Cupped Jointer Results

Vaughn McMillan

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Refresh my memory...if a jointer is making cupped boards (both ends touch a flat surface, but the middle doesn't), what's the likely cause?

I noticed this evening that two light passes edge jointing a 21' long board left about 2 sheets of paper's worth of gap in the middle when the edge was placed on a flat reference surface. I was tired and didn't feel like debugging it tonight, but I suspect the tables aren't coplanar any more. (Too much moving it is my guess...even though it's on a mobile base, the carpeted shop floor makes it tough to move. Can't help but pull on the tables some.)

I'm pretty sure my technique is correct, so I figure it's gotta be the machine. I haven't checked out the tables since I first assembled the jointer a bit more than a year ago. I've got a good 4' Veritas straightedge, although I don't have any real feeler gauges. If the tables are coplanar, I think I know the rest of the setup process. If they're not coplanar, then I'll be heading into unfamiliar terriroty. Any jointer table setup tips?
 
Vaughn, I am not sure either so I won't steer you wrong by posting something I am not absolutely sure about. What I do know though, is that a regualr sheet of say computer paper is roughly .004 of an inch thick. You have the right idea, using paper shims you can get a great idea of how close you are. In your case, you are .008 thousanths off.

When I worked for Lie Nielsen, we often used masking tape (.004) to shim our milling machines to get things just right. You would think that for a 200 dollar plane they would come up with a better system, but sometimes its the simple stuff that works the best.

Just thought I would toss this out there so people can use paper as a feeler guage and shims. Just be sure its regular notebook paper, and not thick photo paper or some other exotic paper.
 
I'm going with your outfeed table being low although other things could cause this. That was my problem when I had that symptom; as the leading edge drops to the lower outfeed the rear of the board has a variable pivot point as it trails through the cut. Tweaked the tables and all is now well.
 
I think Glenn is on it. If not it's the easiest to check. Just lay a straight edge on the outfeed, rotate the cutter head and see if the blades just touch it or lift the straight edge up.
 
A low outfeed table will yield snipe at the end of the cut, as the board drops onto the cutterhead when the infeed table no longer supports it. The rest of the board will be straight, however.

That is a different result from tables being out of parallel, which will cause a concave or convex cut along the length of the board, depending upon whether the tables are high or low at the ends. I had this problem, and with the help of the link posted earlier, got it straightened out (pun not intended, but I'll live with it).

Dan
 
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What kind of wood? Maybe try a different board too, as some pieces move enough to fool you (like my recent use of birch, some pieces bowed 1/16" over 36").

If it is the jointer, the end of one table will seem low relative to the other. If all your gib screws are still tight, and the ends are low, then shimming is in order. I've never had to do that myself, so can't help you there -- I've always managed to get it by tightening the screws. The top two gib screws do more adjusting than the bottom two, so I tighten them first, then do the bottoms, then come back and check the tops. If the bed is hard to move then they're too tight.
 
What kind of wood? Maybe try a different board too, as some pieces move enough to fool you (like my recent use of birch, some pieces bowed 1/16" over 36").

Good point, this could be the problem as well. One way to check is to joint an edge and check it for bow. Similar bow will mean problem is with jointer, a flat result will mean wood moved.

Dan
 
What kind of wood? Maybe try a different board too, as some pieces move enough to fool you (like my recent use of birch, some pieces bowed 1/16" over 36").
Edge jointing 6/4 dry hard maple and walnut, and both showed similar amounts of curvature.

.008" over 21' -- fuggetaboutit!
But if I'm butt joining two boards that have the same concave surface on mating faces (on a big cuttting board, in this case), the .016" gap is more than I like to close up with clamps. In this case, that's exactly what I ended up doing, but I'd like to avoid it next time.

Thanks for all the suggestions everyone, and thanks for the link Dan...I'll study the site and go from there.
 
But if I'm butt joining two boards that have the same concave surface on mating faces (on a big cuttting board, in this case), the .016" gap is more than I like to close up with clamps. In this case, that's exactly what I ended up doing, but I'd like to avoid it next time./QUOTE]

Am I reading it right? A 21 foot long cutting board? How did you handle the stock, how long are your jointer beds? If the boards were that long you would have plenty of flex for clamping it out.

If we are talking a 21 inch board, different story.

Here's the basics for jointer set up:

Concavity on a board: Outfeed table a hair too high (if it was more than a hair you would bump into it on the leading edge.)

Snipe at the trailing edge: Outfeed table too low.

Convexity: One or both of the tables are sloping towards the cutter.

Wedges: (or the cut trails off to nothing being taken on the trailing edge) One or both of the tables are sloping away from the cutter.
 
Just to update, I got around to playing with the jointer last weekend, and got it dialed back in. Not sure what fixed it, but in the process of moving the tables around to confirm they were coplanar, then setting it back up for jointing, the problem went away. I moved the outfeed tabe up 1/8" or so above the cutter head, did the same with the infeed table, then checked with two drafting triangles back to back. It was spot on. Then I dropped the outfeed table so it was a barely perceptible whisper below the knives, and moved the infeed table down appropriately. After that, the cuts were as perfectly flat as I can measure. I'm guessing that I'd tweaked the outfeed table in moving it around, and it just needed to be re-set. I was glad no shimming was necessary.

And Dan, it's not a 21 foot cutting board, it's 21 yards. Had to cut a hole in the side of the garage just to rip the boards. I'm goin' Guinness Word Record, baby! :rofl:
 
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