[samualt]

Does anyone here have experience aligning cnc-tables via laser? There must be some kind of halfway simple procedure. Like with an interferometer or something.

1. How can I measure the distance between two aluminum rails precisely, if they are around 4 feet apart?
2. How can I measure if the two rails are precisely parallel?
3. How can I measure flatness of the rails, precisely?
4. How can I measure if the rails are level to the ground precisely?

I have many lasers, both pen lasers and tube lasers. And I also have beam splitters and mirrors. There must be some way to use them for alignment checking. Do you know of any good tutorials or references?

[HuFlungDung]

I'd like to know about this, too.

Tell me, would the seperation distance, and parallelism not be taken care of by the gantry itself? If not, a rod and a set of feeler gauges will work to space the rails precisely

Is parallelism to the ground actually relevant? Parallelism to the table is more important and can be measured between the gantry and the table with a micrometer.

This leaves just a possible twist of the rails out of one plane as the problem. So would the following procedure work?

Mount your laser at one extreme end of your gantry, mounted on the cross slide. Point the laser roughly parallel to the long X rails, shining on a target some distance away, the farther, the better, I suppose. Run the gantry back and forth full travel of the X rails and adjust the aim of your laser so that it never moves off of a spot on the target. When this is achieved, then you should move the cross slide across to the opposite side of the gantry. Without touching the laser alignment whatsoever, set up a second target and examine the position of the spot on it. If it moves up or down as you move the gantry full length down the rails, then the rail on this side has some twist out of the plane of the other rail.

You tell me if this is easy to do

[samualt]

I was thinking more of the following:
1. Roll - Is either rail twisted. Like a plane doing a barrel roll.
2. Pitch - Is either end of the rails pointing up or down. Like a plane doing a dive or climbing.
3. Yaw - Parallelism. Are the rails exactly parallel to each other. Or do they form a slight V when viewed together. Yaw would be like a plane wagging its tail back and forth.
4. Distance - Are the rails the correct distance apart.
5. Straightness - Is each rail straight? Or do they make a roller coaster for the gantry?
6. Levelness - Is the whole thing level to the ground. Does the gantry have to climb or go downhill when it travels from one end to the other?

The idea of using a laser is that you can measure using extreme precision. The shiny aluminum rails I mentioned are what I'm mounting my THK rails on. I need the aluminum rails to be perfect before I even think about mounting the THK rails that the bearings will ride on.

Your right in that you don't really need a laser for much of it, but the accuracy would be much higher if you used a laser rather than a tape measure, rod, or machine level.

[samualt]

I'm not sure if this helps anyone but here a link to how Hamar uses lasers for alignment.

Hamar Laser Alignment

I'm still studying it. I think the way I would do it would be much different and time consuming because they are using very expensive equipment. However, it may give you an idea of how the other half do it.

[NeoMoses]

Ok, this may or may not be helpful... I'll post anyway, feel free to disregard this information.

This assumes you know the wavelength of the laser light you are using. I have done a little playing around with some laser pointers, which IIRC have a wavelength of about 632 nm. Using a known wavelength of light, distances can be accurately measured using diffraction gradients and or reflectors that diffract light. The 2 most readily available are CDs and rulers with 0.01" gradations or smaller. Both have very repeatable patterns for reflecting light.

Now, the physics involved are quite simple. You'll see bright spots where the light waves join in constructive interference, and nothing (aka dark spots) where the light waves are joined in destructive interference. If this theory is new to you, I could dig out some old physics books to show you some diagrams, as I'm sure I'm not explaining it well.

However, I feel the easiest way to get the precision you desire would be the rod/feeler gage method mentioned above.

Good luck!