Thread forming screws
Thread forming screws
(OP)
I'm not very familiar with thread forming screws (not to be confused with thread cutting screws)
How do you find the appropriate drill size for these? I want to attach two thick aluminum plates together, quickly, without having to tap holes. They seem like the best method, but these "odd" threads like 10-16 and 8-18 leave me hanging.
Tap/drill charts list drill size for some % thread engagement... for machine screws.
Any ideas or drill charts out there?
How do you find the appropriate drill size for these? I want to attach two thick aluminum plates together, quickly, without having to tap holes. They seem like the best method, but these "odd" threads like 10-16 and 8-18 leave me hanging.
Tap/drill charts list drill size for some % thread engagement... for machine screws.
Any ideas or drill charts out there?





RE: Thread forming screws
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Cory
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RE: Thread forming screws
I guess I must have been looking at sheet metal screws or something.
RE: Thread forming screws
J
RE: Thread forming screws
For example, if I need to drill and tap 3/8 UNC into an appropriate metal piece, knowing that the pitch for the National Coarse thread of size 0.375 inches is 16 TPI, I would get:
0.375 - 1/16 = 0.375 - 0.062 = 0.313 inches
This corresponds to 5/16 inch drill size.
You would need to use a drill size chart to find the closest corresponding fractional size. This example was presented because of simplicity.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Thread forming screws
You missed the first line of the first post.
"...thread forming ...(not to be confused with thread cutting...)"
Your 3/8-16 needs a .3445 dia. for 75% thread.
This corresponds to a 8.75mm drill size.
RE: Thread forming screws
5/16 drill size is the recommended hole for tapping 3/8-16 UNC-2B listed in the Machinist Handbook. The method I've mentioned is the classical way of deriving drill size.
Typically listing a metric drill size for an imperial thread is not a recommended machine shop practice. Although I have used it on very rare ocassions, it has always generated confusion with machinists. Specifying 8.75 mm drill size for a 3/8-16 UNC-2B box would be unusual, I would wonder if there would be a significance difference in thread performance compared to 5/16.
But then again, that is just me.
Kenneth J Hueston, PEng
Principal
Sturni-Hueston Engineering Inc
Edmonton, Alberta Canada
RE: Thread forming screws
Good Luck
johnwm
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