Chatter while Chamfering

Hello, I need help chamfering aluminum sheet 2mm thick on a CNC router from the CNC experts!

I'm trying to make an aluminum guitar pickguard with chamfers on the outside edges. I made a bunch of test cuts on different but similar geometries to figure out speeds and feeds and on these tests all of the different speeds and feeds made great results with a mirror finish. All the tests were done on the inside of the part and like I siad they came out great with chiploads of 0025in to 004in. The problem is when I try the actual pickguard geometry the chamfer is on the outside of the part and the results are terrible with a lot of chatter and machining marks. I'm using the same speeds and feeds as the tests and the same depth of cut and everything so I don't know why the results are so different. And on the actual part (the pickguard) the inside chamfers on screw holes are great too.

I'm using a 2 flute 1/4 chamfer end mill. Fusion 360 for CAM and Rhinoceros for CAD and designing th part.

I'm using a vacuum table with a 2X3 aluminum sheet. The ends of the sheet are sealed with tape and the hold down seems pretty good.

Prior to chamfering I'm countouring the part with a 1/8 O flute end mill and leaving .3 mm (0.008in) of onion skin so there are no vacuum leaks. I used a .2mm tip offset on the chamfer mill and made sure the tip isn't touching or draging through the onion skin.

I don't know why the tests on the other geometry witht the same material, countour depth, same chamfer speeds and feeds look so good but on the pickguard they are terrible.

Could it be that the CAD model for the pickguard isn't smooth (I doubt it and I've also had this bad result on an outside chamfer on a different project months ago)

Do I need to use conventional milling instead of climbing?

Are the forces greater when chamfering the outside perimeter? On the inside chamfer the tests cut like butter.

I've also tryied it with a different chamfer end mill and I got the same results.

Any ideas before I waste any more aluminum?

I've attached some pictures of the difference between the tests looking good and the outside chamfers looking bad.

I really apreciate the help. Thank you.

Have you turned on smoothing in Fusion? If your trial cuts were straight or circular, you probably won't see the problem there. Splines from Rhino will likely be segmented when brought into fusion and cause "stutter" marks on your workpiece.

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Originally Posted by MARV

Have you turned on smoothing in Fusion? If your trial cuts were straight or circular, you probably won't see the problem there. Splines from Rhino will likely be segmented when brought into fusion and cause "stutter" marks on your workpiece.

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Marv, thanks for the reply. I did suspect that it could be the splines from Rhino being imported into Fusion. They are segmented in some parts. Would it still cause stutter even in parts where the lines aren’t segmented? I’m going to try smoothing on Fusion.

Thank you

Its my understanding that smoothing looks ahead at the code and applies circular tool paths where tolerance permits. A loose tolerance allows longer circular segments, but accuracy may suffer. Let us know how it goes.