Re: Digitizer?

--- In CAD_CAM_EDM_DRO@yahoogroups.com, "Raymond Heckert" <jnr@a...>
wrote:

> I'm thinking of making a digitizer by connecting a fixture in the

mill's

> spindle to a high resolution encoder to a ball-tipped arm that is
> torsionally pre-loaded, and have the mill move around the 'part' in

a

> specified pattern (which would be calculated from the results of the
> 'previous path'), and readout the encoder at every .005" of the

path, then

> calculating the exact position of the ball (via the sine of the

ball's

> radial disposition as measured by the encoder) to get a tangential

envelope

> at varying 'Z' axis elevations on the part's surface. Is this "do-

able", or

> has it already been done, and I'm 're-inventing the wheel'?

Ray, I doubt that you will be able to improve the resolution of your
scanning much more than a contact probe is capable of doing.
Mechanical Vibration will affect any measurement system and this
external noise will quickly overpower the quality you get from the
higher resolution device.

The problem with scanning surfaces has been solved in DeskCNC because
the controller board is external to the PC. This allows a much
higher responsiveness where you need it in the motion control
hardware, without having to try to fight for a turn at the parallel
port or within the PC operating system priority.

The DeskCNC scanning algorithm does not lift the probe from the
surface any more than the one step required to close the switch. It
then makes the next move toward the next surface location, not to a
safe clearance plane. Since this is done in dedicated hardware, it
is very efficient, smooth and fast. I typically can scan 20,000
points per hour on a Sherline mill with no backlash compensation in
the lead screw (In other words, I am very conservative with the
scanning rates specs). This is generally a target that is 2 x 4
inches, with at 1 inch of total Z elevation. That is about a .020
x .020 inch grid ( .5 mm), which is more than sufficient for smooth
surfaces. For a highly detailed target, reducing the grid size will
make a sharper detail, but you also have to use a smaller radius tip
to reach in between the small details. This is what I call the
texture level of a surface scan.

The 5 way switching in the DeskCNC probe also supports high scan
rates, without having to peck peck peck each point with a single
direction switch type probe.

As far as using the massive data collected, DeskCNC does three things
with it:

1) Reverse compensates for the probe tip radius
2) Smoothes the surface, eliminating certain kinds of data like noise
and spikes.
3) Saves it as a standard .stl format file.

The result is a surface file that is ready to process in the DeskCNC,
3D surface machining CAM or in other programs that are capable of
using 3D stl surfaces to create toolpaths.

DeskCNC with the software, controller board, and contact scanning
probe is $575.

Fred Smith - IMService

http://www.cadcamcadcam.com/hobby