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Harold Hall

Workshop Processes

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Hexagonal auxiliary jaws

As it becomes necessary to machine the jaws for different diameters and depths, eventually the jaw will become so reduced in size that it will be impracticable to machine them further. This can largely be overcome by using auxiliary jaws mounted on top of the standard soft jaws.

 

One example of this is to add hexagonal auxiliary jaws as shown in Sk. 2. These can be rotated so that each corner can be used for a different diameter. Provided that the hexagons are appreciably larger than the width of the chuck jaw then the resulting machined portion should provide as much wrap around as the plain jaw, Sk. 3. Where a greater width of grip is required, particularly required at larger diameters, then three jaws as illustrated in Sk. 4 can be used. This method is extensively used by wood turners using nylon add on jaws.

 

Having made and used the jaws for the first application mark them with the diameter they are intended to grip and the jaw number on which each one fits. Soft jaws must be fitted in the correct position in the chuck in the same way as normal jaws and the auxiliary jaws must also be paired with the jaw onto which it fits. When reusing, fit the three auxiliary jaws leaving the central screw such that the jaw can just be turned. Place the workpiece into the jaws and very lightly tighten the chuck thereby aligning the jaws correctly. The central screws can now be tightened fully and the chuck also tightened to enable machining of the workpiece to take place.

 

Making Hexagon Jaws

Sk. 5, showing my suggested design for shop made jaws, shows that each jaw has a boss that locates into a counter bore in the soft jaw itself. It is essential that this is a close fit in the jaw otherwise there will be no guarantee that the hexagon jaw will return to the same place when it is removed and subsequently returned. The size of the boss is not important but would suggest 4mm deep and a diameter of between 1/2 to 3/4 the width of the jaw.

 

Making the hexagon jaws is straight forward other than to say the bosses on each one must be exactly the same diameter. Concentricity with the outer hexagon shape is worth aiming for but is not crucial as any error will be eliminated when the jaws are eventually machined to the required diameter. Whilst geared up for making these jaws, making a second set would be a wise move. Of course, whilst the hexagonal jaws are shop made the main soft jaws can still be those available commercially.