Grinding a Warped Cast-Iron Table Saw Top Flat

It's not unusual for machinery tables to get deformed out of their proper flat shape, but the problem is correctable. March 25, 2007

I have a Delta contractor's saw that I bought back in the mid 80's, which is a pretty heavy duty saw. I had it in storage for about 4 years, and in the last year I put it back to work in my shop, primarily used for dados. I kept having trouble getting the depths of my dadoes consistent, and attributed it to warped panels.

Last night I put a straight edge on the cast iron top, and to my surprise it is crowned up in the middle 1/16"! I tried two other straight edges, and they all read the same. I don't remember this saw having a crown in it like this, and I don't know if it could have happened somehow in storage, but it is a problem.

Is there any way to get this crown out? I imagine that if it was brought to a machine shop, somehow they could resurface it flat, I just don't know that it is worth the cost or the time for this. I was possibly planning to remove the saw from the shop to save space, but I also wouldn't mind keeping it for dados if it were flat.

Forum Responses
(Cabinetmaking Forum)
From contributor S:
I had the exact same problem. Saw was great when new, and then one day about 10 years later I was checking something or other and noticed the hump. I also had a dish towards one side. I phoned the local Delta repair location, and talked to the tech (he was an old hand, very knowledgeable). He said because cast was so malleable, it could be "tuned" - essentially banged flat after supporting on a flat wood surface. This required a pretty experienced hand, though. Didn't get to this however, because they gave me a new top. Pretty amazing considering the saw was 10 years or more old, and well out of warranty.

From contributor H:
Remove the top and take it to a machine shop and have it milled. It's safer than you trying to hammer it out.

From contributor R:

You would want it ground, not milled. I've had many jointers ground. In my area, Blanchard grinding is more common for bigger tables. The table must be supported correctly or the magnet will pull it out of shape and be in worse condition after grinding. Blanchard grinding gives a circular pattern to the finish as opposed to linear from a precision grind.

From contributor O:
Contributor R is the man to listen to here. I have every shaper top Blanchard ground, and every two-piece adjustable fence belt ground. Brand doesn't matter they're all junk when you first get them. They claim that cast iron must season, and that it was correct when manufactured - yeah right! GM doesn't season a 350 engine block, and that's one hell of a magnet. I've watched it in use - they must shim the underside before grinding. And if your top holds up any amount of weight, it needs a little crown. Then when the motor and spindle or trunion is bolted to it, it will pull down. I have a super 27 with 40'' top within .002. Nice to see a woodworker that knows machinery. It takes good woodworking machinery to do good work fast. Get your machinery right and anyone can do good work without a 100k CNC.