Datum measurement fluctuation?

[thefitter]

I'm using a Hornady Cartridge Headspace Gage Bushing with a MITUTOYO CD-S6 CT caliper. When I'm taking a measurement on a case the measurements can fluctuate several thousand depending on how I have the case aligned in the bushing. It can be frustrating. I try and look at it vertically and make sure that it's not cocked over.

This does not seem to happen when I'm measuring the ogive or OAL.

Is there any other attachment I can use opposite the bushing side of the caliper?

 

[MitchAlsup]

Re: Datum measurement fluctuation?

Did you clean th gauge bushing well, and did you remove dust and small dirt particles from the sholder?

I have a tool that is similarly sensitive to little particles.

 

[thefitter]

Re: Datum measurement fluctuation?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: MitchAlsup</div><div class="ubbcode-body">Did you clean th gauge bushing well, and did you remove dust and small dirt particles from the sholder?

I have a tool that is similarly sensitive to little particles. </div></div>

I don' think it's that, rather it's that the case can shift right left, up down in the bushing holder because the part of the caliper that pushes against the case bottom is narrow. If it was as wide as the case it would not do this. Thanks

 

[427Cobra]

Re: Datum measurement fluctuation?

Remove the primers from the cases first, then measure

 

[thefitter]

Re: Datum measurement fluctuation?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: 427Cobra</div><div class="ubbcode-body">Remove the primers from the cases first, then measure </div></div>

Interesting. I'll have to try that. But aren't the primers seated below the case head?

 

[Victor N TN]

Re: Datum measurement fluctuation?

Twist the case back and forth maybe 1/4 to 1/2 a turn as you hold tension on it. This will help the case center in the guage. I normally don't decap first. If you change something... what else has changed? If the primer is high and causing a rub against the caliper jaw, then yes I would decap, ONLY, first.

Remember when making die adjustments, new brass needs to be fired at least twice, if not three times to get a true reading. The first time or two it's fired the brass will be getting shorter as it flows to fill in the chamber. To prove this all you have to do it look at or measure a new piece of brass and compare to a fired piece of brass. MOST chambers are bigger than the fresh brass enough that it has to expand a bit to fill the chamber. This will cause the brass to be shorter as well.

Good luck.

 

[Fuzzball]

Re: Datum measurement fluctuation?

MY primers are always seated below flush.

I know what you mean about it being a little difficult to get the case head truly flat on the caliper jaw but rotating it a bit while applying a bit of pressure on the other jaw seems to allow things to line up for me. Usually.

Actually, I got tired of dealing with it. So, I made up a base centering tool to fit the second jaw in the same manner as the shoulder tool fits on the first jaw. The bottom of my new base unit was simple drilled with a normal 9/16" twist drill that leaves a fast taper cone for the case head to sit in so everything aligns instantaly and repeatable now. (Wonder why Hornady doesn't make that too?)

 

[thefitter]

Re: Datum measurement fluctuation?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Victor N TN</div><div class="ubbcode-body">Twist the case back and forth maybe 1/4 to 1/2 a turn as you hold tension on it. This will help the case center in the guage. I normally don't decap first. If you change something... what else has changed? If the primer is high and causing a rub against the caliper jaw, then yes I would decap, ONLY, first.

Remember when making die adjustments, new brass needs to be fired at least twice, if not three times to get a true reading. The first time or two it's fired the brass will be getting shorter as it flows to fill in the chamber. To prove this all you have to do it look at or measure a new piece of brass and compare to a fired piece of brass. MOST chambers are bigger than the fresh brass enough that it has to expand a bit to fill the chamber. This will cause the brass to be shorter as well.

Good luck. </div></div>

That I did not know. I thought the once fired brass was giving me an accurate representation of my chamber. Thanks

 

[thefitter]

Re: Datum measurement fluctuation?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">MY primers are always seated below flush.

I know what you mean about it being a little difficult to get the case head truly flat on the caliper jaw but rotating it a bit while applying a bit of pressure on the other jaw seems to allow things to line up for me. Usually.

Actually, I got tired of dealing with it. So, I made up a base centering tool to fit the second jaw in the same manner as the shoulder tool fits on the first jaw. The bottom of my new base unit was simple drilled with a normal 9/16" twist drill that leaves a fast taper cone for the case head to sit in so everything aligns instantaly and repeatable now. (Wonder why Hornady doesn't make that too?) </div></div>

Picture I can work off? Thanks

 

[ChrisGarrett]

Re: Datum measurement fluctuation?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: thefitter</div><div class="ubbcode-body">I'm using a Hornady Cartridge Headspace Gage Bushing with a MITUTOYO CD-S6 CT caliper. When I'm taking a measurement on a case the measurements can fluctuate several thousand depending on how I have the case aligned in the bushing. It can be frustrating. I try and look at it vertically and make sure that it's not cocked over.

This does not seem to happen when I'm measuring the ogive or OAL.

Is there any other attachment I can use opposite the bushing side of the caliper? </div></div>

Things don't have to shift much, to make a .001"-.002" difference, so it's common.

Also, on my Hornady HS and Bullet Comparator, I make a 'witness mark' on the insert bodies that I have, then I make a corresponding witness mark on the HS gages and the bC inserts. This allows me to install them in the same position each and every time.

They can give differing readings depending on where they're cinched down within the bodies/holders.

Chris

 

[Fuzzball]

Re: Datum measurement fluctuation?

"Picture I can work off? Thanks"

No photo but you already have a virtual duplicate in your present Hornady red alum part for your caliper. In fact, I could more easily have used another Hornady gage thingy and just turned a cup for it to set the case head into but when I had the idea I just made it then rather than wait.

I set it up for use by seating the two parts against each other so the centerline is correct.

 

[thefitter]

Re: Datum measurement fluctuation?

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: Fuzzball</div><div class="ubbcode-body">"Picture I can work off? Thanks"

No photo but you already have a virtual duplicate in your present Hornady red alum part for your caliper. In fact, I could more easily have used another Hornady gage thingy and just turned a cup for it to set the case head into but when I had the idea I just made it then rather than wait.

I set it up for use by seating the two parts against each other so the centerline is correct. </div></div>

Well I'll be damned! You are right. Thanks!!

So how did you zero your caliper?

 

[427Cobra]

Re: Datum measurement fluctuation?

My primers are always seated flush too, but when the firing pin hits the primers they expand backwards and most often flow sightly rearward around the firing pin hole, by removing the spent primers first I can measure 20 fired cases and all will measure the same

 

[vman]

Re: Datum measurement fluctuation?

+1 on the brass sometimes needing to be fired more than once.

I find that brass never expands to meet chamber dimensions exactly after the first firing... at least not with any rifles I have reloaded for.

When im taking measurements using the Hornady guage, I tend to hold the two caliper ends together with one hand, and spin the case inside the guage with the other. This levels out the case inside the insert and I find my cases usually only vary by 0.001 to 0.0015 between different measurements. Regardless I set up my dies and check things from time to time to make sure everything is still set up as intended.