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Post by samcrow on Sept 13, 2007 21:03:37 GMT -5
how do you tell when a carbide bit is bad? if it gets hot and black is it ruined even if the black comes off with a wire brush?
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Post by romaxxcnc on Sept 14, 2007 9:31:35 GMT -5
Sam, Thats a tough call. A few things that could cause that are a dull cutting edge to begin with or feeding too slow for the RPM, friction heating. Which would degrade the cutting edge. Remember the cutting edge of most tools is thin and gets thinner on out to where it actually does the work. The keener the edge, the better the cut and shorter the life. If the core of the tool got hot, one can only speculate how hot the actual cutting edge got. In high speed steel tools this is a huge consideration. Carbide is much more durable and tolerant to high heat, but it can be destroyed as well by excessive heat. This is why we have to use some sort of continuous liquid cooling in metal working operations. The word "Continuous" in that last sentence is very important. It's a common practice to use "squirt bottles" on metal cutting tools. Very bad practice. This allows the cutter to over heat then be overly quenched in an almost heat-treat like fashion during the cutting operation. The analogy I have used over the years: Would be like draining the cooling system of a car, let it idle, and occasionally throw a bucket of water on the engine" It wouldn't run long like that. And neither will cutting tools. Wood that is not seasoned completely with alot of sap can cause blackening of the cutters, friction and over heating as well. Cutting feed is important, the material itself actually cools the cutter. Allowing the cutter to dwell or moving too slow allows the material to heat up around it and the cutter gets hot. Nick added the "Cold shot" gun to his system and that seems to work really well in keeping the cutter cool. And it doesn't consume much compressor air. He posted a pic of it on the forum here: romaxxcnc.proboards59.com/index.cgi?board=discuss&action=display&thread=1183078455In the case of carbide tools and their cutting egde, it varies much between vendors. Some are sharp like a knife and some seem dull right out of the tube. In wood working, tools can't be sharp enough. It's important to find some good sources for tooling that one can rely on. The carbide I get from J&L industrial seem good quality and are usually always very sharp. It's disappointing to pay 30 or 40 dollars for a cutter that feels used right from the get go. "Whiteside engineering" is supposed to be one of the foremost suppliers of woodworking router cutters. If you guys have some links to good cutter vendors, would be nice to post them on this thread. Then I'll get them put on the website. Hope this helps out, Ron
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Post by samcrow on Sept 14, 2007 20:18:31 GMT -5
Thanks for the info Ron, I tried the rough pass on a larger version of the "leaf bowl" using a ball nose 1/4" carbide endmill. I was running pretty fast, 200 to 300 ipm. I was making .1" passes, the wood is new kiln dried cherry, pretty hard for cherry. I got blackened after an hour or so. I am thinking I should be making shallower cuts, perhaps .05" while keeping the speed up.
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Post by romaxxcnc on Sept 15, 2007 9:53:56 GMT -5
Sam,
Cutter strategy is very important, of course nowadays it's all handled in the CAM side of most drawing software. Ballnose endmills inherently heat up because the tip really isnt cutting anything and is continually being plunged into the material. Most Cam programs it seems they assume some sort of external cooling will be used. And probably isn't even taken into consideration when designing the software. Of course one could program some dwells into the G-code routine and allow the cutter to cool at intervals on exit of the material.
For long 3D operations, I would definitely have a look at the cold-shot gun like Nick is using.
And it's possible you received a bad/dull endmill. When checking tooling for sharpness they should "grab" when lightly dragging a finger across the edge. And that really varies between vendors and even batches from vendors.
Thanks, Ron
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