Hello i am creating an automatic saw that can cut Medium Density Fiberboard i will have a keyboard that i input the size and the saw have to do the cutting motion automatically i am currently in product design but i struggle to find i way to measure precise milimeters the size of the bed is aroun 3.5 meters and the measurements need to be linear. Is there a way to measure very precise.
You could look at how 3D printers do it. They generally use stepper motors, which are accurate enough for <1mm resolution.
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google LASER distance sensor 1mm accuracy.
This is so much more of a mechanical problem than an Arduino problem. Tell us or show us how you want to make the measurement.
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What environment will this be in. Is the area dust free? Is it vibration free? How about the humidity? How do you plan on moving the table? etc??
Not for long, LOL.
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I'd suggest that safety is the most important aspect here - even moving to a position to cut could be risky, but powering up a saw automatically scares me 
Does "automatic saw" refer to something like a circular/tablesaw/bandsaw thingie to cut in just one direction?
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Idea is quite close to CNC-router (with blade instead of router bit). There you know the relative position of cutting head all the time, so you don't need to measure anything.
You can have pretty much whatever you want as long as you have deep enough pockets. I bought LASER Measuring systems from u-Epsilon which were extremely accurate and extremely costly.
Ron
Yes i mean table saw and i dont want to rely only on stepper mottors because they can miss a step so i need a way check if there are any steps missed basicaley i will drive the table just like 3d printer
Are you doing cuts on multiple sides of the board?
3d printers just use steppers, powered well enough to move their expected loads.
For accuracy in a 3d-printer-like stepper scheme, If you are just doing one-D cuts, I think I'd consider using a stepper driven arm to push the board into position, then clamp it, then move the arm back to home to check if it skipped steps.
Take a closed-loop-stepper or servo. LinuxCNC + MESA FPGA and you are good to go.
Oh, I forgot: I once made such a thing for a stone mason, he used a 4 axis machine to cut stone tables (~ 4000x2000x30mm) with a circular diamond saw. cutting round corners was challenge 
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Also my stepper mottor have gearbox on it do step from the gearbox shaft sre different ration
If the slop in the gear box is near to the same precision you are asking for, there is a problem!
I am thinking of drilling small holes every 1mm and using a light sensor to count the lights
Oh. I didn't realize that from your post. I'll hold back until I understand more.
If you are a really clever programmer, you can use your mm measuring device as feedback and you don't care about counting steps.
Didn't you say above that there is dust in the air?
Just asking the obvious: why do you want to reinvent the wheel? There are proven to work solutions out there. There are reasons why these solutions exist. Why don't you pick one?
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I agree. CNC router is able to do heavy continuous cutting for hours without missing one step with precision of 1/10mm without any xyz distance sensors. You just have to build it correctly.
Optical sensors in dusty environment is a hassle.
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