OK I am getting different dimensions on front to back from a sheet plywood cut.
Must be the fence on table saw is not straight.
When I set it for size and tighten it down there is some movement before it settles and locks.  
I was thinking of using a dial indicator running along the push slot to check this.
Is this a common problem?
Randy

It is fairly common, and you are on the right track for checking it. Also make sure the factory cut is square to start off with, I've gotten sheet goods before that were cut off square at the manufacturer level which can really throw a kink into a project. I tend to always check it with a rafter square before making any cuts now, just to be sure.

Randy,
  I would start off by checking to see if the table is square to the blade, using an indicator and the miter slot then check the fence to the miter slot.

Randy,

Eric gave you the first spot to check, then once you can verify that the table is aligned with the blade, you can go back and check and adjust your fence.  Your fence may have a couple of screws right by the clamping handle to finely adjust your fence. 

Um, I can't wrap my mind around blade straightness being a problem.
That would only make a wider kerf.
I think the fence is the problem. I checked it with travel indicator and depending on how I tightened it the movement from back of fence to front could move .030" to the push guide.
Saw is a cheap Rockwell . Used. Hard lock sets it but there is a knob that adjusts tension on front rail also. This has to be released to move fence.
Please excuse me for being so picky. I have built houses and 1/8" on siding/framing is nothing. Want to get into fine woodworking with what I have if I can.

Randy

Randy, you may also want to check the bar rails that the fence locks against with a straight edge and make sure they have not been bent, also check to make sure the height of them is the same throughout their length from the top of the table, if either is off it will throw off the fence.

Maybe a new/aftermarket fence? I had a Makita table saw that worked well but the fence was a piece of junk.

Thanks guys on the tips.
I will check the rails. Didn't think about that.
Fence has a sacrificial board on the blade side for angle cuts and I can shim board to metal fence.
But it is not perfect surface. Wish I had a planer.
Anyone used formica or another real smooth surface for this?
I have cut 2" thick aluminum with my circular saw so I know plastic would be no problen on table saw.
Maybe I am asking too much precision from a woodworking tool but after 25 yrs. as a machinist I want it to be right.
What kind of tolerances are normal for a table saw?

Randy

With the saw UN-PLUGED measure from the farthest blade tooth at the front of the blade over to the fence. Now do the same at the back of the blade. Double check by measuring from the Miter slot to the fence front and rear.
Now if this won't work for you it's time to make yourself some cut-off Jigs using a straight edge factory cut board attached to a same length but wider scrap piece of Plywood. Measure the distance from the foot of your circular saw to the blade and add 2". Run your circular skill saw the length of the plywood using the attached factory straight edge cutting off the excess plywood. Now clamp this Cut-Off Jig on your sheet of plywood that should be resting on a few 2"x4"s straddling a couple of sawhorses. This Jig was featured in our Magazine a while back. Not sure of the issue.
Make several of these jigs as I did in a variety of lengths from 18" long up to 8'. That 18" jig gets the most use.

If all else fails and you have a second blade try it. I have had the odd blade that wouldn't cut straight for whatever reason. I know it was the blade because I changed the blade, without moving the fence. The blade was all the way up so I didn't even have to change the height. Different blade gave me a straight cut. Worse blade I have ever seen had the set of the teeth set differently on left side of the blade compared to the right side. The blade wanted to cut in circles.

Well I guess a 4 x 8 foot sheet of plywood being cut on a table saw hasn't got a chance in swowball hell of being unstable.
For accurate cuts I use a sheet of foam board laid flat on the floor with the plywood on top. Using a framing square, a straight edge, and a circular saw.
For cuts that don't need to be as accurate I use two saw horses, chalk line, and a circular saw. And of coarse a tape measure and pencil for both.

Oldman- I have been getting magazine for 15 yrs. and don't recall or understand that. Sorry.

Jdeere- I think you might have the answer there.
I did a bunch of test pieces for fence. Measured slot to fence at a certain spot to .001" .
I think the amount of pressure on the push, and the not really flattness of the ply made cuts vary.
A duller side of blade could make it bend with more pressure.
Running it through twice seems to straighten cut as there is not so much force.
Thanks guys.
Randy

Randy,
This is just a WAG (Wild A$$ Guess), but I had a 'problem' with the very first circular saw I had ever bought after a few years of use.... it wouldn't make a 'straight cut' on longer cuts being made to 'rip' plywood or dimensional lumber.  After changing blades a few times, WITHOUT ANY SUCCESS, I happened to notice that the main output BUSHING on the motor shaft had almost 1/8" of wobble in it.  This would allow the outer edge of the saw blade to 'drift' or 'wander' in the cut. 

