Re: DIY probing

Hi Chris,

Yes, I guess the bells would not necessarily be "true", or aligned with
the mill/router.

Then my first guess (how many guesses do I get?) would be that you'd
want to generate points as you went around a supposed axis, and then
drop down and take another contour. The points would first have to be
adjusted for the probe's dia, and then a tool path generated for the
OTHER SIDE (outside) of the horn bell. That's tricky!

I have a very old Starrett "last word". Although I can use it to "tram"
a line or surface, I don't know how you'd use them TOGETHER, to check
out the probe. I did write a little Gcode to just move the probe tip up
to a rail and then back away again, recording the "location" of the rail
contacts. That seemed to be very consistent, seldom varying more then a
.0001, as I recall. Seemed encouraging! I'd think the probe would be
more then adequate for your intended use. You'd have more leadscrew
errors in your router then probe errors, IMO.

The angle of contact of the probe tip to the surface could make for some
interesting calculations! A simple "copy" Gcode file, assuming the same
ball end mill, gets away from that problem (I think)! But how to "flip
it inside out" (cut an outside instead of an inside)!

Would you machine this big "lump" of iron on a rotary table? Would you
first cut a mold part (oversize) out of wood or something? If you could
assume the same size ball cutter, in the same orientation as the
digitizing, you MIGHT be able to just use a simple radial offset to
adjust the tool path. Maybe not. This would take some thinking! HELP!

HTH

Alan KM6VV
(ramble II)

ccs@... wrote:

>
> > Hi Chris,
> >
> > I could TOTALLY see moving the probe into the approximate center of the
> > horn's bell, and then determining the center and diameter, or "walking"
> > down the sides. Do you assume it's symmetric? Do you digitize circles
> > at various depths? Do you "walk down a side, rotate around the circle a
> > defined amount (angle), and then do another walk down the side? Or
> > combinations of the above?? Lottsa interesting movements!
>
> As I'm trying to measure antiques, I have to assume that the bells
> will be somewhat oval in shape and probably have dents. I'm also
> assuming that they won't be perfectly centered on the machine and
> their axis will be skewed from teh machine Z axis if it is even self
> consistent. I originally thought of a rotary machine, but realized
> that I would need to align the probe carefully, wheras with a
> cartesian setup a bent probe simply makes the whole object seem
> shifted a little bit in position - it still gets measured accurately.
>
> I would like to be able to generate cleaned up shapes using both an
> equal area symmetric shape (which should reproduce acoustic properties
> of the instrument's current condition) and/or equal perimeter (which
> would yield the shape before any ovaling).
>
> The resulting data could be used to compare horns from different
> makers, regions, time periods, etc. But the primary purpose is to
> produce G-code to turn a mandrel for making reproductions of these
> historic instruments. About 70-100 lbs of cast iron and
> steel... really worth getting the shape right before I start cutting.
>
> Do you have a test indicator you could run next to the MAXCNC
> digitizer while hand cranking the mill, in order to determine its
> repeatability? I'd be very interested in the results of any test you
> could run.
>
> Chris