Headspace Vs Datum Line ????

[RLP]

I just got thru reading the topic on " Dies that don't size enough" and the question that I have is about the Datum line. All the drawings that I have just indicate that it is somewhere in the middle of the shoulder and the neck. So how is the correct gauge chosen ? Hornady recommends that a .350 gauge is used for a 22-250. Why can't you use a .330 or a .375 ? Does it make a difference on where the gauge sits on the shoulder taper as long as the measurements are all taken in the same place. This has been nagging me in the back of my mind for quite a while and would appreciate your input. Thanks, Bob

 

[CatShooter]

I just got thru reading the topic on " Dies that don't size enough" and the question that I have is about the Datum line. All the drawings that I have just indicate that it is somewhere in the middle of the shoulder and the neck. So how is the correct gauge chosen ? Hornady recommends that a .350 gauge is used for a 22-250. Why can't you use a .330 or a .375 ? Does it make a difference on where the gauge sits on the shoulder taper as long as the measurements are all taken in the same place. This has been nagging me in the back of my mind for quite a while and would appreciate your input. Thanks, Bob

The location of the datum line is picked by SAAMI when the standards are chosen - the smaller diameters are chosen for the smaller case bodies (for obvious reasons).

So Hornady is not "recommending" the 0.350" line, that IS the datum line for the 22-250 case.

Using other datum lines will give you useless measurements.

 

[RLP]

Catshooter, Where can you find this information, none of my manuals (I have several) show the datum line. Thanks

 

[jelrod1]

Sometimes in machining the target feature can't be measured on the machine for whatever reason. Bore gage won't reach etc., so another feature done with the same tool is sometimes used to "assume" the target is correct. The target then has to be verified offline with another method. I use this as example because I think that is kinda what you would be doing by measuring somewhere else other than the datum. May in fact work and theoretically I think would but I would always wonder if it were correct at the correct spot. That and there is no reason not to use the correct sized gage when they are readily available.

 

[jelrod1]

http://www.saami.org/specifications_and_information/publications/download/206.pdf

 

[243winxb]

Click rifle here for cartridge. http://www.saami.org/specifications_and_information/index.cfm?page=CC

 

[Lazydays]

before I got sucked into a Windows seven PC, I had a full bore CAD system (and how I miss it) that was very accurate. Actually as accurate as you wanted to program it. I used to make case drawings and setup my own gauge line deminsions. That way I knew exactly what the diameter was on the bushing, and could then adjust from there. I often made my own bushings, and honed them to size on a Sunnen hone to fit my needs. But I never was completely happy with finding a way to actually get a good measurment off an existing chamber. Had some ideas, but also knew they had their own set of issues.
gary

 

[fguffey]

Datum and head space, an impossible concept, except for those that do not have an elementary understanding of a mechanical drawing. The datum is ‘measured from’ outside of reloading forums everyone involved must use the same ‘measure from’

Insert Quote

I just got thru reading the topic on " Dies that don't size enough" and the question that I have is about the Datum line. All the drawings that I have just indicate that it is somewhere in the middle of the shoulder and the neck. So how is the correct gauge chosen ? Hornady recommends that a .350 gauge is used for a 22-250. Why can't you use a .330 or a .375 ? Does it make a difference on where the gauge sits on the shoulder taper as long as the measurements are all taken in the same place. This has been nagging me in the back of my mind for quite a while and would appreciate your input. Thanks, Bob

“while and would appreciate your input”

http://www.saami.org/PubResources/CC_Drawings/Rifle/22-250%20Remington.pdf

The datum, round hole. circle that locates the datum as in measured from for the 22-250 is .347, if everyone is on the same page, being on the same page is not necessary of you are using a comparator, the diameter of the datum can be any diameter you choose that is less than the diameter of the shoulder between the case body/shoulder juncture and the shoulder/neck juncture.

The datum is not a line, it means 'measured from'.

Middle of the shoulder is not the location of the datum, 25/06, 6mm06, 270 Winchester, 280 Remington, 30/06, 8/06 all use the measure from datum of .375, it is impossible to measure from the middle of the shoulder on all those chamber/cases.

“Why can't you use a .330 or a .375 ? “ You can, but, you can not use SAAMI specs, back to the concept of .000: (zero), you can use a head space gage to zero your gage, the datum is a means to locate a location on a tapper, again, the case body is tapered, and round, same for the shoulder of the case, the round taper creates varying diameters, in the beginning they chose one diameter to measure from, if the reloader can understand the ‘from’ is created by the round hole when drilled through a plate, or as in the case of the Wilson case gage the datum, round hole in down inside the gage. The Wilson case gage datum has a radius.

F. Guffey

 

[jelrod1]

The datum is not a line, it means 'measured from'.


F. Guffey

In geometric dimensioning and tolerancing there are "theoretical datums" and "datum features". Right or wrong most design engineers nowdays seem to have migrated to datum features because something has to be measured to establish a "datum" in alot of cases. Creating perfect datum reference planes, primary, secondary, and tertiary off of features requires the features be "perfect" and we know that can't happen. Qualified measuring fixtures with datum target points are not always available and would have to be spec'd on the print also. Sometimes that's where form factor tolerance takes over.

