Datums for Dummies

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Yosemite Steve

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I light of the recent shortage of reloaders (except Guffey) that know how datums work I will now dedicate this thread to the subject so that others (besides Guffey) might be enlightened and enter into the datum nation.

da·tum
ˈdādəm,ˈdadəm/
noun
noun: datum; plural noun: data

1.
a piece of information.
an assumption or premise from which inferences may be drawn.
2.
a fixed starting point of a scale or operation.

British Dictionary definitions for datum
datum
/ˈdeɪtəm; ˈdɑːtəm/
noun (pl) -ta (-tə)
1.
a single piece of information; fact
2.
a proposition taken for granted, often in order to construct some theoretical framework upon it; a given See also sense datum

How it pertains to headspace applications:
http://www.mssblog.com/2016/02/19/reloaders-corner-setting-cartridge-case-headspace/

A useful device:
https://www.forsterproducts.com/pdf/catalog/Datum_Dial_Cat82_24-25.pdf

Now that you have read all of this be relieved to know that more than one person on the face of the Earth knows what a datum is and how to use one!
 
Tsk, tsk, tsk. The linked Forster blurb refers to "your ammunition's headspace". Everybody who's anybody knows that ammo doesn't have headspace. Hrrrumph.
 
I see your point. It's interesting that Forster did not have the designer of the flyer change the wording. Cartridge size is not so complicated to say rather than cartridge headspace. Seems like good tools to have... for a dummy like me anyways.
 
Q. What is "headspace?

A. Put in very simple term headspace is basically the distance between the a front datum point in the chamber and the face of the breech bolt. With cartridges having a rim the headspace is measured from the back face of the barrel to the face of the breech. With rimless cases it is measured from either the mouth of the case (if a straight walled case) or from a datum point on the shoulder of the case.
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This material was originally published and copyrighted by John Schaefer, www.frfrogspad.com, and is used here with permission."

http://www.frfrogspad.com/miscelli.htm

scroll about halfway down the page and it even has some nifty drawings
 
Now that you have read all of this be relieved to know that more than one person on the face of the Earth knows what a datum is and how to use one!
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Thank you, All of the material that is available has been available, many reloaders that choose not to make an effort are still posting drawings of "The datum line" and they finish with " and that is how they do it.


F. Guffey
 
I would use the definition provided by 243winxb since it is the SAAMI definition.

DATUM
A reference plane, point or diameter that provides a base for calculations and measurements.

Just my thinking on this is that when we refer to something firearms or gun related we should all work from the same glossary of terms and interpret them the same way. The SAAMI ( Sporting Arms and Ammunition Manufacturers' Institute) is an association of the nation's leading manufacturers of firearms, ammunition and components. While the published standards and specifications are voluntary they tend to get things in line.

Geometric Dimensioning and Tolerancing (GD&T) is another good place to look when we get into drawings of cartridges and chambers for example. The ANSI definition would be along the lines of: "A datum is theoretical exact plane, axis or point location that GD&T or dimensional tolerances are referenced to. You can think of them as an anchor for the entire part; where the other features are referenced from. A datum feature is usually an important functional feature that needs to be controlled during measurement as well".

Regarding RELOADERS CORNER: SETTING CARTRIDGE CASE HEADSPACE by Glenn Zediker it's a good read. I have read quite a bit by Zediker and for the most part enjoyed the read and the guy is a wealth of information. Thanks for sharing that article.

Ron
 
Next thing will be, TFL launching a re-loaders satellite. Getting real deep here for the average re-loader. That is why they make head space gauges. I have to ask being I finished my full time working career in a machine shop. Does the average re-loader on this site really need to understand datum and as another posted GD&T? Just my opinion a bit of over kill when head space gauges are available and case length of rifle cases are understood as well as OAL and one having the means and capability to measure both. Sometimes too much info can be a bad thing.
 
Don P:
I have to ask being I finished my full time working career in a machine shop. Does the average re-loader on this site really need to understand datum and as another posted GD&T?
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Nope. That said the subject constantly comes up in this and other gun related forums. While I strongly agree with you that making consistently good ammunition or building high power does not require a complete understanding of chamber and cartridge drawings the subject forever comes up and we have dozens of manufacturers out there convincing the new hand loader then need every gauge on the market to make ammunition correctly and using all of their tools will assure constant X ring. Anyway, you are right on target and nope, in my opinion they only need to learn how to use a basic press with basic good dies to make good ammunition. As to tools? A vernier or digital caliper has served well for many. many decades.

