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Form Tapping - Drill Size Formula


SuperHoneyBadger
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Had a machinist come to me and ask if I had a form tapping drill chart, and if there was a formula he could use to determine the drill size. Told him it was at the bottom of the page, and he was stunned to see how easy it was. Now. Question time for the community: where does the constant 0.0068 come from in the formula? I have called Emuge, OSG and 2 university mechanical engineering depts. No one on the other end of the phone has a better answer than, "engineers sometimes come up with something that works, and we don't often ask about where it's derived from if everything is going well". Most useful was "...it's related to truncation values, P values, and the include angle of the thread..." 

% of Thread Desired = Whole number - 65, 70 or 75

Imperial Form Tap Drill Size = Basic Tap O.D. - (0.0068 x % of Thread Desired)  / TPI

Metric Form Tap Drill Size = Basic Tap O.D. - (% of Thread Desired x Pitch) / 147.06

 

So the 2 constants are the inverse of one another, one uses pitch the other TPI so that's easy enough, but I'm scratching the old noggin trying to determine where they come from in the first place. Just have to keep calling professors and AE's at tap companies until I get an answer, and I'll post it here if something comes up.

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43 minutes ago, SuperHoneyBadger said:

Had a machinist come to me and ask if I had a form tapping drill chart, and if there was a formula he could use to determine the drill size. Told him it was at the bottom of the page, and he was stunned to see how easy it was. Now. Question time for the community: where does the constant 0.0068 come from in the formula? I have called Emuge, OSG and 2 university mechanical engineering depts. No one on the other end of the phone has a better answer than, "engineers sometimes come up with something that works, and we don't often ask about where it's derived from if everything is going well". Most useful was "...it's related to truncation values, P values, and the include angle of the thread..." 

% of Thread Desired = Whole number - 65, 70 or 75

Imperial Form Tap Drill Size = Basic Tap O.D. - (0.0068 x % of Thread Desired)  / TPI

Metric Form Tap Drill Size = Basic Tap O.D. - (% of Thread Desired x Pitch) / 147.06

 

So the 2 constants are the inverse of one another, one uses pitch the other TPI so that's easy enough, but I'm scratching the old noggin trying to determine where they come from in the first place. Just have to keep calling professors and AE's at tap companies until I get an answer, and I'll post it here if something comes up.

Problem with form tapping it I don't find the one size fits all for all materials. Harder metals I might open it up a little more where as with Aluminum or Brass I will hold it almost perfect to the size needed. I have done 12mm x 1.5 mm Rounded Threaded recently and what a nightmare that turned out to be in 7075-T7651. We had to have the Taps EDM drilled so the High Pressure coolant would get to the threads. Without the coolant we could only do two holes before the taps got gummed up with aluminum.

Then I have rolled formed Titanium 6al4V #2-56 holes and run 1000 holes with no issue then hit a batch of material no matter what we did they wouldn't thread form and had to be cut tapped.

We were working with a customer in the last year having quality issues with a #8-32 thread formed features in HRS. They were not 3 flute drilling or reaming the holes to get a superior finish before form tapping them. They were losing one part in 10 because of this. Once they switched over to a 3 flute drill no more issues.

If goes back to there is much art and figuring this stuff out sometimes as there is the science and math of what should work that sometimes doesn't. It takes some good old trail and error to figure it out.

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4 hours ago, Tim Johnson said:

I use this software. It will find inch and metric drill sizes based on thread percentage.

https://www.guhring.com/Tech/tapdrill

"Many variables affect the performance of threaded holes and the taps used to generate them. One very important factor is the drill size used to produce the hole that will be tapped. Most tap drill charts call out only one tap drill size, and that will produce an approximate 75 percent thread. In general, tap tool life can be increased significantly by using a lower percent of thread and we suggest using values between 60% and 70% for most applications. Thread strength is not directly proportional to percent of thread. For example a 100% thread specification is only 5% stronger than a 75% thread specification but requires 3 times the torque to produce."

 

One of the best things I've read in a while

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