Disks or Plugs
Above I have spoken about using a plug to set the position of the jaws for machining.
However, for diameters larger than the through hole of the chuck a disk will be more
than adequate. The disk can be rested against the chuck's body to ensure that it
is accurately positioned before the jaws are tightened onto this. Using a disk in
this way will make it possible to machine the end of the jaws almost to the chucks
face, enabling longer parts to be held, not just disks.
Having made and used your jaw supports, disk of plug, these should be kept with the
soft jaws for possible future use. If you drill these centrally, then as concentricity
is unimportant, a suitable mandrel could be made and kept on which they could be
mounted for reducing to smaller diameters as required by subsequent projects.
Holding inside diameters.
At first, machining the jaws for holding inside diameters may seem no different to
holding outside diameters, that is grip a plug but machine the outer ends of the
jaw for the part eventually to be held. Where accuracy is not of paramount importance
this would no doubt suffice and be the easiest option. However, when machining the
jaws the grip is inward but will be outward when the workpiece is eventually held.
Therefor, even though the jaws will be at the same position in both cases, due to
the backlash between scroll and jaw teeth the scroll will take up a different position
and the accuracy cannot therefor be guaranteed.
Where accuracy is important then the jaws will need to close into an outer ring whilst
being machined and due to the size of this I adopt the method seen in Photograph
4. In this I have wrapped around the three jaws four loops of wire, twisting the
two ends together to make it firm. The jaws are then tightened outward into this
loop prior to machining. To avoid the jaws being marked by the wire they are protected
using a piece of copper wrapped around their ends.
Recording the diameter
Having machined your jaws for inside or outside holding, scribe in the recess the
diameter for which the jaws are intended to work just in case when the next task
surfaces it is for the same diameter. Whilst it is probable that the next workpiece
will be a different diameter, you will be pleased that you have marked it should
it be the same. This is because at this stage it is difficult to determine precisely
the diameter held from the jaws themselves.