CNC Contour Milling and Bit Breakage

      Complex milling with lots of direction changes requires some adjustment of bit speeds in order to keep the cutters cool. August 24, 2008

We are doing some 3-D contour milling of some laminated sheet ply. Our current settings are the following:
1/2" bit (carbide tipped) Onsrud 48-181
200 ipm
16000 rpm
80% stepover
1/4" depth of cut

I am noticing the bit is "howling" which I am starting to think is bit flex, but nonetheless the bit broke midway through and I am a bit perplexed as I thought the settings I am using are conservative. I was hoping someone might offer some advice. Could it be the bit choice? Do HSS bits flex more than carbide and therefore have a tendency to break easier? Thanks for any advice.

Forum Responses
(CNC Forum)
From contributor F:
Your tool's CEL is too long for the cut you're taking. If you're cutting 3-D contour, try a ball-end tool.

From the original questioner:
This issue is happening in horizontal rough cut mode actually and I need a minimum 2" clearance due to the surface geometry I am milling. Is there a bit you recommend that has shorter CEL but same length?

From contributor F:
Vibration causes "howling" which results in broken tool. Use a 3" solid carbide cutter with a 1" flute length.

From the original questioner:
Something else I have been thinking of with horizontal roughing and on this part in particular is that the bit is constantly changing directions and probably hardly ever reaching programmed feedrate. To get proper chip-loading, I would probably then need to reduce rpm (10-12000) and increase the depth of the cut? Does this sound like good logic?

From contributor B:
I bet your tool is over-heating also and this can cause it to break. HSS just cant handle the heat like carbide. Plus HSS does "Flex" more so than Carbide. I only use Carbide Bits except for my 3/4 in diameter roughing bits. Yes your last post is a logical approach and an answer to the HSS bit metal but you are forced to run slower because of the HSS bit.

From contributor C:
Your last post is correct. When your cut has little or no straight runs, it is (almost) impossible to get up to full speed and thus calculated chiploads. I typically run a 2 flute rougher, and when cutting out an irregular shape 2-D, my ideal RPM is 11-12k, though my machine is on the lighter duty side. 3-D tends to be faster as it is typically not taking as big a bite and heat hasn't been an issue. The best thing you can do besides listening, is to collect and measure the chips coming off the tool, and play with settings until you get the desired chip size.

From the original questioner:
To contributor C: which 1/2" rougher are you using?

From contributor C:
I lied, its a 3 flute, not 2 flute, don't know how I goofed that up. Anyway, I use Courmatt's CRU-500-2 for 98% of my roughing cuts, the other 2% is with a larger or smaller diameter. I don't cut cabinet parts very often, primarily MDF for molds/prototypes/carvings, solid wood components, and solid wood/engineered material door parts. Regardless, I always start with a rougher to get rid of most of the bulk.

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