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  • table flatness

    Can someone explain to me why it matters if a table saw table is not completely flat? Why would a dip here and there of a few thousands of an inch matter? Wouldn't a piece of wood span such a small gap anyway? If it is a big deal, how come the miter slots and the holes in the extensions are not a source of concern? Please help me understand this concept.

  • #2
    flatness makes a difference

    Because, the blade is set at 90 degrees, in order to make a cut that is square the table must be flat.(laws of geometry) look at an equal lateral triangle. The miter slot is cut so that it is perdedicular to the table surface so it will not effect the angle of the cut. small differences proably are not for major concern,but the flatter the better in my opinion.


    • #3

      Everything about a table saw matters when it comes to cutting quality. If any part is not well machined and flat, straight and properly algned, there is no way you'll get quality cuts. With the table having the miter guide slots, blade arbor fastened to it and the fence fastened to it, even slight warping can make a big difference. I personally would hate to have a table saw with a cast pot metal table or a molded plastic one. Have you ever tried sliding the miter guide in one (cheapo model) only to have it flop all over the place? We wonder why a good cabinet style table saw costs so much. After we study how it's made and see good machining, we can appreciate all the work that goes into making it. Next when such a saw has been carefully setup and a good blade installed, we see very nice cuts. (If the operator is careful and know what he/she is doing) Try to get such from a junky made saw. Now that does take work and skil from the operator, if it can be done at all. I should add that sometimes what looks like a well machined cast iron table can warp, or be incorrectly machined. It pays to check them carefully.
      Last edited by Woussko; 01-11-2007, 04:21 AM.


      • #4
        checking table flatness

        I appreciate the well worded answers to my question about flatness. To check for flatness, will a good quality framing square suffice or should I see about obtaining something else, like an expensive straightedge?


        • #5
          David, you are correct in one respect, a couple of small dips here and there of a couple thousandths are not a problem. I believe others have contacted some of the various manufacturers of cast iron topped saws (including Ridgid) and the standard allowable deviation is 0.016" (1/64th"). If you polish out a scratch you are probably down 2 or 3 thousandths (which is one reason I just polish off any burrs/rough edges and don't try to polish them out completely). That said, 1/64th over a broad area can result in an off square cut on a smaller piece of wood, and 1/64th difference between the extension and the main table, especially at the front or back edge, can result in gouges in an expensive piece of wood (The hardwood veneered plywood at lowes and HD probably have less than 1/64th" veneer on the surface.) A wide dishout around the insert area can make it very difficult to get an accurate blade height measurment and can really mess up bevel cuts. Its not so much the depth of the defect, it is the cross-sectional size and location that can make it unacceptable to the woodworker.
          As for a straight edge to check, a good framing square will work. I use my 4' level (aluminum box beam type) as it seems to be the truest edge I have of any length. The bottom line is that the saw delivers the quality of cut you need and can expect from that grade of saw.

          Practicing at practical wood working


          • #6
            GoFer has the right idea.

            When doing work with wood, humidity and temperature will cause slight changes in the wood dimensions. Over time wood tends to warp and twist some too. There is no use in trying to get things down to 0.001 inches. If you did buy high precission straight edges, squares, feelers and the like, you may well end up spending more for them than what your table saw costs. If you have a 3 or 4 foot well made aluminum or steel rule, that should work for checking if there are any serious humps or dips that do need you to work on them.

            Are you working on a true cabinet style table saw, a contractor saw, a job site saw or a benchtop model? It's one thing to spend time and money on a $2000 machine, but for a $500 or less one I can't see getting into high precission tools and many hours of your time. Then if you just want to get the best you can from your table saw, maybe it's worth the efforts to you.

            Once done, a good waxing and buffing job on the table really makes a big difference. I'm sure there are some special waxes you can buy, or the old school way was to use good paste floor wax. Please do not use auto wax on the table. It does work on the painted parts to make them look pretty, but with the machine indoors there's no need for it. If you search the forum, I'm sure you find posts about what's a good wax to use on your table. You do need to give it a good buffing. This can be done with a power buffer (using care), or just use yourself and enjoy the workout.

            For what this may be worth back some time ago when Powermatic made the model 66 cabinet saw here in the USA they would ruff machine the table and then let it age a year or two. Then they would check for any warping and do the final machining. They went for no more than plus/minus 0.004 inches flatness over the main table surface. They also took time to be sure the miter gauge slots where true and square. That all took time and skilled workers. It was also why skilled woodworkers were willing to pay good money for one of them. There were/are many other good cabinet style table saws and while maybe not quite as well machined (I would not be surprised if some were/are even better), they are more than good enough for 99-1/2% of their users.

            Many people took pride in owning and using such a machine, but it really was overkill for most.

            Please for now don't worry about small imperfections, just try to get rid of anything major.
            Last edited by Woussko; 01-11-2007, 04:15 AM.


            • #7
              To help with setup and doing work, a good combinationi square comes in very handy. You can check 45 and 90 degree angles (basic) and with some of the better ones, you can add additional heads including a precission protractor. Here is some info on them. I recommend that anyone needing to check cuts or using machinery look into getting one and if you have the $$$ think of investing in a good one with a ground surface cast iron head(s) and well made steel rule. You can buy them as a kit with what heads and ruler you want. There are many brands ranging from cheap but maybe good enough all the way up to some very well made precission models. What you go for depends on your needs and budget. When setting up machinery, you'll most likely want a pretty good one.

