What other difference may there be, well variations in the method that the central
screw is made captive to the jaw maybe but otherwise differences will only be very
small. However, there is one possible shop made variation, Photograph 2, that can
have considerable benefit in some cases. For the reader who would rather purchase
tools, I know of only one supplier of this type and unfortunately they do not have
a web site that I can give a link to. They are also only available in the smaller
sizes to my knowledge.
Toolmakers Parallel Clamps
It was when investigating the supply of this clamp form that I came across an industrial
catalogue having a variation in the name for these that more precisely describes
their use. This is, "Toolmakers Parallel Clamps" and clearly defines their use as
being for clamping parallel surfaces only. That having been said, no doubt they would
cope with an out of parallel of a degree, maybe two, but not much more.
Sketch. 1 shows the three possible conditions when setting the clamp but with only
one being acceptable. The situation in Sk. 1A shows the clamp having been set so
that the point of contact is at the workpiece's edge, that being close to the central
screw. In this situation the clamp could easily slip off the workpiece's edge if
it were knocked, or as a result of vibration if being used on a machine tool, as
only a small movement is necessary for the clamping action to be totally lost.
Sketch. 1B is a vast improvement on the above as the clamping action is much more
reliable, it is though rarely acceptable as I will explain shortly.
Sketch. 1 C Shows the clamp set so that the clamping action is present along the
full length of the jaws contact with the workpiece being held and is the only acceptable
method of using the clamps in the vast majority of cases. How then does the user
determine that the clamp has been correctly set?
The above sketches have the unsatisfactory conditions (A and B) shown exaggerated
so that they can easily be seen, in practice though, the errors can be very small
and certainly then not visible. Unfortunately, even very small errors are not acceptable.
However, the conditions are easily detected by attempting to move the clamp side
to side and observing the result. Sk.2 A and B show that the clamp will pivot at
the edge of the workpiece if set as in "A" but will be at the end of the clamp is
set as per "B".