Centre drills
It is worth taking a moment, here, to discuss centre drills, also known as Slocombe drills as they were first patented by the Slocombe company of America. I had not encountered these before working on this project and they are a terrific invention.
They are short, stubby and above all, stiff. This means that they won’t ‘wander’ as much when attempting to start a hole, even if there is no indent (such as made by a centre-punch) in order to guide them to the right place. Indeed, they are so stiff that they are likely to ‘ignore’ such an indent, even if one is present!
The narrower section is small enough to start the hole and the wider section is then intended to form a tapered guide (countersink) into which a conventional drill’s tip will locate precisely. In this way, a deep hole may be drilled in a precise location, with ease.
fly-cutter would then, ideally, have been removed in the sawing process.
The final step was to apply a metal lacquer to the finished plate. I used Rustins – this is a specialist clear varnish for use on metals and gives a lovely finish, showing through the polished brass almost as if the lacquer was not there. It also saves a lot of future effort with Brasso or Autosol, as the base plate, at least, will never tarnish. Clearly this only works on non-rubbing surfaces (the flywheel being another possibility here, along with three sides of the cylinder), but it is certainly worth doing.
The assembled base, the base plate and nameplate (a bit of indulgent CNC milling on another piece of the 10mm aluminium of the type used for the upright) are shown in Photo 4, with the underside, including the countersunk bolt-holes, in Photo 5. Once assembled, this side can be covered with some felt. The more observant of you will notice that I didn’t quite get one of the holes in the right place in Photo 5 – this is not visible, once assembled, and so is not an issue. It’s all part of the learning, or so I was told when I spotted the original error...
The recesses in the wooden base
“It’s all part of the learning, I was told when I spotted the original
error...”
were made using a wood-router bit (1/8-inch) in the mill, running at 5600 rpm – not many mills are capable of such speeds, but a lower speed can be used with a suitably slow feed-rate. A small wood chisel was then used to square-off the corners of the recesses.
I could have used a larger router bit, followed by the 1/8-inch tool (for the corners), but then I would have had to do a tool-change and re-set the depth and such. At least with a single tool, whilst it may be a bit slow, the depth is guaranteed to be constant across the whole area (no chance of setting the depth slightly wrongly, following a tool-change).
■ Next month Matthew tackles the flywheel and its components.