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Thread: ID thread measurement

  1. #1

    Default ID thread measurement

    Hello all,

    What would you use to measure an ID thread during machining?

    I have thread mics for OD thread measurement and have seen the caliper anvil attachments for ID thread measurement.

    Short of machining a go-no go plug gauge or using an ID groove gauge, I am out of ideas.

    The threads are in the 3 3/8" range.

    I need flexibility in the measuring, as this is a one of part, and need a solution for ID thread measurement in the wide range of sizes one might encounter.

    Thanks in advance,

    bgmnn

  2. #2
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    Default

    the cheapest and most readily available (gage) is a screw of the size you're cutting. If close or other tolerances are required, then the (commercial)thread gage is the only alternative. If it is a special, its most likely going to cost more for inspection equipment.
    gvasale

  3. #3
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    Default

    If you need quantitive pitch diameter of an internal thread and you don't want to invest in a go/no-go plug gage set, here's how.

    Make a plug gage a bit undersize of any convenient material that's readily machinable. Measure and note the actual pitch dia using wires. Make sure the form is good. Leave a short round dia to extend for indicating against.

    After boring the internal thread clean the threads thoroughly. Screw in the plug gage. Measure with a dial indicator the diametral play of the plug in the threads Be aure the thread is fully engage, parallel to the internal thread axis, and the full max and min readings are taken. Add the diametral indicator reading to the plug gage's actual PD to determine the actual internal PD of the newly cut thread.

    This is not foolproof even when carefully done. Plug gages and calibrated dial comparator gages are the definitive method for PD. Sectionng and optical inspection are definitive for thread form conformance. A lead following inspection is proof for pitch and drunkenness. Etc but the "sloppy plug gage check" is good enough for non-critical short batch thread production

    I have sacrificed an ID mike by silbrazing balls on the measuring tips. Measure over the balls; note for later calculation. Take a reading with the balls engaged in a thread. Work the calculating using the two wire formula compensating for the ball dia by using the odd sized wire formula. This trick is limited to situations when the mike can screw down to clear the minor dia. it's also expensive because the ID mike will be ruined for normal use by brazing heat.
    Last edited by Forrest Addy; 08-01-2009 at 04:55 PM.

  4. #4
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    Default "Nuts!!"

    Its as simple as falling off a log.

    A pair of internal spring dividers, a few small steel balls a good external micrometer and a cheap CAD system and access to a good oxy/fuel/acet welding set and you are there.

    A good internal screwing tool that can be accurately set up is a requirement as well.

    A good "nut" or mating part is always a useful tool too - just to be sure and just in case!!

    I will do the sketches and post it later.

    This old shibboleth has come up lots of times and with the same predictable results. I have left it alone until now.

  5. #5
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    Default "Bump"

    This is "bump" to keep it where I will (hopefully) see it.

    I have the answers, but need to pull all the strings together.

    I am bogged down in another couple of threads.

  6. #6
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  7. #7
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    Default

    Quote Originally Posted by Forrest Addy
    I have sacrificed an ID mike by silbrazing balls on the measuring tips. Measure over the balls; note for later calculation.
    Great post Forrest -- thanks!

    By the way, I cheat and use Cerro-Safe. It's a Bismuth alloy that's very similar to Cerro-bend ("fixturing alloy"). A lot of gunsmiths use it to measure chamber bores -- it's about $14/lb, but you only need about 4 ounces. McMaster carries it.

    You heat it to 180* (hot water), pour the melted Cerro-Safe in the threaded region, let it cool for 15 minutes, and unscrew. Let it settle for 30 minutes, and then you have a perfect impression of the internal threads.
    "Twenty years from now you will be more disappointed by the things that you didn't do than by the ones you did."

  8. #8
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    Default

    That's a neat trick. Not sure if it's the same stuff, but McMaster want about $40 for a stick of 1.5bs.
    Last edited by lakeside53; 08-03-2009 at 10:37 AM.

  9. #9
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    Default GP

    Quote Originally Posted by lazlo
    Great post Forrest -- thanks!

    By the way, I cheat and use Cerro-Safe. It's a Bismuth alloy that's very similar to Cerro-bend ("fixturing alloy"). A lot of gunsmiths use it to measure chamber bores -- it's about $14/lb, but you only need about 4 ounces. McMaster carries it.

    You heat it to 180* (hot water), pour the melted Cerro-Safe in the threaded region, let it cool for 15 minutes, and unscrew. Let it settle for 30 minutes, and then you have a perfect impression of the internal threads.
    Lazlo.

    There is quite a range of similar - but varied - types and grades:
    http://www.amacgroup.com.au/Alloys/Fusibles.html

    http://www.amacgroup.com.au/Alloys/Product.html

    http://www.google.com.au/search?hl=e...cr%3DcountryAU

    Many years ago, the burned cavities in gun breech-chambers and the start of the rifling were measured by using "gutta-percha" on a device that was similar to that used to expand foot-wear. The GP was heated in hot water than placed on the "expander" and left there to cool. The expander was collapsed and the now-hardened GP used to check the "wear" and defects. It worked very well indeed.

    This post also acts as a "bump" as I have yet to complete my skecth referred to previously.

  10. #10
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    Default Getting the message

    I have completed the sketch - including methodology.

    My next post - which will include the sketch - will not only take a bit of time but should get really stuck into some more old "fairy tales", "legends" and plain bull-$hit as regards the sacred "3-wires" method of measuring screw threads.

    It should be easy to follow.

    Having a CAD system is a requirement. Just a "cheapie" or "no cost" one will do just fine. All else that is needed is an internal spring caliper, some steel balls and access to a gas welder and some silver solder.

    I hope to move some of the discussion and methods (and people and thinking) from the 15/16/17/18/19/20th. centuries and into the 21st. as regards internal thread-cutting measurement.

    Anyone with a closed mind or just looking for an argument or afraid of some "tradition" being challenged may well choose to "give it a miss" and let those who have an open mind and who want to join what I hope will be a good discussion.

    I am not saying that I am 100% - or any %-age - right, but I am open to new ideas and sensible discussion.

    Later.

    So - we - or I - will give it a go shortly.

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