Homemade (diy) CMM arm?

[hotponyshoes]

Hi,
We could do with some way of checking suspension location points on a car chassis.
We had a demo of a cmm arm the other day and it was pretty impressive,
This thing had what looked like a ruby-tipped edge finder mounted to the end of an arm and ran on a roof mounted gantry.
Just wondering if it is possible (or worthwhile) to make one of these yourself?

We thought make a gantry like an inverted torch table (this would give us 2 axis's) and hang the arm from it. That way we could have a couple of 'refrence' points on the car that you touch so the thing knows where the car is parked and mesure the rest from there.

Atcuall construction, thinking along the lines of a swivel where the arm meets the gantry and as few joints as needed along the arm. I presume you would need some sort of friction to hold the arm in place and some sort of encoder (could you use stepper motors) on each joint (and the gantry).
Once the controler got a sign from the edge finder all the information from the sensors would be stored to a pc and, if possible, given a % away error from a known reading.

We will be using this on chassis we make ourselves so we will know the refrence points from a master chassis and would like to use it to check production chassis's and also check any accident damaged chassis's.
Thanks!

[TCSpooner]

Hi Interesting Idea. Instead of steppers you could use digital encoders which are designed for counting in steps per revolution. Some way to convert the motion of the arm into rotary motion would be needed.

Hope it all works

[Graham S]

You need very high resolution encoders, say about 50000ppr after quadrature, you can't use steppers as encoders!

Avago (was agilent) do some nice 16bit absolute encoders that could be used.

The real problems are mechanical precision and calibration,the latter really requires some niftly software to do the corrections over what you thought you built and what you actually built.

Not an easy project but there was a long thread about it in this forum.

Graham