I could absolutely dimension the end of the case from a "datum" of my choosing up the taper.

Naturally what I'm saying has nothing to do with headspace of a chamber or gun making, but in general with regards to GD&T.

 

[357Mag]

RLP -

Howdy !

Reknowned ballistician Homer Powley gave a wonderful treatise on this very topic, in the 1974 " Guns & Ammo " Annual.

I $$$ a copy of this publication off Amazon, and @ < $20.00... it was a steal !

The article is called " The Powley Papers ". G & A claimed they published all of Homer's tech articles, therein.

Powley uses " balls " in figuring out datum lines for individual cases, and included a list of results for popular rifle cases.


With regards,
357Mag

 

[bigedp51]

RLP

We live in a plus and minus world of manufacturing and headspace varies with each rife and each reloading die set. The datum line is a mythical place in the chamber like the paradise of Shangri La.



All that matters when your done resizing the case is the amount of head clearance or the "air space" between the rear of the case and the bolt face.

Below when you start at the suggested starting load your primers will back out until they contact the bolt face and the case will headspace on the primer. Resizing and shoulder bump controls the amount of head clearance.



As the chamber pressure increases the case has to stretch to meet the bolt face, and bumping the shoulder back too far can cause case head separations.



Every time I read one of these postings I keep thinking of what a shooting friend of Germán A. Salazar said to him at "The Rifleman's Journal".

"I get the best accuracy when my cases fit the chamber like a rat turd in a violin case."

In over 46 years of reloading I have never had a case head separation and the datum line was a mythical point in my chambers for many of these years because I didn't have any gauges.



Below from our plus and minus world the .303 British and datum line hell.

 

[jo191145]

As long as your using a comparator to compare between fired, unfired, sized etc any datum line will work. Just don't compare that number to anything you see in a cartridge print.
I have in a pinch used a .40 S&W cartridge to "questimate" fired/unfired/sized headspace variations on a 308W. Obviously not as accurate as a machined comparator but in a pinch lots of averaging and practice it can do the job good enough for field use.

The datum line is for gunsmiths and soldiers in foreign lands. Thanks Biged,

 

[bigedp51]

The datum line is for gunsmiths and soldiers in foreign lands. Thanks Biged,

No problem, during the Viet Nam war in 1969 a bottle of booze was $1.75, a six pack of beer was .60 cents and a pack of cigarettes was .17 cents and the Military said we were over paid. (and you could trade a bottle of Johnnie Walker scotch for any "datum line" item)

 

[RLP]

Gentlemen, Thank you for all the input, I had all my questions answered to the 10th degree and then some ! Kudo's to you all and my thanks.

( especially to you fellow "Old Farts" from Nam)

 

[K22]

The Sinclair bump gage measures off the shoulder not datum line. I don't have one but that's what their catalog claims. They claim this method produces "better, more easily assessable measurements of how much you are actually sizing your cases."

Do know how valid this claim is but these guys seem to know what they're talking about.

 

[fguffey]

K22,

“bump gage measures off the shoulder not datum line”

The round hole in the gage is the datum/round hole that locates the datum on the shoulder as in measured from, the measured from is the top of the disc with the round hole, again, I do not care what the diameter of the tool is when I am using a comparator, all I have to keep up with is to make make sure the case does not fall through or is too small in diameter for the neck, meaning anything between the the case body/shoulder juncture and the shoulder/neck juncture is for consideration, again If I am using R. Lee’s book on modern reloading is going to be used for case length from the datum/shoulder a reloader must use the same datum/round hole, same for SAAMI data,

I make datums, I collect datums and I have been know to purchase datums, the round hole, a true datum has a very sharp edge, not good? There are case friendly datums with a radius, L.E. Wilson case gage used a a case friendly datum, they use a radius, to test for the radius datum in the Wilson case gage, place the case gage on a block of lead, then drop a case into the gage and with a round drift that is smaller in diameter of the case drive the case into the gage with a hammer, The case will drop below the top of the gage when the case shoulder collapse/conforms to the datum.

Getting the case out of the gage is made easier if the case is lubed first.



F. Guffey

 

[Lazydays]

Datum and head space, an impossible concept, except for those that do not have an elementary understanding of a mechanical drawing. The datum is ‘measured from’ outside of reloading forums everyone involved must use the same ‘measure from’

Insert Quote

I just got thru reading the topic on " Dies that don't size enough" and the question that I have is about the Datum line. All the drawings that I have just indicate that it is somewhere in the middle of the shoulder and the neck. So how is the correct gauge chosen ? Hornady recommends that a .350 gauge is used for a 22-250. Why can't you use a .330 or a .375 ? Does it make a difference on where the gauge sits on the shoulder taper as long as the measurements are all taken in the same place. This has been nagging me in the back of my mind for quite a while and would appreciate your input. Thanks, Bob

“while and would appreciate your input”

http://www.saami.org/PubResources/CC_Drawings/Rifle/22-250%20Remington.pdf

The datum, round hole. circle that locates the datum as in measured from for the 22-250 is .347, if everyone is on the same page, being on the same page is not necessary of you are using a comparator, the diameter of the datum can be any diameter you choose that is less than the diameter of the shoulder between the case body/shoulder juncture and the shoulder/neck juncture.

