Can Pitch Diameter of thread be measured on and Optical comparitor?
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    Default Can Pitch Diameter of thread be measured on and Optical comparitor?

    If yes, how is it done?

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    Sure it can be measured on an O.C., if you have 60 degree included lines on yours or you have a 60 degree included overlay. I used to do it all the time.

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    No it can't, but it can be compared.

    R

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    Crap. Gonna need some thread wires then.

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    Quote Originally Posted by Dave K View Post
    Crap. Gonna need some thread wires then.
    Or a Thread measuring micrometer. Stop by if you need me to measure something.

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    The "how" isn't difficult. Extend the lines so you have a sharp pointed thread, both crest and root. The pitch diameter is measured from the 50% points. You should be able to make an overlay on clear plastic that's good enough to get the job done.

    There is a subtle pitfall, but everybody seems to ignore it and it probably doesn't matter for many things. Ideally you should tilt the thread to the helix angle. Now one side will be perfectly sharp on both flanks, but the other side will be a bit fuzzy. You do the 50% thing on the sharp side, then establish the center-line of the part. Measure to that and double it.

    This is way easier with a video microscope and some software to draw lines and do distances. Remember that thread specs are only meaningful if the thread is cut to the right angle.

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    What Conrad Said.

    Also I have two comments: We used to print our own comparitor overlays from CAD drawings onto over-head projector stock using an Ink-Jet printer. We were extremely surprised how accurate the lines were from that $50 printer at the time. The over-head stock is available from Staples or Office Max. It's like cellophane that accepts the ink. Probably works with laser jet as well. Would be way better than scribbling on your comparitor screen.

    My second comment is more of a question: I thought J & L developed Optical Comparitors to inspect threads and that was their original function. So why wouldn't you be able to inspect threads on one now?

    P. S. Some of the shops I worked in used only optical inspection equipment. They were calibrated regularly and often the only reference we had to NIST for QA. They never used surface plates.

    Best Regards,
    Bob

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    The problem with optical comparators is that they are comparing. And it's a 2-D profile, but a Thread is a 3-D feature.

    If the customer accepts that, great. No Aero-Defense contract is going to buy it. Hard Gages only.

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    Depends how you use it. Precision leadscrew moves part in relation to crosshairs. As good as the leadscrew if the feature is clear. Micrometer- compares to a precision leadscrew. No Aero-Defense allows comparator inspection? How about video inspection? Somebody's still in the 1940s.

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    Quote Originally Posted by Conrad Hoffman View Post
    The "how" isn't difficult. Extend the lines so you have a sharp pointed thread, both crest and root. The pitch diameter is measured from the 50% points. You should be able to make an overlay on clear plastic that's good enough to get the job done.

    DaveK -

    Actually, ( following up ) I've some thread overlays, as well. You are more than welcome to borrow them. I never use them any more since purchasing Thread Pitch mics. Is it burger time?

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    Quote Originally Posted by Conrad Hoffman View Post
    Depends how you use it. Precision leadscrew moves part in relation to crosshairs. As good as the leadscrew if the feature is clear. Micrometer- compares to a precision leadscrew. No Aero-Defense allows comparator inspection? How about video inspection? Somebody's still in the 1940s.
    Just not on Threads Conrad. Other stuff yes absolutely.

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    Well, I've spent a lot of time measuring very small threads (<1 mm) with a video system because they can't be done any other way with any sort of accuracy. In fact, the 3-wire method would be apt to damage them (if sufficiently accurate wires could be had) and no thread mic can handle the tiny pitch. It may not be allowed for the gov, but there's little question about accuracy of the method if it's done correctly. OTOH, my guess is any other method would be less expensive in terms of time and equipment.

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    I work in Micro Machining Conrad.

    01616_picture_1__46436.1446672537.jpg

    Small picture, but that is an 000-120 Ring Gauge
    Attached Thumbnails Attached Thumbnails 01616_picture_1__46436.1446672537.jpg  

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    Holy mother of micro stuff, how do they make those? That's right about the size I use.

    Now, just to rant a bit, why can't we get anything like that made in the US? Almost everything I do, if you can get it here the costs make it a non-starter. The affordable specialty items all come from overseas.

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    Quote Originally Posted by Conrad Hoffman View Post
    Holy mother of micro stuff, how do they make those? That's right about the size I use.

