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  • Rotary Broaching

    A while ago I asked about "drilling/making square holes. I was directed to a site with instructions on how to build a rotary broach. Except for the heat treat and final grind the tool is finished.

    So the question is, "How fast do I run the milling machine?" The author, in his rotary broaching on a lathe, suggest starting at a "fairly slow speed".

    What are your thoughts?

    Thanks in advance

    Pete

  • #2
    You could check Slater or Somma sites (makers of rotary broaching tools) for feed and speed recommendations.

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    • #3
      When I used one at a place I once worked, I ran it at around 100 rpm cutting hex in steel. If you feel that is too slow after you have tried a few pieces, speed it up. Be aware your drilled hole needs to be deeper then the square/hex for chip clearance. Not to worry on through holes. The tool needs to be centered with the hole or the square/hex may "spiral" on it's way down hole.
      I have yet to use the one I have in my shop. A new tool, just got two square cutters for coming jobs.
      Krutch


      Mentally confused and prone to wandering!

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      • #4
        Hi Pete,
        The speed must be slow enough to prevent the tool heating up and losing its temper. The speed will therefore depend on the diameter of the tool. It will also depend on the clearance the tool has in the pilot hole. The pilot hole should be slightly larger than the across the flats measurement of the tool. If there is a slight clearance and the tool is less that 10 mm across the flats then a speed of a few (100-300)hundred rpm should be OK.
        Commercial rotary broach tools will be HSS and can be operated at higher speed.
        Also if the hole is a blind hole then be careful not to over lubricate the tool or the hole since excess lubricant can produce a hydraulic lock that inhibits, or even stop, the progress of the tool.
        I hope this helps
        Mike ( of mikesworkshop.weebly.com)

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        • #5
          The Polygon Solutions technical support page has a downloadable speed and feed chart.
          http://www.polygonsolutions.com/technical-support/

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          • #6
            I made a rotary broach and made the cutter from silver steel. I used it today but in my lathe and on a piece of brass. It is one of the tools I have made that gives me a thrill everytime I use it.

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            • #7
              Originally posted by krutch View Post
              ......The tool needs to be centered with the hole or the square/hex may "spiral" on it's way down hole...
              I seem to recall that having to do with the relief angle of the cutter; saw something about it on Slater's website several years ago. A guy at the company i worked for back in the day made a synchronizing gizmo that synchs the workpiece / chuck to the cutter though, so i made one for myself too.

              .

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              • #8
                Originally posted by Dr. Rob View Post
                I seem to recall that having to do with the relief angle of the cutter; saw something about it on Slater's website several years ago. A guy at the company i worked for back in the day made a synchronizing gizmo that synchs the workpiece / chuck to the cutter though, so i made one for myself too.

                .
                You may well be right there Doc. It has been a while since I did that job and I have yet to use the RB I have and so don't remember all the details of avoiding the "spiral" effect. That effect is more like a slow twist and not actually a "rifling" feature, but such as a square or hex won't enter all the way.
                Not sure a synch device is needed if the tool is applied with positive action and pressure. Could be for aligning the broached hole with some other feature. At least for the job I did all those years ago the hex was not tied into any other feature and I just loaded the parts and applied the broach. This was done on a BP and I was so impressed with the results I bought a RB when I had my shop up and paying. As I noted I just recently acquired two cutters for some upcoming tools to be made.
                Krutch


                Mentally confused and prone to wandering!

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