the steel in hardening, thus destroying the alignment and displacing the holes in their relation to each other.

ACCURATE JIGS.

When large quantities of accurate work are to be done in jigs, the tools, of course, should be carefully made. In such jigs drill guiding holes should always be bushed with hardened,

lapped, and ground steel bushings, made to standard external diameters, so that they may be easily replaced when the inside has been worn by the revolving of the drills while working. Such bushings are usually forced tightly into reamed holes in the jig bodies. For producing accurate work in small quantities interchangeable bushings are used, a full set of them being kept on hand. These bushings may be used in any of the large jigs in the shop indiscriminately.

TOOL-ROOMS AND THEIR EQUIPMENT.

Naturally, in a chapter devoted to the value of tools and the evolution and development of tool-making, one expects to find

something on tool-rooms; at all events, a few remarks on the subject will be timely.

Tool-rooms are of two classes-those in which tools and fixtures are made and those in which they are kept.

In those of the first class the most important item is the lathe. An approved type of a modern tool-maker's lathe is shown in Figs. 9 and 10,

the general features of which are apparent. It is a ten-inch, tool-maker's lathe, and its design and construction represent the attainment of perfect and complete convenience. It is one of the most complete precision-lathes ever produced for the tool-maker or model-maker. Now, of all machine tools, for either tool-making or manufacturing, the lathe is king. If a machine-shop or a tool-room is to have only one tool in it, it is obvious to all that the tool should be a lathe, and it should be a good lathe. With a good lathe and a skilled mechanic to operate it and bring out all its capabilities, almost anything in the line of tool-making and machine construction may be accomplished. As to-day lathes

are being built in the most astonishing variety of capacities, from the delicate precision-lathe to the ponderous three-hundred tons gun-lathe, no difficulties should be experienced in procuring one for any special line of tool-making or manufacturing.

After the lathe, next in importance comes the drill-press, the selection of which depends upon the class of work to be done. Usually there should be two-a small sensitive drill and a large column machine. Next we have the universal milling-machine, with its boundless possibilities. In order of importance the shaper and planer come next, and in their choice the nature of the work to be done is also the chief factor to be considered. Vises and small tools, of course, follow; then the speed-lathe, for hand-tooling, polishing, and lapping. Lastly, the modern tool-room is not complete without a tool-grinder. All of these machines are sufficiently well known and a detailed description of any would only take up valuable space.

In regard to a tool-room of the second class, it must be obvious to all that its chief requisites are that it shall form a convenient place where tools and appliances may be systematically and handily distributed. In a small establishment only one toolroom is necessary, but in any extensive establishment, where there are several buildings and several floors in each building, it is necessary that there shall be a number of tool-rooms in order that there shall be convenience in the distribution of the tools.

In order that the reader may understand what a tool-room should be like, it is essential that a short description of a model one should be presented. I know of no better way of doing this than by describing those in the shops of Brown & Sharpe, Providence, R. I., U. S. A. In their shops the different tool-rooms are much alike, the largest one being on the second floor of the main building, where all lathe tools are ground on a Seller's grinder before being given out, and other work of like character done. Like all shops in which large numbers and varieties of tools are in use, the check system is in use. Ten checks are given each workman, one of which is placed opposite the place reserved for any tool that he has out. One noticeable and excellent feature of these tool-rooms is the good supply of parallels in each. To save checks, when a workman requires several paral

lels the system shown in Fig. 11 is in use. The parallels are placed in pigeon-holes, those of one size in one row, the next larger in a row below, and so on. At the right is a board, on the side of which is marked the size of the parallels in each row, and at the top of which are the numbers 1 to 6, to indicate the number of parallels in use. Checks in the positions shown would indicate that a workman had out four 14-inch parallels and three 24-inch ones. In a great many shops it is common to keep the tool-rooms supplied with sets of taps and tap-drills together in two blocks,

only one check being necessary to secure the whole. In the Brown & Sharpe tool-rooms the tap blocks are more completely equipped than usual. Each set or block consists of a full set of drill, tap-drill, starting, sizing, and bottom-tap, two counter-bores for holes where countersunk head-screws are used, one counterbore having a tip the size of a standard hole and the other to fit. a tap-drill hole; each block also contains a test plug, giving the size of a standard head for screws of that size and a tap-wrench.

In regard to keeping track of workmen's supplies, there is a novel system in use. It consists of a six-sided case, one in each tool-room, on the sides of which hang an extra size of ten checks for each man. The top of the case is divided into several compartments, marked, respectively, "Oil," "Waste," "Towels," "Emery Cloth," etc., and when a man wants a ball of waste one of his checks is dropped into the receptacle bearing this name. Thus after a certain time has elapsed the checks may be removed, counted, and a record taken of the amount of supplies which each man has used. The checks are then put back on their pins.

CHAPTER III.

Fundamental Principles, Processses and Practical Points for Jig Design and Construction.

BEFORE taking up the various types of jigs and fixtures used for the production of repetition parts by drilling and milling, and illustrating them and describing their construction and use in detail, I have thought best to devote a chapter to a presentation of the fundamental principles, various processes, and practical points which are required to be understood in order to successfully design and construct drilling jigs and fixtures or similar special tools used for the machining and duplication of machine parts. If the rules laid down are followed, much unnecessary labor and expense will be avoided and the best of results attained. The descriptions are given from an entirely practical point of view, the theoretical not being touched upon and anything purely speculative being omitted.

FACTORS INVOLVED.

In the first place, let it be understood that there is no one other branch of the machine business that requires more thought, · wider knowledge, and broader experience of shop conditions than the designing of jigs and fixtures, and in order for one to be competent to do this work successfully he must possess this essential knowledge of shop conditions. To those who are not so equipped a close study of the chief factors and the fundamental principles involved will be of untold value.

In jig and fixture work there are six highly important factors to be considered: 1. The course the work is to follow during manufacture. 2. The locating and securing of the work in the fixtures. 3. Keeping of the locating points for the work free from chips and dirt. 4. Self-contained tools. 5. The class of help that will use the tools. 6. Convenience and ease in hand ling the tools during their operation.