The satellite would be a nice touch though. :)

Ron
 
Don P,

Loading tools usually work smoothly enough that the average handloader can get good ammunition without understanding dimensions other than case length and COL if he follows manual recipes specific to the bullets he is loading. As soon as you get reloading for precision match shooting with a bottleneck rifle cartridge, given the proliferation of case comparators and measuring tools that rely on locating the case datum, you are well advised to be informed about it, especially if you have a measuring problem you are trying to chase down.


Q. What is "headspace?

A. Put in very simple term headspace is basically the distance between the front datum point in the chamber and the face of the breech bolt. With cartridges having a rim the headspace is measured from the back face of the barrel to the face of the breech. With rimless cases, it is measured from either the mouth of the case (if a straight walled case) or from a datum point on the shoulder of the case.
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Unfortunately, the first sentence of that definition is exclusive. Only modern rimless bottleneck chamberings whose cartridge and chamber shoulders have the same angle use the shoulder datum to determine true headspace measurement. A more concise statement would be that each cartridge has a location in the chamber that is designated to limit how far a cartridge may be inserted into it without crowding, and the distance from the breech end of that chamber to that designated stopping location is the chamber's headspace. 'Cartridge space' would be a more descriptive term. "Headspace" is a holdover from early metallic cartridge days when there was always a rim that was part of the pressure head at the breech end of the case. Thus, the space available for that rim to fit into the chamber was literally the headspace. When later designs by Mauser and Browning and others didn't use a rim or any other part of the head to determine how far the cartridge could go into the chamber, the term was no longer descriptive, but it was retained out of either tradition or sloth. I don't know which was responsible, but it certainly causes confusion today.

In a SAAMI drawing, both bottleneck chambers and cartridges have a datum located at a specific shoulder diameter and that diameter is marked with the letter B (for Basic dimension) and with no tolerance given. The significance of there being no tolerance is that regardless of what other combinations of length and width the tolerances allow, the datum will still be at that exact same diameter, however far forward or backward along the case it may then be. Another datum (again, a fixed value designated by the letter B), but this time along the case instead of being a diameter, is located a specified distance forward of the breech end of a chamber or cartridge. For most rifle cartridges the distance is either 0.200" or 0.250" forward from the breech. This datum is used in conjunction with a third datum in the same direction that defines a fixed distance forward from it, thereby to determine the slope of the taper of the sides of a bottleneck rifle chamber or cartridge.

The 300 Winchester Magnum, for example, headspaces on the step that constitutes the forward end of the belt recess in the chamber profile. That step is 0.220"-0.227" from the breech end of the chamber, so that range constitutes the headspace range for that cartridge. There is no datum there. Instead, the sidewall slope datum is 0.250" forward of the breech end of the chamber and the shoulder datum is 2.2791"-0.2891" forward of the breech end. It seems funny to say a smaller cartridge like the .223 Rem has a minimum headspace of 1.4636" while the much larger 300 WM has a minimum headspace of 0.220", but that's because the former headspaces way forward on the shoulder while the latter does it with its much shorter belt.
 
Unfortunately, the first sentence of that definition is misleading. In the case of a pistol case that headspaces on the mouth, a rimmed case headspacing on the rim, or a belted case headspacing of the belt, there is no datum involved in the headspace measurement. That is only for rimless bottleneck cases.
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there has to be a datum point involved if you are measuring a distance. To take a measurement you have to have someplace to be measuring from. Datum points are not just arrows on a SAAMI drawing pointing to a spot on the shoulder of a bottleneck cartridge.

In the case of the .300 Weatherby magnum the headpace is set to
.219 - .006. The datum point is the front of the belt which is what stops the cartridge from moving forward in the chamber. That is the point you measure from to the base of the case which is the definition of headspace Here is link to a .300 Magnum go gage.

https://www.brownells.com/gunsmith-...RED--CENSORED-=itwine&utm_content=513-100-321

In a rimmed cartridge such as a .22 rimfire the datum point would the front of the rim and headspace is the distance to the base of the cartridge. In other words the headspace is determined by the part of the chamber the rim sets in because it is the front of the rim that stops forward movement of the cartridge. I would think that too short a headspace and a rimfire cartridge would slamfire as soon as the bolt closed
 
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DATUM
A reference plane, point or diameter that provides a base for calculations and measurements.
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:eek:

Again: I was at a gun show checking out tables of miscellaneous items; I said "DATUMS!". The gun show customer behind me asked "WHERE?", The gun show customer in front of me asked; "WHAT DOES A DATUM LOOK LIKE?" And the dealer claimed he did not have any datums.

