Equipment exists to get very accurate and very careful measurements of just about anything you can imagine. Small shops tend to use hand tools, such as micrometers and calipers; time was, a master machinist might not have his car keys or his wallet in his pocket, but he’d damned sure have a Micrometer. When I bought Dad his own 6″ dial caliper in 73, he felt as if he’d Arrived; he made a leather shoulder holster for it and it never (Literally) left his side.

medium sized shops or shops doing extremely high precision machining will often use a device called a CMM or Coordinate Measuring Machine. For almost everything, the CMM is the last word in measurement, because it uses scales and math to calculate diameter, depth, concentricity, all of the things you need to know about a manufactured part.

CMM’s are not cheap. A good one will set you back a hundred K, and the unfortunate reality is, a cheap CMM is not a good cmm. You could end up with a measuring machine whose resolution is less than your machining center, and the information you get from it is worse than useless.

For large shops, there is usually a combination of small handheld tools and CMM’s. Large production shops also use what are called “Hard gauges” to bridge the gap between calipers and micrometers (Which take some skill to read) and CMM’s which are costly and usually not used directly on the shop floor (Though this is changing).

A Hard gauge is a way for a relatively unskilled person to test if a parts dimensions are in tolerance without having to know too much about the process, or think too much about the gauging. A hard gauge can be as simple as a “Go/Nogo” gauge, used to check the size of a hole. This type of gauge has one pin at the stated size of the hole, and another pin at just slightly beyond the maximum limit of the hole size. They look like this.

It’s pretty simple to use. If the ‘Go” doesn’t go in, it’s too small. If the “No go” goes in, it’s too big. Cletiii everywhere can use them like masters.

Another type of hard gauge that is very common is the Flush Pin gauge. This is an interesting type of gauge that is based on the ability of people to feel things they cannot see.

Take a hair. A blonde one, if you can get it. Lay it flat on a piece of glass. Now rub your finger over it. If you don’t have serious nerve damage in your hands, you can easily feel it. The hair is about four thousandths of an inch thick. Most people can feel about a half a thou. That’s .0005.

A Flush Pin gauge is often used to check the chamfer of a hole. it involves a hollow cylinder with a ground pin captive inside it. On the bottom is a flat ground surface on the cylinder and a cone on the pin. The gauge is used by putting the cylinder on the surface and pushing the pin down into the chamfer. The top of the gauge has a “Step” surface in the cylinder while the pin is ground flat.

This is what one looks like. You can see the “step” on the top (left, in this photo)
The magic to this little gauge is that the step all0ows you to tell, just by using your finger, if the chamfer has been cut to the correct depth, and do so within .0005. You check to make sure that the pin sticks up above the first step, which you can easily do just by sliding your finger across it, and below the second step, likewise. And those two steps can be as small as .0005 apart, and most people can still feel it easily. If you have very sensitive fingers, you can feel .0001.

This type of gauge can be used to check the depth of a hole, the length of a shoulder, and many other critical dimensions, with remarkable accuracy, very quickly, by a moron.

IN large production operations, rather than tie up an expensive machine like a CMM checking every part, most parts are checked with hard gauges, (Or, in automated systems, the electronic equivalent of hard gauges) and only representative samples are tested in the CMM.

The CMM is used for another reason as well: as the tools in a machine wear, they get microscopically smaller. This changes the part by very small amounts, too small to be caught by the hard gauges, but they will be significant in time. When the parts are checked in the CMM, offset data is passed on to the machines that cut the parts, so the machines stay well within tolerance until the tools life has expired, and in time, the process will be monitored so closely that the tools can be changed predictively, knowing exactly how many cuts each one has before it will wear excessively or fail.

In automated systems and in some manual systems, electronic hard gauges are used so that the data can be passed directly to the machines, and in very critically dimensioned parts, this is an important part of the modern manufacturing process.

I remember setting up a cell on the northside in the winter of 1998 that had to meet 2.00 CPK requirements at a nearly impossible bore tolerance. We were doing OK, and halfway through the runoff some idiot opened the garage door ten feet away from the cell, and it stuck open.

We figured we were fucked. As the machines cooled from room temp (around 69) to a nice 38 degrees, the coefficient of thermal expansion of the iron made the lathes make smaller and smaller parts, but the hard gauges sent offset data back to the machne, and we made 2.00 CPK like falling off a log. THe gauges corrected for the machine’s thermal shrinkage as it happened, each part was machined a bit smaller but the offset made a bit bigger, and despite miserable conditions we got the job done.

The company who made that part is long gone, but the process is still around, and the parts still being made, having been sold to another company as a unit in the company’s closing auction.

Another type of gauge that is common across the industry is the air gauge. In operation, it’s extremely simple. A bore- say a bearing bore- is to be tested to a specific diameter.
Now, you can use mechanical bore gauges or calipers, but two cletii will have two different “Feels”. One will tighten an ID mike until it barely touches, then read it, and another will tighten it as if it wewre a clamp, and read it. These two cletii will produce dramatically different readings for the same part, quite literally.

Anyway, back to the bore. An air gauge consists usually of a plug that is almost the size of the bore, with several holes of a specific size bored into the edge. Air is plumbed to these holes, and when the plug is put into the bore, the holes are partially occluded. The air pressure goes up. An air column gauge shows the rise in pressure as LED’s on a column. Those gauges look like this:


There are many types of air gauge but the ones in common usage are green when the part is in spec, yellow when they approach upper or lower tolerance, and red when they’re out of tolerance. The air can only leave the holes at the velocity allowed by the gap between the plug and the bore; a tight bore lets less air through, and the pressure rises, a loose bore lets more air through, and the pressure drops. No “Feel” is necesary. No talent is required. Cletiii over the world use these to measure the most stringent tolerances, because most of them can more or less drive, and they understand the whole distinction between “Green, yellow, and red”

The other thing Air gauging does is to let you measure things that can be measured in almost no other way. Springfield Armory, years back, made ‘Star Gauged” barrels, which were and are very highly sought after. The manufacturing of that gauge was difficult and it was easily damaged. Air gauges do the job so much more efficiently and are incredibly easy (By comparison) to manufacture.

Modern rifle barrels are made by any number of methods, and each method has it’s hardcore adherents. Compared to the manufacturing methods of even 100 years ago, the way mfrs put those twists into the barrels today is- if not better, at least more reliable. No matter what firearm you choose to shoot, no matter how the rifling is made, the liklihood that the barrel will shoot far more effectively than your ability to point it at anything is great- because of the way they’re measured and how that measurement is used.