Earlier, I referred to accuracy being the fourth consideration. The holes in the
small jig above were placed using the mill's micrometer dials not so much that accuracy
was crucial but at such a small size even an error of only 0.2mm off centre, if
marked out manually, would be visually very apparent on such a narrow item, being
just 3mm wide. Some larger components would also need their holes accurately placing,
say if setting out gear centres, but for others marking out the holes in the jig
by the rule, scriber and square method would be adequate.
If the quantity and size of the holes to be drilled is considered to warrant it,
then hardened bushes should be fitted to the jig, Sk. 4 shows the normal form though
they can be made without the head. Making these is straight forward with silver steel
being the obvious choice of material to be used in the home workshop. The sketch
shows the shank having two diameters, the smaller being a close sliding fit in the
hole in the jig enables the bush to start in line with the hole. The top 1/3 rd of
the shank should though be a tight fit in the top plates but not so tight that it
distorts the plate which could be particularly a problem if the jig is quite narrow.
Having made the bush they require to be hardened.
Hardening and Tempering
Those who are experienced in hardening and tempering steel components will need no
help, others though should see my pages detailing the method.
Three drilling jigs
Earlier, I stated that drilling jigs could be used for a very wide range of tasks
and that as a result it could only cover the subject generally. However, to encourage
the viewer to consider their use more often I will finish briefly with three actual
examples, two quite common and the other very special.
Without doubt, no other workshop task has produced more designs for a drilling jig
than the task of cross drilling a shaft or collar, illustrating that in most cases
there is not just one design for the job. The methods also show that designs can
range from the very simple to the quite complex. Photograph 11 shows a very adaptable
jig that can be used either on the lathe top slide, Photograph 12, or, with it having
a flat back it c an be used on the drilling machine. Construction of this is shown
elsewhere on the site.