SO, I purchases a box of stuff the dealer did not know he had and a gun show customer saw a box of datums and still does not know what a datum looks like.

And then there is L.E. Wilson with their case gage, I have been told they have been with us for over 70 years and still there are reloaders that refer to the case gage as a drop-in gage. From the beginning the Wilson case gage was a datum based tool, some reloaders choose to use their thumbnail. Very few read the instructions. I used a straight edge and a feeler gage, I have always felt Wilson was very cleaver and creative when making tools.

F. Guffey
 
In the case of the .300 Weatherby magnum the headspace is set to
.219 - .006. The datum point is the front of the belt which is what stops the cartridge from moving forward in the chamber. That is the point you measure from to the base of the case which is the definition of headspace Here is link to a .300 Magnum go gage.
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Very good, and now that everyone knows the definition and they know how to use one I ask: Why is it so difficult to determine clearance in thousandths? Reloaders claim they have to have three head space gages, one is the go-gage, the next one is the no go-gage and the last one is the field reject length gage. There can be .014" difference between the shortest and longest; I honestly believe if reloaders would simply relax and calm down they could figure how to do it with one gage.

Elmer Keight was surrounded with snarky smiths, they knew he was doing something different, they just could not figure out 'WHAT'.

F. Guffey
 
I honestly believe if reloaders would simply relax and calm down they could figure how to do it with one gage.
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does not take a rocket surgeon to figure out that you would use a go gage and add shims to the base. I have seen recommendations on another forum to use everything from pieces of feeler gage to scotch tape. I just keep it simple and buy a go and a no go when I re barrel to a new caliber. I can swap a barrel or a bolt face out on a Savage in 20 minutes and know when I pull the trigger it will go bang and I will not be picking shrapnel from my face

Edit - just to make it more confusing I discovered belted cartridges have a datum point on the shoulder of the SAAMI drawings. That is the diameter that you would measure from to determine shoulder setback

Also found out that only five datums (diameters) will cover almost every cartridge from the .17 Rem to the 375 H&H
 
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hounddawg said:
there has to be a datum point involved if you are measuring a distance. To take a measurement you have to have someplace to be measuring from.
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All you need is another dimension to measure from. It's called a relative measurement. It may not have the absolute accuracy of a measurement taken straight off a datum, but it is used all the time. Think of it as using a secondary standard.

hounddawg said:
In the case of the .300 Weatherby magnum the headpace is set to .219 - .006. The datum point is the front of the belt which is what stops the cartridge from moving forward in the chamber. That is the point you measure from to the base of the case which is the definition of headspace Here is link to a .300 Magnum go gage.
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Alas, that is incorrect in every detail. First, you've confused the cartridge and chamber dimensions. Cartridges have headspace clearance, but no headspace as there are no cartridges that have to fit inside them in order to achieve function. Only chambers have to have room for a cartridge to fit inside (headspace). The gage you linked to has to fit inside a chamber, not a cartridge, in order to gage headspace.

I will explain the dimensions: First, headspace is a linear chamber dimension. It will, therefore, be given as a minimum value with a plus tolerance only. It is only cartridge linear dimensions that are given as maximums with a minus tolerance. This follows standard engineering practice for unilateral dimensioning, which always treats a worst case tight fit as a critical dimension and the sign of the tolerance goes away from it in the non-critical direction. In our case, that's the direction that makes fit looser.

The classic example is a shaft and journal bearing. If the shaft is too big, you can't even assemble the machine, much less operate it. But if the shaft is too small, you can still assemble and run, even if it doesn't work ideally. So, for the shaft, the largest diameter that still allows assembly into a minimum diameter journal is the critical maximum and the dimension you are given for its diameter is that maximum critical value with a minus-only tolerance. The journal is the other way around. Too small and you can't assemble it to a maximum diameter shaft, so the critical dimension of the journal is its minimum inside diameter that allows assembly with the maximum diameter shaft and the dimension you are given that minimum value with a plus-only tolerance.

In guns, the cartridge is the shaft and the chamber is the journal. The values given are those that are the tightest fit that still allows chambering and firing, and the signs of the tolerances are in the direction that makes fit looser. Minus for the cartridge and plus for the chamber.

