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  • Forming Threads

    I just ordered my very first Thread Forming Tap from MSC.
    I do a lot (millions and millions) of tapping 8-32 threads in 1/8" Aluminum (6061).
    I haven't been able to find a table for recommended hole sizes.
    I assume they would be slightly larger than for Thread Cutting.

    Tom M.

  • #2
    http://www.natool.com/engdata/data04.php
    Free software for calculating bolt circles and similar: Click Here

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    • #3
      <font face="Verdana, Arial" size="2">Originally posted by mayfieldtm:
      I just ordered my very first Thread Forming Tap from MSC.
      I do a lot (millions and millions) of tapping 8-32 threads in 1/8" Aluminum (6061).
      I haven't been able to find a table for recommended hole sizes.
      I assume they would be slightly larger than for Thread Cutting.

      Tom M.
      </font>
      Tom,

      Greenfield recommends a #25 drill, .1495, for a #8 with 65% thread.

      Theoretical hole sizes are .1523 for 55% and .1484 for 75%. As you can see it doesn't take much difference in the hole size to make a fairly large difference in thread percent. It's a good idea to keep watch on your hole sizes, if a drill starts cutting oversize you can lose thread percent fast.

      With thin, soft materials if the part permits I like to go small on the drill size and the material will extrude out on the back side during tapping giving a longer length of engagement.

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