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    Default Threads and tolerances.

    This won’t be the thread to do it in but all are welcome to ask screw related questions or give advice in Metrology. I know a good bit about the subject just as there are others that do too.

    No thread inspection or measuring system is “best” but there are many different ways to do things. It often very much depends on how many you make (repairs, single, serial or mass production) and if your threads are standard or special.

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    Your thoughts on optical vs. mechanical checking of threads?

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    Quote Originally Posted by CarbideBob View Post
    Your thoughts on optical vs. mechanical checking of threads?
    If by "your" you mean me then to me it's not one against the other. Depends on what you want to do and how much time and money you want to use.

    Optical though as goods as rules out internal threads.

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    I made this years ago and there is also a page 2-2. Page 2-2 is my own product so I choose not to show it. I have though just made something for optical thread inspection.

    There are of course other methods to inspect and measure threads than the few I show here and they range from inexpensive to very expensive. Again, it's only a question of what you need and are willing to "invest".

    If any find my advantages and disadvantages incorrect or lack information state what and I'll update if they are relevant.

    threads1.jpg threads2.jpg

    I have and use this frequently and it certainly often saves me time on thread calculations.

    ThreadPal - dimensions and tolerances for inch and metric screw threads

    I'm avoiding single thread wires (3 pcs.)
    They are the cheapest way to measure external threads but if time is money then they drive me nuts. I prefer the wires that clip on to the micrometer anvil and spindle but those are expensive and if the spindle rotates tend to fall off. The more chips and cuttings there are the faster they fall off.
    Last edited by Gordon B. Clarke; 12-10-2019 at 05:35 AM.

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    We use optical checking quite a bit. For this we use Reprorubber Putty as the primary means of checking our threads. It's fairly expensive stuff but works well as long as you set it up properly. The time it takes to set up and check isn't as much a concern for us as it is a fairly small cost in time compared to the parts themselves.

    For our more common/smaller API threads we have gauges for them, however for our larger parts it's not very practical. We also do quite a few custom threads that preclude using any sort of standard gauging so we have to rely on checking the thread profile to match the print.

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    Quote Originally Posted by CAMasochism View Post
    We use optical checking quite a bit. For this we use Reprorubber Putty as the primary means of checking our threads. It's fairly expensive stuff but works well as long as you set it up properly. The time it takes to set up and check isn't as much a concern for us as it is a fairly small cost in time compared to the parts themselves.

    For our more common/smaller API threads we have gauges for them, however for our larger parts it's not very practical. We also do quite a few custom threads that preclude using any sort of standard gauging so we have to rely on checking the thread profile to match the print.
    Sounds as if you do "unusual" threads. API threads and Texas seem to go hand in hand.

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    Ok, here's one. Double lead tapered threads at odd angles through organic surfaces. How would you inspect those?

    7537-1.jpg

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    For 1st article and final inspections on o.d. threads we use 3 methods
    Comparator to check form and root rads as well as profile.
    then gages for fit and pd size.
    we do use wires to verify thread pd's and taper especially if its small threads on o.d.s like 4-40 2-56 etc we use wires front and back to to check taper. due to small dia and single pointing. we check in process with wires as well as the above.

    for i.d threads we use plug gages as well as molding it for thread form using the comparator.

    I generally wont run a job with out thread gages either they supply them or I do. if its a big thread mainly i.d. threads then they either supply a mating parts and OK it before we run any more. its also put in the quote and our billing.

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    Quote Originally Posted by mhajicek View Post
    Ok, here's one. Double lead tapered threads at odd angles through organic surfaces. How would you inspect those?

    7537-1.jpg
    Questions apparently directed to me wasn't the purpose of this thread. What I'd like to see were thread on threads started by members asking advice or exchanging know-how.

    To answer the specific question asked I'd personally do as I already do with "standard" (NPT, R and Rc) tapered threads. R and Rc were formerly known as BSPT.

    What I'd require from a customer making tapered threads as described to measure would be the taper, pitch and the pitch diameter at a specific depth. I'd then have the thread inserts (external or internal) made to measure at the specified depth. Tapered threads (API are exceptions) normally have large pitch diameter tolerances.

