Long Stem Indicator or Alternative Solutions
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  1. #1
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    Exclamation Long Stem Indicator or Alternative Solutions

    I work with a company doing custom rifle chambering. We recently purchased a new machine to increase production and I am looking to upgrade the entire process along with it. I am in need of a solution to measure TIR to an accuracy of at least .00005"- .0001" 3" deep into the rifle bores with an internal diameter as small as .215".

    We currently use an Interapid .0005" dial test indicator with a 2.5" stem. Looking to buck the bore to read .0001" TIR with a .0005" indicator with that long of a stem just is not possible. I want to make this process read true. The 2.5" stem also requires us to rough out the ID in order to re-indicate further into the bore where we need to. If I can reach in to about 3" then that would remove most of the need to re-indicate my barrels. When indicating... the indicator lays horizontally, stem inserted into bore, and hand spinning the chuck, measuring off the grooves cut into the bore, so the larger ID diameter. The each groove is cut approximately .003"-.006" larger in radius from the base ID diameter.

    I have been unable to find a solution thus far. I had a thought that a dial bore gauge would be the perfect style shape to reach in... however I am unaware of dial bore gauges being able to measure TIR or any similarly shaped instruments that would. If anyone has any clues, I'd love to hear about it. Thank you.

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    What about a set of custom pin gages to fit the minor diameters? Extend the inner diameter enough to get the desired indicator on the the gage. 3" deep makes me think you are chambering BMG.

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    Looking at MSC, It doesn't look like anyone makes a .0001" indicator with a long measuring tip like that.

    You could try swapping that tip over to a tenth indicator and calculating the error, but my feeling is they don't make indicators like that because they wouldn't be repeatable enough to measure tenths.

    Alternatively, you could true up the outside of the barrel to the bore by turning the whole thing between centers. That way any subsequent operation can reference the Od you cleaned up instead of trying to indicate the inside of a tiny bore.

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    So we indicate at the throat point of the caliber. Many calibers are within reason, however on let's say a 300 Win Mag the throat point is about 2.835" deep. It is very common for gunsmiths to use a rod as you have described, however we prefer dialing off the throat point physically.

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    1. It seems like a good air gauge application, but I'm guessing that's outside budget.
    2. I've seen custom versions of the Starrett 670A, 670B, and 671 that repeat at the sort of accuracy you're looking for. You might have to make it yourself, but it isn't terribly complicated if you can properly handle lash and the stiffness of the arm. I've never used the Starrett versions to say how well they work.

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    I'm not a gunsmith but it seems that turning the thread and the barrel OD between centers would make the closer chamber to bore accuracy because the reamer would follow the bore.. and trying to correct any error after the barrel was turned would be near impossible. The inspection to .003 or 50 millionths might grade the barrel service or match grade, but would be tough to correct.
    Again, I'm just talking through my hat because I'm not a match shooter or a gunsmith.

    Here find a youtube on the same...Yes, I don't know if this guy is a good gunsmith or just a hack.
    precision rifel chambering - Bing video

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    The closest thing I think you'll get to what your trying to achieve is use a 1 micron indicator with a long stem installed. 1 micron is less than .00005 so the resolution is off to a good start. At least thats what I'm aiming to do.

    Another option. . . . I don't 100% agree with the full gritters method of indicating going back and forth between 2 points close together to dial the throat and muzzle 20-30" away BUT. If you use a gritters rod with a tight fitting bushing, some downward pressure to ensure any clearance between the rod and bush is consistent, and leave it set up at the throat then use a 1 micron indicator with a short point on the indicator rod close to the back of the barrel you will likely achieve similar accuracy of an indicator with long point but with more rigidity.

    People will cry "but your not indicating on the groove and that's where the bullet rides so that's not accurate" well. . . . Unless your running without a pilot bushing what surface is your reamer being guided by?

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    Default This is the way

    20210325_111421.jpg
    .0001 almost 4" in a .221 bore .

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    Quote Originally Posted by Hbjj View Post
    20210325_111421.jpg
    .0001 almost 4" in a .221 bore .
    That's a nice tool! How did you make the bearing in the middle of the probe bar? A flexture might work well there.

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    It's just a couple of 6/32 screws that I ground a radi on that fit into a smaller
    cross drilled hole nothing special at all.

    A flextur might work here but there is horizontal force on the tip as it has to ride up and over the lands as the barrel rotates.

    Its pretty easy to see the variation in the individual grooves with this tool.

    I've noticed that the depth of the grooves is not consistent it is often deeper right next to a land...

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    With the same idea as Hbjj a long teeter-totter type thing can get you in a very deep and small hole.
    Spring load the ass end. I prefer one to one on the pivot to ends. The above pic confuses me on this but I assume it is compensated for in the readings.
    As he noted the pivot has problems in part rotation as it crosses tool marks or lands so stiffness in this axis counts of both bearings and shaft length.
    A second sideways spring helps here when very deep as in 8 to 20 inches.
    Yes that is a 40+ inch long teeter-totter. Breathe on it hard and the reading changes.
    Super long tips on a DTI simply do not work. For one DTIs do not measure distance they measure angular change of the tip. That is how they work.
    Second the pivot joint is not designed to handle sideways loading so any rotation in this axis to the gauge is questionable.

    Here I must give a big ass plus to Hbjj for the advice given as this is very slick and outside the box thinking.

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    Quote Originally Posted by CarbideBob View Post
    A second sideways spring helps here when very deep as in 8 to 20 inches. Yes that is a 40+ inch long teeter-totter. Breathe on it hard and the reading changes.
    Bob, if you have some tools like this, please post pictures. I am particularly interested to see the springs and bearings.


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