Cutting a Block Square, version 2
By
R.G. Sparber
1/9/2012
Copyleft
protects this document[1].
The following procedure is probably as old as
the milling machine. It has been presented in many articles yet it took me a
while to fully appreciate why it works. Writing this article clarified the
facts for me.
Before you start to true up a
block, verify that the fixed jaw's face is parallel to the Z axis. Error here
will cause you to cut a parallelogram rather than a square. Thanks Martin.
I
start with a block of metal with an irregular profile. Before I can cut to a
precise size, it must be made square. I have greatly exaggerated the shape to
make it easier to see how this procedure works.
The first step is to clamp the block in the
mill vise with your smoothest face of the block up against the fixed jaw. If
necessary, use a file to remove roughness that prevents solid bedding of face 4
onto the fixed jaw. A parallel supports the bottom along face 3. The parallel must
be snug. Face 2 is compressed by the movable jaw acting through a short length
of round stock. The round stock should be at least as long as the block is
wide. Then proceed to cut face 1. Note that the cut face 1 is our first
reference plane.
The block
is now rotated so face 1 is against our fixed jaw, face 4 is supported by the
parallel, and face 3 is compressed via the length of round stock by the movable
jaw. Face 2 is now cut. Face 2 is perpendicular to face 1.
The block is again rotated. Now face 2 is
against the fixed jaw, face 1 is down on the parallel. Face 4 should be
reasonably parallel to face 2 so we remove the round stock and let the movable
jaw come in contact with the block. You may notice that my parallel is larger
in this figure. I was just too lazy to redraft the figures to be consistent.
You should use the same parallel throughout in order to avoid error associated
with using different parallels. We now cut face 3.
Our final step is to cut face 4. We now have all
4 faces perpendicular to each other. The ends are still uncut so assume they
are not true. One way to true an end is to cut it with the side of the end mill
used to cut face 4 before block is removed. You will have to make a few very
light cuts in order to minimize bending of the end mill. Another method is to
place a 1-2-3 block onto the ways of the vise and clamp it down. Then set the
block on end with say face 1 against the fixed jaw and face 2 against the 1-2-3
block. Tighten the vise and take a light cut. This should put the first end, call it face 5, perpendicular to faces 1, 2, 3, and 4.
You can now turn the block over and rest face 5 against the parallel. Make your
final cut on face 6. You should now have a block with all 6 faces perpendicular
to each other.
Brad Peters suggested another way to cut the
end. Side mill it but only cut enough to fit on a
parallel. Then you can rotate the part 90 degrees, put it on a parallel and end
mill the opposite end of the block. Then use the standard approach of turning
the block over and milling the first end that had the small step in it. Thanks
Brad for a great idea!
Comments and questions are always welcome.
Rick Sparber
Rgsparber@AOL.com