Breaking end mills

So, I’ve just used a 1/32" Solid Carbide Downcut Fish Tail Spiral Bit for the second time, after successfully cutting out a shallow pocket yesterday, to cut a series of pockets in 18mm plywood to test cut three pockets and some lettering, with the following settings:

Stepover: 0.397mm
Depth per Pass: 1.270mm
Plunge rate: 203.2mm
Feed rate: 254mm
RPM: 10,000
Max Depth: 3mm

But the bit snapped (my first one!)

Are these settings a bit too much for such a small end mill?

It waited until the second of the three pockets to break and, of course, I don’t have another one!


depth per pass looks a bit too much to me: 1.270mm per pass is 1.6 times the endmill diameter, at about 50% stepover, which is higher than my goto rule of thumb of 50%D max (as a starting point at least)

You could input those values in @gmack’s spreadsheet to figure out how much deflection you had, for a more precise answer.

And then there is the matter of runout which becomes predominant when doing micro-machining (which I consider using a 1/32" endmill to be). If you have e.g. 1/64" runout in your router, it won’t matter much when using a 1/8" endmill, but when using a 1/32" endmill it would be 50% the endmill diameter, which at that scale is a huge deal (breaker).

TLDR: go shallower and measure your runout?


[quote=“Julien, post:2, topic:30312”]
[/quote] ?

Thanks, @Julien, good point (erm, what does TLDR mean? :blush:)

I guess it’s “TL;DR” in Internet lingo, which means “Too long; didn’t read”, as in, here’s the summary.


Got it, thank you. It seems to resemble a similar abbreviation: RT(F)M!

Except in the case of TL;DR, the (F)M is too many pages, so please just give me the cliff’s notes! :laughing:

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