Enlarge the punch marks using a hand held punch and drill 2.1mm, tapping drill size
for 7BA. This can now be used as a drilling jig for both cylinder covers, Photograph
25, also, together with fixture 2 and the dual purpose 3/4” hole gauge, for both
ends of the cylinder, Photograph 26. DO NOTE that a pair of holes are parallel to
the flat face of the cylinder.
Open up the holes in part 15 to 7BA clearance, for part 6 though the operation is
different. In this case, locate the 5/8” boss in the standard and clamp in place
using a long screw and drill both parts clearance for 7BA Photograph 27. Again, the
position of the holes is important with one pair of holes being parallel to the standard's
If eventually you can assemble these parts in any one of the five positions you have
done well but do not be surprised if you have to find the position where all five
holes line up. Whilst not perfect it will be visually acceptable. For the cylinder
this just leaves the 8BA tapped holes, made later.
Valve Chest Cover 28
For this you will need a small non slotted faceplate but if one is not available
then a piece of steel placed in the three jaw chuck and faced will do equally well.
Using a piece of emery cloth, say 180 grit, on a flat surface lap the rear of the
cover to remove any high spots and achieve a reasonably flat surface. Do not expect
to remove the surface completely, only to remove the high points. With that done
cover a suitable area of the faceplate with thin film double adhesive and place the
rear side of the cover on this, using the front end of the tailstock barrel to apply
some pressure. For added support add some strips around its edges, then machine the
first side, Photograph 28. Having completed the first side, carefully prise the
cover from the faceplate, reverse and refit, again using the tailstock to provide
pressure, then machine the second side.
If you are apprehensive regarding the method you may be reassured by the fact that
I was machining two covers, one of which I replaced as it was a little too thick,
making a total of five sides being machined, all completed without a problem. This,
even though the adhesive was looking very dirty by the time the fifth was placed.
Finishing the edges and drilling the holes will be carried out later.