Breaking Cutters With Tool Deflection: An Anecdote

This is article is part of the new Tool Deflection page in the Milling Feeds and Speeds Cookbook.

Not long ago I got a call to go visit a shop and check out their new Volumill HSM module for GibbsCAM. Being a fan of HSM techniques, I couldn’t resist. Volumill is indeed very slick, though we noticed it was leaving some pretty severe nicks in the walls and rough spots in the floor of a pocket. My friend commented that the dealer had suggested Volumill was focused on roughing, and so not having a smooth finish was really not an issue. It certainly did radically increase the MRR’s on his job, which I think he said went from 26 minutes to 8 minutes or something similar.

I asked to see the job run, and while he was teeing up the tool in the changer, I remarked that it seemed like he had a lot of stickout for the 3/16″ EM he was using. Perhaps tool deflection was the reason for the rough finish? So, not only did we change the stickout, but we went to a more rigid collet chuck while we were at it and fired up the job. We couldn’t see much through all the coolant, but we distinctly heard the tool break near the end of the job. Darn!

Note the little wall caused by tool deflection (red arrow)…

After pulling the part out of the machine it was pretty easy to see what had happened. The program for the pocket contained two passes. First was a pass using VoluMill to rough out the interior of the pocket. The machinist had then programmed a second pass that was a standard constant stepover that would work from the outside inward to clean up the rough finish of Volumill. The immediate cause of the failure is the thin wall I’ve pointed to in the photo with the red arrow. That wall turned a partial stepover cut into a full slot which reduced chip clearance and likely shrouded the cutter from getting enough coolant. The cutter didn’t get very far along the wall before built up edge caused the aluminum to start welding onto the cutter. You can see the typical debris from that all along the left side–it looks like mud, but it’s aluminum. Most machinists have had this happen before and they know it doesn’t go on very long before a tool breaks.

The key question: Why was that little wall there? At first I blamed Volumill because the thickness of the wall seemed on par with some of the other little dips and divots of the toolpath. However, that blame turned out to be misplaced. After I got home with the g-code, I plugged the parameters into G-Wizard and determined that the toolpath had been severely deflecting. The cutter was climb milling, so as we mention above, the deflection would tend to be perpendicular to the direction of cut. In this case, the cutter deflected deeper into the cut and left the wall. If it had deflected away, we never would have seen a problem.

In retrospect, a lot less stepover probably would’ve improved the performance and finish of the HSM path as well as eliminating the deflection that eventually took out the cutter.

It pays to be aware of and control your tool deflection!

Related posts:

1. Tool Length Offsets and Tool Data Management: A New CNC Cookbook Recipe
2. Climb Milling a Must for Thin-Walled Parts
3. Part 2 of Tool Data Management with Tool Presetting, TDM Software, Chatter Management, and More
4. G-Wizard Custom Tool Cribs
5. Too Much Stickout Can Ruin Your Day (And Your Cutter & Job!): Use G-Wizard to Check It

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• 12 Great Posts for 2011 « « CNCCookbook CNC Blog CNCCookbook CNC Blog 2012/01/30 at 7:06 pm

  [...] 2011:  Spudgun Madness.  Love the spudguns.  FWIW, readers preferred, “Breaking Cutters with Tool Deflection.”  Go figure.  Machinists are a serious [...]

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