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Posted: 8/25/2016 12:15:23 AM EDT
| Most of the dimensions listed on an official diagram make perfect common sense. The few which I have yet to understand, and aren't explained in the SAAMI manual, are the diameters of a cartridge near the base and behind the shoulder as well as the distance between these. Why are they shown as being in the middle of the case body with no real reference point other than the rear dimension is almost always 0.200" from the base of the case? Can anyone explain what these numbers represent? |
| Those four dimensions (two DIAs and two lengths) define the taper of the case/chamber. Without any one of them, the taper is undefined. There is a length from the case base or bolt face to the bottom of the shoulder, and a DIA of the shoulder, but those are reference dimensions because the location of the bottom of the shoulder is very difficult to find (it's theoretically the intersection of the two lines). Does that make sense? |
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Does that mean the diameter at the shoulder is a theoretical value which would only exist if the shoulder was a perfect corner instead of a radius, or is the diameter at the shoulder actually accounting for the radius using the midpoint or something? [It just seems like it would be redundant if the shoulder diameter doesn't account for the radius, because then the taper is already defined by that reference dimension.]
Also, if the diameter adjacent to the shoulder on the body is simply to define the taper, why do so many diagrams have very specific lengths to that point? Some seem to arbitrarily choose 1.000", but others are defined to the 10 thousandth (such as 0.8710"). Is this value supposed to go right up to the point the radius should begin on the shoulder, or is it actually arbitrary as long as the taper is consistent? |
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Quoted:Does that mean the diameter at the shoulder is a theoretical value which would only exist if the shoulder was a perfect corner instead of a radius, or is the diameter at the shoulder actually accounting for the radius using the midpoint or something? Quoted:Does that mean the diameter at the shoulder is a theoretical value which would only exist if the shoulder was a perfect corner instead of a radius, or is the diameter at the shoulder actually accounting for the radius using the midpoint or something? Yes, the DIA at the bottom of the shoulder is "theoretical" in that it is impossible to measure directly due to the radius. Since it's impossible to directly measure to that point, the other dimensions give a way to measure case body taper. Quoted:Also, if the diameter adjacent to the shoulder on the body is simply to define the taper, why do so many diagrams have very specific lengths to that point? Some seem to arbitrarily choose 1.000", but others are defined to the 10 thousandth (such as 0.8710"). Is this value supposed to go right up to the point the radius should begin on the shoulder, or is it actually arbitrary as long as the taper is consistent? The taper of the case body is just one important parameter. Reference DIAs are needed not only to determine the taper amount, but also to determine case diameters along the tapered portion. The reference DIAs could be anywhere along the taper, so the reference lengths to those DIAs are arbitrary. You can calculate the DIA at any point on the taper knowing the two reference DIAs and the two reference lengths. Since they are all basic dimensions, you can create your own reference dimensions at alternate distances, should you need to. Does that make sense? |
| It doesn't make sense why there is a diameter listed in between the shoulder and the base if it has the same taper. The base and shoulder diameters already define that taper, so it seems redundant, but I'm glad to know I now understand all the dimensions listed on the case diagram. Thanks! |
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You need to understand that SAAMI (and like drawings) are trying to identify required dimensions ONCE (and only once) while providing every dimension required.
If you provide the same measurement more than once it requires two changes to alter the dimension, and the two occurrences WILL eventually get 'out of sync' and not match. |
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