Bit Breakage, Cut Depth and Bit Diameter

With a thin bit working in thick material, multiple passes may be needed to keep from breaking bits. October 26, 2012

Question
I am running multiple passes of straight runs on thin veneered MDF. I am using a 1/8" down spiral to just break through the veneer at 2.5mm depth, and following with a 1/8" upcut to a depth of 14.5mm. The feed rate is 250f/m, and the rotation is 19500rpm on both tools. My problem is that the upcut bit keeps breaking. The speed and feed fall into recommended chip load rates, but I am wondering if I should speed up the feed rate to avoid overheating.

Forum Responses
(CNC Forum)
From contributor J:
I would try cutting in 3 tool paths, the first with the down cut, and split the second up. 14.5 mm is a lot for a .125 bit to take.

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From the original questioner:
Thanks. I just changed to the three passes, taking 5mm out with the down, then 7 with the up, and 14 on the final. Seems to be working. Appreciate it.

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From contributor S:
I usually limit the cut depth to, at most, 2X the bit diameter.

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From contributor J:
Even 7 mm is pushing it, but glad it is working.