A drill jig is a device for holding work so that one or more holes may be accurately drilled; the locations of the holes may be governed by hardened bushings (guides) through which the drills run.
The design of a jig depends entirely on the shape of the piece and the nature of the work to be done, but it must be such that work may be placed in them and taken out as quickly as possible. The fastening device should allow rapid manipulation, yet be capable of holding the work without danger of a change of location.
The construction of drill jigs calls for as great accuracy as any branch of the tool-maker's business, but no undue accuracy should be indulged in. If the location of a hole is near enough when within a limit of variation of 1/16 inch, it is a waste of time to attempt to get it within .0005 inch; yet if the work is of such character that it is necessary for the holes to be within a limit of variation of .0001 inch or even closer, every effort should be made to locate the drill bushings as accurately as possible.
While the design of the jig and the character of the work to be drilled must necessarily determine the method of construction, a few general directions may not be amiss. The amount of finish given the exposed surfaces of a jig must be determined by the custom or requirements of the individual shop. In many shops it is not considered necessary or advisable to finish the surfaces any more than to allow of their being wiped without the waste sticking to the jig.
Under other conditions the surfaces are machined as smooth as possible, and the surface finished by placing a piece of J- or 3/5-inch wood dowel in the drill-press chuck, so that the dowel projects 3/4 inch or so from the chuck. The surface of the metal is covered with a thin coating of oil and fine emery, and the dowel, revolving at high speed, is brought down upon the surface, allowed to run for a few seconds, raised, and again lowered so that it cuts part way into the first circle. This is repeated until the whole surface is covered with the part circles. The effect is pleasing and the surface is not easily marked by light scratches that would show plainly on a highly finished piece of steel. It is an economical method of producing a fairly good finish.
A jig should be constructed so that it can be easily cleaned. Chips or dirt between the piece of work and the seating surface, or between the work and the stops, or locating points, throw the work out of true, and, as a result, the holes will be at a wrong angle to the working surface, or they will be improperly located. Either condition would make the pieces unfit for use on most work; consequently, bearing surfaces should be cut away, wherever possible, leaving several small seating surfaces, rather than one large one. In A, Fig. 255, is shown a piece of work .resting on its entire seating surface, while B shows a surface cut away to leave six bearing points. If the seating surface is to be cut away, the raised portions should be so located that the article cannot be sprung by the action of the cutting tools or from any pressure that may be applied by any fastening device; otherwise the work will be thrown out of true as badly as though chips were lodged between the work and the seating.
Fig. 265. Seating Surfaces for Drill Jigs.
It is advisable, whenever possible, to divide a long locating bearing into several short surfaces, and thus to decrease the chance of holes being inaccurately located. When making jigs for pieces that are likely to have burrs at any given point, it is well to cut a depression in the seating or locating surfaces for the burr, thus preventing the work being incorrectly located.
Seating surfaces should be made smooth so that chips and dirt will not stick to them; but they should not be polished or finished, as this would involve unnecessary cost and might throw the surface out of true.
A jig must be handled by the workman, and a clumsy jig is difficult to manage. Sharp corners should be avoided wherever possible, and all handles or similar devices should fit the hand; if they do not, the amount of work done will not be the maximum, as the operator cannot do so much work with a jig which tires the hand and wrist.
As already stated, the accuracy with which a jig should be constructed depends entirely on the nature of the work to be done; yet it should be borne in mind that any inaccuracy must of necessity be duplicated in the work.