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Harold Hall

Workshop Processes

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Mandrels for Work holding on the lathe, Harold Hall

Of the four common methods of work holding used for turning, mandrels are by far the most economic, invariably made by the user, and probably from scrap material. They cannot though eliminate the need for the other methods but, using a mandrel, probably purpose made, is frequently the only practical method for the task in hand. They are therefore an essential item in the turner's kit. In simple terms a mandrel is a spigot, turned to fit a hole in a workpiece. The methods of fixing the workpiece to the mandrel, typically being using adhesive, or just being a tight fit.

 

Advantages

The main advantages of using a mandrel are twofold. Firstly, as the workpiece is held internally the whole outer diameter is available for machining, and often the end faces also. Secondly, perfect concentricity is possible, providing the mandrel is turned in situ prior to placing the workpiece onto it. If a four jaw chuck is used to position a previously made mandrel, and care is taken, errors in concentricity should be very small but not exact as when made and used whilst still in the chuck holding it.

 

Methods of work holding

Tight fit

Probably the most used mandrel is one that uses a tight fit to provide the drive to the workpiece. This mandrel is turned parallel for part of its length and with a very close sliding fit for the hole in the part to be machined. The mandrel then tapers to a larger diameter using a taper of around one to two degrees internal. The workpiece can then be pushed, with a twisting action, onto the mandrel and a sufficient grip will result for quite substantial cuts to be taken. If the component to be machined is fitted without the mandrel having been removed from the lathe, perfect concentricity will result. This mandrel is illustrated in Sk.1 whilst photograph 1 shows the method in use.

The photograph shows that not only the outer diameter, but also the end faces, are accessible for machining. If machining the face nearest to the chuck is carried out, care will have to be taken to ensure that the workpiece is not forced from the mandrel. A restraining bush with grub screw and fitted on the outer end of the mandrel would avoid this.

All pictures can be clicked on to provide a larger view

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Drawings

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