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Old 08-27-2006, 10:40 PM
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Originally Posted by Rob Keller
What is the whole purpose of this?

What is the end result?
I gave a couple of examples in the next to last paragraph. Once you have an accurate 3D model of the exterior surface of the car, you can manipulate that model and use CNC to make plugs for fiberglass or metal parts to make those manipulations real.

Or, replace the measuring laser with a CO2 laser and etch your name a few ten-thousandths of an inch deep into the whole top of your car.

That's just off the top of my head. Take a look at some of the CNC machines at

Originally Posted by scholman
I think it would be better to bolt the car down to a solid frame and then have the laser move back and forth and all around the car. Step motors are used on computer controlled machines. No worm drives because they are not accurate enough.
That would require either a box-like frame or cylinder-like frame that went all around the car. Quite a bit more complexity to get the head/trolley to move around the car to obtain straight-on views of top, bottom, sides, front and back.

Having the laser travel along the same axis as the rotisserie gives you just two lines to make parallel. Even then, any inaccuracies would be measured at the same time the measurement from laser head to car is taken, so they're compensated for in the software.

The reason I said worm gear is because of the fact they don't coast; they stop when you stop the input. The worm could be driven by a step motor and the backlash could be measured with each step. Since it would be the computer doing the measuring, it doesn't matter that it would be a tedious job. This avoids using expensive, high torque step motors and puts it back in reach of a garage engineer.

The computer could do the calibrating for a full rotation, producing a +/- deviation for each step of the motor. That compensates for any slop in the gearing. As long as it can measure the error, the data will accurately reflect the real surface.

Essentially, it's taking the same rotisserie that a lot of people custom make for body work and turning it over to your PC to rotate in tiny increments. At each stop, the laser measures the distance to the car surface at however many points along that axis that you choose and measures how far off from the reference target it is at each of those points. Same priniciple used to align industrial machines or to level dropped ceilings.
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