Holes are too small. Fusion 360 Bore

[Auarhau]

Hi.
I am trying to make some parts with my SO2 and Fusion 360, but I am having some trouble getting my holes the correct diameter. Especially the smaller holes!

When using the CAM bore function I consistently get slightly too small holes. 0,15mm too small on average. I have tried to run the operation every way I can think off, with different speeds, different helix pitches, different finish passes/ step overs etc. And the result is pretty much the same no matter what. I ran this with feeds from 400mm/min to 1200mm/min and plunge 150mm/min in 12mm plywood. My cutter is measured to be 3,1mm diam with digital calipers. And I checked my axis travel with a dial indicator last night, they are all spot on as far as I am concerned. When I try to cut the same hole as a pocket made in V-carve pro or as a Circular toolpath in Fusion I get a better result (as good as I'm going to get it on this machine I think). This leads me to believe this is a CAM problem with Fusions Bore function. Any ideas? I would like to use the Bore function in stead of making circular pockets, because Bore is a much neater and quicker tool path, with no annoying up and down movements etc. And it is the indtended toolpath for making holes, right?

Settings: http://imgur.com/a/NjTik

Code: Select all

%
(1001)
(T1  D=3.1 CR=0 - ZMIN=-12 - flat end mill)
G90 G94
G17
(Metric Units - mm)
G21
G54
G0 Z15

(Bore2)
M9
T1 M6
S20000 M3
M9
G0 X9.666 Y10.344
Z15
Z2
G1 Z0.31 F400
Y10.351 Z0.241
X9.664 Y10.374 Z0.175
X9.662 Y10.411 Z0.117
X9.659 Y10.46 Z0.068
X9.655 Y10.519 Z0.031
X9.65 Y10.584 Z0.008
X9.646 Y10.653 Z0
X9.635 Y10.808
G2 X9.924 Y11.137 I0.309 J0.021
X10.076 Y8.863 Z-0.5 I0.076 J-1.137
X9.924 Y11.137 Z-1 I-0.076 J1.137
X10.076 Y8.863 Z-1.5 I0.076 J-1.137
X9.924 Y11.137 Z-2 I-0.076 J1.137
X10.076 Y8.863 Z-2.5 I0.076 J-1.137
X9.924 Y11.137 Z-3 I-0.076 J1.137
X10.076 Y8.863 Z-3.5 I0.076 J-1.137
X9.924 Y11.137 Z-4 I-0.076 J1.137
X10.076 Y8.863 Z-4.5 I0.076 J-1.137
X9.924 Y11.137 Z-5 I-0.076 J1.137
X10.076 Y8.863 Z-5.5 I0.076 J-1.137
X9.924 Y11.137 Z-6 I-0.076 J1.137
X10.076 Y8.863 Z-6.5 I0.076 J-1.137
X9.924 Y11.137 Z-7 I-0.076 J1.137
X10.076 Y8.863 Z-7.5 I0.076 J-1.137
X9.924 Y11.137 Z-8 I-0.076 J1.137
X10.076 Y8.863 Z-8.5 I0.076 J-1.137
X9.924 Y11.137 Z-9 I-0.076 J1.137
X10.076 Y8.863 Z-9.5 I0.076 J-1.137
X9.924 Y11.137 Z-10 I-0.076 J1.137
X10.076 Y8.863 Z-10.5 I0.076 J-1.137
X9.924 Y11.137 Z-11 I-0.076 J1.137
X10.076 Y8.863 Z-11.5 I0.076 J-1.137
X9.924 Y11.137 Z-12 I-0.076 J1.137
X10.076 Y8.863 I0.076 J-1.137
X9.924 Y11.137 I-0.076 J1.137
X10.254 Y10.849 I0.021 J-0.309
G1 X10.264 Y10.694
X10.269 Y10.625 Z-11.992
X10.273 Y10.56 Z-11.969
X10.277 Y10.501 Z-11.932
X10.281 Y10.452 Z-11.883
X10.283 Y10.415 Z-11.825
X10.285 Y10.393 Z-11.759
Y10.385 Z-11.69
G0 Z5
X9.646 Y10.653
Z-10
G1 Z-11.69 F400
X9.645 Y10.661 Z-11.759
X9.644 Y10.684 Z-11.825
X9.641 Y10.72 Z-11.883
X9.638 Y10.769 Z-11.932
X9.634 Y10.828 Z-11.969
X9.63 Y10.893 Z-11.992
X9.625 Y10.962 Z-12
X9.615 Y11.117
G2 X9.904 Y11.447 I0.309 J0.021
X10.096 Y8.553 I0.096 J-1.447
X9.904 Y11.447 I-0.096 J1.447
X10.233 Y11.158 I0.021 J-0.309
G1 X10.244 Y11.003
X10.248 Y10.935 Z-11.992
X10.253 Y10.869 Z-11.969
X10.257 Y10.811 Z-11.932
X10.26 Y10.762 Z-11.883
X10.262 Y10.725 Z-11.825
X10.264 Y10.702 Z-11.759
Y10.694 Z-11.69
G0 Z5
X9.646 Y10.653
Z-10
G1 Z-11.69 F400
X9.645 Y10.661 Z-11.759
X9.644 Y10.684 Z-11.825
X9.641 Y10.72 Z-11.883
X9.638 Y10.769 Z-11.932
X9.634 Y10.828 Z-11.969
X9.63 Y10.893 Z-11.992
X9.625 Y10.962 Z-12
X9.615 Y11.117
G2 X9.904 Y11.447 I0.309 J0.021
X10.096 Y8.553 I0.096 J-1.447
X9.904 Y11.447 I-0.096 J1.447
X10.233 Y11.158 I0.021 J-0.309
G1 X10.244 Y11.003
X10.248 Y10.935 Z-11.992
X10.253 Y10.869 Z-11.969
X10.257 Y10.811 Z-11.932
X10.26 Y10.762 Z-11.883
X10.262 Y10.725 Z-11.825
X10.264 Y10.702 Z-11.759
Y10.694 Z-11.69
G0 Z15
M9
%

