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Milling a slot

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  • #46
    Nice job JC and goes to prove that there is nothing special about it.

    Industry is doing this millions of times per day.
    If there were a problem it would have been addressed by now by brains greater than our collective ones.

    Industry has now moved onto the over centre type cutting mills as opposed to the centre hole of years ago.

    If there is a problem in cutting slot the same size as the cutter then look elsewhere other than the cutter.
    Spindle bearings, collet or holder run out, rigidity of the setup, in fact rigidity full stop.
    Don't expect to do a 1/2" slot in one pass on a mill drill, the machine will be the limiting factor here, not the cutter.

    If you are limited by your machine, and lets face it not everyone can have a #7,000 vertical at home, then you have to find a way to work round.

    Cuting out with an undersized cutter and then two clean up passes or swap cutters and do an on size cleanup pass or whatever suits.
    You just have to learn to live with the limitations you have and work round them.
    OK it will be slower but I'm sorry that's the price you pay.

    John S.

    Sir John , Earl of Bligeport & Sudspumpwater. MBE [ Motor Bike Engineer ] Nottingham England.


    • #47
      I did a little experiment this afternoon, mainly to satisfy my own curiosity & certainly not to prove a belief, but thought others might be interested in the result.
      I was cutting a slot similar to the OP's aim, 3/16" wide x 10mm deep in brass. I did 3 slots, nominally 25mm long. First (right in the pic) was a plunge cut with a 3/16" dia 2-flute slot drill (Call it what you like )

      Middle one was a 3/16" dia 3-flute centre cutting FC3 throwaway endmill.
      Left hand slot was a conventional 4-flute 3/16" endmill, not centre cutting hence the open end.

      All were done with the same setup, dry, nominal 3000 rpm, 25mm/min feed, reasonably rigid machine. All cutters were new or as new, UK made (Osborn/Clarkson)

      Measured with decent digital calipers, & the calipers checked with a micrometer, the first, 2-flute cut measured .186". The second, 3-flute, at .188/.1885, and the 4-flute at .189". This was at the bottom of the slots, but no real variation on any of them with depth.
      By far the worst finish was with the 3-flute, the others were both pretty good with the 2-flute just marginally the better.
      I don't pretend this is scientific, there may be variations in actual cutter sizes etc, plus I had no air on hand to blow away the swarf - finish might have been better if I had. Feed was just a guess at something which wouldn't cause the cutters distress, optimum feeds might also have influenced the result.



      • #48
        Well Thanks for the explanation John S.,I am very happy to know that us dumb Americans have not been missing out on that magically ground slot drill, it seems to be just a different name.


        • #49
          Thanks for showing the experiments! There might still be help for me yet. As soon as there's not little children running around getting into the chips, I'm going to give the 2 flute mills I have a try. Its going to be tricky getting the feed down that slow when I'm hand cranking. Practice makes perfect.