Re: [CAD_CAM_EDM_DRO] Digitizing

Gordon Robertson wrote:

> It seems to me Roland (as well as Centroid) has the right idea.
> since any digitizing to a high degree of accuracy would take a long
> time due to the literally thousands of XYZ or XZA points to be
> tabulated, the best solution might be to have a program work as
> follows:
>
> 1. The user would plot the area to be digitized thru the simple
> expedient of using jog buttons. This would indicate the starting (0-
> 0)and ending (#-#)points of both the X and Y (or A) axis. The
> maximum up and maximum down positions for the Z axis can be set at
> the same time. User also enters the desired resolution. The program
> then creates an array based on the desired resolution and the area to
> be digitized. The X & Y or X & A parts of the array would be
> automatically filled in.
>
> 2. Program start would initially move the probe to the 0-0 position
> with the probe in the home (full up) position.
>
> 3. Digitizing begins with a rapid Z move to the maximum up postion.
>
> 4. The probe now down feeds until the probe contacts the part and
> breaks a circuit. This provides the Z dimension to be placed into
> the array at position 0-0. The probe is returned to the max up
> position.

No need to do this! With the type of probe that has been talked about here,
it could raise just enough to break contact, then start moving to the next XY
position. If the probe shows no deflection when it gets there, it lowers it until
it gets contact. If it shows deflection, it raises it to clear contact, then lowers
until it gets contact, just to eliminate backlash from the mechanism. As long as
the XY steps are smaller than the maximum deflection of the probe, no damage
will result. For many generally rounded surfaces, this might only need movement
of .050" in Z per XY step.

>
> 5. Program continues indexing in steps along the X or A axis. After
> each X or A move, the probe repeats step #4.
>
> 6. After all points in the array have been filled, the machine
> returns to its home position.
>
> I've watched my Roland machine work, and I've seen it make a LOT of
> unnecessary moves. With a system I've just decribed, the probe
> should not need all those 3D movements everyone is trying to build
> in. All the probe would need is simple up and down movement.

This will only work for rounded objects, or objects with relatively gentle
slopes. If you had a block that you want to determine the dimensions of
the top perimiter, your program would have errors that could be as
large as the step size. So, you also need an edge following routine that will
walk all around any 2-D feature and compute the true shape, rather than
just sampling the 3-D elevation at a grid of points. I think most people could
live quite well with these 2 programs. For complex shapes, it might be
necessary to do the digitizing in several passes, acquiring the elevation over
part of the area, and then feeling its way around the part at several elevations.
Then, you'd need to merge the data, but that might not be too hard.

Jon