Hi John,
"With a self levelling laser hitting a 2mm square target over the entire machine shouldn't be too hard assuming you have levelling feet on the machine, we all do this or better when we put up a shelf. This assumes we can weld the frame to within 2mm."
Assuming a vertical arrangement the only alignment required is to align the rough frame he two counter weighted wires. When a work piece is clamped in position it is also aligned to the wires.
The concept of using a "metrology frame" in this case for one axis is not new.
https://www.google.com/search?q=metr...5sCCs1M:&vet=1
It is important that the alignment wires are in line with the tip of the cutter at the point of cutting. This will greatly reduce any error caused by any angular misalignment of the z axis to the work piece. Abbes principal must always be honored.
https://www.google.com/search?rlz=1C...71.zasns7Fk5YU
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"Should also be noted that it is only the vga (640x480) web cams that have this small sensor area I think these are 1/6" diagonal sensors, other similarly priced but higher resolution cams have larger sensor areas. For example the raspberry pi camera (1/4") has a 4.6mm diagonal and another sensor I have the OV2640 has 4.5mm. It seems that the sensors are classified by the diagonal size of the sensor of which the active area with the pixels is smaller, this is why 1/4" > 4.6mm."
I think sarting with a low cost solution first then if necessary upgrading to a more expensive camera.
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"I like the idea of levelling a jig then grinding the surface as an alternative to shimming. What sort of grinder would you use for this?"
I had been reflecting on grinding but had substituted milling, strangely I should have written milling!
Grinders have a lot of inertia and take a long time to stop, they also make a lot of noise. A Small motor driven milling cutter ideally around 3mm can be fed relatively a lot slower and the motor can stop quickly. I suspect having a running spindle near the wire alignment system may cause the wires to vibrate. For high accuracy results it may be necessary to stop the spindle before each alignment cycle.
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I can imagine a jig short jig say 500mm that clamps around a piece of box section that can be micro adjusted for twist and level. I we bolted aluminium to the top of the box section you could probably skim it with a hand router with a small fly cutter in it. Skim the section under the jig then move the jig along, re-align it with the laser/wire then repeat. When you finish one rail then move the assembly to the other without moving the reference laser to bring the other rail into plane. Maybe a wood power planer could be used for the skimming, would be scary though.
The jig itself needs to be planar as you say but this should be straight forward on a milling machine.
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"What are you thinking for differentiating twist from height variation? I think with the laser two image sensors stuck out each side could detect twist same when you tram a mill with two dial indicators on a T bar in the spindle."
Two weighted wires suspended vertically will have no twist between the at all. These two wires are our meteorology frame. if we position our cutting stage relative to the wires using two cameras, make a peck cut then move X or Y,, check the cameras are still perfectly centered, make a new cut and so on our accuracy will be determined by the accuracy achieved by the camera system.
No system is without error however if you consider the above scenario the surface generated will have a surface that comprises a series of physical levels some high and some low. representing the accuracy of the measurement cameras and the positioning system.
Using a vertical alignment system, the exciting part is that the wires represent a plane that is far flatter than an inspection grade surface plate. The final result will be a surface that is extremely straight overall. With a surface similar to a scraped bearing surface.
Upon reflection I will make my test setup vertical.
As you may have guessed I have been thinking on this problem for a long time.
Cheers, Joe