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Harold Hall

Workshop Processes

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Close fit

Some parts when assembled may  be secured using a grub screw. for this a plain parallel mandrel can be made and the part fitted using the grub screw. A flat on the mandrel for the screw to tighten onto would be a good idea. If there is some clearance between workpiece and mandrel the action of the grub screw would be to force the part off centre and concentricity would suffer.

 

Adhesive can be used to fix a part to the mandrel, the type that weaken with heat being ideal as this aids eventual removal. However, the part, with mandrel, would require to be removed from the lathe to have the heat applied. It is therefore only suitable for a single part if concentricity is vitally important.

 

Close fit, end fixing

The methods described thus far, also permit with care, turning the end faces fully as well as the outer diameter. However, if this is not required, an end fixing screw, as illustrated in Sk.2, works well. The sketch shows that the workpiece also contacts the end face of the raised portion of the mandrel and this is obviously a requirement for using this type of mandrel. Photograph 3 though, shows a slight variation on that method in that the end of the mandrel mates with the bottom of the bore in the part being machined. This is a die holder as shown in Photograph 4 and is being machined in this way to ensure that its shank is both concentric to the dies housing and true to its base.

 

The viewer may suggest that this could have been turned with the bore made and the shank then turned with a left hand knife tool(one that cuts on its right hand side). This would though leave a stub in the chuck and as a number of holders were being made much more steel would end up as scrap. Using a fixed steady would avoid the scrap but I preferred the method I have illustrated. Should the viewer like more detail on the alternative, see  using lathe steadies.   

 

Between centres mandrel

It has been explained that for perfect concentricity the mandrels, so far described, must be machined and used without being removed from the chuck. Reuse will, even if returned to the four jaw chuck, not achieve that precise result obtained in the first instance. However, if a mandrel is known to have use more than once, making it for use between centres will provide concentricity comparable to the original situation. This providing that the bore of the lathes spindle and the centre used in it are accurate enough for the centre to run perfectly true even if removed and replaced.

Stub Mandrel, holding a die holder being made.
Die Holders, shop made.
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 3

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 4