    This is best shown by the following and the principle can also be used on 2 start threads. It "only" requires a decent knowledge of trigonometry.

    http://f-m-s.dk/3.06.19.pdf

    To measure the type of thread(s) asked wouldn't be cheap but could be done. The price would probably scare most customers off.

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    Quote Originally Posted by Delw View Post
    For 1st article and final inspections on o.d. threads we use 3 methods
    Comparator to check form and root rads as well as profile.
    then gages for fit and pd size.
    we do use wires to verify thread pd's and taper especially if its small threads on o.d.s like 4-40 2-56 etc we use wires front and back to to check taper. due to small dia and single pointing. we check in process with wires as well as the above.

    for i.d threads we use plug gages as well as molding it for thread form using the comparator.

    I generally wont run a job with out thread gages either they supply them or I do. if its a big thread mainly i.d. threads then they either supply a mating parts and OK it before we run any more. its also put in the quote and our billing.
    That doesn't seem cheap but it does sound as if what you make is to spec.

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    thread.jpg

    I wonder how often people do this? Just by looking at a thread can make alarm bells go off. Diameter d (external thread) or diameter D1 (internal thread) are very easy to measure. Too large a flat is a warning sign.

    Personally I wouldn't accept a thread with d below tolerance or D1 above tolerance.

    Thread gauges aren't made or designed to catch those two defects.

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    Quote Originally Posted by mhajicek View Post
    Ok, here's one. Double lead tapered threads at odd angles through organic surfaces. How would you inspect those?

    7537-1.jpg
    Interesting challenge! The immediate method that comes to mind would be a qualified gauge with a mounted ball centred on the gauge line. That would allow a cmm to measure the intersection of the gauge line and the surface along the axis of the thread.

    Without a CMM though...

    Are the threads oriented normal to the surface or at completely arbitrary angles?

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    Quote Originally Posted by gregormarwick View Post
    Interesting challenge!
    Your PM box is full.

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    Quote Originally Posted by mhajicek View Post
    Ok, here's one. Double lead tapered threads at odd angles through organic surfaces. How would you inspect those?

    7537-1.jpg
    Picks it up, squints, yeah, they are good........

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    Question about 'theoretical' accuracy -

    thread wires and regular mics or pitch mics? (assuming both mics are calibrated and thread wires are "good")

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    Quote Originally Posted by Gordon B. Clarke View Post
    Sounds as if you do "unusual" threads. API threads and Texas seem to go hand in hand.
    You can't go 1 mile with out running into oil business around here. API and Buttress threads are the most common for us.

    Unusual is one way to put it. There are times when customers send us an old piece of pipe from some outdated equipment from a manufacturer that either; doesn't exist any more, got bought out, just doesn't support it any more, or they just want too much to make what they need. So we have to get all of our measurements from it, whip up a profile and do our best to get them to fit together.

    I've mentioned it before in another thread, but the largest thread we've run was a 655mmX17mm triple lead. I'd like to see a wire kit for that.

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    Quote Originally Posted by Mike1974 View Post
    Question about 'theoretical' accuracy -

    thread wires and regular mics or pitch mics? (assuming both mics are calibrated and thread wires are "good")
    That post probably better suited to another (new?) thread as it's more about accuracy than threads. Calibration labs always use wires for external threads.

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    Quote Originally Posted by Gordon B. Clarke View Post
    Your PM box is full.
    Made some room in there.

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    Quote Originally Posted by CAMasochism View Post
    I've mentioned it before in another thread, but the largest thread we've run was a 655mmX17mm triple lead. I'd like to see a wire kit for that.
    I know a couple of companies in Finland that make threads in that diameter and pitch range. I believe they are in the timber industry.

    Forest industry in Finland - Wikipedia

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    Quote Originally Posted by mhajicek View Post
    Ok, here's one. Double lead tapered threads at odd angles through organic surfaces. How would you inspect those?

    7537-1.jpg
    What is a "Double Lead" thread? By definition a thread can only have one lead.


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