Thread forming screws

[gijim]

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?

 

[CoryPad]

TAPTITE is the most common thread forming screw. Here is their brochure that shows recommended hole sizes:

http://www.taptite.net/pdfs/documents/TAPTITE2000-CONTI-REMINC.pdf

Regards,

Cory

Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[gijim]

Excellent, thank you!

I guess I must have been looking at sheet metal screws or something.

 

[JsTyLz]

Machinery's Handbook is also a goo place. It give different values for different material types, sizes, etc.

J

 

[Cockroach]

Typically the drill size is given as 75% theoretical thread height, such as in the Machinist Handbook. Then the drill size is simply the external thread diameter of the screw minus the multiplicative inverse of pitch.

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

 

[JDana]

Cockroach,

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.

 

[Cockroach]

No, I didn't miss the first line. A thread is a thread regardless of the method employed to make it. Would it not have the same geometric size, tolerance, allowance and profile as an equivalent "UN" class cut thread?

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

 

[johnwm]

The difference in recommended drill sizes for TAPPING and THREADFORMING can be visualised by thinking about the chips! Clearly the hole for threadforming will be larger than for threadcutting as no chips are produced and the metal displaced forms the peaks of the internal thread. Don't have the data to hand at the moment, but as I recall hole size for M6 ISO threadformer (Taptite) is around 5.4mm (varies with material thickness) as opposed to 5.0/5.1 for tapping.

Good Luck
johnwm
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