How uniform is a good rip on a good saw?

Assuming that you have a factory plywood edge against the rip fence on a cabinet grade table saw, what would expect the width variations to be on a 2" rip over a 4 foot length?
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Ok first off, a Cabinet saw is not a cabinet grade saw. It's called a cabinet saw because the table is on top of and the motor is inside of a "cabinet".
You should expect + or - .005" or better if you are using a top quality blade.
If I need a good clean edge when making the first cut on a factory edge I cut a smidge wider and then cut the factory edge off. The factory edge is pretty crappy compared to the what the saw and a good blade will produce..
All of this is also assuming the saw is properly set up.
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I would expect to be able to take my tape measure, measure at any two points on the cut and get the same reading. IOW, 0.
If you're measuring small variations, like 1/16" or so, make sure the factory edge is actually straight. They usually are, but there are no guarantees.
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I wouldn't trust a store bought factory edge. It's good enough for building construction but often not good enough for precision work. It depends on the factory.
A quality panel saw or high quality slide on a cabinet saw will get you the best rip for lengths over 4'.
Thin blades can wobble at high speed, especially when hot from cutting friction and under variable load, knots, grain alignment, etc.. Use dampeners when using thin blades. I can measure the difference of the kerf with and without dampeners.
A table cannot be trusted to remain in alignment if there is any movement; play in the bearings, warped blades, loose top, etc..
Given that the table saw is of quality construction, all bearings, arbor, blade, trunnions, etc. are in good condition and all bolts are tight, the saw could still be out of alignment but in a fixed stable amount.
Parallelism of blade to miter slots and a true 90deg stop are the most common problems I've seen on "good" tables saws. These are often bad in store demo units and even at many commercial shops. Many people (or just me) adjust the 90deg stop past 90deg and use a guide to manually set rather than relying on the stop. If angles are cut often, better results can be had by hand setting the angle vs. using the saw angle ticks. These are just used as a rough first position to start blade alignment.
I don't use a parallel fence to avoid kickback so I don't count bad fence alignment as an issue. The fence must be set and checked manually for every critical cut. A good fence system should stay within a few hundredths of an inch between position changes without manually checking or re-alignment.
Positioning is very stable on my Unisaw. I don't have a reference surface long enough to verify long cuts. Measuring long cuts appear to be <0.01". Well within 0.003" for 12" or less. The majority of what I cut is very small. I have only fully aligned it twice, once on purchase and once after a "bumpy" shop relocation. It needed it both times. I have purchased blades that were slightly warped, these go on the radial or chopsaw.
There are many variables when cutting; feed speed, stalling in the blade, smoothness of sliding table or wood against top surface, knots, grain, splits. Once the cut goes off the saw's main top surface (>3'), movement and deviation of the cut is common.
After alignment cut 10 pieces 2' in length and if you can't get more than 7 pieces within 0.01" of each other the saw may have alignment stability problems, play in the bearings, warped blades, etc..
Remember that you can easily push more than 0.005" out of alignment by sloppy cutting methods on a well tune saw. At it's best, it can only ever be as precise as you are when cutting.
Also if you do anything else to the cut edge, plane, scrape or sand, a few thousands off to start won't really matter. Long sanded edges will often be much worse than .005" deviation.
Theoretically a slow start motor without the startup torque could keep alignment longer on flimsier built saws. Although high end shops I've been to don't use them on saws over 3hp.
Short answer: Most books and manuals I've read state 0.003"-0.005".
http://www.woodshopdemos.com/align-1.htm http://www.newwoodworker.com/basic/tsalign.html http://www.thewoodshop.20m.com/calibrate_sled1.htm
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