# Thread: Calculating the Size of the Tap Drill

1. Senior Member
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## Calculating the Size of the Tap Drill

I need to drill and tap a hole for 1x28 threads. To determine the size of the bore hole for the threads, I looked for the formula to calculate the size of the hole so the threads would be a 75% Full Thread. In the Machinery Handbook, I found this formula.

Hole size = Major Diameter - (1.08253 x % Full Thread) / (Threads per inch) Where % Full Thread is entered as 0.75

As a test, I used this formula to see how close I could match the table listed in the Handbook. As an example I picked 10-32 bolt, which according to the table requires a #21 drill (0.1590"). Using 75%, the above formula calculated a hole diameter of 0.1646". This is 0.056" larger than the hole specified in the table. This seems to be excessive.

In another machining book, I found the following formula for calculating hole diameter.

Hole size = Major Diameter - (1.299 x % Full Thread) / (Threads per inch) Where % Full Thread is entered as 0.75

Again as a test, I used this formula to see how close I could match the table listed in the Handbook. Again, I picked 10-32 bolt, which according to the table requires a #21 drill (0.1590"). Using 75%, the above formula calculated a hole diameter of 0.1595". This is 0.005" larger than the hole specified in the table. This seems to be more in line with the numbers in the table.

Why are there two different formulas? Which one is considered to be correct? Am I taking things out of context and this is why there is a difference? Are the tables based upon a different percentage? There was no percentage give for the table. Am I doing something wrong in the calculations? Appreciate any assistance. Thanks.

2. Senior Member
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Usually in charts they give tap hole to 90% of a thread.

Ie. #7 .201 drill for 1/4-20 tap is 90%

75 % would put it at .209 dia

.005" bigger that the chart gives. Which is what it should be.
Looks like you ve messed up the decimal point
And second formula is wrong for both metric and UN threads.

More knowledgeable guys may say what that second formula is for
Last edited by Zero_Divide; 04-09-2013 at 06:32 AM.

3. DR
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Bill,

The most important factor in sizing a tap drill is basing it on the length of the thread engagement. "Thread engagement" being the thickness of the tapped material for through holes. Using a chart or formula with no adjustment in drill size for length of thread engagement could lead to tap breakage or at least unnecessary wear on the tap.

Get a chart that shows tap drill sizing based on this. I know it would be nice to have a formula, but a chart is handier. The best one I've found is the Greenfield "Screw Thread Manual", for both common inch and metric threads, male and female. Back in the day they gave these away, I think they sell them now.

4. vpt
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I never go to formulas for tap drills. I use a hole gauge. Take the tap and start sticking it in holes until the first two threads fit down into the hole and than find the appropriate drill that fits the same hole.

If I want a tighter fit (more thread engagement) I find the hole that only one (first) thread of the tap fits in, and likewise for a looser fit (less thread engagement) with a hole that 3 tap threads fit in or more.

Simple and works every time.

5. Dr Stan Guest
Another method is to look up the double depth on a fish tail (AKA center gage) and multiply it by .75. Subtract the result from the major diameter. Especially on something like a 1" dia thread that should get you very close.

6. Senior Member
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The simplest method is to divide 1 by the thread count and subtract from major diameter. Use the drill closest to this size to drill tapping hole.

For example; 1/4-20: 1/20=0.05". 0.25"-0.05"=0.20". Use a number 7 drill.

7. Senior Member
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Tap drill size. The diameter doesn't matter. Find another thread of the same pitch, different dia. In your case, 1/4-28. According to the charts, in steel and Iron, tap drill is a #1, or .228 dia.
.25 dia thread -.228 tap drill dia = .022. For a 1-28 tap drill the dia would be 1"-.022= .978 dia. tap hole dia.

8. Senior Member
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The formula with the 1.299 constant (yields TD = 0.1596) is for the American National thread form.
The formula with the 1.0825 constant (yields TD = 0.1646) is for the Unified thread form.

1.08253... = (5/8)*tan(60)
1.299... = (6/8)*tan(60)

Note that, in the formula:

TD = MD - 1.299 * %DOT/TPI

if we set:

1.299 * %DOT = 1 (or %DOT = 1/1.299 = 0.77)

we get the oft quoted approximation:

TD = MD - 1/TPI

which says that this approximation will always yield a 77% depth-of-thread. It's these values that are commonly found in thread charts.

9. Senior Member
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Originally Posted by Zero_Divide
Usually in charts they give tap hole to 90% of a thread.

Ie. #7 .201 drill for 1/4-20 tap is 90%

75 % would put it at .209 dia

TD = 0.25 - 1/20 = 0.2
#7 drill = 0.201
My Starrett tapdrill chart says to use a #7 drill.

90% depth-of-thread would require a tapdrill of:
TD = 0.25 - 1.299 * 0.90 / 20 = 0.192

10. Senior Member
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Just subtract the pitch. Inch or Metric.

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