Lathe – workholding – turning between centers
What might be thought of as the simplest way of holding a workpiece goes back to the primitive lathe. It is to hold the workpiece between two centers. To do this one center it fitted in the socket in the spindle and another in the tailstock.
On a primitive lathe the centers might consist on ordinary nails hammered through two pieces of wood. One nail sticks into either end of the workpiece.
When this is done on a modern lathe the centers are centers on the end of a taper. One center fits in the taper in the headstock the other fits in the taper on the tailstock.
Whenever fitting something into a Morse taper it always pays to give it a wipe with a piece of kitchen roll. It also pays to check the socket and to give it a wipe from time to time.
The workpiece is to be fitted between these two centers. To do this means that centers have to be cut in each end of the workpiece.
A very long time ago a center could be put in the end of the workpiece simply by banging a punch in the appropriate position at either end of the workpiece. However the center (ie hole) will not be deep enough or of the optimal shape. To get the best results the workpiece can be held in a three-jaw chuck and the center is cut using a center drill held in a drill chuck held in the tailstock.
If the position of the center (hole) is not central or if there are more than one and they have to be in the right place then the workpiece can be held in a four jaw chuck and drilled as needed.
If any pretense to accuracy is needed then the workpiece should be mounted in a three-jaw chuck (see later) and a drill chuck holding a center drill should be mounted in the tailstock. This is used to drill the center. What is needed here is the conical shape, which should match the conical shape on the center that will be used to hold the workpiece.
The workpiece is turned round and a center put in the end. The two centers will not be quite perfect but will be good enough to get good results.
A center is fitted into the morse taper in the headstock spindle. For the very best possible results it is traditional to grind this so it is perfectly concentric with the axis of rotation of the spindle. In practice it will be found that the error with most centers in good condition will be good enough.
To rotate the workpiece it is necessary to fit a dog to the workpiece. If possible it is useful to have a flat on the workpiece to give the screw of the dog a good grip. It is also necessary to fit a catchplate to the spindle. This usually fits the same way that a chuck would be fitted. A bolt is then fitted to the catchplate that will engage the dog as the catchplate rotates.
Fig workpiece between centers with dog and catchplate
The turning of the spindle will push the workpiece round. But it does not stop the workpiece from moving backwards. This can happen if the cut is intermittent. This can be solved by tying the dog to the pin on the catchplate with a piece of string.
It is not possible to machine the workpiece where the dog is. However one of the key features of turning between centers is that if the workpiece is removed from the lathe and turned round and refitted (having moved the dog to the other end of the workpiece) everything should still be perfectly concentric.
This simplest of turning methods is, ironically, one of the best ways of getting the best possible accuracy.
If a cylinder is made then the sides should be parallel. If this is not the case then it is probably because the tailstock is not aligned with the headstock. In fact, the above process, is exactly the procedure used to align the tailstock but done for the opposite reason – the tailstock is moved till the workpiece has parallel sides.