Free 186 More Best Homemade Tools eBook:  
Get 2,000+ tool plans, full site access, and more.

User Tag List

Results 1 to 1 of 1

Thread: Calculating tap drill size

  1. #1
    Supporting Member mklotz's Avatar
    Join Date
    Aug 2015
    Location
    LA, CA, USA
    Posts
    3,434
    Thanks
    357
    Thanked 6,380 Times in 2,117 Posts

    mklotz's Tools

    Calculating tap drill size

    The suggested tap drills on most charts are the size needed for a 77% depth-of-thread (DOT). If you're working with hard to tap materials, you can significantly reduce the tapping effort and chance of tap breakage by using a smaller DOT with only a very small loss of joint strength. The DRILL program on my website will suggest DOTs and calculate the required tap drill but it's not hard to do it yourself with a desktop calculator.

    The generalized formula for 60 degree threads is:

    TD = MD - 0.013*DOT/P

    where:

    TD = tap drill size
    MD = major diameter of thread
    DOT = desired depth of thread expressed as a percentage
    P = thread pitch expressed as TPI

    As an example, for a 1/4-20 thread with 75% DOT we have

    TD = 0.25 - 0.013*75/20 = 0.201"

    which corresponds to a #7 drill.

    Some of you may be familiar with the simpler formula

    TD = MD - 1/P

    Here the product 0.013*DOT has been taken equal to 1. If we solve for DOT, we have:

    DOT = 1/0.013 = 76.9%

    so the simpler formula will always produce a drill that will yield a 77% DOT which is a bit too much for many situations.

    The suggested DOTs used in my program are...


    MILD AND UNTREATED STEELS 60-65
    HIGH CARBON STEEL 50
    HIGH SPEED STEEL 55
    STAINLESS STEEL 50
    FREE CUTTING STAINLESS STEEL 60
    CAST IRON 70-75
    WROUGHT ALUMINUM 65
    CAST ALUMINUM 75
    WROUGHT COPPER 60
    FREE CUTTING YELLOW BRASS 70
    DRAWN BRASS 65
    MANGANESE BRONZE 55
    MONEL METAL 55-60
    NICKEL SILVER (GERMAN SILVER) 50-60

    186 More Best Homemade Tools eBook
    ---
    Regards, Marv

    Home Shop Freeware
    https://www.myvirtualnetwork.com/mklotz

  2. The Following 2 Users Say Thank You to mklotz For This Useful Post:

    Karl_H (Jan 31, 2020), WmRMeyers (May 25, 2023)

Thread Information

Users Browsing this Thread

There are currently 1 users browsing this thread. (0 members and 1 guests)

Bookmarks

Posting Permissions

  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts
  •