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May 4, 2010 6:34 PM   Subscribe

How do I measure the thread pitch of a mason jar?

I am trying to get an alternative lid attachment manufactured for a wide mouth mason jar.

I am trying to get a machinist to make them for me, and i need to find the thread pitch, so that i can get a functional screw-on attachment.

How do i go about finding this information OR how on earth do I go about measuring it? I have a pair of nice digital calipers at my disposal, along with your normal slew of tape measures.

Bonus points for telling me HOW to talk to a machinist to make this as clear as possible. I'm finding their language a little hard to decipher.
posted by furnace.heart to Home & Garden (6 answers total) 1 user marked this as a favorite
You don't measure it yourself, you give the machinist a sample lid. The machinist can measure this stuff more accurately than you can.
posted by caddis at 6:49 PM on May 4, 2010 [1 favorite]

However, if you insist on doing it yourself get thee to Home Depot or some such place and purchase a thread pitch gauge. Still, I recommend letting the machinist do it, then the responsibility for getting the right measurement is in his lap not yours, and he (or she) is going to be more skilled at making the measurement.
posted by caddis at 6:51 PM on May 4, 2010

Seconding caddis, the machinists I know insist on taking the measurement themselves.
posted by mlis at 6:53 PM on May 4, 2010

Threads per inch, measure the thread peak to peak and measure the width of the land between the peaks, how many turns of the lid to move one inch.

found: "Standard jar threads are 5, 6, 8, and 12 threads per inch. The different threads are not geometrically similar. You probably are looking at the standard narrow mouth canning jar. It is 5tpi.
The height of the 5 tpi thread is 0.060". The radius of the thread top is 0.044". The radius of the base corner roots is 0.030" max. The sides of the threads are thirty degrees from vertical."
posted by hortense at 6:54 PM on May 4, 2010

Here's a helpful link.
posted by Floydd at 7:39 PM on May 4, 2010

I agree with the easiest, and most efficient, is to take either the jar and/or the lid to the machinist. He has the equipment to measure it all (thread gauges, thread forms) and you have the added benefit that you can't be blamed if the part doesn't fit because you measured the thread wrong!

Most machinists that I know like to do little projects - just saying "make me this", which they get all day, is kind of boring and mundane - so "hey, can you make me a lid that does this and this to this jar?" is actually a more interesting project and they may be able to suggest some advantages (materials, extra widgets?) for your application than you might not initially be able to think of.

I'd just give the guy the jar, chat with him about what you wanted and see what he suggests. Even when I produce full engineering drawings for low volume or one off parts (like tooling and holding brackets for production/build) I'll often take it to the person who's going to make it and run through it with them to see if there is anything that will make it easier/more elegant/cheaper/more useful. Even if I'd drawn the most perfect part ever (well, of COURSE!), it's often useful and interesting.
posted by Brockles at 6:58 AM on May 5, 2010

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