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Cutting flutes in a homemade tap?

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  • Cutting flutes in a homemade tap?

    I am ready to cut the flutes in a homemade acme tap that I am making. It is a 5/8-10 LH. I have a 5/16" ball mill that I am going to use for the flutes and I am going to cut 4 of them. Based on a regular 4 flute 5/8-11 tap that I have, it has the flutes around .163 deep and a 5/32" radius for them.

    Should I cut the flutes exactly on center? That tap I have looks like they are, but I thought I remember reading that the cutter should be off center. This will be cuting delrin, but it may see bronze at some time.

  • #2
    I have done both ways on taps and they have worked well in steel, so you should have no problem.
    Cutting Delrin may require a more positive tooth form in my opinion !
    What you want to look at, regardless of the flute size is the angle formed at the tooth face by the ball cutter/depth.
    I know you mentioned that you were duplicating a mfg tap, and that is fine.
    Be sure to stone the cutting face with a stone the size of your ball mill if possible . They make small tapered stones for such work.
    What is important is to not make your flutes too thick....since the thread form is not backed off !
    A 1/8 " thick flute works in steel..Delrin is more forgiving however


    • #3
      I tried making an acme tap for delrin and it worked, but barely. Took an incredible amount of torque to get it though, even with a long taper. You might try making a roughing and a finish tap.

      If you really are going to do bronze, you might want a real tap, not a bad price here:
      Last edited by macona; 11-15-2012, 07:08 AM.


      • #4
        commercial ones are made with profile cutters/grinders so the geometry can be misleading. Cut the flutes with an offset ball end mill cutter, the offset gives you positive rake....if you want it, or engineer it to be neutral for brass. I find it easiest to draw it out, cad or manual in a large scale to figure out the offset. The mistake I made at first was leaving too much profile and not enough flute area - you want lots of flute and little profile or it binds easily. I'm made lots that were odd ball (left hand, 2 start etc) and they've work well in steel but I haven't yet made an acme....I'd be thinking of making several that cut progressively.


        • #5
          I just cut flats when I made one. I cut the flats a bit off center so the cutting edges had more of a angle on them.

          Cut the taper:

          Cut the flutes:

          Cut the threads:



          • #6
            Thanks for the replies. I did cut one of the flutes today on center so I am commited to this. I will have to see. I am going to try it in some scrap first and if need be I will make a rougher or try something all together. How much undersize should the rougher be?


            • #7
              I make taps all the time to repair mechanical antiques. I just file them to a tapered square on the cutting end,or a tapered triangle. That's how they were made in the 18th.C.,and they somehow cut just fine!! I also sometimes grind a flat side tapered down to the centerline,and then grind a relief on the tap. All these methods work quite well,have strong cutting edges(except for the last,which has a single edge),and leave nice,smooth threads. It doesn't take long to make a square tap. They are stronger than fluted taps,too,if you think about it. Those flutes remove a lot of metal from the tap's body.


              • #8
                gwilson, can/would you do a write up or post some pictures of your tap making procedure?



                • #9
                  I tried making an acme tap for delrin and it worked, but barely. Took an incredible amount of torque to get it though, even with a long taper.
                  That's normal. The plastic is flexible enough that it expands a bit when cut with a tapered cutter, especially with a long taper. It then clamps down on the tap. It also develops a lot of heat very quickly since plastic is a good insulator. Don't try to power tap plastics with a home made tap even at low rpm and even with most factory made taps. It will melt the plastic and getting it back out is no fun at all.
                  Free software for calculating bolt circles and similar: Click Here


                  • #10
                    Note taken on power tapping. I was originaly going to do it that way, but had since decided it might not be a good idea. At least I was on the right track that time.


                    • #11
                      Note taken on power tapping
                      That's called "learning from the mistakes of others".
                      Free software for calculating bolt circles and similar: Click Here


                      • #12
                        Don't try tapping any flexible plastic with a tap that is at all dull... or neutral/negative rake. It will just push plastic out of the way, which will then grab as mentioned above.

                        You want to CUT as near to the full profile as possible directly and no foolin around.

                        Keep eye on ball.
                        Hashim Khan