I bought one of those really really cheap (relatively) (and demonstrably inaccurate) CMMs some years ago.
It had a simple serial interface that could be used to send coordinate position numbers to AutoCAD's command line when the button was clicked. So you could start a line command, click twice, and have a line in 3D, or start a 3 point circle command, click three times, and have a circle in 3D, or just start a point sequence command, click click click... and have a point cloud.
The CMM also included a copy of Rhinoceros, which had slightly more useful functions built in, but the deal was the same, pretty much.
For reverse engineering at least, the process was agonizingly slow, and when you were done you had a lot of wireframe objects in 3D space that lined up only as accurately as you could touch the point to a recognizable feature, which was not really all that accurate. I.e., you couldn't directly produce a usable 3D model from the digitized data; you had to fudge new objects in place, aligned like they should be.
I have also watched inspectors use expensive CMMs with whatever metrology software came with the machine in order to inspect parts 'per drawing', and that was also agonizing to watch. The software appeared to be capable of establishing reference planes by touching features and measuring an arbitrarily placed part, but the inspectors seemd to forgo that, and bump and slide and jack the part into alignment with the machine, then use the machine as a digital caliper.
I have had designers tell me that their particular borderline magical CMM could run unattended, e.g. overnight, and produce a point cloud model of an object by probing it, as if the CMM had powered axes. When challenged, they couldn't make the machine do any such thing, and their inspectors told a different story, so I don't know if such machines actually exist, or if they would be useful if they did.
Frankly I'm not convinced that CMMs are any faster than an experienced inspector with a big surface plate and a reasonable collection of ordinary metrology tools.
They do serve nicely as a centerpiece for dog and pony shows, so I guess you'd want to buy whatever looks most flashy on a big color monitor.
We do use the CMM to establish reference planes etc, but I notice that our American colleagues prefer Mike's method. We have designated measurement points on a car that define the plane, so once you measure them you are all set. haha.
I used to work on a surface table for technical inspection and I know damn well that on an engine block i'd be more accurate with slip gages etc, but on a whole car the CMM is probably better.
Either way CMMs are incredibly time consuming, we have 3 people who do CMM and seem to be good at it, it requires a lot of practice.
Cheers
Greg Locock
New here? Try reading these, they might help FAQ731-376
we would specifically be using it to digitize clay surface, in a stying studio, we use tokyo boeki towers. my current program is "putty", just a "logging" program, so xyz points are printed out, then re-entered into alias, then built, from there it goes to inventor and soon catia. a huge consupmtion of time. finding a good program would emliminate the need for expensive laser scanning.
As Greg says you can get scanners that will give you surfaces rather than point clouds however if you are looking for A class surfacing you will not get it that way.
Basically A class surfacing is not only tangent to itself but is constrained within the Cad data, you will never get that from a piece of clay, or at least not for the foreseeable future.
