Go Gages, Datum Lines, and Reamer Prints...

Z

Zebra13

Member
Gents,

Hoping ya'll can help me understand something here...to wit:

In looking at a couple of reamer prints for the .223, one by PT&G, the other by JGS, I noted the datum line measurement on both was 1.4636. So I grabbed my PT&G Go gage for .223 and took a measurement of it with my Stoney Point tool with the .330 insert in place (.330 datum line diameter for the .223). It measured 1.455. I expected to see 1.463 or so. Both prints list 1.4636 as a minimum. .223 factory ammo measured 1.455 to the datum line.

Head scratching...

So I check out a JGS .308 reamer print, and it shows the datum line measurement as 1.630 min and 1.640 max to a .400 diameter datum. So I do same as above with a .400 datum insert and a .308 Go gage. It measured 1.6155. The No Go gage measured 1.6195...4 thou longer and to be expected. Federal .308 Match measured 1.610 to the datum line.

I expected the measurements on the Go gages to jibe with the datum line measurements on the prints, but the closest I got was .008 on the .223. The .308 was .0145 shorter than the print's min.

The bores on the inserts aren't chamfered...their pretty sharp.

Why aren't these measurements the same, or a whole lot closer? What am I not getting?

Thanks for your help,
Justin
 
ammo should be shorter than min chamber, and in both cases that was true...so i would suggest you send the gages back and ask that they be checked for accuracy.

i do not have the stuff in my hands do not have your tools and with out it is my only suggestion.....well you could check the dia of the stoney point gages with pin gages....they are aluminim and could be off...but the ammo measured ok..so i do not think that is it.

mike in co
 
Trust the people making the gages.
They use precision ring gages with crisp sharp edges to measure a gage, not anodized aluminum.

Dave
 
The gauges that fit on calipers should have their names in quotes, for they are not made to measure rifle chamber headspace as an absolute standard, but rather are intended to measure how far back case shoulders are pushed during sizing by comparing fired and sized cases. The term headspace gauge was probably chosen because it sounded better than universal shoulder bump measurement kit. For their intended use, they work well, but they are not really headspace gauges.
 
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Dave Tooley said:
Trust the people making the gages.
They use precision ring gages with crisp sharp edges to measure a gage, not anodized aluminum.

Dave
Click to expand...

Roger that, Dave. I don't think it's the gage...I think it's me missing something.

So, given the proper measuring equipment, a measurement of the Go gage at the datum line should match the bolt face to datum line measurement on the chamber print? Correct?

Justin
 
no
the chamber can be anything...
hopefully somewhere BETWEEN min and max..of chamber drawing.

and yes it MAY be min which would be the go gage.

mike in co
Zebra13 said:
Roger that, Dave. I don't think it's the gage...I think it's me missing something.

So, given the proper measuring equipment, a measurement of the Go gage at the datum line should match the bolt face to datum line measurement on the chamber print? Correct?

Justin
Click to expand...
 
If you really want to see a big difference, measure a factory belted magnum versus the reamer print.
 
Zebra13 said:
Roger that, Dave. I don't think it's the gage...I think it's me missing something.

So, given the proper measuring equipment, a measurement of the Go gage at the datum line should match the bolt face to datum line measurement on the chamber print? Correct?

Justin
Click to expand...

Yes it should. I suspect the gauges, while a good enough comparison tool, are not precise enough for a positive measurement. Getting a precise measurment, even with a certified ring gauge, is not that easy. Temperature of either the measuring instrument or the piece being measured can make a significant difference when you are measuring to a datum line on an angle.
I don't know what the tolerance is on a typical headspace gauge but I would expect it to be pretty close. Regards, Bill.
 
I don't believe that the cartridge datum line is relative to a headspace gage. HS gages are simply go and no go plugs that tell you if the chamber is to proper length. They could do that at either end of the shoulder, or anywhere in between as long as the go and no go use the same datum or point of reference. Also, comparing the cartridge to the chamber is apples and oranges. If they were the same, they wouldn't fit. And new brass almost always runs small.
 
mwezell said:
I don't believe that the cartridge datum line is relative to a headspace gage. HS gages are simply go and no go plugs that tell you if the chamber is to proper length. They could do that at either end of the shoulder, or anywhere in between as long as the go and no go use the same datum or point of reference. Also, comparing the cartridge to the chamber is apples and oranges. If they were the same, they wouldn't fit. And new brass almost always runs small.
Click to expand...


