Re: repositioning parts on fixtures

Jon,

So you're saying that center drills in collets are accurate enough;
shallow registration holes in the parts slip over dowel pins in the
jig. Good! Then it's just a simple hold-down problem.

I'm getting braver, I run gcode to center drill a set of holes in a
part, change to a drill bit, run it again (deeper), and then change to a
tap held in a "loose" jacob's chuck in the mill's spindle to get the
threads started after "commanding" each hole location. Came out good!

I haven't found out yet what MAXNC PRO does with G81 commands, I can
generate them in Vector, but after I read Tim's [Denver, CO] comments on
drilling holes on the Vector user's list, I used "connect at Z" (I put
points in the drawing at the depth I wanted) to generate drill moves.
Works! Thanks Tim! And I'd like to see more white papers like Tim's,
for those of us just starting up on CNC (and machining too, for that
mater).

Alan

Jon Elson wrote:

>
> 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