Harold Hall

Workshop Processes

First Page

Even the end fixing mandrel could be adapted for this approach if the end fixing screw is replaced by an external thread and nut, then the end of the thread can be centred.

Expanding mandrel

The most complex mandrel is the expanding type. Its complexity really means that it is only practical if it is intended for reuse, or at least for a single batch of sufficient parts to make it worthwhile to make one. Its main advantage is its ability to cope with workpieces having holes that differ in size, though not as much as may first be considered as the expansion, and therefore grip, will tend to be at one end only. It is preferable that the workpiece is mounted on the outer end of the mandrel for best effect. The method is seen in Sk.3.

A useful variation would be to include a flange to support thin components, as is shown in Sk.4. The flange will support the workpiece ensuring that it runs true. The support will also enable the part to be machined both on its outer diameter and on the accessible face. Due to the short depth of hole and the

supporting flange, the mandrel of this type will cope with a larger range of hole sizes, probably size up to plus 1mm., maybe even more.

Concentricity cannot be guaranteed with either type the individual segments may not expand equally. Also, accuracy will be effected by removing and replacing it in the chuck between batches. It should though be adequate for many applications.

Limitations to workpiece size

Definite rules are difficult to set, but with experience the lathe user should gain an idea of the limitations. The only real limitation is that the mandrel must be robust enough to permit the machining to be carried out. Therefore, the user must consider length and diameter of mandrel and outer diameter of the workpiece. To extend the capabilities, the tailstock centre can be brought into use as seen in Photograph 5. This can either support the mandrel or sometimes the end of the workpiece itself.

Photograph 6 shows that the mandrel in Photograph 5 was reused for another part. Whilst concentricity was vital for the flywheel, demanding that the mandrel was turned and then used whilst still in the lathe, concentricity for the part in photograph 5 was of little importance. Because of this, it was acceptable for it to be removed and replaced at a later date for the machining operation shown. The mandrel was even pressed into use for eventually finishing parts away from the lathe, Photograph 7.