I do not make out to be an expert on cross drilling jigs but do know there have been
many designs for carrying out what appears, though falsely, a simple operation. Having
made and used this item in this article, Photograph 1, I can confirm that it performs
the task with considerable ease and to a high standard that must make it amongst
the best available.
This is the main item and is made from a cast iron casting that machined well. The
taper, necessary when making castings, makes securely holding the part for machining,
a task that must be approached with care. The casting is largest at the base and
smaller at the top, the top and bottom surfaces being nominally parallel.
To securely hold the part in the four jaw for machining the top face first, place
small pads of soft copper a short way up the chuck's jaws, making sure that the casting
is gripped over a wider area, not just the lower edge, see Sk. 1. However, before
fitting into the chuck, using the coarse wheel of the off hand grinder, make a substantial
chamfer, say 2 mm, around all edges. This prevents the lathe tool having to break
through the outer faces that may have hard spots present, especially important if
Having machined the top, Photograph 2, reverse and refit the casting with the now
machined top held cleanly against the chucks face and machine the base. We now have
two parallel faces that will begin to make holding it more secure. There is though
a problem that can be overlooked.
I do realise that non UK viewers may find the transport costs for the casting that
makes up this kit prohibitive, as a result, making this project a non starter. However,
I would though suggest that the project is worth studying as some of the methods,
and the principle behind the design, may be of interest. For those interested, details
of the castings can be found on the "Hemingway Kits" web site. Also, whilst the casting
is the best approach, for those for whom transport costs rule this out then the design
can be adapted to be made from a block of bright mild steel.
The photographs that were taken when the jig was first made are no longer available.
Because of this, the photographs that follow have all been taken with the parts that
have already been made and finished.
Do take note of this fact when studying them for each task being illustrated as
some will show latter operations also completed.