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As mentioned in detail above, CNC machines & processes result in neither perfectly uniform nor uniformly perfect parts.
CNC IS much more precise, on the whole, in turning out to-spec consistency than a bank of skilled machinists who used to do them on machinery that demanded more operator skill as opposed to programming skill.
But, tolerances do vary, and the "stacking" term simply means an accumulation of variances from part to part in a given assembly (here, a gun).
As examples of extremes at both ends, if ALL of the parts in a particular gun are manufactured/machined/CNC'd/cast/MIM'd to exact specs & assembled together, that gun would come closest to the older individual hand-fitting that produced guns prior to the CNC & sintering/MIM process introduction.
Minimal to practically zero tolerance stacking.
On the other end, if ALL of those parts were individually outside specs, either under or over, as the result of cutting head wear, mold wear, or whatever, the combined sum of those variances adds up (or stacks up) to result in a gun that may be loose as a goose (relatively speaking) in general, or may have this part here not interacting ideally with that part there, because four other parts surrounding them being off-spec don't allow them to mate correctly.
Major tolerance stacking.
Most guns are somewhere in between, and there's no way to have every gun made by any given company always come out in the first category using CNC & casting technology without hand fitting.
One of the contact guys I dealt with at S&W (recently retired) had worked his way up through the ranks to an executive position.
He started out as an assembler/fitter, and while he didn't do the mill work, he did use the parts produced by those skilled machinists in building S&W revolvers.
That process took a part from a parts bin, and fitted it into the "puzzle" (gun) by installing it & checking to see how close it was.
If close enough, it'd require very minor finish work (a stone here, a stone there).
If not quite as close, maybe a little more filing, a little more dressing.
If too far out of spec, rejected.
Today, Smith & Wesson doesn't have skilled fitters, for the most part, in the regular production lines.
They have assemblers.
They pretty much adopted Colt's production model when Colt developed their first sintered revolvers with the MKIII Series.
Pick a part out of a bin, stick it in, if it fits & functions reasonably well, that's it.
If not, keep pulling parts out of the bin till one does fit & function.
With Colt's MKIIIs, their sintering process (and later their cast process in the MKV) spit out parts close enough to finished form to be pretty much drop-in assemblies.
The advantage to those non-fitted methods was in saving manpower hours, cutting back on salaries paid to highly skilled labor, and speeding up production.
The disadvantage is that we don't get the hand fitting that used to produce such well-built revolvers.
CNC processes can come close, but still don't achieve the same end result.
Tolerance stacking is simply built into the program nowdays, where-as in the past it was adjusted, removed, or compensated for by a very skilled pair of human hands.
Denis
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