Re: [CAD_CAM_EDM_DRO] one for the math gurus.....Squaring an Axis

CL wrote:

>Setting up an Axis to be truly square on a CNC machine has always been a
>challenge. It obvioulsy gets a little easier if one is fortunate enough
>to have a friend or a day job where you can borrow thier precision
>squares and straight edges.
>
>Them babies are big bucks, and I used to be able to borrow one from
>where I used to work, but,.....
>
>Obviously, a "circle" can't be cut into a true circle if an axis is
>skewed, so,
>
>Is there a mathematical equation where one could:
>
>Engrave a "Dial" with degrees, cut it out into a circle, and then
>measure and "map" its actual circular dimensions.
>
>

You don't need to do this to determine axis orthogonality. A ball-bar
test detects
about a dozen different sorts of errors. But, just to detect orthogonality,
you just need some way of measuring a distance accurately. If you mounted
a ball bearing known to be round to the table, and another one in the
spindle,
and got them right above each other, and then moved the table some equal
distance
away, first, say +X +Y, and then the same distance away +X -Y, and measured
between the balls at each point with a good 6" micrometer, you should be
able to detect this error. Making measurements at all 4 diagonal corners
and summing the results might help cancel small errors and magnify the
real orthogonality error.

Now, getting the two balls centered exactly over each other takes
several steps.
First, you'd tram the mill spindle carefully to be parallel to table
movement
(not necessarily parallel to the table surface, itself). Then, using a
dial test
indicator or co-ax indicator in the spindle, center the spindle over the
ball
on the table. Finally, install a ball attached to a shank in the
spindle. This
would have to be made carefully so the ball is exactly on center, but that
shouldn't be too hard. When the balls are moved apart, you can then lower
the quill or raise the table to put the centers of the balls at the same
height.
You can't pass through (0,0) without raising the quill, or detouring around
where the balls would hit.

Alternatively, I've thought about making a simple version of a ball-bar
tester. The balls are set up like above. But, one more piece you make
is a 2-piece bar with ball-sockets at each end, maybe spring-loaded to keep
the ends on the balls, with a dial test indicator mounted to the middle.
it would be set up to read the compression of the bar ends. You could
go to the corners of some convenient square dimension and read the
dial. All 4 readings should be the same. If diagonally opposite corners
are shorter than the other two, you have an orthogonality error.

Jon