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concave cuts in sheet goods7/8
I have trouble cutting straight cuts on my cabinet saw. I start against the fence, but for some reason the sheet 'loses' contact with the fence after the blade. The sheet is alway s against the fence before the blade. The result is invariably a concave cut. Often I cut oversize and rectify the 'hollow'. A rip of 1 or 2 ft ride against the fence no prob, but a full width sheet wanders. Any ideas? I feel I should need only one cut. Thank you
Sheet goods will have an internal stress due to the outside edges picking up moisture. As the edges pick up moisture from the surrounding environment, they try to expand...get longer. They can't, so there is a built-up tension along the edges. When you make a cut, the tension is released, and you end up with a banana shaped strip.
If you are using a table saw and rip fence only, you will never get an absolute straight edge, as the fence just traces what is already there. You can probably produce a straight enough edge for most applications, however.
The only way to get an absolutely straight edge, is to use a sliding table saw, or a vertical saw. Make the first cut over-sized, then cut the factory edge off, the re-trim the first cut to the proper size.
I have used a vertical saw a lot, and the best way when cutting 24 or 12" strips, is to make the fist cut dead center of the panel (24 +/-), turn the panels so both cut edges are up, and make a second trim cut off the top. Now you have one absolutely straight edge on both strips to work with.
I would crank the blade all the way up and take measurements to make sure that the blade is perfectly parallel to the fence. Over time they can get a little off and cause that. The above poster has a good response as well.
I think you need to start at the beginning. True your saw! Remove the blade. Use a magnetic base & dial test indicator to check the run-out on the fixed blade flange. A decent quality saw will have less than .002" in 180 degrees of shaft rotation. Any run-out here is magnified by the diameter of the blade. Put a good blade on, mark on a tooth with a Sharpie. Raise the blade all the way. With the best most accurate measuring device you have, (digital caliper?) measure from the miter slot to the marked blade tip near the front of the table. Rotate the blade to the back and measure again. Should be exactly the same. If not adjust the table to the blade, if the trunions are attached to the cabinet by loosening the top to cabinet bolts. If the trunions are attached to the top you will need to loosen that connection and adjust (more difficult and more common on cheap saws.) Tighten bolts and recheck. Make sure the fence rails are straight and solidly attached to the table. Move the fence very close to a miter slot. Adjust it to match the slot. As a double check use a feeler gauge and the marked tooth, checking between the fence and the tooth at both forward and rear positions. While you are at it adjust the fit of the miter gauge in the groove. Beyond the scope of this, you might want to check table flatness.
The blade: Quality blades have been tensioned so they still run true as they warm or speed up. Not much on a 10" blade. Uniform tooth marks on the cut are good. Variations can indicate bade spindle bearings or a blade that has lost its tension. An evenly repeating deeper tooth mark means blade damage or poor sharpening.
About sheet stock: it isn't necessarily square or straight. That's why panel saws are used for cutting it. Some sheet stock has built in tension from when it was manufactured. As you cut into it that tension is relieved and causes the part to bow. The only solution is to make a stress relief cut on a panel saw or to cut over size and then attach a straight edge to the part to get a true reference edge. Then cut the 2nd side. If your parts are narrow enough you can cross-cut them on a sled.
A 10" table saw only has about 14" of table ahead of the blade and a bit more fence. Not much for guiding and 8' cut. But if the saw is true you should get a reasonable reference from most of the length of the fence.
Sawing technique: don't push too hard on the fence when feeding. It may have some flex and could cause a bowed cut.
Have you considered a track saw?
Really, I didn't start out to write a dissertation!
Last batch of plywood I got all the long edges were curved, and I don't mean a little. At least 3/16" of concave or bow depending on which side you were looking at. Not borg plywood either. Columbia brand. Had to use a track saw on every pc to get a straight edge. The first pc I cut sounds exactly like what happened to you.
I stopped using Columbia core plywood because it cupped so bad off the router. The parts would literally pop after pump released. This was a Birch B-2 plywood.
Do you notice that you have to hold your sheet really tightly against your fence for it to stay tight?
If so, it could be that your fence may not be running parallel to your blade. I lay a perfectly straight board gently against the blade, touching the teeth, not the body. Set your fence and measure to this board at both ends. The measurement should be exactly the same.
I don't use these "Board Buddies" much anymore since I cut most sheet goods on the cnc, but I used to use them a lot, they do work well on sheet goods. The downside is that you remove them to rip narrower pieces because they're in the way. They remove quickly and are easily replaced though.
I think it's most likely your fence is splayed outward from the miter slot. Could be the blade is splayed in the opposite direction, and/or your splitter isn't lined up with the blade. For me, the plywood itself would be the least likely source of the problem. I try to have it in my shop for at least a couple of weeks before using it, and I sticker the sheets so that it can come to equilibrium with my shop. Dense stacked sheets brought into an unconditioned shop and used right away can lead to the same kinds of problems as with solid stock.
Woodpecker makes a handy little micrometer gauge for trueing up a table saw. It slides it the slot. You true your top to the blade and then true the fence.
I use a track saw and cut off at least one factory edge of a sheet of plywood to get it straight. Most of the time I will make my cuts 1/8th to 1/4 over if there is room.
It's a lot easier than picking full sheets up and running them through a table saw.
Someday CNC. Someday.