I've been using a jointer to flatten some reclaimed 6x6 posts that are 8 feet long, but they end up slightly bowed after jointing. This doesn't happen with 4/4 to 8/4 boards, which come out perfectly flat. I'm not applying much pressure since these are thick boards. Could the weight of the heavy posts be causing the outfeed table to sag a bit? It's tough to measure while feeding the boards through. Would using roller workhorses make a difference?
3 Answers
You might want to check if your jointer’s tables are coplanar. A sagging table can really mess things up, especially with longer boards. Clean your jointer and then run a straight edge across the tables while shining a light from behind it to spot any sag. That might reveal the root of your problem.
I can’t remember the exact numbers, but there’s a formula about how long a board can be relative to the jointer's table length. It might be worth looking into that to see if you're pushing the limits with those long posts.
It sounds like the weight of those heavy boards might be causing a little flex in your outfeed table, just like you suspected. If the table is giving even a bit, that can definitely lead to bowing. Consider bolstering your infeed and outfeed tables for extra stability; a sturdy setup could fix the issue! It’s a strange problem, but you wouldn’t run into this with a handplane since it's more controlled.
But isn't it unlikely that a heavy board could flex a cast iron table? If it's sagging, it'll need a fix or replacement!