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Capstan – examples
Example – making washers
Making washers is important because though they may be cheap they do not necessarily exist in the sizes required for scale models. An example of a part where the wrong size washer has been used is shown:
Fig wrong size washer
The difference can be seen if a better size washer is used.
Fig specially made washer
It might seem that making washers cannot be done usefully on a capstan. If we start with a round bar, we need to set it to length, we need to center drill and drill a hole down the middle and then part off the washer.
Though usually we make one part at a time on the capstan there is no reason why any other way of using it if it suits the problem in hand should not be used. In this case washers could be made in batches.
A batch might consist of setting to length, center drilling and drilling a long enough hole for the batch.
If the diameter of the rod is sufficiently large so as to make it rigid enough, it is quite possible to just part off the washers as required. The trick here is to use a variable stop. After parting one washer off the stop is turned by one complete turn. This means the parting tool will be in just the right position to part off the next washer.
This works because we can control the width of the parting tool.
The thickness of the washer + the thickness of the parting tool =
The variable stop uses a thread whose pitch is the width of the washer. By putting a head with a suitable number of sides it is easy to make washers of any necessary thicknesses.
After the batch has all been parted off the stop is unscrewed with the number of turns that equals the number of washers parted off.
If the rod is too small to be rigid enough then it is possible to only turn down enough for one washer.
The parting off cutter can be made very thin. This reduces the cutting force required which could reduce the need for a steady. It also reduces the amount of material that is wasted.
Using the capstan as a steady for a cross-slide cutter
If the washers are being parted off without a steady the diameter that can be cut is limited. However it is quite possible to use the capstan to hold a steady. This steady will be the same “diameter” as the steady that is needed to reduce the size of the feedstock. If the feedstock is already the right size a steady is still needed.
Fig using the capstan to hold a steady
In real life washers are usually pressed out from a sheet of metal. This means the washers has a round edge on one side but not the other. This effect can be obtained by using a piece of emery paper glued on a piece of wood to take the egde of each washer just after the previous one has been parted off.