# Measuring threads



## workinforwood (Sep 25, 2008)

How do I measure threads to order a tap?  The diameter, would be the thread diameter or the before thread diameter?  I need a tap to fit nib coupler on cigar kit.


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## hilltopper46 (Sep 25, 2008)

For outside diameter of the bolt or screw, include the threads, so threading a 3/8-inch rod uses a 3/8-inch die.

For inside diameter, you use the size of the threaded part that threads into it, even though the size of the hole that you drill for your tap is smaller.  So for 3/8-inch, you drill a hole ~5/16-inch in diameter and then tap it with a 3/8-inch tap.

3/8-inch used strictly as an example.


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## rherrell (Sep 25, 2008)

The easiest way is with a screw pitch gauge. One of those little thingies that looks like a feeler gauge with teeth.
If it's Metric you'll need to determine the "pitch". That's how FAR a part will travel when turned one revolution. Common pitches are M.5, M.75, M1.0 and M1.25.
If it's English then you'll need to know the TPI or threads per inch. This has nothing to do with travel but rather how many "peaks" or threads there are in 1". Common TPI's are 32, 28, 24,20, 16, 13.
Pen parts are usually Metric. Usually when you measure an outside thread it will be a little smaller than the nominal or actual size. If you measure a threaded part at 12.85mm it's a 13mm. 
What you call a "before thread diameter" would be the "Major" diameter on an outside thread and the "Minor" diameter on an inside thread.
In your case, since you have the male part, I would measure it to get your tap size. If you only had the female part you could measure the minor diameter and then add the pitch to get close enough to the correct size. I hope this all didn't cofuse you, it did me!:biggrin:


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## randyrls (Sep 25, 2008)

workinforwood said:


> How do I measure threads to order a tap?  The diameter, would be the thread diameter or the before thread diameter?  I need a tap to fit nib coupler on cigar kit.



A thread gauge is a good tool to have for matching threads.  Cheap too!

I checked a cigar pen from my case.  The thread on the nib is metric 0.75 x 8mm  This is an odd metric thread.  I measured the thread with a thread gauge and it is definitely .75mm thread. At first I thought it might be .8mm.  The major diameter of the coupler thread is 7.84mm 

It is a single start thread.

You should be able to find this at one of the major Industrial suppliers like MSC, Enco, McMaster-Carr, etc.

As always your mileage may vary, your car may not run!


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## rherrell (Sep 25, 2008)

Randy is right in that this sounds like a common thread. Keep in mind that alot of these pen companies don't use so called "normal" threads. Half the time you won't find anything close
Try these guys http://www.e-taps.com/ They have alot of oddball stuff. Be prepared to PAY for it though.:biggrin:


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## workinforwood (Sep 25, 2008)

I'll be threading wood, so I think I only need to be really close, unlike other materials that have no give to them at all.  Thanks all for that info.  I think I kinda understand some of it.  I am assuming if I ask for a single start tap of .75 x 8mm, that's all I need to know?

Oh..never mind that, thanks for that hook-up, I called and ordered that. It's 8mm x .75, in stock, single start $11.70 plus shipping.  I would have bought the die too but it was $26 and change.  Hope it all works out.  I'm sure the numbers are good or at least good enough, but the person on the phone is very difficult to speak to.

I did caliper my part to teach myself how ya'll come up with that number.  The .75 part is a tough one.  I understand what is being measured, but to properly measure with a caliper is impossible.  I'll have to invest in one of those thread gauge tools some day, add it to the list.


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## rherrell (Sep 25, 2008)

Glad it worked out for you, Jeff.


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