0.911677 Y 0.816214 Z -0.100000 R 0.125000
X 0.886012 Y 0.796520 Z -0.100000 R 0.125000
X 0.866319 Y 0.770855 Z -0.100000 R 0.125000
X 0.853939 Y 0.740968 Z -0.100000 R 0.125000
X 0.849717 Y 0.708895 Z -0.100000 R 0.125000
X 0.853939 Y 0.676821 Z -0.100000 R 0.125000
X 0.866319 Y 0.646934 Z -0.100000 R 0.125000
X 0.886012 Y 0.621269 Z -0.100000 R 0.125000
...
[/pre]

 

I looked at the gcode files, and while technically they are producing holes that are too small, they are only undersized by 0.001 - 0.002". So, you might be right about my machine. It seems strange that the outside cuts are good, but then again, I'm only noticing the inside cuts now because I'm trying to fit a precision milled steel rod into the holes I'm milling.

(In the first example, I subtracted the smallest X value from the largest and added the bit diameter - the sketchup circle was 0.46875" diameter, the bit will cut a 0.467671" hole).

I will double check. The machine is a scratch built, so it is quite possibly too loose or something.

 

I agree with Shaun's first comment that it is a calibration problem. If it was a backlash the holes would be not round.