Friday, November 11, 2016

Is This Normal For an End Mill?

I bough a  5/16" x 1 5/8" long Kodiak end mill and finally removed it from is sleeve to install it.  Every end mill I have ever seen has a flat spot on the non-cutting section so that the set screw in the mill holder can lock it down.  This did not.  It appears to matter; the mill would not stay concentric and of course cutter wider than 5/16".  I think I will send it back.


  1. I'm not sure what determines whether a make puts a flat on a milling bit. I have lots of them without a flat, more without than with, IIRC.

    I suspect they may be intended for those installed in adapters, like those used in large CNC machines with high HP ratings, and a carousel to hold all those adapters. No setscrews in normal collets, of course.

  2. Many end mills have no flat, especially carbide mills. Bear in mind that an end mill will nearly always cut a bit oversize, if you want a slot 'on the nose', you must use a smaller mill and make multiple passes. I measure the mill's diameter and use that if it's different from the nominal size. Start on the centerline of the slot, then move side-to-side by 1/2 the difference between the mill diameter and the desired slot width.

    If it's really critical, you must measure the slot width and adjust accordingly. Best way to measure is with a gauge; a drill bit will do if it's the appropriate size, otherwise turn something on your lathe.

    DROs on the mill help a lot, otherwise you can use dial indicators.

  3. BTW, you can always put your own flat on it, using a grinding bit or wheel. Dremel, etc. Try to keep it perpendicular to the radius if you do this. Doesn't have to be very deep/big, as long as the screw tip fits into the area.

  4. Or just drill a dimple into the mill shaft. Might be easier than grinding a flat.

    A problem with a setscrew may (I don't know for certain) be that by locking the mill to the collet, you might break something if it jams during cutting.

    If no flat, it may slip before breaking.

    John Henry

  5. John, I think his little mill is belt driven. Might be o-ring type belt. I don't think he can hurt the mill itself. Usually the part is what takes a beating if it bites, which is what typically happens in most belt drive systems, even the 1-2hp Bridgeport types. If the bit is small enough, it breaks off flutes, or shears the shaft, sometimes.