Re: [CAD_CAM_EDM_DRO] Coordinate Measuring Machine

Unlike some of the other replies, in my mind a CMM involves "real"
resolution similar to, but not necessary equal, to those found on
commercial machines.

As such, at the low end, you would need to think in terms of microns for
resolution.
It is normally seen as a given that your mechanical resolution, be it
movement or measurement, must be 10 x whatever you are trying to do.
At a very low minimum, a cmm should have a 0.01 mm resolution and
repeatability, both well within any amateurs abilities, and within
victorian engineering and manual tools.

Almost all probe designs I have seen are not, in my mind, adquate.
Basically, because their premise is that creating a "touch" is
sufficient without any regard to the plane, direction, etc of the contact.
Thus they make a point cloud in a fairly repeatable, but by no means
accurate manner.

The basic problem is lack of adjustment of the make or break point in
all plains, x,y,z.
Likewise, they are typically not assembled on a flat surface (lapped
plane, optical flat, surface plate) and no effort is made to measure
bend of the probe or repetability.
The most important problem, in my mind, seems to be that they make no
effort to measure errors.
If an erro is, say, x:0.002, y,0,004, z:0.003 all +/- 2 microns, this
would then make an excellent base for a high quality solution.
All above are easily solved problems.

Screw-type mechanisms, can easily resolve and repeat to 1 micron, think
of microscopes, optical stages, astronomy, sine bars etc.
The keys will be repeatability,adjustment, etc.

Some ideas can come from differential screws, and or special fine pitch
screws.

Making a mechnical machanism that is repeatable to few microns is not hard.
I proved this with my 12x cnc lathe.
Basic repeatability is about 2-3 microns, to an optical home switch,
with lathe mechanical resolution of 1 micron.
This was achieved by use of 0.1 micron resolution mechanical motion
train (30.000 counts/turn servos).

But this needs a build based on quality as a premise (ie the optical
switch triggers an electronic gate for repeatbility) rather than a
simple switch-makes-contact.
A 2-sensor optical switch assy cost about 40$, with real-world micron
resolution, +/- 2-3. Plenty good enough, imo.
As one manufacturer of machine tooling stated "we cannot build stuff to
1 micron, but can adjust stuff to it".

I would make a probe similar to those suggested, but would try to
measure it for error.
Also, I would make it adjustable, as far as possible, for near-zero error.
A fine (0.25 mm) screw offsetting a 1 m rod of drill rod can easily
adjust a point to better than micron levels, with 40:1 mechanical advantage.

Then measure a mic std, eg 1 inch, using say 0.1 micron resolution.
Adjust until your results are as close as possible to 25.400 mm.
Use screws to fix mounts, with a 0.1 mm oversized mount hole. Adjust
x,y,z separately.

Maybe 50 hours to make a good one, and results probably about 50x better
than the suggested probe examples.
Error stack is x * y * z, ro 3 microns = 27 microns error.
Typical suggested probes are likely about 10 * 15 * 10 = 1500 microns.

Accuracy, resolution and repeatability are not the same !

>
> I m making a CMM trainer as my final year engineering project... i m
> facing problems.. reinshaw probes are very expensive and not
> economically feasible for a trainer..
>
> any idea?
>

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