I have a Delta/Rockwell 6" jointer I have owned since I bought it new about fifty years ago. Upgraded it with a Byrd Shelix head. I notice it seems to produce a very slightly concave edge. I was jointing pieces about twenty inches long and they were "hollow" about 1/100". Just enough to hold two of them edge to edge and see light through the crack at the center. With a straight edge on the outfeed table the knives just snick the edge.
(If I were jointing two fairly long narrow boards this would not be a problem, the clamps would pull the tiny gap closed and I have seen discussions of doing this on purpose to close the ends tightly but I was trying to join two halves of a circle together and could only use one clamp.)
Where do I start looking for whatever is going on?
The final check I use when setting up a jointer is to lower the out feed table and get a snipe on the end of the board. Then I raise the out feed table until the snipe just exactly goes away. That's where I like it.
Are the in-feed and out-feed tables co-planer? I'd think you could create either a concave or convex curve depending on which way they are out. As little curvature as you indicate you have, they are probably close but maybe not quite perfect.
I recently did the same thing with my JET/Byrd 6".
1 Raise the infeed table to the top, then check both tables for co-planer across the full width. I use an 18" straight edge (+ a feeler gauge if needed)
2 Lower the infeed and use a 12" combo square on edge on the outfeed table to check the outfeed/cutter position across the width. All cutters should just "rub" the rule's edge. This is different from the old "hop/skip" method used for knives.
3. Loosen the outfeed's locking screw and raise or lower the outfeed until the cutters all just rub the rule's edge. This is a good time to rotate one or more cutters for fresh edges.
4. Tighten the outfeed's locking screw.
5. Raise the infeed all the way and check to see if both tables match for height and raise or lower the infeed so they do.
If the workpiece you're jointing has internal tension that may be the reason the edge comes out concave. Removing wood from one edge will make the wood change shape a little to rebalance the internal forces.
A large majority of kiln dried wood has some degree of case hardening, which means that the outer parts of the board are in compression and the interior is in tension. This results from the way wood shrinks and the fibers lock together during drying. If the wood is in storage for a long period the case hardening decreases.
While I was working we generally got wood fresh from the supplier for the project we were about to start. After learning how to deal with case hardening I was always glad it was there because it helped me straighten crooked boards and keep them straight. The few boards without case hardening were the ones that ended up not quite straight.
After jointing the boards I would put them through the planer, which removed a similar amount of wood from the other face. This caused the wood to bow slightly the other way, leaving a straight board. If I had to remove more thickness I would sight down the board to see which side was convex and remove wood from that side, which would help straighten it again.
When setting up a jointer to produce a slightly concave edge (as was done in shops producing cutting boards, for example) the outfeed table was set slightly high. If your outfeed table is exactly even with the knives or slightly low and you're still getting a concave edge it's the wood that is causing it, not the jointer.
This was a piece of oak I had glued up and made a twenty inch square. I rough cut a circle out of it and then tried to pattern cut a 20" circle. I got distracted and the pattern slipped, slightly mangling one side. I cut it in half and was trying to glue in a filler strip. I jointed the the two half circles and the strip, over and over. I can't imagine how the half circles flexed unless it was some sort of internal tension. Even the strip was 20" long and three inches wide. The wood was salvaged from some window valances that had been made many years ago. You just don't get any wood any more acclimated.
I think I will check the jointer following Mark's suggestions and see what I get.
Thanks for taking the time to type that out and explain it to me. I have a good straight edge, 24" long I think and some feeler gauges. Now if someone could tell me where I put the feeler gauges I just happened to buy a month or two ago I am ready to start. They are right over...
You only need the gauge if one of the tables is a little higher than the other. If they're co-planer (and not tilted ;( ) you move on to the next step. You'll get them level after you get the outfeed table and cutters set.
I got out my straight edge and my feeler gauges and checked out my jointer. I was happy to find the tables were co-planer left to right and front to back. What I found was that somehow the outfeed table was very slightly high, above the knives. I lowered the outfeed table and got some snipe, raised it until the snipe barely disappeared and it seems to be good. I jointed several boards and could not see any daylight between the straight edge and the board. The straight edge is two feet long. The boards were slightly longer. So I think I am good. Waiting for the next complicated glue up as the real test.
Tomorrow I will change my planer blades. (Sometimes I work in the shop and sometimes I work on the shop.)
I have thought about upgrading the planer, but it is maybe twenty years old and its the old two knife version of the Dewalt planer. I am a little skeptical of investing that much money into an old machine. Considering a new one, maybe an Oliver.
A concave edge is normal, to a degree, and the default for all handplanes also. However, if the machine works properly, which is to say it is properly set-up, then you should be able to adjust the concavity to be less by raising the outfeed.
You can measure this on an unplugged machine by putting a straight edge of wood on the outfeed table, and slightly over the head, and rotating the head by some safe means. This will move the piece forward or back by an increment, and when you have things properly adjusted to your liking, you can use knowledge of that skip to return a machine to it's correct baseline condition.
You've described the "hop-skip" method that is conventional s.o.p. for jointers' 1-piece knives. Using a Byrd segmented head is a little different since you can't adjust the height on the individual cutters except to rotate them to get "fresh" edges. Having tried both methods, I've found the mere rub to to give a better/straighter cut.