* * * The marks which are placed on any bolt face are accidental in nature. That is, they are not placed there intentionally in the first place. They are residual to some machining operation, such as a milling machine, in which each cutter of the milling tool cuts away a portion of the metal; then the next tooth comes along and cuts away a little more, and so on, until the final surface bears the combination of the various teeth of the milling cutter. In following that operation, then, the surface is additionally scratched--until you have numerous- -we call them microscopic characteristics, a characteristic being a mark which is peculiar to a certain place on the bolt face, and of a certain shape, it is of a certain size, it has a certain contour, it may be just a little dimple in the metal, or a spot. of rust at, one time on the face of the bolt, or have occurred from some accidental means such as dropping the bolt, or repeated use having flattened or smoothed off the surface of the metal.

* * * As the blade of a milling machine travels around a surface, it takes off actually a dust--it is not actually a piece of metal--it scrapes a little steel off in the form of a dust---or a very fine powder or chip--that tooth leaves a certain pattern of marks--that edge. That milling cutter may have a dozen of these edges on its surface, and each one takes a little more. Gradually you wear the metal down, you tear it out actually until you are at the proper depth. Those little pieces of metal, as they are traveling around, can also scratch the face of the bolt--unless they are washed away. So that you may have accidental marks from that source, just in the machining operation.

Now, there are two types of marks produced in a cutting operation. One, from the nicks along the cutting edge of the tool, which are produced by a circular operating tool--which produce very fine scratches in a circular pattern. Each time the tool goes around, it erases those marks that were there before. And when the tool is finally lifted out, you have a series of marks which go around the surface which has been machined, and you will find that that pattern of marks, as this tool goes around, will change. In one area, it. will be one set of marks--and as you visually examine the surface of the metal, these very fine marks will extend for a short distance, then disappear, and a new mark of a new type will begin and extend for a short distance. The entire surface, then, will have a---be composed of a series of circles, but the individual marks seen in the microscope will not be circular, will not form complete circles around the face of the bolt.

Q. Have you had occasion to examine two consecutive bolt faces from a factory?

A. Oh, yes.