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What are the different ways of measuring the pitch diameter of a screw thread?

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Nels

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#1
How do you use a screw-thread micrometer or three-wire method of checking the pitch diameter of a screw thread?



:tiphat:Nelson
 

Ed Hoc

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#2
Good Question. Let's start with some basics.

USA V-threads are basically 60 degrees. The amount of clearance between bolt and nut determines the class of thread, Class 1 looser and Class 3 is closer together. Machinery Handbook has tables for this info. Many other sources too. Mics and wires measure only male threads. Solid gages measure female threads.

Thread mics I have used are direct reading. Anvils are changeable, to fit the pitch of the thread. My boss got a new thread mic from MSC last month, It was about $150 US Money. Mitutoyo or B&S were over $500 each. The chinese one was terrible, we sent it back. We did the thread measuring with wires.

Wires come in a set, they are precision tools. My set of Pee-Dee wires cost under $20 many years ago. Plenty good for most work. The chart included has a "best wire size", determined by the pitch. There's alot of math involved but use the chart it's well accepted. The chart has a column "ADD" and a column "CONSTANT". (I'm doing this from memory as my tool is at work, so please forgive me if I go wrong.) Take your nominal thread OD, add the "ADD" figure, this is what you should measure over the wires, when the thread is finished. Use a regular mic. Subtract the figure "CONSTANT", that will give you the Pitch diameter. Keep cutting til you are in the tolerance you need.

The tricky part is holding the wires on the work piece. I usually place a rag under the part to keep the wires out of the chip pan. I don't do anything special, just balnce two wires on top, hold them in place with the mic's spindle. With my left hand I get the third wire in the appropriate thread and adjust the mic. Takes alot of practice, but it's not impossible.

I'm sure there are others who can add to this or clarify / correct my efforts (please?)

Good Luck
Ed Hoc
 

Tony Wells

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#3
With a bit of math, using the actual OD of the thread, you can calculate the measurement over a single wire.

The Machinery Handbook has a good explanation of the method of calculating for wires, including a compensation for the thread's helix angle, which has a bearing on the measurement. I cheat, GageMaker software will calculate any thread and recommend a best wire size, which is as close as possible to contacting the thread flank at the pitch diameter. That way the error possible in the UN thread form 60 degree is minimized.
 

Ed Hoc

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#4
Yes those triangle things are pretty cool. I've used them a time or two. They are a little tricky to use, but once you get the feel for them they work well.
Watch out for those rubber bands on your wires, they will fling those little wires across the shop if they get the chance!

This is a really good 'thread' nuk nuk nuk

Ed Hoc
 

British Steel

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#5
I've always held the three wires poked in either a wine cork (good justification for a bottle of Chateau Lamothe...) or a pencil eraser, easy peasy!

Dave H. (the other one)
 

brucer

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#6
I usually use the rubber band method.. i have also used thread micrometers on specific jobs at work..
 

TOOLNUT

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#7
How do you use a screw-thread micrometer or three-wire method of checking the pitch diameter of a screw thread?



:tiphat:Nelson
I have read the replies and they are all pretty much right on target I can only add my agreement with the rubber band method and mention something that I have also used many years ago....A small bit of modeling clay applied to each end carefully to hold the wires together. Might be especially good for beginners with only two hands.
Good Luck
Jerry
 

Tony Wells

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#9
There is an instrument that no one has mentioned yet, marketed by GageMaker. Essentially, the simple version uses a contact ball fixed on one end of an adjustable "C" clamp shaped rig with a 1" dial indicator on an adjustable arm. The indicator also has a ball mounted in the stem. They offer software to calculate the setting distance between the two balls to the exact PD to the basic PD, then you read the variation, either plus or minus from where it is set. The software compensates for the helix error, and can be adjusted for different ball sizes, in case you don't have the recommended ball size. Just like wires, there is an ideal ball size that contacts the flanks close to the pitch line. They offer contact balls for Acme and Stub Acme with precision ground flats, since the 29 deg flank angle would allow the ball to hit the root, or minor diameter before it made contact with the flank at the pitch line. This is also offered in an internal version.

Additionally, they make a similar instrument that used what they call "rolls" instead of balls that contact several threads at once. Helps detect lead error instead of a separate lead gage,which they also offer.

