how to measure the diameter of this o-ring groove
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  1. #1
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    Default how to measure the diameter of this o-ring groove

    solidworks-premium-2020-sp1.0-m3707-rev-b-sheet1-_-9_17_2020-4_31_53-pm.jpg
    I'm looking to measure the .147 diameter +-.002 . I can kind of get in there with the tips of my calipers, but I'm thinking thats not the right way. btw this is a round part. I'm thinking maybe an outside caliper gage with ball points like this https://www.mscdirect.com/browse/tnp...SABEgK-vfD_BwE
    Last edited by Bluejeep; 09-17-2020 at 04:32 PM. Reason: added an idea

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    Picture doesn't get bigger when you click on it so hard to see for sure.

    Blade mic if it clears the thread.

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    Quote Originally Posted by mneuro View Post
    Picture doesn't get bigger when you click on it so hard to see for sure.

    Blade mic if it clears the thread.
    blade mic still has square ends. The groove is round. .044 thread clearance to the step. though I could turn the step back for clearance on a test piece to get a measurement..

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    How about a point mic? that way you could get the smallest point on the radius.

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    I'm only curious, did the designer specify +/-0.002" tolerance for a rubber oring groove diameter?

    and once the oring is squished, it is meant to touch the non-relieved end of the threads?

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    Quote Originally Posted by jz79 View Post
    I'm only curious, did the designer specify +/-0.002" tolerance for a rubber oring groove diameter?

    and once the oring is squished, it is meant to touch the non-relieved end of the threads?
    My thoughts the same - with this design it will more than likely compression set the seal because there's nowhere for it to "go".
    I'll add, rightly or wrongly, I've never seen a design like it.

    OP - ref measuring it, you could use an Interapid Groove Checker - there's various types of interchangeable tips and some are very "pointy" with small rads on the ends.

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    If you do not have enough clearance for a blade or point mic - which it seems you don't - check our Mueller Gage groove diameter gauge. Has replaceable legs and does OD and ID.

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    I've seen fittings with oring in that exact spot, but the thread ended couple turns before the oring, so that when it is compressed, it has no chance of getting damaged by the thread, which will happen in this case

    and if the thread relief was added, one could make the thread relief diameter the same as the oring seat ID, making it much easier to measure if they indeed need the tight tolerance with something like the blade mic mentioned previously

    the other reason for thread relief would be that the mating part at that place has to have a face for the oring to seal anyway, so the "missing" 1, maybe 2 turns wouldn't have engaged anything in the mating part anyway, I'd extend the fitting a little bit to add those 1~2 turns in the front if possible

    sorry, just thinking out loud when I see these "impossible" parts...

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    You can use your blade mike you’ll just have to calculate the h dimension off the cord .The formula is in the machinist hand book bit of a pain to do all the math but it works. As for changing the design unless your customer goes along it can come back to bite u in the ass if you do it on your own and there’s a problem.
    Pete

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    Another option is to measure it over pins. A pin mic would have one of those pins fixed, make it less fiddly. Mueller gage with customer jaws a good idea if you're doing zillions of these. As others have said, the o-ring is going to deform more than .002" and the design itself subject to change . . . I'd wait a while to see about things like those near-interfering threads before paying for a set of custom anvils.

    On edit - as Gregor says - have to be tiny balls, if that tiny drawing has as tiny an o-ring seal as it seems. Bad idea.

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    Quote Originally Posted by PeteM View Post
    Another option is to measure it over pins. A pin mic would have one of those pins fixed, make it less fiddly. Mueller gage with customer jaws a good idea if you're doing zillions of these. As others have said, the o-ring is going to deform more than .002" and the design itself subject to change . . . I'd wait a while to see about things like those near-interfering threads before paying for a set of custom anvils.
    No way to get pins on that geometry that I can see, drawing is too small to see clearly, but looks like it would have to be balls.

    If you can get balls in there, then two balls and a mic, although there is only .012" from the face to the centre of circular cross section, so not much to measure.

