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Calipers, micrometer, & dial indicator - one not like the others

 
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  #1  
Old 03-15-2009, 09:20 PM
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Calipers, micrometer, & dial indicator - one not like the others

Hello,

So... I've got a couple sets of calipers - one Lyman dial caliper, one Mitutuoyo digital caliper, a Mitutuoyo ball micrometer, and a no-name import (Grizzly) dial indicator.

The dial indicator is trying to tell me that Winchester .308 Win brass is about 14 thou in thickness, while the others all say ~12 thou. They seem to all agree pretty well about where the variations are, and how much they are, but the dial indicator seems to read a bit thicker than the others.

Anything to be done for it, or pitch it and get a new one?

Thanks,

Monte
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Old 03-16-2009, 12:14 AM
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Re: Calipers, micrometer, & dial indicator - one not like the others

All of the digital as well as dial type calipers I have can be zeroed, as I assume yours can be.

But, what is the dial indicator on, how is it mounted and can it be zeroed?

And, the calipers probably have flat faces on both jaws. The dial indicator probably has a rounded type point on the indicator rod at least. Don't know about the opposite side or how it's mounted. This would possibly mean that you are trying to read a rounded case neck's internal dimension with a flat faced jaw that doesn't actually touch the inside of the case neck with the center of the caliper jaw.

Is it possible that the difference in flat faces and a rounded face can be giving you the difference in thicknesses but still indicating the same spots for variations.
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Old 03-16-2009, 12:49 AM
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Re: Calipers, micrometer, & dial indicator - one not like the others

Everything that can be zeroed easily (the exception being the tubing micrometer) was.

I see what you're saying about the different jaws contacting the rounded contour differently... but normally (in my limited experience) when thats the case, the calipers would be reading *thicker*, not thinner. And the ball mike (a model I purchased from Russ Haydon years ago) has a specially modified anvil to allow closer contact to the neck on smaller cartridges - the flat is on the outside, the inside contact surface is a fairly small ball. It agrees with the calipers, but not the dial indicator.
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Old 03-16-2009, 06:01 AM
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Re: Calipers, micrometer, & dial indicator - one not like the others

I would only believe the ball mic.
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Old 03-16-2009, 10:03 AM
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Re: Calipers, micrometer, & dial indicator - one not like the others

I agree.

Quote:
Originally Posted by Mikecr View Post
I would only believe the ball mic.
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Old 03-16-2009, 11:20 AM
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Re: Calipers, micrometer, & dial indicator - one not like the others

No measuring instrument needs to be zeroed to be used accurately. It IS necessary to adjust the reading by whatever error at zero exists. For what you are doing (measuring wall thickness of the brass necks), the ball micrometer is the appropriate instrument; additionally, it is the most accurate of the selection of instruments.

The calipers are the next most-accurate instrument type. The uniformity of the brass and your skill in making the measurement will influence the accuracy of their measurement.

I don't know how you are using a dial indicator to make this type measurement. It isn't surprising at all that the measurement doesn't agree with the others. Measuring wall thickness requires the use of a fabricated jig to provide an anvil and to hold the indicator. The design of the anvil would need to like that of the ball mic for the instrument to compensate for an unevenness in the metal's surface.

As mentioned, the ball mic is the proper instrument for making your measurement. If the anvils are at zero, then your measurement is correct as read. If the anvils are not at zero, then the measurement should be adjusted by the amount of error.
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Old 03-16-2009, 12:24 PM
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Re: Calipers, micrometer, & dial indicator - one not like the others

The dial indicator is being used in a Sinclair case spinner jig - one of those little 'L' shaped brackets that holds a rod w/ a caliber-specific mandrel and when you slide the case in over the mandrel so it stops on the case mouth, a dial indicator should give you a reading of the neck thickness. More to the point, when you spin the case (gently) on the mandrel, the dial indicator should give an idea of how much variation in neck thickness a particular case has.

Basically a much older version (with no base) of their current rendition (shown below):



I don't think it was ever intended to replace a ball mike; I know that when I'm neck turning, that is the tool I rely on. I also know that there are some tools set up to make taking readings around the neck easier, with a micrometer instead of something as 'coarse' as a caliper or dial indicator, such as these two:




but for what I'm trying to do (sort a reasonably large number of cases in a timely fashion)... the spinner seems (seemed) like a better option. Granted, I've had it for years and haven't used it for several, but if the dang dial indicator isn't going to read at least somewhat close it severely undermines my confidence in the tool.

Looking at 'good' dial indicators from Mitutuoyo or Starrett they seem to run in the $125-150 range (this one cost like $20); at that point I would be very tempted to get one of the micrometer gauges shown above. Either that or figure some way to take the measurement with my NECO gauge; darned thing measures everything else known to man but I can't find a description of that particular setup in the manual.
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