Hi there, have they definitely bowed, as I’ve experienced cast iron distorting slightly through temperature change?
I have heard of cast iron tables and tops bowing in a few instances, and coincidentally they were Felder machines too.
Some say it is temperature change that causes it, or uneneven floors too but, I’ve read experts say, that the way the cast iron was processed is the true reason. Apparently, if not processed and cooled in the right manner, problems can arise, many years down the line?
If it does need correction, there are some methods including a similar way to flattening a hand planer sole. This would probably be with a block, and various grit abrasives. Then some wet or dry paper, and a final polish and waxing.
This would be quite painstaking, and would mean a lot of checking, a machine (sander) would work too but, you risk removing too much material.
Personally, I would take the tables to an engineer or metal worker, and get them to grind and polish them professionally.
