Re: repositioning parts on fixtures

Alan Marconett KM6VV wrote:

> Jon,
>
> Once I locate a corner of the stock, I'm OK. There were times when I
> used a jig to hold parts, and generated gcode based on a location on
> the
> jig. The part blank extended past the jig's origin (maybe that's the
> problem), and I couldn't get back to it. I've wanted to be able to
> re-locate the jig, or a "known" feature on the stock. If I contour
> the
> outside of the part, I've lost the stock's edges.
>
> I don't have a "home" on the Sherline mill, and seldom have a vice or
> fixture located the same exact place on the bed. I'd like to be able
> to
> check that the work is still located correctly conveniently. On the
> needle, looks like one could just turn a needle point on a piece of
> drill rod, and eyeball it to the work, probably as accurately as the
> wiggler?

With the spindle rotating, you can pretty accurately judge the
eccentricity of the
point, and detect offsets of a few thousandths of an inch. Just a
needle in the
chuck won't be quite as good, but noticing it deflect when it enters a
prick
mark would work fairly well.

> Can one use a dowel pin (shank of a drill bit) to re-locate a same
> sized
> drilled hole accurately? Seems like a center drill will re-locate a
> center drilled hole, if you leave the work loose, and lightly hold it
> temporarily with the center drill while you tighten down the work (or
> make a 60deg end on a piece of drill rod). But that only gives one
> point of location, good only for drilling that one spot. Or is that
> bad
> shop practice?

Center drills, held in Jacob's chucks, rarely drill a hole exactly the
same
diameter as the drill point. Usually, there is some runout in the
chuck, and
the hole is bigger.

A technique I have used is to put a few special holes in a part first,
to
allow the part to be relocated on the fixture later. For instance, I
was
center drilling, then drilling, then tapping 238 holes in 19 pieces of
thick-wall tubing (that's 6 holes on each side of these 'rings').
There is a fixture with a 1/2" bored hole in the center, for
establishing
a reference point, and a large round step that pretty tightly constrains
the
rings to be concentric to the reference hole. But, I need to also align

the rings to the same rotation every time. So, first I drill a shallow
hole
of just the right diameter in one side of each ring with a center drill.

The fixture has a hole of the same diameter, but deeper, into which I
fit a piece of dowel rod that same diameter, so that just a little
sticks
up. This can now fixture the hole in the ring to align it. So, I now
drill
another alignment hole in what is now the top side. Now, I have two
holes, with a known alignment between them, so I can fixture the part
with either side up, in the same alignment every time. Now, I can
spot drill all the holes on all the rings, change tool and drill all
holes,
change to the tapping head and tap all the holes. I only change tools
3 times in the entire run. If your fixture makes it easy to slap the
part
in and out, this is the fastest way, rather than changing tools 3 times
per part.

Jon