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End mill corner radius (Bullnose), What is enough to make a difference.

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  • End mill corner radius (Bullnose), What is enough to make a difference.

    I have a question about endmills with a corner radius, not roundover cutters for cutting a radius.

    I have two issues that I'm trying to solve.
    1) When cutting steel it seems like the tips chip off of carbide end mills long before the rest of it has worn out.
    2) When cutting UHMW-PE the corners dig in and create swirls that are hard to remove.

    What is a ballbark number for the minimum amount of radius that I need to help with edge life and finish? What I'm looking for is a rough guideline like 1%, 5%, 10%, etc of the diameter will make a noticeable difference.

    I normally buy fractional endmills but saw a a deal on two metric ones and they didn't work out as well as I thought for obvious reasons. One was a 4mm endmill online that had a 1mm radius. That's half way to a ball end mill. The other was a 6mm endmill with a 0.5mm rad. It has so little radius that it looks like a regular endmill unless I check it with a loupe. Between the two it looks like that extra 0.5mm made a huge difference. Or it could be that the 6mm one is the wrong spec and is really 0.05mm rad, it's cheap online stuff. I dug through the old threads and found a lot of metions of people grinding them freehand. Most just said "grind enough to knock the corner off". That works for hss, but given how cheap coated carbide is these days it's not really worth it.

  • #2
    Hi,

    You can get nearly any corner radius you might want. Generally I've found the more you push the endmill, the larger the radius I prefer. Or more likely, how rounded the corners you can get away with. But testing under use is the final arbiter.

    For UHMW, I wouldn't think you would need a lot of radius for strength. Something in that .5mm to 1mm should meet your needs. You can always call a tooling rep to ask for recommendations.
    If you think you understand what is going on, you haven't been paying attention.

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    • #3
      For tip life in metal a small radius like the .5mm you found would likely make a big difference. We're talking about erosion or chipping on the actual metal of the end mill in this case. not the part being machined. So amounts similar to the nose radius used on lathe tools used for similar hard use would not be out of line.

      For improving the finish on plastics and other jobs like that I'd want to see a little bigger tip radius. So I'm thinking more like 10 to 15% of the diameter. So a 10mm end mill would have a 1mm to 1.5mm radius. Or sort of act like an insert cutter of 10mm with either 2 or 3mm diameter round "inserts" if that aids with getting a feel for the issue. I'd expect such a cutter to provide a pretty nice surface finish.
      Chilliwack BC, Canada

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