Saami

[rvenneman]

On page 28 of the SAAMI manual, it states the lenght of a 9x18 Makarov case should be .713-.010. Does anyone know if the -010 means that .010 is a tolerance range to trim the case between .703 to .723? I am confused.


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[jcren]

That designation should denote a max of .713 to a min of 7.03. At least that is how they were written when I had to use engineering diagrams.

[NSB]

Some tolerances are called out in one direction only. The reason being you can go smaller or larger depending if it's a plus or a minus, but going the wrong way makes it a "no/go" dimension.

[country gent]

On most blueprints shafts and bosses and lengths are called out in a - only tolerance and holes bores and inside grooves in a + only tolerance to save on the possibility of stacking tolerences and having parts that wont fit together. A 1" shaft with a plus or minus of .005 and a 1" hole with a plus minus of .005 could actualy end up with both parts in spec and .010 interference in them. 1" shaft and +.005 tolerance = 1.005. 1" hole and -.005 tolerance =.995 hole or .010 interference fit. The above dimension should be designated .713 +.0000 / - .010 or .713 /.703. Manufacturing tolerences can be tricky and sometimes confusing

[EDG]

Tolerances for ammo and chambers are given from worst case condition AKA maximum material condition or MMC.

MMC means the largest possible cartridge and the smallest possible chamber. With MMC you have an external object - the cartridge at maximum size with the internal mating feature - the chamber at minimum size. These 2 combine to create the most likely condition for ammo that will not chamber/function.

When tolerance is applied to the cartridge it always makes the cartridge smaller. Tolerance applied to the chamber always makes the chamber larger. The added tolerances always makes the fit of the ammo in the chamber have more clearance to avoid ammo that will not chamber.

in the example of .713-.010 the .713 is the most desired dimension however it has NO positive tolerance so .714 would be a reject.
All tolerance applied to the .713 case length is negative and permits the case to be as short as .703. While ammo with brass at .703 might not be as accurate it will still fit the gun and go bang.