How flat should a cabinet saw top be?

[Ben Grefe]

I thought I recall that SawStop's spec is 0.01 inch.

That being said, I'd be more concerned about whether it's causing issues with performance as opposed to a set number. And that is perhaps more closely related, as Jim said, to where it's "not flat".

I distinctly remember this spec when I setup my SawStop many years ago too, but I looked through the current manuals online and it doesn't seem to say anything. I had to shim my wings with hvac tape in the joint to get them dead flat.

[Myk Rian]

How flat? All depends on how OCD you are.

[jim gossage]

Just checked my Saw Stop. The main table is generally within 0.002" but near the rear of the table it dips a bit to 0.006" relative to the front. I had an old dewalt table saw that cut pretty good except when I was making some tissue boxes, about 6" square on the sides. They were completely off. Pertinent to what Jim said, it turned out that the table dipped at the throat by about 0.015-0.02" as I recall, and this error really affected the smaller pieces because they weren't held up by the flatter parts of the table. I just built a sled that I clamped the pieces to and all was fine.

[roger wiegand]

I'm curious about what Bad Thing happens when a table saw top isn't perfectly flat? Having used saws with distinctly non-flat tops, like the one I described earlier, saws with stamped steel tops that were no where near flat (indeed, incorporated ribbing patterns with ~1/8" depressions to stiffen them), and even a homemade one with a plywood top that probably sagged a half inch at the edges, all of which could produce acceptable results, I'm not sure why it's important to have a top that's dead flat to within a few thousandths over several feet.

For sure, I often use mine as a reference surface when I'm trying to get something really flat and wouldn't intentionally buy a saw now that didn't have a pretty flat top, just on general principles, but I'm unconvinced how big a deal it is compared, say, to having a flat jointer top.

[Maurice Mcmurry]

Someone posted a neat old document a while back that listed acceptable industry standards for various machines. It seemed very sensible to me. My saw is fairly flat on the left side of the blade and within 16 inches to the right. When I acquired the saw it had a very distinct ridge where the kerf line exited. I worked on flattening the 3 main casting's with wet or dry sandpaper. My homemade wing on the right has not been flattened. I do not think it needs to be. I appreciate having a good flat saw top most when making batches of small parts that need to fit well.

[Jim Becker]

I'm curious about what Bad Thing happens when a table saw top isn't perfectly flat?

I think that it's a variable risk based on the size of the workpiece and the type of cut being made. Large things are likely not going to be affected all that much. Small things, as someone already alluded to, could be affected more relative to quality of cut including the cut not being perpendicular to the face of the material...it might be a small amount, but could be magnified for some kinds of joinery and noticeable.

[johnny means]

In 20 some years as a professional in the field, I've never run into a situation where table flatness was an issue. Table flatness seems like the polished chrome exhaust of the woodworking hobby.

[Bill Dufour]

Far more likely the blade insert is not flat to the table. A little sawdust under the leveling screws will raise it high enough a board will not advance past the front of the insert. When I cross cut I apply downforce over the table not the insert or I use a sled to average it out.
Bill D

[Keegan Shields]

Yeah but that polished chrome gives you an extra 5hp easy.

[Maurice Mcmurry]

After 15 years the out feed area of my saw is back to looking like it did when I got. Performance does not seem to be affected in any way. I should not have checked, now I am going to be thinking about lapping the top again.

IMG_1068.jpg

[jim gossage]

I agree that it does not matter for most woodworking. I've only noticed the problem once, when I was ripping a 45 miter on 6" pieces. The table dipped about 0.015-0.02" to the right side of the blade in the vicinity of the throat. As the piece got to the blade it tilted into the dip which threw off the miter angle. Multiply that by 4 pieces and its hard to close up your box, especially if you like tight miters. I could fairly be accused of having a bit of a machinist mentality when it comes to woodworking. Dewalt actually came out and replaced the top.

[Todd Worstell]

Thanks for all the thoughts on this issue. I just got a new table saw and I put a straight edge on it and checked various areas, sides, angles, etc. Some were pretty flat but a couple I could see light under the edge and rock the straight edge slightly. If I could find my stupid feeler gauges, I could post some numbers. Might have to go out and get some new ones.Anyway, I am only checking the main table. No wings installed and throat plate remove. Just seeing light, it is awfully hard to estimate the gap. Rocking the straight edge a little too. The light I saw was on the right side of the blade, past the miter slot, and dropping down slightly. So, at least its away from the throat. When I get my miter gauges I will have more info.Now, as far as how much is too much, I realize there are some factors like where, how much, size wood, using a sled, etc. But I just got a Rigid jobsite saw for my dad and trying to get the blade at 90 was impossible. My accurate square hit the top of the blade first from both the left and right side of the blade. There was a noticeable gap at the bottom on both sides. Returned it and got a Bosch. Blade hits 90 from both sides right on the money. It does have some small gaps under straight edge, but further away from throat. I suspect it is always preferable to have the flattest area around the blade and my new table saw appears to be that way, but even so, a big enough gap even far away from the blade could cause issues. Will be posting numbers/areas when I get them.

[Steve Rozmiarek]

In 20 some years as a professional in the field, I've never run into a situation where table flatness was an issue. Table flatness seems like the polished chrome exhaust of the woodworking hobby.

Yes!!! It's similar to lapping the soles of handplanes before you ever even see if they work as is.

[Cameron Wood]

Yes!!! It's similar to lapping the soles of handplanes before you ever even see if they work as is.

IKR- even sharpening plane irons is a waste of time.

[Steve Demuth]

I'm curious about what Bad Thing happens when a table saw top isn't perfectly flat? Having used saws with distinctly non-flat tops, like the one I described earlier, saws with stamped steel tops that were no where near flat (indeed, incorporated ribbing patterns with ~1/8" depressions to stiffen them), and even a homemade one with a plywood top that probably sagged a half inch at the edges, all of which could produce acceptable results, I'm not sure why it's important to have a top that's dead flat to within a few thousandths over several feet.

For sure, I often use mine as a reference surface when I'm trying to get something really flat and wouldn't intentionally buy a saw now that didn't have a pretty flat top, just on general principles, but I'm unconvinced how big a deal it is compared, say, to having a flat jointer top.

I would agree, particularly with respect to low spot that aren't overly large. If you're cutting wood with characteristic dimensions in the plane of the table that are many inches, it's not going to "see" dips in the table that are a couple of inches across. If you're cutting wood that's only a couple of inches in each dimension on a table saw (unless it's clamped to a sled or other floating surface), the flatness of the table is the least of your problems.

To some extent I feel the same way about jointers. A serious bow or warp that spanned the length or breadth of a jointer top is obviously not acceptable, but a .01" low spot that's an inch or or two in breadth and 10" away from the cutters? I don't see how that is in any way going to affect operation, accuracy or safety of a jointer. In fact, on my 6" jointer, the fence is easily 1/16" out of flat, and is bowed at least that much from end to end. But the fence is everywhere very square to the tables measured over a 3" height from the tables. It squares to within a thousandth or two on edges up to 3" wide on any stock with a flat face notwithstanding.