dowel pins & drilling
Using dowel pins to align components
The four hole in the casting above had been drilled and tapped on the workbench and the corresponding countersunk holes in the bracket drilled in the correct places at the same time. That was the easy part.
When the casting was bolted under the flywheel and the bracket hand held around it, it became quite difficult to align it so the screws could start in their tapped holes and without cross-threading them.
The trick was to fit them together on the bench and between two of the screws drill a ¼ inch hole for a dowel pin of sufficiently light fit that it could be removed by hand.
A dowel pin was then made with a bullet shaped end so that when the casting was installed and the bracket offered up the dowel pin bullet head found its hole with a bit of wriggling and when inserted fully the screw holes were then aligned.
Once the screws were started the dowel pin was removed until the next trial fitting of the bracket.
One dowel pin was sufficient, but more complex jobs may require two or more.
Drilling in alignment
Sometimes it is necessary to locate existing tapped holes to a new casting for drilling exactly in alignment.
It is almost impossible to find the centre of a tapped hole by measurement, even if studs are fitted to the tapped holes and measurement made from the outside of the studs, then deducting one stud diameter from this measurement.
There are three reasons this may not be very accurate:
- The studs may not be full diameter, especially at the thread
- The original holes may not be in a line
- It seems that centre-punching introduces its own errors in positioning
An effective way is to make up false studs which are a loose fit in the original tapped hole. Do this in a lathe, then turn a sharp point at the outer end and the stud just long enough to protrude about ½ mm above the surface.
Fit the studs as above, locate the undrilled casting exactly, the tap it with a leather hammer or wooden mallet so that the sharp points make centre-pop marks in the casting.
If the studs have tiny flats filed next to the sharp points they can be unscrewed with a small needle nose pliers.