Breaking Bit on Purple Heart

We’ve successfully engraved over 100 small items in maple, walnut, padauk, and purple heart using the 1/32" diameter Downcut Fish Tail Spiral Bit. Cut settings are 30 in/min Feed, 12 in/min Plunge, and 0.015 Depth per Pass.

Blanks are 3 x 24 x 1/4" from Rockler, so the thickness is pretty uniform per piece (0.254 - 0.261) and they are evenly supported using six points of hold down clamps.

This morning, we broke 2 bits in a very short time frame, the first bit had cut all of the previous pieces so I assumed fatigue or dulling, but a new bit broke in about two minutes of cut time.

Thanks in advance for any suggestions on my cut settings to prevent breaking of more bits.


I know you mentioned the pieces are regular, but have you used calipers to check the problem piece? It would not take much extra thickness or an irregularity to snap that bit.

Wood hardness can vary quite a bit within a species depending on where it was harvested. You may have an especially tough piece, or perhaps an entire lot of them.

Would it be possible to use a tapered 1/32 bit to do the job?

These are much more robust than straight bits, but they are pricey. However, the carbide and coated carbide bits are fantastic for extremely hard wood and last much longer than HSS bits.

Thank you sir!

I do check thickness on each piece prior to clamping it down, so the thickness for this piece is 0.254 - 0.251 checking in a couple places.

That $35 dollar bit is the same cost as the two I just broke, so I’m ordering now…I owe you one my friend.