    Now, just to rant a bit, why can't we get anything like that made in the US? Almost everything I do, if you can get it here the costs make it a non-starter. The affordable specialty items all come from overseas.
    HOW? you ask. When you quote the job with/from Airbus, Boning, Rockwell Collins, ATK or L-3 or whoever you add the cost of the Gauges to the cost of Manufacturing the part.

    R

    Forgot to add; the really painful part is when the noob is using it. And the PD is too big for the GO member, and busts the stud off in the Gage. If you can imagine, it's already a Bear to get it out, but now your maybe messing with the Minor-D of the Gage and compromising the Calibration, hence you need to replace it. Also very light hands, which I know Conrad has, but many many of us do not. I'm talking feather light hands.

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    Yes you can measure pitch diameter this way.
    Simple thought, make an overlay with your thread wires on it.
    BUT measuring pitch dia is not the same as all the things a go/no set checks.
    The same goes for thread wires. Most experienced people have seen a wire mic check ok but it fails the go/no gauge.
    Hence the deal for not accepting just an optical and the requirement for the go/no check on fussy stuff.

    Simple optical methods do give you a number to adjust from that a go/no won't but they don't check for overall form fit so may pass threads that a go/no will not.
    A vision system correctly set up for thread checking makes lots more measurements than just PD and gives numbers on true to form.
    Such systems are normally custom builds for high volume production in a family of parts. Read that as expensive. I have built some for very picky people making many wheelbarrows worth of parts every hour.

    Yes you can measure pitch diameter on your optical comparator if you have the right mag and do the table offset twist angle thing, but there is ton more things to get right when making threads. You can check those too but it becomes very time consuming on a manual compartor.

    Threads are a form fit, worse yet you can violate the PD and pass a go/no set so everybody becomes fat,dumb, and happy with a bad thread.
    So common that you would think it easy but screw threads are in fact a nightmare to inspect and qualify.
    I learned that it is a rabbit hole that once you go deep into.............

    What sucks to no end about go/no gauges is that they tell you nothing about what to move or how much so the guess and check model becomes the way out. I'm not a fan of the go/no alone technique but I know it's SOP.
    One should trust your inserts/dies etc. to be right. Nail the PD and length spacing and all is good, What could go wrong?
    Bob

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    Quote Originally Posted by Zahnrad Kopf View Post
    Or a Thread measuring micrometer. Stop by if you need me to measure something.
    Yeah, can't get one soon enough for the job. I would stop by but it's only a 12 pc order, might not be worth the trip. I have thread mics but only up to 2". I'll pick up a 2-3 soon, just gotta get this job out. Thanks for the offer.

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    Quote Originally Posted by CarbideBob View Post
    Yes you can measure pitch diameter this way.
    Simple thought, make an overlay with your thread wires on it.
    BUT measuring pitch dia is not the same as all the things a go/no set checks.
    The same goes for thread wires. Most experienced people have seen a wire mic check ok but it fails the go/no gauge.
    Hence the deal for not accepting just an optical and the requirement for the go/no check on fussy stuff.

    Simple optical methods do give you a number to adjust from that a go/no won't but they don't check for overall form fit so may pass threads that a go/no will not.
    A vision system correctly set up for thread checking makes lots more measurements than just PD and gives numbers on true to form.
    Such systems are normally custom builds for high volume production in a family of parts. Read that as expensive. I have built some for very picky people making many wheelbarrows worth of parts every hour.

    Yes you can measure pitch diameter on your optical comparator if you have the right mag and do the table offset twist angle thing, but there is ton more things to get right when making threads. You can check those too but it becomes very time consuming on a manual compartor.

    Threads are a form fit, worse yet you can violate the PD and pass a go/no set so everybody becomes fat,dumb, and happy with a bad thread.
    So common that you would think it easy but screw threads are in fact a nightmare to inspect and qualify.
    I learned that it is a rabbit hole that once you go deep into.............

    What sucks to no end about go/no gauges is that they tell you nothing about what to move or how much so the guess and check model becomes the way out. I'm not a fan of the go/no alone technique but I know it's SOP.
    One should trust your inserts/dies etc. to be right. Nail the PD and length spacing and all is good, What could go wrong?
    Bob
    Thanks Bob. I understand all you're saying. These are just electrical adapters to change the connection from one size to a different size, so the pickiness of these threads is not very hi-tech. If they screw in, customer will be happy.


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