Breaking bits in brass. What am I doing wrong?

Hi All,

I’m having an issue that I’m hoping someone might know more about. I’ve broken two bits so far. Both broke and I left the rest of the stock attached so I could order another and continue the job. I have a hunch but wanted to ask others before I snap another. I’m trying to cut these little luchedor masks out of 320 brass. I’m using Amana’s ZrN coated 1/16th bit. (#46290) I’ll post all my feeds and speeds at the end but the odd thing is that all goes well until the very last second, when the “mask” separates from the stock. It cut the pockets first, then the drilled holes, and 99.9% of the profile with no problem. Each bit broke at the exact same time. It’s secured to the spoilboard with double-sided tape first, and clamps at each end. I suspect that, despite how well the tape holds, at the moment of separation, the mask moves just enough to knock the fragile bit and break it. With that hypothesis, the next time I try it, I’ll secure the masks to the spoilboard with a screw through the already drilled nose hole. What does anyone else think? Thanks for any help in advance!

Shapeoko XXL
Amana ZrN coated 1/16th, 3 flute EM
RPM: 16000
Feed: 20 ipm
Plunge: 10 ipm
DOC: 0.008"
Stepover: 40%

I think I’m doing a pretty good job of keeping the chips cleared.

Hi Jeremy, I’m a newbie and I’m sure there are many more qualified users who can help but I have cut a bit of brass and thought I’d share my observations.

I use a similar diameter cutter but mine is a 2 flute. My feed, however, is much slower than yours - I use 56mm/min or 2.2 ipm. I based this off of an app called FSWizard (mines for IOS but I think it’s also available for Android).

I noticed that your spoil board has several grooves in it from previous projects. If you use double sided tape, I don’t think it would hold very well. All my brass is secured to the spoil board using blue tape on both the brass and the spoil board with CA glue in between. There’s a thread on this set up and a youtube video too. I find it works incredibly well. I cut small pieces so every square inch of the brass needs to be well secured. The only time something similar happened to me, was when I didn’t fully cover a section of the brass with tape and CA.

Bear in mind that my spoil board is absolutely smooth, no grooves at all so perhaps adding another smooth board on top of the existing one might help with that.

It’s difficult to tell from the photos but your brass looks around 5mm thick - 3/16 I think in imperial. I’m assuming the length of your cutter is adequate.

Hope this is of some use to you.

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20ipm in brass with an endmill that small is pretty fast, and at that DOC, it’ll still take forever. Gwizard is solving to 6.8 ipm, .008 depth of cut, and it still has both a chatter and a rubbing warning. I can’t really see the scale here, and I have some extruded bar like that 6" wide… I’d use a larger tool if at all possible.


I’ll definitely slow it down then. Unfortunately, I can’t really change bit sizes for this instance. The masks are nested with the bit diameter taken into account and part of them is already cut. I’d like to try and finish the three that are left. They’re about two inches high and 1/8" thick.

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