Drill jigs are used for drilling holes which must be accurately
located, both in relation to each other and to certain working
surfaces and points; the location of the holes is governed by
holes in the jig through which the drill passes.
The drill must
fit the hole in the jig to insure accuracy of location. When the
jig is to be used in drilling many holes, the steel around the
holes is hardened to prevent wear. If extreme accuracy is
essential, or if the jig is to be used as a permanent equipment,
bushings, made of steel and hardened, are used to guide the
General Considerations in Designing Jigs
The design of a jig should depend altogether on the character of
the work to be done, the number of pieces to be drilled, and the
degree of accuracy necessary in order that pieces drilled may
answer the purpose for which they are intended. When jigs are to
be turned over and moved around on the drill press table they
should be designed to insure ease and comfort to the operator
when handling, and should be made as light as is consistent with
the strength and stiffness necessary.
Yet, we should never
attempt to save a few ounces of iron, and thereby render the jig
unfit for the purpose we intend to use it for. The designer
should see that the jig is planned so that work may be easily
and quickly placed in and taken out, and that it can be easily
and accurately located in order to prevent eventual mistakes.
As it is necessary to fasten work in the jig in order that it
may maintain its correct position, fastening devices are used;
these should allow rapid manipulation, and yet hold the work
securely to prevent a change of location. Yet, while it is
necessary to hold work securely, we should not use fastening
devices which spring the work, or the holes will be not only
improperly located, but they will not be true with the working
surfaces or with each other.
When finishing the surfaces of
drill jigs and similar devices used in machine shops, the
character of the finish depends entirely on the custom in the
shop, for while in some shops it is customary to finish these
tools very nicely, removing every scratch, and producing highly
finished surfaces, in other shops it is not required, neither is
it allowed, as it is considered a waste of time and an
unnecessary item of cost.