Understanding SAAMI Dimensions

[footpetaljones]

I've been looking at the SAAMI dimension for different cartridges, but without a background in machining/engineering it's a bit hard for me to discern their meaning. For example, I'm looking at the cartridge specs for 22 LR here: http://saami.org/specifications_and_...wnload/208.pdf on page 13 in the document/20 in my browser.

On the rim thickness, it lists "0.043-0.007". The way I am interpreting that is that the minimum size is 0.043" and the maximum size 0.05".

Another thing mentioned is "unless otherwide noted body dia -.004". Would that dimension be similar to the minimum/maximum I mentioned earlier, and would the same rule apply for the "unless otherwise noted all dia +.002" and "length tol +.015" statements?

Thanks for the help.

[NSB]

This drawing is a bit different than the several hundred thousand I've looked at over the last forty-plus years. Unless you're familiar with reading drawings this one can be a bit confusing. Most drawings list the +/- dimensions for all the dimensions and call out the exceptions to those tolerances. On this drawing they list all the +/- dimensions on each listed dimension. It looks to me as though the single call out for diameter is a minus dimension only. I'd guess this is so an oversized won't pass inspection since it won't fit into the match chamber. It will fit if undersized but it's not to be undersized more than the minus .004 listed on the drawing. I've dealt with drawings that are a bit different than normal machine shop callouts, particularly some Japanese drawings. Without some introductory training, reading blueprints can be somewhat confusing at times. We'll probably get some machinists chiming in with a better explanation.

[oneokie]

I've been looking at the SAAMI dimension for different cartridges, but without a background in machining/engineering it's a bit hard for me to discern their meaning. For example, I'm looking at the cartridge specs for 22 LR here: http://saami.org/specifications_and_...wnload/208.pdf on page 13 in the document/20 in my browser.

On the rim thickness, it lists "0.043-0.007". The way I am interpreting that is that the minimum size is 0.043" and the maximum size 0.05".

Maximum size would be 0.043" and minimum size would be 0.036".

Another thing mentioned is "unless otherwide noted body dia -.004".

Maximum body diameter would be the stated dimension, minimum body diameter would be 0.004" less than stated dia.

Would that dimension be similar to the minimum/maximum I mentioned earlier, and would the same rule apply for the

"unless otherwise noted all dia +.002" and "length tol +.015" statements?

In this instance, minimum dimension would be the stated number, maximum deminsion would be the stated measurement + the tolerance.

Thanks for the help.

[footpetaljones]

Thank you both for your answers. Hindsight is definitely 20/20.

[John Taylor]

Cartridge print and chamber print will be different.

[1hole]

Cartridge dimensions are maximum minus the stated tolerance.

Chamber dimensions are the minimums plus the stated tolerance.

They must be that way to insure that any in-tolerance cartridge will fit and fire in any in-tolerance chamber.

[EDG]

>>>Another thing mentioned is "unless otherwide noted body dia -.004". Would that dimension be similar to the minimum/maximum I mentioned earlier, and would the same rule apply for the "unless otherwise noted all dia +.002" and "length tol +.015" statements?<<<

The cartridge drawing is a MAXIMUM MATERIAL CONDITION (MMC) DRAWING. It is the maximum size the cartridge can be.
The tolerances in the notes will be minus meaning any deviation from the dimensions let the cartridge get smaller. This insures the cartridges will always fit the chamber.


The chamber being something of a female feature is also an MMC drawing. But since it is a hollow feature its MMC is the smallest it can be,
All tolerances for a chamber let it get larger so the tolerances are plus tolerances. This also insures the cartridges will always fit the chamber.


Big disclaimer here

Some of the older rounds had poor engineering before SAAMI ever came along.
SAAMI adopted the older dimensions and did not correct the errors.

That is why you can take a maximum SAAMI 38-55 round and find that it will not chamber is most 38-55 rifles.

[W.R.Buchanan]

NSB: it is not uncommon on DOD drawings to have every dimension called out with it's specific tolerance included. Especially if it as a critical part that interfaces with many other parts

I have the Drawings for the M14 Receiver and Flash Hider. Every single dimension has it's own tolerance called out.

The best reason why they do this that I have came up with is that there are so many different requirements for how the interfacing parts interact with the main part that individual tolerances are needed for every dimension. The drawing's "Title Block General Tolerances," we normally work with, wouldn't be accurate enough to satisfy all the different fit requirements of the parts that have to interface with the Main Receiver. Thus each dim needs it's won tolerance.

Over a long mass production run (millions of parts) of a complex part like a Rifle Receiver, everything really needs to run as close to nominal as possible so that the tolerances for mating parts don't stack up and cause problems. However they don't want to throw away and waste serviceable parts that are close to nominal.

The headspace on an Enfield #4 Mk1 rifle is .064-.074... However for Wartime Emergency Production they opened that up to .064-.084! So even though they wanted it to be exactly right they weren't about to scrap any guns that were outside of the earlier production tolerance, but still perfectly safe. They did this simply because it didn't matter when using the ammo they were making at the time. IE they changed the Production Requirement so that a broader range of ammo could be used. Effectively they loosened up one tolerance so that they could loosen up another one that would speed production and got two for one.

The dimensions in the OP's example are abbreviations. IE: Normally a dimension would be 2.350 +.000/- .005. If it only says 2.350 -.005 then it is understood that the nominal dimension is the max allowed. They just left out the +.000. Probably for Clarity?

Randy