              Stanley has some basic ones and Starrett has several that are very well made precission measuring instruments. There are many others too. Please see pictures below.
              Attached Files
              Last edited by Woussko; 01-11-2007, 04:39 AM.


              • #8
                Thank you to all the kind people who take their own time to respond to my questions. The saw in question is the Rigid TS3650 that I bought after reading so many good reviews. It replaces an older Sears model that I had been using and before I got it, my uncle built a lot of cabinets with it. I bought the Rigid because I liked the dust port idea and the herculift appealed to me too. I'm still working on final assembly and my schedule allows some time for that this weekend.


                • #9
                  Astronomically flat

                  Good luck with your new saw. Bear in mind that you will never get your table "perfectly" flat. There will always be a degree of dip, bulge, or twist. If you want to maximize flatness, you'll need to manufacture a massive new support system for the table made of a low thermal expansion material, such as carbon fiber, and support that massive support structure with an even more massive foundation system that is properly anchored, such as 36" reinforced concrete piers anchored to bedrock.

                  Then you'll need to buy two more cast iron reference surfaces of the same size as your table. Apply a micro-grit abrasive slurry and polish those surfaces against one another, alternating regularly and testing frequently. This "one against the other" method is how optical flats are made for telescope mirrors, but it works for cast iron just as well.

                  All in all, you can expect to spend somewhere in the range of 2-3 million for all this work and equipment, just like those big research observatories do. Or, you could throw a straight edge across your table and get it as close as you can.

                  Seriously, though, you can pick up a good straight edge for $30.00 or so. Any dip, bulge, or twist that is noticeable with a straight edge will cause problems with your cuts. Those problems can include burning, binding, kickback, and inaccurate cuts. You'll just have to decide what degree of accuracy and precision you require, and invest the effort accordingly.

                  For what it's worth, I used 3 different aluminum levels as a base for a dial indicator that I use for the telescope mirrors I grind, and averaged the differences to adjust my table. I estimate that it is within .002" flat and true PTV (peak to valley, the highest point to the lowest point), and less than .001" RMS (root mean square) across the entire surface, which is close enough for the work I do. Still, with the cold front that's moving through the area today, that might all change. I'll recheck flatness in the spring, again when it starts getting really hot, again in the fall, etc. It takes me about an hour to check for flatness and make adjustments.
                  Last edited by greenandgrowing; 01-11-2007, 01:28 PM. Reason: Corrected procedure for checking table for flatness.


                  • #10
                    How to make corrections for flatness?

                    For what it's worth, I used 3 different aluminum levels as a base for a dial indicator that I use for the telescope mirrors I grind, and averaged the differences to adjust my table. I estimate that it is within .002" flat and true PTV (peak to valley, the highest point to the lowest point), and less than .001" RMS (root mean square) across the entire surface, which is close enough for the work I do. Still, with the cold front that's moving through the area today, that might all change. I'll recheck flatness in the spring, again when it starts getting really hot, again in the fall, etc. It takes me about an hour to check for flatness and make adjustments.
                    If I may ask, what sort of adjustments can you make? Now that the cold weather is coming, it's going to be fairly cold in my garage - not quite as cold as outside, but still around the freezing mark and colder on the really cold days. After seeing this thread, I went out and laid a long aluminum ruler across the top of my saw, and now I see a VERY noticable gap. I hadn't made this sort of check before, so I don't know if it happened because of temeratures or what.

                    The gap that I'm seeing is really disconcerting. I checked the center table, and the center table itself is reasonably flat. Each of the wings seems to be flat too, however, along the join line between the wings and the main table, the wings seem to be angled upwards! The angle is high enough that if I run a 4' level from the left edge of the left wing to the right edge of the right wing, there is 1/16" to 3/32" between the bottom of the level and the surface of the main table. It seems to be the same elevation from front to back, so I wonder if it's not square along the edges of the table...

                    No idea what caused it, and I'm not sure when it happened or if it was there from the start. Any ideas on what I should do to fix it (or perhaps diagnose a cause)? Is it possible that it's the temperature doing this? It has been unseasonably warm around here lately, so I can't imagine what it'd do when it drops 30-40 degrees over the coming months....


                    • #11
                      Testing iron with aluminum

                      The first thing I'd do is turn your aluminum ruler over to see if you still have the gap. I measure with three aluminum levels, and I measure off each side to ensure that they are flat. If you get convex on one side and concave on the other, then it's not really a straight edge, is it?

                      I only mention this because aluminum is not particularly well known for its thermal stability. It can warp fairly easily.

                      As for the adjustments I make, I keep it pretty low tech. I have "beams" (for lack of a better term) that I made from plies of MDF and baltic birch that I clamp across the table and span the extension wings. I clamp them rather loosely at first to the extension wings (just enough to help support the weight), loosen the bolts holding the wings to the table, place clamps on the joint between the extension wing and table, then tighten down the clamps on the "beams".

                      This, in theory, is supposed to bring the top of the extension wing perfectly in line with the top of the table while you re-tighten the bolts. In practice, you have to hold your mouth just right while softly humming "How Great Thou Art" to get them to line up the way you want them. The only way to absolutely positively guarantee that you're going to get them adjusted to optimal flatness is for you to be more stubborn about it than the saw is.