[Hans]

What makes you say the holes are too small, measured or something doesn't fit? If you are measuring the ID with your calipers, you should expect an inaccurate, undersized measurement especially in smaller hole sizes.

[Auarhau]

That's what I did yes Hans. I know it's difficult to measure this way, but this does not explain why the holes made with bore are slightly smaller then if I make them with different CAM strategies.
The parts did not fit with it's rods and bolts etc either by the way. This is always difficult for me But this time I think the CAM made it worse.

[McOtis]

Any time I have had this problem, it was due to flex of the bit, spindle, machine etc...
It also can have everything to do with the CAM strategy...
Climb vs conventional cutting, speed etc...
The bits usually flex toward the center of the hole, making it smaller.
Try a slower feed, or the direction (change conventional to climb).

[Hans]

In that case yes, it's probably just the machine flexing differently like McOtis said depending on the machining strategy. I'd be really surprised if they didn't come out the same if the finish pass is the same thickness and full depth in both cases. If you can characterize how undersized the holes are you can correct for it in CAM, or you can add a single full-depth pass before retract to make sure the hole is opened all the way up. If you have clearance holes for bolts and .15 mm under is enough to cause binding then I'd say you should change your design, but that's a separate issue

[Auarhau]

Just curious how much over sized you draw your holes to get a nice easy slip fit with as little wiggle room as possible? The 6mm holes I made now fit a 5,3mm rod (back end of drillbit) with a tiny bit off play. So I guess my 6mm holes fits 5,4mm rods pretty nicely. That's ~0,5/0,6mm over sized diameter to get the fit I want. Is this a normal, seems a little high?

Edit: Just to mention it. My preferred way to get accurate holes is to mill it and then manually ream it. But I only have reamers in whole mm increments, so this problem occurs when I have to make small holes with strange decimal diameters. Like it is on the Shapeoko Z plate for instance (5,2mm and 7,137mm etc).

[Auarhau]

I am also curious to what the "Compensation type: computer" parameter does. Maybe it's doing some number on me?

[Auarhau]

I asked the guys over at the autodesk forum. And they verified that my code was correct. That leaves only my shapeoko.. I have to blame my wobbly Z axis I guess, it's about as rigid as a noodle Well not quite, but it is a really annoying weak point to an otherwise capable machine. I guess I just have to experiment more! On a side note, the SO3 webpage says that real life parts will be within 0,15mm. So I guess most of my results are still within spec. Still curious how much clearance other SO2 owners use to get a nice fit on parts, inlays etc?

[Gadgetman!]

I got +/- 0.15mm on my SO...
It just took an awful lot of adjustment and a low feed speed.
I also hand-coded it all.

Are the dual rails on your SO2 bolted together properly?

[McOtis]

I asked the guys over at the autodesk forum. And they verified that my code was correct. That leaves only my shapeoko.. I have to blame my wobbly Z axis I guess, it's about as rigid as a noodle Well not quite, but it is a really annoying weak point to an otherwise capable machine. I guess I just have to experiment more! On a side note, the SO3 webpage says that real life parts will be within 0,15mm. So I guess most of my results are still within spec. Still curious how much clearance other SO2 owners use to get a nice fit on parts, inlays etc?

From your post, it appears you are using a 1/8" bit. Is that correct?
How long is the bit?
The bit itself is probably first, then the Z axis itself.