All belted magnum chambers have a minimum headspace of 0.220" in the SAAMI drawings. Some have differences in the tolerance. For the 300 Winchester Magnum, it is +0.007. For the 300 Weatherby Magnum, it is +0.004.

All datum dimensions in the SAAMI drawings are appended with the letter B, for "Basic" and have no tolerances. That's because they are a starting point and if they changed at all, it would alter the result the toleranced dimensions actually gave you. If you look at the chamber drawings for the belted magnum, with plain dimensions in inches and parenthetical dimensions in mm, coming away from the base is one Datum labeled .250/(6.35) B. That is the datum nearest the breech. The shoulder has one labeled .4276 (10.86) B. The length from the first of those to determine the side slope is labeled 1.750 (44.45) B. Those three data cover all the B dimensions in the 300 Weatherby Magnum. That is due to the curved shoulder shape. Cartridges with straight taper shoulders also have the shoulder angle given as a B dimension or fourth datum. Again, since a datum is a reference dimension, it has no tolerance. If you see a length or a diameter with a tolerance, it is not a datum.

As the late Senator Daniel Patrick Moynahan once said, "everyone is entitled to his own opinion, but not his own facts." There is no point in arguing about a fact. It's either so or it isn't. You just look it up or measure it. In this case, go to the SAAMI drawings and look for yourself. On each page, the cartridge is the top drawing and the chamber is the bottom drawing.

For straight-sided rimfire cartridges, all dimensions except throat angles are relative and no datum is declared for any of them but the ball throat taper. Bottleneck rimfire cartridges have the usual shoulder datum diameter and shoulder angle datum, but because the sides of the case body below the shoulder are cylindrical, there is no taper slope to be established, so the other two you have in a rimless case are not present.
 
Based on the constant reference to datum or data I believe next dog I get I will name him Datum. A nice short and simple name easy for the dog to learn and me to remember.

Again: I was at a gun show checking out tables of miscellaneous items; I said "DATUMS!". The gun show customer behind me asked "WHERE?", The gun show customer in front of me asked; "WHAT DOES A DATUM LOOK LIKE?" And the dealer claimed he did not have any datums.
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But it was just established and has been covered in past threads the plural of datum is data. Then again...

datum - Computer Definition. The singular form of data; for example, one datum. It is rarely used, and data, its plural form, is commonly used for both singular and plural.
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Unfortunately that is referenced as a computer definition.

Now I have no idea what to name the next dog? Maybe overwhelmed?

Ron
 
Hah! Now I was wrong about something! I was going to direct you to Webster's dictionary, but there it says "data" is the plural when a datum is the starting point for reasoning something out, but "datums" is the plural in mathematics where measuring is done between points. Now I have to go put the thread title back the way it was. I'd never seen that before and am gobsmacked, but probably shouldn't be because -um and -a singular and plural are Latin and everyone is forgetting that language these days.

Mr. Guffey, you win again!!!!!!;)
 
I won't debate definitions of a material control dimension. Just know where you are measuring from and to.

But if you consider the prospects of getting a .213" belt and a .227" recess, a head SPACE of 0.014", you will understand why we are commonly recommended to ignore the belt and load to the shoulder. Except for dangerous game where reliability is more important than MOA.

F.W. Mann's "Hamburg Rifle" had helical locking lugs that screwed the bolt right up against the case head for zero head SPACE. What he called "backleash". But I doubt he allowed much variation in .25 Krag rim thickness, either.

I felt no need for anything but a Go gauge to change Savage barrels. Screw the barrel in against the Go gauge for zero head SPACE and tighten the nut. Adds about .0015" by thread slop and stretch.
 
Jim, I'm sure you know this, but that's actually head clearance you are referring to (also in the SAAMI glossary). Headspace is the total space for a cartridge to fit into, so it includes the space occupied by the cartridge from breech end to the surface that stops on the chamber headspace determinate. Head clearance is what is left over after you subtract the space occupied by the cartridge from the headspace in that area.
 
Uncle Nick I think the confusion on your part is due to my first language being hilbilly. No one has ever used the words great and writer about me in the same sentence. Of course I am aware that a headspace is the distance the distance from a datum point in the chamber to the breech of the barrel. Also pretty much common sense that headspace gages go into the chamber. What I am saying is that it is measured from the physical part of the chamber that stops the cartridge from moving forward in the chamber. That would be the forward edge of the belt in the case of a belted magnum

BTW thanks for the .220 info on the belted magnums and the tapered case info. Not that I plan on owning any but nice stuff to put in my trivia locker
 
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