Gunsmithing Need precise measurement ... engineers, techs help

[GentleBen]

Like a lot of reloaders, I have several digital calipers ranging in price from $20 to $100 and was using them at will for just about anything reloading. But then I noticed some were more accurate or inaccurate than others -- particularly giving me different readings on headspaces when using a Hornady L-N-L setup.

Do any of you know of a common household object, such as a nickel or a quarter coin, that has a precise width or thickness that I can check all of these digitals against.

BTW, I try mimicking new, virgin Lapua brass measurements when setting up my sizing dies in .223 and .308. That is when I swapped out a digital caliper and got a widely different reading on the new Lapua. And it's not the brass.

 

[jonaddis84]

Re: Need precise measurement ... engineers, techs help

No household object will have anywhere near the tolerances you need for verifying your calipers. You can buy a gage block fairly cheap though from a local tooling store or useenco.com

 

[waste_knot]

Re: Need precise measurement ... engineers, techs help

That what gauge blocks are for.
Grizzly has some 123 blocks for $14
http://www.grizzly.com/products/T24799
You should also check the top two slide tension screws on the top of the calpers and adjust them to a tension where the head just begins to slide then put nail polish on them to lock. When these get loose the encoder loses counts on my mititoyos.

 

[Manning5R]

Re: Need precise measurement ... engineers, techs help

Drill bits are usually pretty darn close to right.

 

[ShtrRdy]

Re: Need precise measurement ... engineers, techs help

Bullet diameter can be very close to published values.

 

[300sniper]

Re: Need precise measurement ... engineers, techs help

a man with one clock always knows what time it is, whereas a man with two is never sure.

 

[turbo54]

Re: Need precise measurement ... engineers, techs help

Using virgin Lapua brass is not a precision method.

Gage your chamber with a HS gage and .001" shim stock, or, anneal a piece of brass and fire it 2 or 3 times, only neck sizing between firings.

THAT will be your chambers HS.

Use a set of calipers with HS attachments or a precision mic to compare sized brass to your formed "gold standard". Doesn't matter a lick if the measuring tool reads 9 angstroms, 1.360", or 5.56 light years.... you only need to COMPARE to the gold standard.

If you still wish to calibrate your calipers, you need gage blocks (aka jo-blocks). Then you'll need to consider what grade Jo block you're referencing!

 

[GentleBen]

Re: Need precise measurement ... engineers, techs help

All very good advice, thank you as always.

 

[cal50]

Re: Need precise measurement ... engineers, techs help

Feeler gauges or a quality bullet are good if you do not have access to gauge blocks or standards.

Calipers do have a "feel" quality to them for use and you can spring the jaws slightly if measuring at the tips and applying excessive pressure. You can see this on a well used set of calipers. Clean the jaws by clamping a piece of paper and pulling out then close the jaws and hold up to a light. If the jaws are warped you will see daylight between the jaws or tips depending on how warped they are.

 

[mcfred]

Re: Need precise measurement ... engineers, techs help

<div class="ubbcode-block"><div class="ubbcode-header">Originally Posted By: GentleBen</div><div class="ubbcode-body">Do any of you know of a common household object..
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Pennies are 0.750" in diameter. They are usually pretty close +/-0.001" and if you measure several different ones and get the same numbers with your calipers that should be good enough.

If you are trying to set up dies to bump back the shoulder, I've found that a good way to fine tune it is to put black soot on the case shoulder and lower the die in stages until you can see contact marks in the soot from the die. Then you know it's touching, and you can go slightly lower to bump the shoulder back. Function testing in the particular gun is also a good way to check.

Another easy way to do this is with a stub gage. It's a short block with the front part of the chamber cut into it. Ideally this is cut with the same reamer that cut your barrel. You drop a fired case in the gage and measure it with your calipers. Then size the case and measure it again until it's 0.002" shorter (or however much you want it shorter). You can measure factory ammo the same way if you're trying to duplicate that dimension.

 

[dunes450r]

Re: Need precise measurement ... engineers, techs help

as was mentioned: tighten the adjuster screws. i've seen $200+ calipers being off by .003" just because they weren't adjusted. that said, calipers are really only good for +/- .001" you need micrometers to be very precise

 

[seanh]

Re: Need precise measurement ... engineers, techs help

every time I use mine I zero them...

Buy a set of gauges and don't worry about it.

 

[WindDoper]

Re: Need precise measurement ... engineers, techs help

They are called standards. You should be able to source them from any machine tool supply company. They will be precision manufactured to precise dimensio : as in 1" exactly or a metric if you wish etc. I Use them to check micrometers all of the time.

 

[stangfish]

Re: Need precise measurement ... engineers, techs help

Make sure you install the LNL gauges correctly. Calipers are gear driven and if cheaply made seems reasonable to expect that you would want to check calibration near the dimension that you are checking. If you check it at 1 inch and it is being used a 4 inches there may still be an error. The 123 blocks are a good suggestion as they give you multiple dimensions to check your calibration.

 

[Quicksilver]

Re: Need precise measurement ... engineers, techs help

Buy a set of "standards" on ebay from one of the following companies:
Starrett
Mitutoyo
Brown & Sharpe

Since your measuring overall length and diameter, try and find a standard that is CLOSE to what you are measuring to minimize the "localized" error buildup (tolerance stackup of sorts...)

Or, buy a set of micrometers (they come with standards usually) and use them instead. If your really trying to be precise (+/- .001) you need micrometers, not dial calipers. Dial calipers are really for "roughing" out I.D./O.D./Length, not making "accurate" measurements. A good new set of starrett or mitutoyo micrometers will go a LONG way and only need the carbide faces re-polished every few years or so (depending on frequency of use).