With the edge located, traverse the table by 38/40mm plus half the finder's diameter
and lock the table in place. Next, similarly locate the rear edge and repeat the
operation, this time locking the other axis and centre drill the lug to position
the eventual hole to be drilled, Photograph 12. As the photograph shows, using a
long centre may be beneficial in avoiding the need to raise the head on the column
to allow a longer drill to be fitted that would then loose position.
Having marked the holes position now drill the hole 16mm diameter, Photograph 13,
followed by removing the castings and turn over and drill the second side. Do not
move the machine table as this will permit the second end to be drilled without having
to repeat the edge finding stage. Do though, when drilling the second end, add a
short length of 16mm bar in the holes already drilled as this will be an aid to accuracy.
If, like me, you are working with relatively short angle plates do take particular
attention to the requirement for the faces of the casting to be fully in contact
with the angle plates to which they are secured
Ideally, the method should be to drill 15.75 mm and ream 16mm (as per the drawing).
I suspect that these would be items not available and would cost far too much to
purchase for a one off task. Probably costing as much as a finished budget adjustable
angle plate of the same size.
However, using just a drill, as I did, has its problems as when a drill drills oversize,
as many do, then the hole will reduce to size as it breaks through. In this case,
with a large drill, the tip will break through well before the full size of the drill
passes and the change in hole size will occur gradually over a distance of a few
millimetres, as it did in my case. This making it difficult to make a close fitting
pin, or bush in my case.
The alternatives are briefly therefore. 1. Using a boring head to finish the hole,
a time consuming method. 2. Make the pin/bush with a reduced end to accommodate the
locally reduced bore size, not very professional, and 3. Purchase one of the budget
adjustable reamers that can be had for a few pounds, My bushes, as a result, have
a little clearance, perhaps one day I will obtain an adjustable reamer and remake
the bushes, as it is though it appears to be reasonable.
Clamping a piece of steel under the lug to prevent the drill breaking through into
fresh air would also be worth considering but there is not much room for the clamps
to be fitted.