The datum is not a line, it means 'measured from'.

Middle of the shoulder is not the location of the datum, 25/06, 6mm06, 270 Winchester, 280 Remington, 30/06, 8/06 all use the measure from datum of .375, it is impossible to measure from the middle of the shoulder on all those chamber/cases.

“Why can't you use a .330 or a .375 ? “ You can, but, you can not use SAAMI specs, back to the concept of .000: (zero), you can use a head space gage to zero your gage, the datum is a means to locate a location on a tapper, again, the case body is tapered, and round, same for the shoulder of the case, the round taper creates varying diameters, in the beginning they chose one diameter to measure from, if the reloader can understand the ‘from’ is created by the round hole when drilled through a plate, or as in the case of the Wilson case gage the datum, round hole in down inside the gage. The Wilson case gage datum has a radius.

F. Guffey

The very concept of a gauge line (that's the correct term) is to help a machinist set up a taper and be able to accurately measure it. Your shoulder angle on a case is a form of taper, but just a lot more radical. Now the length of the gauge line is setup by the diameter your measuring off of. Say you had a 2" gauge line dimension with a .340" diameter at the shoulder, or whatever worked out for you. The thing is, is to be consistent. I always liked to choose the number that was half way between the shoulder O.D., and the neck O.D. I also know that it's near impossible to duplicate the shoulder angle, so I sort of average it out. I don't like using aluminum for my bushings these days as that alloy seems to change a lot with temperature extremes. I use steel whenever possible Brass and bronze are just as bad, and maybe even worse. I also like to start out with a once fired case that is untouched. Then work from there.
gary

 

[Lazydays]

K22,

“bump gage measures off the shoulder not datum line”

The round hole in the gage is the datum/round hole that locates the datum on the shoulder as in measured from, the measured from is the top of the disc with the round hole, again, I do not care what the diameter of the tool is when I am using a comparator, all I have to keep up with is to make make sure the case does not fall through or is too small in diameter for the neck, meaning anything between the the case body/shoulder juncture and the shoulder/neck juncture is for consideration, again If I am using R. Lee’s book on modern reloading is going to be used for case length from the datum/shoulder a reloader must use the same datum/round hole, same for SAAMI data,

I make datums, I collect datums and I have been know to purchase datums, the round hole, a true datum has a very sharp edge, not good? There are case friendly datums with a radius, L.E. Wilson case gage used a a case friendly datum, they use a radius, to test for the radius datum in the Wilson case gage, place the case gage on a block of lead, then drop a case into the gage and with a round drift that is smaller in diameter of the case drive the case into the gage with a hammer, The case will drop below the top of the gage when the case shoulder collapse/conforms to the datum.

Getting the case out of the gage is made easier if the case is lubed first.



F. Guffey

agree again. I like a .005 / .0075" radius on my bushings, and adjust for that. I once tried to actually put the correct shoulder angle on a set, and that became a nightmare!
gary

 

[Turbulent Turtle]

Interesting discussion. For the .308 Winchester the standard datum/guage/mythical line is located on the shoulder at the point where the outside diameter is .400 inch. You can of course, make up any other such line for your own measurement pleasure.

 

[K22]

K22,

“bump gage measures off the shoulder not datum line”

The round hole in the gage is the datum/round hole that locates the datum on the shoulder as in measured from, the measured from is the top of the disc with the round hole, again, I do not care what the diameter of the tool is when I am using a comparator, all I have to keep up with is to make make sure the case does not fall through or is too small in diameter for the neck, meaning anything between the the case body/shoulder juncture and the shoulder/neck juncture is for consideration, again If I am using R. Lee’s book on modern reloading is going to be used for case length from the datum/shoulder a reloader must use the same datum/round hole, same for SAAMI data,

I make datums, I collect datums and I have been know to purchase datums, the round hole, a true datum has a very sharp edge, not good? There are case friendly datums with a radius, L.E. Wilson case gage used a a case friendly datum, they use a radius, to test for the radius datum in the Wilson case gage, place the case gage on a block of lead, then drop a case into the gage and with a round drift that is smaller in diameter of the case drive the case into the gage with a hammer, The case will drop below the top of the gage when the case shoulder collapse/conforms to the datum.

Getting the case out of the gage is made easier if the case is lubed first.



F. Guffey

Not disputing anything you're saying, just curious about the claim made by Sinclair because they compare their tool to the Hornady Lock-N-Load Headspace Tool albeit indirectly by stating that "rather than measuring to the datum line, these gages [Sinclair] indicate off the shoulder of the case to get a better, more assessable measurement..."

Is that true and if so why wouldn't one want to purchase the Sinclair tool instead of the Hornady? Would appreciate your thoughts on this.