All in all, it's a little awkward a system, but if you don't have a ring or plug gage, it's better than nothing. One advantage is that you can measure many,many threads with the system and not have to have a huge gage inventory. They also offer an instrument for working with tapered threads as found on API tool joints.

Oh, and it's rather expensive, so not likely practical for the hobbyist.
 

4gsr

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#10
Yep, I'm a big beliver in using Gagemaker, been around and used their equipment for many years. We only send work to shops that are equipped with their equipment for machining parts with threads. Finnaly convienced them they had problems with the thread rolls for Acme and Stub Acme threads, now fixed. Like you said Tony, "not for the home shop!" unless you have lots of money to throw around.

One word of though on using three wire method, use a dab of grease to hold the wires in place while taking measurements. Works everytime!
 

Tony Wells

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#11
I first became acquainted with GageMaker in the 80's, as I remember it. Bought everything from Deterco Sales. I know Gary Deterling, who started that distributor and he helped get me set up with all the ID and OD stuff. Later came the API stuff. I still have a thread height gage and a lead gage with a few standards, but I don't have the whole setup any more. I do have the software, and use it quite frequently.

I think they changed the company name now to something else, but I can't recall it. Houston Precision maybe?
 

4gsr

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#12
I first became acquainted with GageMaker in the 80's, as I remember it. Bought everything from Deterco Sales. I know Gary Deterling, who started that distributor and he helped get me set up with all the ID and OD stuff. Later came the API stuff. I still have a thread height gage and a lead gage with a few standards, but I don't have the whole setup any more. I do have the software, and use it quite frequently.

I think they changed the company name now to something else, but I can't recall it. Houston Precision maybe?
Use to deal with, the company I worked for, Gary back when I was at Huntsville years back. He used to come in and work on our prehistoric J & L comparater we had then, later retired and a newer one took its place. He still around, I haven't dealt with him since then. At Huntsville, we had over 20 PD internal and external sets, with 100's of rolls for the instruments and one mic track. They used it for calibrating everyone's mics, calipers, etc.

Back when I first started using Gagemaker, we dealt with them direct, back around '87, '88?

I believe they have a dealer network they deal thru today.
 

Nels

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#13
Anyone have a picture of the Gagemaker?


Nelson
 

Tony Wells

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#16
Thanks for the links, Gordon. I know you from another site, that will remain un-named. We do have a section for member sites, so I may move this one, or at least copy it to that sub-forum.

Welcome to the quiet side of the river.
 

Nels

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#17
Welcome Gordon!

Your link is appreciated. If you have a list of links, just post them here or in the site forum and we can sticky them for people to have handy.

Nelson
 

bfd

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#18
I use thread wires. taping the wires to a light piece of cardboard 1 wire at one end and 2 wires at the other end. fold the cardboard in half lining up the wires. stops you from dropping the wires into the chip pan never to be seen again. bill
 

kd4gij

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#19
Where I used to work I used a comparator. We also had A Johnson Thread gage.

upload_2017-6-19_21-56-11.png
 

scwhite

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#20
How do you use a screw-thread micrometer or three-wire method of checking the pitch diameter of a screw thread?



:tiphat:Nelson
My thread wires have three wires , two on one side and one 180 degrees opposite side .
And it also has a chart with the set that tells you what to add to the Outside diameter of the part you are threading .
I will take 1" masking tape and lay two wires next to each other about the thread pitch then the other wire 3" or 4" and fold the tape back over to meet the end . Tear it off and Wright on the tape the Dia. The chart calls for . For example if you are threading a
5/8 - 11- tpi. You would use the .055 and you would add to the diameter of you part .0273 = .6523
Is what you will thread to I would cut .652 or .651
The masking tape makes it easy to work with
And if you drop one you drop all of them in to the chip pan you want have any trouble finding your thread wires . If you drop one in the chip pan with out the tape on it . It can be very hard to find in a chip pan half full of chips .
 

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Silverbullet

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#21
I have a set of triangles which fit on the micrometer. I had some weird threading job forty years ago , I was going to buy the wires but these were easier to use and the job passed inspection so I guess they were done right. Anyway they cost a bit more but the ease of use made up for it. Don't even know if they still sell them . Pretty sure they worked for any 60 degree thread.
 
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