After publicly complaining about the warped top on my Taiwanese table saw, I was dismayed to discover that the table on my beloved MiniMax bandsaw had sagged over .020" perpendicular to the blade cut-out. SCMI USA would not replace the table, and local heat-treating companies wanted as much as $200 to try to stress-relieve it ("with no guarantees, and you'll lose your smooth ground finish").
After talking some more with SCMI's technical service department I learned that their more expensive saws have table-flattening adjustments that bend the cast iron tables, so I felt confident enough to try clamping and shimming the table flat. About an hour after I began sawing and filing pieces of scrap steel I had the flatness to within .005".
Apparently most of the warping occurs within the first year after cooling, so some manufacturers age their castings for a year before machining. However, it is expensive to store castings for a year rather than sell them right away, so many manufacturers skip this step. Even when they do age the castings, machining away stressed metal can cause further warping.
Better results can be obtained by clamping the casting to a rigid jig to hold its shape, heating the casting to a temperature below melting but high enough for the metal to yield (and, thus, to relieve internal stresses), and then cooling the part in a controlled manner to avoid introducing new internal stresses. This stress relieving takes as much as ten hours or more to perform for cast iron. Sometimes a part is stress-relieved, rough-machined, stress-relieved again, and only then is final machining performed. Such heat treatment is expensive to perform correctly and is rarely done for woodworking machinery.
Another form of warping can be caused by tightening the mounting screws. If either the table mounting bosses or the support bosses are not machined flat, the table will be bent as the screws are tightened, as shown below.
The flattening technique makes use of the fact that, despite its being a rather brittle metal (see caveats), cast iron is flexible over a small range of bending, as is cast aluminum over a larger range.
Consider the set of cubic splines shown in the figure below.
Even though machine tables with reinforcing ribs, varying thicknesses, and cutouts are much more complex than cubic splines, we can still apply these principles and make corrections away from a point where we can apply bending forces. This is important because the places where we want the table to be its flattest are usually near a blade or cutter where there is no room to shim the table directly.
Consider now the specific case of my bandsaw table.
To measure the sag, place a straight-edge on the table in several different orientations and insert feeler gauges between the straight-edge and the table. As there can be humps as well as sags, check for the straight-edge rocking over high spots as well as bridging gaps.
If the warping is not simple, you may want to mark the high and low spots with a permanent felt pen. The marks can be removed later using alcohol.
In this case the point of maximum deflection is shown by the red arrow. Although we would prefer to shim the table up at this point, this would block access to the adjustments for the lower blade guides and, in addition, the trunnion casting was quite thin here and might have cracked from the deflecting force.
Fortunately the ledge next to the right Allen screw was clear of the blade-guide adjustments and was thick enough for the load. In general, the thickest part of the table and support should be chosen for the shimming point. If no suitable shimming points are available, a beam may need to be attached under the table. If this is done, be sure it clears all mechanisms under the table and try to make it about as deep as the table ribs so that it will be stiff enough.
Loosen the attachment screws and insert the over-sized shim or shims, then tighten the screws just enough to hold the shims in place. Place a straight-edge over the region to be corrected and slowly tighten the screw or screws, watching the gap between the straight-edge and table change. Adjust the screw tensions until the overall error is minimized where it is most important, usually closest to the blade or cutter. The following figure shows the shim in red and the straight-edge in blue.
In some cases a tapered washer around a mounting screw must be used instead of a shim, as shown in the figure below.
To determine the tapered shape, place a shim between the table and support and touching one side of the mounting screw (here the shim would be placed touching the left side of the right mounting screw. Tighten the screw until the table shape is best, then measure the gap opposite the shim and subtract the gap from the shim thickness to arrive at the maximum thickness of the washer. Taper the washer from this maximum thickness down to zero thickness. Note that a washer can be made by drilling a hole in a shim.
I was able to take out a .020" sag over about 7", and I feel confident that I could have taken out at least twice that much without damaging my table. However, my castings were very smooth and, apparently, of high quality (even though they were improperly stress-relieved). Lower-quality castings, particularly where casting defects are present, are more susceptible to cracking. If in doubt, live with a little warpage.
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Flattening Warped Machine Tables using Shims/ firstname.lastname@example.org / revised 1996 September 30