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Harold Hall

Workshop Processes

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Positioning

When using the three jaw positioning is automatic and with the four jaw it can easily be set by the adjustment that the jaws provide. Such a luxury is rarely available with the faceplate and locating the workpiece is very much a case of trial and error. To add to the problem the vertical surface means that the item being positioned has first to be clamped to hold it approximately in place. With the item lightly clamped it can be encourage into position, typically by being tapped with a soft hammer. The clamps though being only very lightly tightened can easily loose their grip with this action and the part falls from face plate. It is far from an easy operation!

 

In view of the problems with a vertical faceplate, fixing the workpiece away from the lathe with it in the horizontal position may seem like a good idea. This though is a non starter as there is rarely any means of determining that the part is correctly positioned, later in the article though a method of overcoming this limitation will be described.

 

The most common methods of determining the positional accuracy of the workpiece are by means of a centre finder, or a DTI (dial test indicator) testing some part of, or an item added to, the workpiece.

 

Using the centre finder

In the case of the centre finder it will be located into a centre punch mark or a centre drilled impression and the faceplate rotated by hand when any off centre error will be evident by the finder swinging in a circle. The position of the workpiece will be adjusted until the finder remains static as the faceplate is rotated and if precision is not necessary then this can be judged visually, otherwise, movement of the finder will be checked using a DTI as seen in Photograph 1.

 

If the reader is not familiar with this simple device it can be seen in the photograph that it is supported between the workpiece and the tailstock centre and it is at this end that the finder has a sprung loaded drilled centre. With this compressed a little by feeding the tailstock it ensures that the working end of the finder remains firmly in contact with the workpiece as the faceplate is rotated. Similarly, it will also find a use to position parts in the four jaw chuck. If you do not posses a centre finder then this will make a simple project for an hour or so, see the drawings for the device.

Centre Finder for use on the lathe
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All pictures can be clicked on to provide a larger view