(This is only a part of the material on capstans – go to the page above to see it all)
These pages are not about how capstan and turret lathes used to be used in industry. It is about how techniques that use to be used universally in industry can be used by the model engineer in the home workshop.
The whole idea of using a capstan lathe is to produce many copies of one particular part as efficiently as possible. This gives two aims in the design of the system. It should be able to produce the widest range of different parts. Yet for any particular part all of the parts must be identical. The process must be accurate.
Capstan lathes intro
Before the advent of cnc machines there were several classes of specialised lathes that were designed to produce vast quantities of turned parts. One of these was the capstan lathe. On this, instead of a tailstock, there was a system for rotating tools that could have been held individually in a tailstock.
A similar lathe was the turret lathe. This did a similar job was but was usually a larger, more robust and more powerful machine. It was also more complicated. On a capstan all that was automated was the capstan moved forwards to a stop and then rotated when it was pulled back.
One important difference is that the movement of the capstan is limited by the sliding mechanism in the capstan. On a turret lathe the movement of the cutter can be far greater.
The capstan lathe is of interest to the model engineer because it is quite possible to make or buy capstan type attachments that will fit some of the larger lathes a model engineer might have.
A capstan is a device that fits on the bed of a lathe instead of the tailstock. It consists of a rotating toolholder. Each tool can be used in turn on a workpiece held in a chuck in the headstock. It is designed to automate operations so they can be done more quickly, consistently accurately and with less skill.
Some capstans are very simple and fit in the socket on the tailstock. The tailstock is firmly fixed at a suitable point on the lathe’s bed.
Fig 1 capstan that fits in the tailstock socket
See A Tailstock Turret, Peter Rawlinson, Mew 78 p48
In this case the tooling moves towards the workpiece by turning the handle on the tailstock. It also has to be withdrawn by turning the same handle the opposite way. The tool then has to be rotate to the next tool by the user.
For each operation the user has to determine how far the cut has to go. There are no stops to set this distance. Of course there could be a rotating set of stops but the user has to turn these himself.
Since the capstan is fitted to the tailstock it is always on the right hand side of the saddle and cross-slide. This is important since, invariably, the parts will need to be parted off and the parting tool will only work if fitted to the cross-slide.
A more useful arrangement is the capstan attachment that fits on the lathe instead of the usual tailstock. An example of this is the capstan attachment that fits a Colchester Bantam lathe. In this case the tailstock is removed and is replaced with the attachment.
Fig 2 – Bantam lathe fitted with capstan attachment
This capstan can be fitted anywhere along the bed of the lathe but it has to be fitted to the right of the saddle. It is then firmly bolted to the lathe’s bed. Notice it has an inbuilt system for aligning it so that it can be aligned accurately with the axis of the spindle.
In this case the cutting tool is moved towards the workpiece by turning the large handle in the anticlockwise direction. For each position on the capstan there is a stop on the capstan.
Fig 3 screws for adjusting stops on the Bantam capstan attachment
Each of the stops sets the limit of movement of the cutter towards the workpiece. When the cutter has hit the stop any cutting is finished. The user feels this and then turns the handle the other way. This not only retracts the cutter but after a certain distance it also rotates the capstan by one position. It also rotates the stops by one position.
As before the cross-slide will hold a parting tool to part the finish workpiece off.
It needs quite a lot of very skilled work setting up a capstan properly so is only cost effective when making large numbers of identical components.
In both of these types of capstan attachment, the capstan is an alternative to using the tailstock. But anything that could have been held in the tailstock can usually be held in one of the sockets on the capstan. The lathe can be easily turned back into being an ordinary lathe by removing the capstan attachment and replacing it with the tailstock.
In both cases it will be seen that the axis of the capstan is not vertical, it tilts towards the headstock. This is no accident. Some of tooling can be quite bulky. There is room for it in front of the capstan because there is nothing there to be in its way. But as the capstan turns there would be no space for it behind the capstan. This is a major difference between a capstan lathe and a turret lathe. On the turret lathe the center height is much higher. The turret rotates about a vertical axis. There is room for the tooling either in front or behind the turret.
Lathes have been made simply for use as a capstan lathe. These are of limited use unless one also has a straight forward center lathe. Where vast numbers of components of one type have to be made the traditional machine would have been a turret lathe.
Fig 4 – a small turret lathe
The turret lathe is designed only to be used as a turret lathe. There is no alternative tailstock. There is just the turret. The turret is far more rigid than a capstan. The axis of the turret is always vertical. The turret usually has 6 sides. Each piece of tooling is fitted to the turret by means of a large flange. The turret can usually move along the whole bed of the lathe.
Fig 5 – fitting tooling on a turret lathe
On larger turret lathes the entire cycle is done automatically. These are often simply called “autos”. It is feed automatically from a stack of bars. Where large castings are held in the chuck the only need is for someone to fit the casting in the chuck at the start and remove the finished workpiece at the end. In all cases it is also necessary for someone to sharpen the cutters as needed and set them properly afterwards. It is also necessary for there to be some sort of system of inspection.
A turret lathe cannot be used to do many of the jobs done of a center lathe in a practical way.
It has been said that an engineer is a person who can produce a widget for one dollar which any idiot could do for 10 dollars. In many cases it is very expensive producing one item but if large numbers have to be made then it is possible to make tools, jigs etc that will dramatically reduce the costs of production.
When a small number of one type of parts need to be turned the ordinary lathe would be used. But as the number of parts rises it becomes worth thinking how a lathe can be made to be more productive.
The movement of any tool on the capstan is limited to this space. In practice the fitting of tooling on the capstan is often constrained one way or the other so the total usable movement in any one job is a lot less than 4 inches.
For several reasons the size of the components tends to be small. The main reason is that to feed the material to make the components it usually comes through the bore of the spindle. This is usually the maximum diameter of the components that can be efficiently made on a capstan lathe.
On a capstan the work piece is usually held in some sort of collet chuck. A three jaw chuck would do but it is less accurate and is slower to use. The collet chuck could use any feedstock that fitted a collet the chuck could hold. This could hold round, square or even hexagonal bar stock.
Ideally the collet would be a dead length sort. These collets always hold the work piece at exactly the same distance along the axis of the lathe. Collets of the ER variety are not dead length collets. The difference this makes is very small.
Use of stops
The basis of any capstan system is that the tool starts cutting at a point outside the envelope of the workpiece. Most tools on the capstan only move from right to left – not in or out. The accuracy of this starting point does not matter. The tool moves and at some point starts cutting. This cutting action continues till the tool hits a stop.
The accuracy of every cutting action is determined by the use of stops. The limit on the accuracy of the system depends entirely on the rigidity of the stops. One way of maximising this is to make the stops as rigid as possible. On a small lathe this is limited. However, if used intelligently, this can be overcome. If a part moves freely and then hits a stop the force moving the part stays constant till it hits the stop. It then rises monotonically as the user forces the movement.
It might seem that to get things right one should push hard. But for accuracy one should only push lightly until the first sign of resistance appears.
Doing this means the operator has no means of affecting the way the workpiece is machined. He simply machines to the stop.