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Capstan – Examples
1 Making screws/bolts
2 making nuts
3 making washers
Example – making a bush
Set to length – center drill
Part off and chamfer
It would be nice to chamfer the inside of the bore at the head end but this can only be done as a second operation. See later.
Example – Making a cylinder drain valve
These are quite complicated for such a small part. Any engine will only need four of them. One way is simply to buy them.
The problems are that it has a tapered hole at right angles going through it. The body of the valve is often shaped. On full size valves this is to minimise the amount of metal needed and the weight of the finished part. On a model it is done to look like that on the full size part.
The tapered hole
This is difficult to do on the lathe. The solution is to make a rod, while it is one piece, and drill the holes in it as the first part of the job. It is essential that a cross drilling jig is used to drill the holes.
The way the capstan works means all the holes must be the same distance apart.
The solution is to machine the feedstock first in the milling machine. A flat is milled on either side. It could be milled all the way along or just where needed.
A parallel hole is drilled for each valve at the precise pitch required for the capstan operations. This pitch will be the length of the valve plus the width of the parting tool. It is then drilled with a tapered drill.
This bar is then fed into the lathe’s headstock.
Set to length/ center drill
Drill/reduce diameter for thread
Relieve end of thread
Use live center to steady workpiece
Cut off slide / capstan 3
Form tool to shape out line/live center as steady – locked?
It might be necessary to do this in two halves
Cut off slide
A drain valve only needs a thread at one end. Most boiler fittings need a thread at both ends. In this case the part will have to be fitted into the chuck the other way round to machine the other end. One way of doing this is to make a fitting from round bar that has a hole with a thread in it. The part machined valve body is screwed into this in or to reduce the diameter at the other end and then put a thread on it.
Many boiler fittings have a part fitted at right angles to the body. In most of these cases the hole for this can be done on the bar before any other machining is done. On the other hand, the part can be made from a bar.
feed to stop, center drill
reduce diameter, chamfer
When the required number of these (plus some spares) have been made the the collet chuck if fitted with a stop inside it. Each part is fitted in the collet chuck.
All of the tooling needed is already on the capstan.
When all of these parts have been machined they are center drilled, drilled or drill with a stub drill on the milling machine. The milling table is locked in the y direction so the hole is drill on the axis of the part. The position of the part is set in the x direction by a stop fitted to the vice. To hold the part securely it is held by a piece of bar with a v-shape cut in it.