Flattening Warped Machine Tables using Shims

© John A. Swensen 1996. All rights reserved.

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".

Why Castings Warp

When a cast part cools from the molten state, thinner areas tend to cool faster than thicker areas and the differential cooling and shrinking sets up unbalanced, residual stresses in the solid metal. When part of the metal is machined away the stresses can become even more unbalanced and the part will warp from the stress.

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.



If the table is flat when the screws are loose, but is warped when tightened down, the mounting bosses are probably responsible. Sometimes filing the bosses square is the best solution, but shimming is non-destructive and reversible, and is usually preferred.

Principles

This flattening technique is successful in part because it is applied to woodworking machines rather than to more precise machinery. Flatness to within .005" or .010" is perfectly acceptable for woodworking purposes, so long as the deviations from flatness are not so abrupt as to catch on the wood.

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.


Figure showing splines with control points in different positions
Each spline is supported at its ends and is deflected by a control point some distance from one end. The position of each control point has been adjusted so that the maximum deflection is the same for all splines. Two important things to notice are that:
  1. the point of maximum deflection can be quite far from the control point, and
  2. the closer the control point is to an end, the less it has to move to get the same maximum deflection.

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.

Assessing the Warp

My bandsaw table had a sag perpendicular to the path of the wood, as shown in the figure below.

The table is supported by and bolted to trunnions using Allen screws through the table. Between the two mounting screws is a semicircular cut-out that allows access to adjustments to the lower blade guides. A reinforcing rib in the table runs directly over each trunnion.

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.

Determining Where to Shim

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.

Shimming and Clamping to Reduce the Warp

For cast iron tables, cut a piece of steel a little thicker than the gap between the support and the table, for each shim to be applied. For aluminum tables and aluminum mounts, use aluminum for the shims so that the relative thicknesses will remain constant over varying temperatures.

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.



Note that there is a gap between the table boss and the trunnion on the right. This gap should be measured and a corresponding amount of material removed from the shim (by filing or grinding) so that the table can be removed and replaced by fully tightening the screws, rather than requiring use of the straight-edge for adjustment each time. Fine tuning each shim may take several iterations, as subtle changes in shim thickness can have a dramatic effect on the correction. Some shims may need to be slightly tapered for best results.

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.

Caveats

The reader is cautioned that cast iron in grades used for machinery tables is brittle, and limited corrections are possible without cracking the table.

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/ jaswensen@comcast.net / revised 1996 September 30