Submitted by - John Whitmore,
Auckland, New Zealand
Mr. Whitmore won the First prize of a SuperNova Chuck and a 35mm Spigot Jaw (His Choice)
"My Cole Jaw set is so useful that I have it permanently set up on a dedicated chuck. However, I suggest that the most versatile use of Cole Jaws is achieved by designing the work piece so that it can be reversed using the chuck's expansion mode, not the contracting mode described in the Teknatool literature. Used as suggested, reverse-chucking is safer; it is easier to perform reshaping and blending operations as part of the base cleanup and the technique allows mounting of much larger pieces than the standard method of use.
The standard method is to use the jaws in compression mode to reverse-chuck bowls of up to 250mm diameter. My method using the jaws in expansion mode makes it feasible to reverse-chuck bowls/platters up to about 400mm overall diameter.
The standard rubber posts can be used in any of the radiating sets of holes except for the outermost set. The reason that the outermost set of holes is not used is to give a surface against which the rim of the bowl/platter will locate and (see later) a surface against which to position a spacer, if required.
The critical part of my method is the shape of the interior of the bowl/platter. This is hollowed out so as to be slightly undercut at the rim. Undercutting at this point is in any case considered a good design point in a turned piece as the overhanging lip creates a shadow that gives texture and depth to the work. Additionally, the depth of the bowl recess at the rim must be at least enough to accommodate the Cole Jaw rubber posts (again, see later for mention of spacers - which could be designed to limit the depth clearance necessary).
With an undercut rim the effect of using the Cole Jaws in expansion mode is to cause the rubber posts to deform slightly inside the bowl giving a firm grip like a dovetail. An accessory set of rubber posts having cone profile could be made available. Alternatively, enthusiastic owners could turn down standard cylindrical rubber posts to cone profile. I reckon that this method can be used on undercut bowls/platters of an internal diameter of between 105mm and 250mm. A bowl/platter of 250mm internal diameter with a wide, flat rim could tastefully be 400mm in overall diameter - hence the 400mm quoted above.
Apart from significantly increasing the size of the item that can be reverse-chucked, my expansion method also improves access to the base of a bowl/platter for sanding and finishing. Because the rubber posts in the recommended contracting mode prevent access to the edge of the reverse-chucked piece, finishing of at least the outer part of the underside has to be conducted before reverse-chucking occurs. That limits options for reshaping the underside as part of a base cleanup. It also causes problems with any resanding and refinishing done to the base as it is impossible (and dangerous) to blend in both shape and finish near to the edge with rubber posts whizzing around. With the internal expansion grip that I am advocating, the rotating rubber posts are covered by the reversed work piece and pose no risk of injury to the turner.
A work piece of greater than 280mm overall diameter will be wide enough to completely clear the outer edges of the rotating Cole Jaw segments - giving access to the whole base for reshaping, sanding and finishing (with buffing, if required) completely unrestricted by considerations of damage to the aluminum jaw segments and risk from the rubber posts. There is also access to whatever part of the rim top surface overhangs the Cole Jaw perimeter.
Slight top rim access (useful for finish blending purposes) can be available with a work piece of smaller than 280mm diameter by placing around the rubber posts a small spacer of thin MDF, or similar, in the shape of a flat washer - that will pack out the work piece slightly from the Cole Jaws. However, care needs to be taken that such a spacer is not so thick as to compromise the grip of the rubber posts. The design of the undercut rim of a piece intended to be reverse mounted with spacers needs to be thought out with the spacer thickness in mind - so that an effective 'dovetail' grip is still possible.
With the larger work pieces possible, standard Teknatool advice on the speed limitations applicable to use of Cole Jaws in the contraction mode would probably be reasonable for the proposed expansion mode, given the peripheral speed of a large platter. For smaller pieces, 600 rpm might be considered conservative given the stronger grip possible in expansion mode - even allowing for the litigious American market!"
These were the comments from our
safety advisor - Trevor Cole:
"Overall a well presented piece of thinking, although we suggest expansion
and contraction holding, we tended to highlight contraction as the grip was more visible -
especially on our videos. You appear to be very safety conscious with what is
suggested. Within the suggested limits, this appears not to present any extra
dangers"