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    Looks like an optical comparator would get right in there ... after you take it out of the machine.

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    Quote Originally Posted by jz79 View Post
    I'm only curious, did the designer specify +/-0.002" tolerance for a rubber oring groove diameter?

    and once the oring is squished, it is meant to touch the non-relieved end of the threads?

    The +-.002 is just a global tolerance for three place decimal dimensions on the drawing. The actual diameter isn't even directly called out just a radial dimension picked up off a diameter dimension. this is just a modification of an off the shelf tube fitting adapter low pressure application. They will not actually Q.C. these parts I was asking more as a learning opportunity for a hard measurement situation.

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    Quote Originally Posted by jz79 View Post
    I've seen fittings with oring in that exact spot, but the thread ended couple turns before the oring, so that when it is compressed, it has no chance of getting damaged by the thread, which will happen in this case

    and if the thread relief was added, one could make the thread relief diameter the same as the oring seat ID, making it much easier to measure if they indeed need the tight tolerance with something like the blade mic mentioned previously

    the other reason for thread relief would be that the mating part at that place has to have a face for the oring to seal anyway, so the "missing" 1, maybe 2 turns wouldn't have engaged anything in the mating part anyway, I'd extend the fitting a little bit to add those 1~2 turns in the front if possible

    sorry, just thinking out loud when I see these "impossible" parts...
    I just talked with the designer and found out that the tiny o-ring mates up with the chamfer on a threaded hole which does not have a max chamfer dia. called out

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    Blade mikes won't do it directly. Neither would point mikes, unless you had them custom made. Non-rotating spindle. The radius is R.035", so the spherical tip has to be smaller than that. the side of the spindle would have to be ground with clearance to get at the root of the cut (almost in half, but not quite). Blade Mike modification would be a radius one one corner, and an angle for clearance. I'd draw a picture, but I'm not there right now.

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    The ID of the o-ring groove is 0.146", no?

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    Well, you know the OD of the part.

    Take it to the inspection plate, Pick up the groove with an indicator on your height gage or transfer stand, and transfer it to a gage block stack.

    Or zero the indicator on the groove, and pick up the OD. Note the difference on the height gage, and measure the OD with a mic. Now you know the groove diameter.

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    Quote Originally Posted by Bluejeep View Post
    I just talked with the designer and found out that the tiny o-ring mates up with the chamfer on a threaded hole which does not have a max chamfer dia. called out
    so it is as I suspected, and it would look something like this: (the red line shows a possible cross section of the mating part
    adfasdf.jpg

    and that would mean the last couple turns of the thread on the fitting aren't necessary and can be turned down to the ID of the oring groove giving you something you can directly measure and at the same time make the part quite a bit easier to make

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    20200918_101330.jpgjancollc thats what I ended up doing. The pic was oriented correct on my computer . The digital height gage is great for this application touch off incremental points.
    Thanks all
    Attached Thumbnails Attached Thumbnails 20200918_101330.jpg  

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    Quote Originally Posted by Bluejeep View Post
    I just talked with the designer and found out that the tiny o-ring mates up with the chamfer on a threaded hole which does not have a max chamfer dia. called out
    Ouch.

    Glad you got it measured, cover yourself pretty thoroughly here since it sounds like you are making this for someone else and based on what I have read here, getting an -007 o-ring to perform its task in this application is nothing short of a small miracle.

    DON'T OVERFILL!!!! Compressed rubber has a poisson's ratio of 0.49999999, it is essentially incompressible. So when it deforms, the total volume available to the seal needs to be more than the volume of the part (especially in temperature swings, there are documented cases of steel hydraulic cylinders being ruptured by overfilled rubber seals that thermally expanded). 85% is a usual top limit, and I have gone as much as 90% fill in certain specialty applications, but if you don't have room for seal flow, it'll damage the seal and be more likely to leak.

    I doubt you have much pull in the design here, but I expect you'll try to stand by your work if there's a problem with the assembly and I think you'd be justified to do so.

    Sorry, just had to vent a bit on the "interesting" design. Best of luck!

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