As generally understood, a tool-maker is a machinist who has a greater knowledge of the trade than is sufficient simply to enable him to make such machines or parts of machines as may be the regular product of the shop in which he is employed.
The business of the tool-maker is to make the tools for producing the different parts of machines, implements, or apparatus. It includes the making not only of cutting tools, but also of jigs and fixtures for holding the work while the various operations are being done, and the necessary gages to determine when the different parts are of correct size and shape. It also includes the making of the models for the different fixtures and gages. In some shops where there is work enough on the gages and models, the tool-makers regularly employed on this latter work are termed gage-makers and model-makers, respectively; yet, in the average shop, it is the tool-maker who makes these tools and such special machinery as may be required.
In order to acquire any degree of success, the tool-maker must have not only the ability to work accurately and within reasonable time, but also a knowledge of drafting to enable him to read quickly and exactly any ordinary drawing. Unless he can read decimal fractions readily and correctly, he will experience much difficulty when working to measurements that require accuracy to within .0001 inch. As most of the measuring instruments used by the tool-maker read to .001 inch, and some of them to .0001 inch, or even closer, it will be readily seen that in laying off measurements for gages, models, drill jigs, and similar work, a thorough knowledge of arithmetic is essential.
A tool-maker should be familiar with the accurate reading of the micrometer and of the vernier, as applied to the vernier caliper, vernier depth gage, and vernier height gage. He must bear in mind, when using the vernier caliper for inside measurements, that it is necessary to add the amount of space occupied by the caliper points AA, Fig. 1, to the apparent reading on the vernier side.
When measuring the distance between the centers of two holes, as in Fig. 2, set the vernier so that the portions of the jaw marked AA, Fig. 1, will exactly caliper the distance from B to B in Fig. 2, To the apparent reading of the vernier, add the space occupied by the caliper points; and from this subtract one-half the diameter of each of the holes. It is necessary to caliper the size of each hole. Do not take anything for granted when accurate measurements are necessary. A reamer ought always to cut an exact size, but experience proves that it does not invariably do so. If the size of the hole is taken for granted and a mistake of .002 inch is made, an error of .001 inch in a measurement would result.
While extreme care should be exercised when accuracy is essential, there are parts of a tool where approximate measurements will do. If within 1/16 inch is sufficiently exact, it is folly to spend time to get a dimension within a limit of .0001 inch.
Approximate measurements are those made with the aid of calipers, dividers, surface gage, etc., set to an ordinary steel rule. Precise measurements are obtained by the aid of the various measuring instruments graduated to read to very small fractions of an inch; also by the use of standard reference discs, and standard test bars, accurate within a limit variation of 1/50000 part of an inch. In using the micrometer, the vernier, or any of the measuring instruments supposed to give accurate readings, it is necessary to exercise great care in setting the tools. In setting the vernier, it is well to use a powerful eyeglass in order that any error in setting may be so magnified as to be readily apparent.
The difference between the two characters of measurements described - approximate and precise - may be readily seen in the plug gage shown in Fig. 3. The gage end A, when ground and lapped, must be exactly 1 inch in diameter, as shown by the stamped size on the handle C. The handle should be 15/16 inch in diameter and knurled, and the neck, 7/8 inch. While the end marked A is necessarily a precise measurement, B and C are approximate, and an error of 1/64 inch or more on either diameter would not interfere with the accuracy of the gage. This does not mean that so great an amount of variation from given sizes should ever occur; but the illustration is given to show that the practical workman will never spend an unnecessary amount of time to produce accurate measurements, when an approximate measurement will do. On the other hand, all care possible should be taken when lapping the gage end A to size.
The working parts of any machine that may be running should be kept as clean as possible. Do not allow chips to collect on the shears, Vs, of your lathe. If the shears become roughed or worn, accurate turning cannot be done. Keep the machine thoroughly oiled; clean the oil holes out occasionally with a piece of wire, in order that the oil may get to the bearings. Be sure the centers of your lathe are in good condition; have them to gage; and make certain that the live center runs true before taking any finishing cuts. Try the center gage on your countersink occasionally to see that it maintains its correct shape. Keep the center punch ground to a good point. It is advisable to grind the prick punch used in locating working points in some form of a grinder having a chuck or collet to hold the punch while revolving it against the emery wheel; if the point is not perfectly round, it will be impossible to indicate a piece of work perfectly on the faceplate of the lathe with the center indicator.
Fig. 3. Plug Gage.