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Since I didn't have a chart, I just knocked myself out trying to find out the proper drill to use with a tap. Maybe the info below will save someone some grief.

For tap sizes use this formula to calculate drill size:

Tap Size - Pitch Size = Hole Size

Tap size should be in decimals. Pitch size for machine screw and fractional taps is determined by dividing 1 by the number of threads per inch.

Pitch size for metric threads is equal to the specified pitch in millimeters.

Example # 1:

1/4 x 20 Tap Size - (1 ¸ 20) Pitch Size = Hole Size
That is: .250" Tap Size - .050" Pitch Size = .200" Hole Size

You can use a decimal equivalent charts to find a drill size that most closely matches the hole size. In this case, the drill size is #7 which has a .201" dia. Because the drill usually cuts a hole larger than itself, tap thread engagement will range from 70-75%. Thread engagement from approximately 60-75% is acceptable for thread strength and tap life.

Example # 2: 6 x 1.00 mm

That is : 6 mm Tap Size - 1.00 mm Pitch = 5 mm Hole Size
For the decimal crowd:  6 mm = .2362"   1 mm = .0394"
That is: .2362 Tap Size - .0394 Pitch = .1968" Hole Size

In this case a #9 drill size is .1960" or #8 = .1990" or 5 mm drill = .1968" These metric guys may be on to something :>)  (Don Schneider)

Rule

There is a table of double depths of threads on the center gage. I've heard the table can be used for determining the size of tap drills. My question is: How? Example?  (Don Schneider)

    Multiply your desired thread depth (75% is typical) by the double depth figure. Subtract the result from the nominal diameter.

    For the example of a 1/4-20 thread

    Double thread depth for 20 tpi = .065 (rounded for simplicity)

    .065 x .75 = .049 (rounded, again)

    .250-.049 = .201

    On a drill/tap chart, .201 corresponds to a #7 drill, the correct tap drill for a 1/4-20 thread with a standard tap.  (Larry Blan)

      I should have added that an easier way to calculate tap drill size is to subtract one 'pitch length' from the nominal diameter.

      Again, for a 1/4-20 thread

      1/20 = .05

      .250-.05=.200, the closest standard drill is again the #7

      For a metric thread, simply subtract the pitch from the diameter

      For a M6x1.00 thread

      6-1 = 5mm, the correct tap drill size.   (Larry Blan)

    Just subtract the double depth of thread from the 'nominal' thread size. eg; depth of thread for a 1/4-20 = .0325", nominal size is .250" minus (2x .0325 = ..065"). .250" - .065" = .185" for 100 pct. thread engagement. 75 pct. engagement (common) uses #7 drill (.201" or 13/64ths, .203 ")  (Vince Brannick)

    What the gauge gives you is the minor diameter. You subtract the decimal number on the gauge opposite from the pitch number on the gauge, from the major diameter of the tap. The gauge reads 20  .065 for example. You have a 1/4 20 tap, so subtract .065 from .250 to get .185 minor diameter.  If you had a 1/4 28 tap, subtract .046 from .250 to get .204 to get .204. However, if you actually used those size drills, you would have 100% depth of thread, which is not usually done. Most machinists go with 75% depth of thread, which makes for an easy fit that is still quite strong. Therefore, take the number on the gauge, multiply by .75 then subtract. In the case of 1/4 20, .065 x .75 = .04875.  .250 - .049 = .201.  .201 is a #7 drill size, which is what is normally recommended as a tap drill for 1/4 20.   (Tom Smithwick)

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