You might want to check the amount of 'play' in the motor shaft by raising the blade to its maximum height and then grasping the blade near the edge (USE GLOVES) and checking to see how much 'side-play' the motor shaft bushings/bearings are allowing.  This has been a COMMON COMPLAINT in many of the reviews being written on some of the 'cheaper model' Craftsman table saws (usually of questionable mfg).  Most reported VERY EARLY or PREMATURE bearing failure or excessive wear/play in these bearings/bushings.

It may be that the amount of pressure or 'rate of feed' you are putting on your wood being fed into the blade is causing any 'excessive play' in the motor shaft to be transferred to the cutting edge of the blade causing it to 'wander' or 'drift' in the cut being made.

Just some 'educated' thoughts.

I had one of the "cheaper" Craftsman table saws, and the fence would not reliably lock down parallel to the blade. I always had to measure, and then hold it in place while I locked it down. When the motor bearings seized after about 4 years, I bought a Grizzly hybrid saw with a Shop Fox fence. Couldn't be happier.

OK -
I got my pieces cut now and they are straight and square.
Biggest is 12" X 9" .
I didn't have room in shop for 4' X 8' so rough sawed it outside with a circular saw.
(My Dad always called them "circle saws" the way I cut. LOL )
Wish I had read about foam under and a straight edge before but it gave me some extra stock to practice on.

The fence still is not exactly straight with push slots but I marked a place on the table to take measurements.
Finally found that if I took a cut, flipped board, another cut, spun board and repeated everything was paralell.
After I adjusted the squareness on the push I bumped it onto the concrete floor. !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
Start over. Luckily I am out of work and have plenty of time......
Had to rebuild that too.

This makes me think that
1) the blade is not perpendicular to the table at the 0 stop. Have to fix that.
2) blade could be not exactly parallel to slot but it only gives a bit wider kerf so may ignore that.
3) blade is probably duller on fence side teeth. Need to get it sharpened.
4) Bearings seem good - no play.
5) it doesn't matter if fence is straight with all these problems.
I can make repeatable cuts with some extra work. Within .005 .

Where the teeth hit the wood is where it cuts.
Some recutting on backside just smoothes it a bit.

Still have some questions not answered

1) Is Rockwell a cheap saw ?
2) What kind of precision should I expect as good - say for cabinetry?
( I know wood shrinks and grows depending on temp and humidity. Metal does too but not nearly as much.)
Is 1/32" ( .032 ) as close as you can expect as normal for a table saw?  
3) Would a radial arm saw be more better for such small work?

Please excuse as I have come from a toolmaker background that is not needed in the USA anymore.
Don't expect .0001 measurements on shrubbery and hope to sell birbhouses or something.

Randy

1/32" is good, anything if you get fussier than that, you are being too fussy

Randy,  so glad you got that figured out and that you had success in cutting your boards.  Good job!

http://www.familyhandyman.com/DIY-P...ep-By-Step

Randy here is info on cutting long wood or boards. The jig shown isn't the one I was trying to describe but will give you some idea's. I will see if I can locate the article showing how to build the Long straightedge cutting jig. It's basically a 1"x 10" x 8' board with an 1"x 6"x 8' placed on top of it, aligned up with the left edge of the 10" board. Glued and screwed in place. Set the Circular Saw on the 10" board with the base resting up tight against the 6" board. Cut the 10" board the full length keeping it tight against the 6" board. Now when you want to cut a long sheet of plywood, line up the cut-off edge of the 10" board on your mark. Clamp it down on the marks. This is where the saw blade will travel cutting off the measured section. These jigs can be made shorter for different applications where a 8' long jig would just be too much to work with.
I hope I made the instructions clear. And I don't remember when but I'm quite sure it was in our Magazine. But I do have issues going back to the late 60's and early 70's up to present day. I drag them out on a rainy day and thumb through them occasionally. It's very interesting what has changed and what remains the same in the Handyman DIYers methods and tools.   

Thanks Oldman, that was perfectly clear.

Randy

Here is the completed project I was working on.
A clean air box for living area.
Randy