The datum line has everything to do with head space. It is the point from which all chamber case body dimensions are referenced from.

Take a 300 Win Mag chamber for example. At a diameter of .420" on the shoulder the length to the case head is 2.2791" A very specific number. That number has nothing to with brass. SAMMI and CIP both have two sets of standardized dimensions. One for the chamber and one for the brass.

Dave
 
Dave Tooley said:
The datum line has everything to do with head space. It is the point from which all chamber case body dimensions are referenced from.

Take a 300 Win Mag chamber for example. At a diameter of .420" on the shoulder the length to the case head is 2.2791" A very specific number. That number has nothing to with brass. SAMMI and CIP both have two sets of standardized dimensions. One for the chamber and one for the brass.

Dave
Click to expand...

That's not the point. My point is that you could make a guage that just chambers, and is faced flat on both ends without regard for the datum line. Make one identical to it but .004" longer and you would have a no go guage for that cartridge. Again, without regard to the actual datum line for that cartridge. The go guage could hit anywhere on the shoulder and the no go would hit .004 further down the shoulder of the chamber. We're not making reamers with hs guages..we are simply using them for reference tools that only have to use the same datum as the other, but not the actual datum line for a given cartridge.
 
So you're starting with a chamber with unknown HS dimensions and making a gauge to fit and calling it a go gauge. Then one slightly longer and calling it a no go. You would have home made gages that fit that chamber, not an industry standard that would stand up to any scrutiny for that cartridge.
 
Dave Tooley said:
So you're starting with a chamber with unknown HS dimensions and making a gauge to fit and calling it a go gauge. Then one slightly longer and calling it a no go. You would have home made gages that fit that chamber, not an industry standard that would stand up to any scrutiny for that cartridge.
Click to expand...

Yes..sort of. The question was why does his hs gage not measure what the saami std is at the datum diameter. I say, why should it? Where is there a std for making hs gages listed that says they must use the same datum line as the cartridge it was designed to check. All that matters is that the end result, including shoulder angle be correct. It doesn't matter how you get there. Although the two datums will be close, I know of nothing where it says that they have to share the same datum line as the cartridge...only that the end result has a given hs at a give datum. If they all have to be the same as their respective cartridges I have a drawer full of them that are junk.
 
Obviously you've never seen or studied SAAMI or CIP specs. The gages are made to an entirely different spec and tolerances. The shoulder bump gages are not even interchangeable from the same manufacturer so using them to measure HS is asking for trouble. Ask the manufacturer of those bump gages if they will guarentee them if used as a precision HS measuring tool. One reamer maker I know uses precision ground ring gages with the proper ID spec'd to within .000050" to measure his gage lengths. A tumbled anodized piece of aluminum is just an approximation.

Dave
 
The short answer is is that what he used to measure the gauge with was not designed for that task, nor is it, or what it is used in conjunction with, the caliper, of suitable precision for accurately verifying the dimensions of a precision gauge. As to the importance of the datum line. As a practical matter, I don't think that closing a bolt on a sharp cornered cylindrical gauge that is of such a diameter that one end registers against a chamber's shoulder is a good idea, because of the potential of damaging the chamber,and I believe that this is why we have gauges that have one end that makes parallel contact with chamber's shoulders. Then there is the problem of conveying the dimensions of that gauge to a machine shop in a fashion that will be universally understood, and having that method of dimensioning be compatible with measuring ammunition. I believe that these are the reasons that the datum line reference was chosen, and as far as I am aware, it works just fine. On the other hand, shoulder bump can be measured all sorts of ways that will all give satisfactory results, as long as the same tool is used to measure before and after sizing.
 
Use an optical comparator to check the HS gages. A local friendly machine shop should have one handy.
 
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