I’m going to be doing some posts on how things are made, but before I do it’s important to make a few things clear.

One of the most critical issues in manufacturing is tolerance stack. Ordinarily this is defined as the possibility that a part can be non functional and still be within spec. I could link to Fritz Scholz, who pretty well has the science down, but it’s a classic TL: DR.

Instead, imagine a 2″ diameter shaft ten inches long. The engineer wanted grooves every inch, .100″ deep and .100″ wide. The width of the grooves isn’t critical, soo he toleranced them .100 +/- .005. Then he dimensioned the lands between the grooves .900″.

The Machinist looks at the print and determines he doesn’t have a grooving tool .100 wide, so he used one he had in his toolbox that was .104 wide. He also saw that the other dimension on the print, the .900″ one, had no stated tolerance, so he was very careful to make sure there was exactly .900″ between the grooves.

The top is “As drawn”. the bottom is “As made”

They look really close to the same, don’t they? Lets zoom in to the last groove on the right.


The addition of the extra .004″ per groove added to each segment until at the end, nine grooves later, the groove is .036″ off.

That’s tolerance stack. Proper tolerancing of engineering drawings is necesary to the manufacture of quality parts.

There’s another, less well understood kind of tolerance stack, and that is the tolerance stack of multiple fixturing. When something as simple as a bolt is made, it has one operation- it gets threaded. A shoulder bolt gets threaded, and then has the shoulder turned or ground, but even this is usually done without removing it from the machine. A pipe fitting, on the other hand, has threads on both ends, and is held in the middle to machine. So one end is cut, the fitting is turned around, and the other end is cut. The act of chucking and unchucking will cause the threads to be out of alignment, and it is the amount of “Slop” or innacuracy in the machine’s fixture that determines how far off this will be. Mostly a non issue with pipe fittings, this is a disaster in more complex manufacturing.

The upper receiver on an AR is machined in several steps, and the brilliance of the original Stoner design is that the critical dimensions are machined all in one operation. This means that the bore that houses the bolt carrier group, the mounting surface of the barrel, and the centerline of the threads are all in precise alignment, or as precise as the stated accuracy of the machine. In other words, if the bolt carrier bore is done in one op, and the part is removed from the machine and placed in a different fixture to do the threads, the liklihood of the threads not being concentric with the bore is accentuated. There are some manufacturers that do this, with varied results. This is the kind of “Tolerance stack” that is due not to bad design, or the misunderstanding of the design by the machinist, but to a failure to understand the process. The critical machining must be completed without removing the part from the machine

Here is an example of CNC machining which includes threading. This is simply a demo, but if you were to take the part out and very carefully measure it, you would find that the cylindrical surfaces are all parallel and concentric at a level that can tax the limit of the ability of the equipment to measure, because they were all cut in the same, new machine, without being removed or disrupted during manufacture. The thread roots are concentric with the shaft and cannot be otherwise.

When an AR upper is manufactured properly, the bore, the OD, the face, and the threads are cut in one operation, without removing the part from the machine, so there can be no fixture related tolerance stack. If the face is not perpendicular to the bore, it can only be because it was manufactured improperly. If the manufacturer didn’t go to the trouble to do that right, the whole thing is suspect, and if accuracy is important to you, you should purchase a properly manufactured part. If the face is not perpendicular to the bore the odds are good that the threads are off as well, and that cannot be corrected in a lathe or with some files.

Lots more on this later. Anyway, I just wanted to talk a little about tolerance stack before moving forward, it’s important to the discussion.