Mr. Eisenberg. Why doesn't a series of the same machines, or repeated use of the same machines, cause the same results, apart from future accidental markings?
Mr. Frazier. In some instances a certain type of cutter will duplicate a certain pattern of marks. In general you will find for a milling cutter a circular mark. And you may find the same pattern of circles. But that milling cutter does not actually cut the steel; it tears it out, it chips it out, and the surface of the metal then is rough—even though the circle is there, the circle is not a smooth circle, but it is a result of tearing out the metal, and you will have a very rough surface. When magnified sufficiently, you can detect the difference even between two similarly milled surfaces because of the minor variations in the cutting operation.
Mr. Eisenberg. Have you had occasion to examine such similarly-milled surfaces?
Mr. Frazier. Oh, yes; many times.
Mr. Eisenberg. Would you go into detail on that?
Mr. Frazier. Well, part of my work in the laboratory is dealing with tool-marks of all types, from drills, mills, files, cutting instruments, and so on. And when you are dealing with filing marks or milling marks and so on, it is sometimes possible to identify a particular mill as having made a certain mark on the basis of the grinding marks on that particular mill. But such as a case like this, where the cutting marks have now been altered through use of the weapon and corrosion, or in wear or in filing, some of the original marks are removed, and other marks are in their place, until eventually you reach a condition where that bolt face will be entirely different from any other bolt face. It is a matter actually—when you get down to the basis of it, it is a matter of a mathematical impossibility in the realm of human experience for any two things to ever be exactly alike.
Mr. Eisenberg. That is because the original markings will not be exactly alike, and then you have added accidental markings on top of the original ones?
Mr. Frazier. That is right; yes, sir.
Mr. Eisenberg. Returning for a moment to the original markings, as I understand it, you have worked with the tools themselves and the impressions the tools themselves leave, as opposed to a tooled surface, such as this.
Mr. Frazier. I have worked with both. In other words, in comparing tool-marks, you examine not only the tool, but the marks they produce.