The second jig is for a common task in the home workshop involved in model engineering,
that is drilling the end flanges and cylinder block of a typical steam engine. Photograph
13 shows the jig and a plug for locating it when drilling the end of the cylinder
as seen in Photograph 14. The jig positions itself directly onto the end covers for
drilling as these have a small boss for locating them when fitted to the cylinder
block, Photograph 15.
If the jig is marked out using a rotary table or a dividing head then the jig is
worth making if only for the accuracy of the results, Photograph 16. Incidentally,
since the holes were drilled the jig, Photograph 13 shows a second set of holes has
been drilled for some reason should you be confused.
The third jig is very special and is unlikely to be required in the home workshop,
unless the owner is going to make the boring head, described here. The task is
to produce a partial hole on the edge of a round component and for this to be tapped
M6, a seemingly impossible task as only 1/3 of the hole’s diameter is in the component
with the other 2/3 in fresh air.
For this to be achieved a drilling jig was produced with a close fitting hole into
which the component being made was fitted. This was then drilled as shown in Photograph
17 and the two, that is the part and the jig, tapped before the workpiece was removed.
Photograph 18 shows the jig after being used together with the required component
but this having been completed with other operations since being drilled and tapped.
The partially tapped hole can though be clearly seen in the component, also in the
jig itself. If required, the jig could have been used for more than one part by drilling
more holes round the periphery of the large hole, though in this case only one part
was needed. Even with there only being one part to be made the jig was all but essential
as an alternative method would have been difficult to find.
Not just a scriber and centre punch job
I do hope these pages have convinced you that positioning holes in a workpiece is
not always a rule, scriber and centre punch task, but that there are other, and often
better, methods for placing a hole in the required position.