PIVOT FRAME JIG
Producing a Circular Disc Work
Surface
The first application for mini-pivot work is
necessarily the construction of a primary disc of
suitable size. For this purpose, the jig will be
required in its beam trammel mode. Initially
however, the router alone is used in conjunction
with a simple jig. This is simply a scrap panel
with a rectangular cutaway at one corner, used
to locate the position of the router.
Where it is desired to maximise the working
range in mini-pivot mode, the diameter of the
primary disc is made equal to the guide rod
length.
MDF is an excellent material for the disc, since it
is flat, smooth, of constant thickness and
dimensionally stable. The minimum thickness
which may be used is 15mm.
A piece of MDF which will accommodate the
required diameter is selected, and the centre
found and carefully marked. A hole 6 mm
diameter is now drilled, to accommodate the
pivot assembly. It may be noted at this point
that the inherent accuracy of the jig is of a very
high order. In the mini-pivot mode, this accuracy
is however, critically dependent upon that of the
disc. It is therefore extremely important that the
disc is machined as accurately as possible. It
may well be that the throat capacity of any
available drill-press is insufficient to enable the
drilling of the centre hole to be carried out by this
means.
Freehand operation is not advised, since it is
important that the hole is drilled precisely at right
angles to the face of the stock material
moreover, it is also possible to drill oversize
when working freehand. Given however, that the
stock material is squared-off to begin with, it is a
simple matter to use the router as a drill press.
The router is placed in the desired position on
the workpiece, possibly with the aid of a scribing
point held in the collet. The jig is then brought
up to the router and cramped in position. The
router is then fitted with the required cutter and
the hole bored. Note that sideways pressure on
the router is required to keep it in position during
machining.
It is equally important, for maximum accuracy,
that the pivot stud fits the hole fairly tightly. Any
play arising from a slightly oversize hole may
however, be removed by wrapping thin plastic
adhesive tape around the end of the bolt until
the fit is satisfactory.
The jig may now be used in the simple beam
trammel mode to profile the workpiece to the
diameter required. If desired, the adjuster may
be used on the free end, but is not strictly
necessary. The disc may be machined directly
from rectangular stock, in which case, a small-
diameter 6.35mm (1/4”) straight cutter should be
used (eg. Trend Ref. C008 or 3/22X1/4TCT), to
reduce loading and minimise dust. A better
method is to profile the disc slightly oversize with
a scroll saw or similar device and use the router
only to generate the final profile. In this case,
finish will be improved with a larger diameter
cutter.
Cutting should be carried out in a series of
passes, gradually increasing the depth of cut. It
is important that the disc blank is fixed firmly to a
flat worktop before routing the profile. This can
be done with two woodscrews on a diameter of
the disc, at some personal ‘standard’ radius.
WORKPIECE
PRESSURE
ON ROUTER
SIDEWAYS
WORKPIECE
CRAMPED TO
SCRAP PANEL
3
2
1
TAPE
90¡
-11-
Summary of Contents for PFJ/SET/1
Page 1: ...PIVOT FRAME JIG...
