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136 
ROAD AND WORKSHOP by HANDYMAN
Page 64
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The Value of Drilling Jigs
ALTHOUGH one would not expect to find any large amount of repetition drilling work in vehicle repair, there are jobs that are worthy of examination and the possible application of any aid to accuracy and speed of turn-round. I refer to the drilling jig, which even in its simplest form can score heavily over marking off and centre-punching.
A jig can be a simple, hardened-steel plate or a complicated, box-pattern job to deal with holes in every side of a work piece. One form of jig plate is of normal steel, but fitted with hardened-steel bushes at the drilling points.
Jigs must be robust in design to stand firm clamping and have enough thickness to centralize the drill. In making one's own jig for a particular job, care should be taken to avoid the jig plate distorting.
In one garage dealing mainly with tippers engaged on factory site work, and where the fleet was almost standardized on one make, I saw a chassis being re-drilled for spring brackets after blacksmith's shop and welding attention, and I must admit to some doubt when I saw the drilling preparations.
First, the chassis area to be drilled was chalked over, then from beneath a bench an old rear-spring bracket was produced and held by one hand over the repaired area. The other hand first wielded a steel rule for a distance check and then produced a small screwdriver and scratched around in each hole. The final operation was to centre-punch the scribed-out ring and then drill the chassis.
It is enough to say that only two of the five holes would accept the bolts when the new bracket was produced, and a further exercise with a in. round file was called for to complete the job. Examination of the old bracket that had acted as a template showed it to have elongated holes, and this no doubt called in the round file.
Why the new bracket was not used to mark off I did not discover; if the bracket had been clamped on and used as a jig, only three of the holes could have been approached by the drill. Therefore, the answer in this instance, and for future use, was a prepared and hardened jig plate for this duty.
The value of the jig plate was that a precision job could be done, using interference fit holes, and a long and secure life for the new bracket guaranteed. But with the other method, the round file had destroyed any hope of bolts really fitting and acting as dowels, and the future of this bracket was uncertain.