Harold Hall

Workshop Processes


Again, we do not want the scroll to contact the jaw at their edges as shown in Sk. 3b but nominally central as in "a".


In this case the curvature of the tooth must be flatter than the flattest point on the scroll, that being at its outer diameter. From this it can be seen that the curvature on one side of the teeth requires to be quite different to that on the other, see Sk. 4. Manufacture is not therefor just a case of running an appropriate end mill across the jaws to make the groove at one pass as the gap varies in width across the jaw.


Sk. 5 shows therefore the configuration, and in "a" in the closing mode, and "b" opening. From this it can be seen the jaw and scroll only contact each other at the crest of the jaw when closing (X) and the crest of the scroll when opening (Y). I should add at this point that all my sketches so far show the results simplified and exaggerated for clarity.


Incidentally, if you have a much used chuck you may see evidence of the jaws teeth being polished just locally, certainly on the outer face where they are most heavily used, that is when closing.


The sketch shows that the jaw and scroll contact each other only at X and Y that is a point contact, why therefore do the teeth have to be so wide? The answer is  to provide added strength. Because of this, whilst beneficial, the actual shape of the jaw either side of the contact points is far from critical.


Making the jaws

Determining the dimensions

With the above considerations in mind it will be necessary to measure both the inner and outer diameters of the scroll from which to determine the curvature to be machined on the two faces of each jaw tooth. Unfortunately, access to the scroll to take these dimensions is not available with the assembled chuck due to there being just three jaws and the only option would appear to be to dismantle the chuck.


To avoid this, make a plug to fit the chucks bore and use this to determine the required dimensions as shown in Sk. 6. Do ensure that you take these dimensions immediately adjacent to the inner and outer ends of the scroll. To accommodate any minor errors in the dimensions taken and those arrived at later these dimensions should be changed by a millimetre of two to ensure the scroll and jaw bed correctly, smaller for the inner diameter and greater for the outer diameter.