Taking the first factor-the course of the work during manufacture-we will say that a part of a machine is given us to design tools for its production in repetition. Now say that the part, in order to complete it, will have to go through two operations, drilling and milling. The question is which should be done first, the drilling or the milling?

In most cases where the part is to be drilled and milled, it is best to provide for doing the milling first; because it is desirable that the drilled holes and milled surfaces shall bear a certain definite relation to each other, and because by having the holes drilled from a milled surface greater accuracy and interchangeability in the parts can be obtained than if the milling were attempted after the drilling of the holes. However, in order to decide the question, a "working point or surface" must be decided upon. Whether the part to be machined is a casting or not, there is always one point which from its position-that is, in relation to others should be taken as a "working point," a point to work from and refer to in all subsequent operations required to manufacture the particular part. The point chosen may be a hole, a plain surface, a slot, or a lug or a boss-it matters not.

THE LOCATING AND HOLDING DEVICES.

Now having chosen the working point, it follows that this is the point to be machined first, and that the first jig or fixture to be made is the one for this operation. This is the secret of successful jig-making. Also use this point for the locating of the work in the different jigs and fixtures for subsequent operations. Never change a working point, as the performing of one operation from one point and the next from another is not conducive to good results.

When designing fixtures for drop-forgings, turned work, punch-blanks, or any part that has been previously put through a cutting, abrading, compressing, or forming operation, the contour of the part is usually such that the holding of it is a simple matter, especially if the first operation is to be a milling cut. With castings, however, through their lack of uniformity in many cases, fixtures of intricate and costly design are required, thus necessitating considerable care and judgment in the devising

of the locating and holding means. If, instead of milling, it is decided that the drilling should be done first, and that the holes so produced are to be used in locating and securing the work instead of using the outline, it will be found that a simpler and less costly fixture can be used. Whichever course is decided upon, the fixtures should be so designed as to allow of all operations of one class being completed before commencing on another class.

Now in regard to locating and securing the work quickly, accurately, and easily, these are factors of the greatest importance, and it is difficult to discuss them properly, for the efficiency of the finished work depends more than anything else upon them.

The various methods in universal use for locating and fastening the work to be machined in jigs and fixtures, such as bunters, cams, set screws, spring pins, slides, flat taper pins, etc., are well known, and I will not attempt to lay down a general rule for their application, as this must be decided by the designer according to the type of fixture and the nature of the work.

One of the most essential conditions necessary to the accurate and rapid production of work in jigs and fixtures is convenience in keeping the locating point free from dirt. This must be evident to any one at all familiar with the use and object of such tools.

When I state that tools should be self-contained, I mean that all devices and means utilized in the locating and securing of the work should be component parts of the tool. When this is the case, the operator is not obliged to use a hammer, wrench, or any other tool in order to operate the fixture.

SIMPLE DRILLING-JIGS.

When drill-jigs of the comparatively simple types are to be constructed for the machining of parts in which no great accuracy is required, the main point to be considered is the interchangeability required in the work after it is machined. With this point constantly in mind, the avoiding of all unnecessary expense and labor will not be difficult. In the construction of simple jigs, which are to be used for the drilling of parts which have been first finished at one or more points, or for rough castings which have not had any previous machining, the most essential points necessary to their successful construction and use are

as follows: First, in making the patterns construct them so as to leave openings in the castings at all points wherever possible, without affecting the strength or rigidity of the castings when finished, for the escape of the chips and dirt. Second, provide spots with just surface enough to allow of their rapid surfacing. Lastly, so design the jig as to allow of the expeditious fastening and locating of the work and its removal when finished, as this is one of the important factors in the operation of such tools.

CONSTRUCTING SIMPLE JIGS.

First, a machined

When this is not

When constructing, after having done the preliminary machining of all necessary outside points, choose the most reliable and positive points for locating the work. surface for the positive points for locating. possible, those points in which the minimum of variation is to be expected in the castings should be chosen. Then, in the fastening of the work within the jig, use means which will be the quickest in operation consistent with all possible simplicity. As there are any number of simple and inexpensive devices which can be adopted to allow this, it should not be difficult.

One point which cannot be too strongly impressed on the designer of simple jigs is to allow excess of metal at as few points as possible; that is, only at the locating and squaring surfaces. The all too prevalent habit of leaving unnecessary surfaces to be finished is expensive and not consistent with satisfactory results.

PROCESSES OF ACCURATE JIG-MAKING.

When drill-jigs are to be made for the drilling of work in which the utmost accuracy is desired, the locating and finishing of the bushing-holes is of the greatest importance, and for that reason I give here descriptions of the most rapid and practical methods for the accomplishment of this part of the work.

THE BUTTON METHOD FOR LOCATING DRILL
BUSHING-HOLES.

In the first place, if the jig to be made is of the box typewhich is the most generally used type-for which the body casting has been secured, after all sides and bearing surfaces have

been planed or milled square and true with each other, including the feet, it should be rested on a surface plate, as shown in Fig. If the feet 12, which should be used only for work of this class. are cast on the jig, they should be scraped until the sides of the

body portion are at perfect right angles with their bottoms and until all legs rest perfectly square on the surface plate. If the feet are of tool steel and are screwed into the jig, they should be hardened and lapped on a flat lapping-plate (as shown in Fig. 13), until the same results are accomplished. This preliminary

work on the jig is the basis for the successful attaining of all other results, and unless done carefully there is no possibility of the remainder of the work being accomplished accurately.

For the laying out or locating of the bushing-holes in jigs, and

the finishing of them, there are any number of methods in use among tool-makers. Some of these methods allow of fair results being attained, while others are useless, and when accurate or satisfactory results are accomplished though their use it is pure luck, not the method that does it. There is only one method for locating bushing-holes in small and medium-sized jigs accurately and expeditiously.

The following method is used by the best tool-makers on this class of work and is known as the "button method": In shops where jigs for accurate production are constructed, a few sets of locating buttons should be kept in the tool-room as standard sizes say, five-sixteenths, one-half, and three-fourths inch in diameter, as shown in Fig. 14. They should be of tool-steel and finished to from one-half to one inch in length, and should have

a hole through them large enough to allow about three-sixtyfourths inch clearance for the fastening screws, after which they should be hardened and then ground perfectly square on each end, and on the outside to standard size, finally lapping them to get them accurate. One end of the button should be slightly countersunk, so that it will rest squarely on the jig when in position. The centres for the bushing-holes in the jig should next be located approximately correct by the dividers and then prickpunched. They should then be drilled and tapped for the button

screws.

To locate the holes positively, first secure a button in position by working from two sides of the jig, using a Brown & Sharpe height-gauge, and fasten it securely by tightening the button screw. Locate the next hole in the same manner, using the height gauge or vernier gauge to get the buttons exactly the proper dis