The height of the packing under the outer end of the clamp is critical and would
suggest this to be within one to three millimetre higher than the workpiece. The
closer to the workpiece the smaller should be the difference. What form this packing
takes will depend on the situation
If the item being clamped has been made from a piece of stock bar then a piece off
the same bar, maybe another part also being made, can be used as the packing by adding
a piece of hard card under the clamp at the packing to provide the difference required.
Photograph 4 is an example of this with clamps at both ends of the workpiece. Do
not attempt to use this approach without the additional thin packing as the inevitable
bow, even if minute, will result in the clamping force being only at the workpiece
edge. A piece cut from a discarded credit card is ideal for the task.
Another form of packing and one that provides considerable variation to its height
is to use a jack. For shorter heights you do not need to spend time or money acquiring
jacks for this purpose, a T nut with a screw threaded into it will provide an adequate
alternative. Facing the top of the screw head to remove the raised lettering often
present would though be a good idea. I would however recommend that the height of
this makeshift jack should be no more than three times the length of the T nut's
base, perhaps a little more if the T nut is placed in the machine's T slot.
Another method that provides continuous adjustment of height and one that I use extensively
can be seen in Photograph 5. In this a stud is screwed into the T nut and locked
in position with a nut. A nut and washer are then place on the stud and adjusted
to the height required (workpiece height plus a millimetre or two). The stud passing
through a hole in the clamp then has a nut and washer placed on top of the bar and
tightened in place. Whilst this nut may be superfluous I consider it adds to the
stability of the set-up. If the clamp is positioned so that the T nut can be placed
in the T slot (right of Photograph) this is preferable. However, if this cannot be
done then standing the T nut on top of the machine table as seen on the left is acceptable.
Once again note how the clamping studs are closer to the workpiece than the packing.