Tuning Up an Out-of-Whack Jointer Bed

      Pros ponder what could be causing a jointer to create a "belly" in every freshly jointed board. (In this case, turns out it was a misalignment of the infeed and outfeed tables.) April 27, 2007

What causes a jointer to create a "belly" on the jointed edge? I've lowered the outfeed table just above the snipe level, and it seems a little better, but the board is still widest in the center of its length, so that two boards edge butted together will rock.

Forum Responses
(WOODnetWORK Forum)
From contributor F:
Assuming your jointer is set up right, you may be starting with a bellied board to start with. Keep the pressure on the infeed side of the cutter head. Are you getting your stock straight-line ripped?

From the original questioner:
Keep in mind that I'm not talking about much belly here - talking 1/64-1/32" at the very most. And it doesn't seem to matter if it's been ripped or jointed a dozen times - it still seems to belly. I'm not ripping it on a straight line rip saw, I'm just ripping it on a powermatic TS. I'm a little confused as to how you can start with a belly and still end up with a belly. Isn't a jointer supposed to fix all that? I'm trying to visualize the path through the jointer, but not quite coming up with the process.

From contributor F:
My only guess is the outfeed table is too low, in relation to the knives. Are you using a 4" jointer?

From contributor D:
The jointer won't fix anything. You have to use the jointer to fix it, and you have to get rid of the belly first, then straighten. Try starting the cut about 3 or 4" back for the first pass, then next pass run the whole edge. If it's straight now, then it was the board and you. If not, may be the jointer.

From Professor Gene Wengert:
Some pieces of lumber have internal stress (growth stress from the tree or drying stress from drying). As you joint one face, this unbalances the stress and the piece immediately warps slightly. So, if you have stress, it will continue to be warped, pass after pass. You can test for stress by ripping a small piece from the lumber - if the ripping is warped, then you have internal stress in the lumber. It is impossible to remove such stress in your shop. It should have been done in the dry kiln.

From contributor N:
The tables might be out of parallel.

From contributor T:
Your outfeed table is too low. A jointer is used to fix edges that aren't square. That is what it is designed for. It will fix this problem if it is set up properly.

From the original questioner:
I certainly understand that some wood does indeed have internal stress. But not every single piece I pick up has internal stress. Any piece I run through this jointer comes out with a very slight belly. This leads me to believe that it's not the wood, but rather the jointer.

As for the jointer not being able to fix a belly, that's clearly not true. If I have to true up the piece before running it through a jointer, why in the world would anyone have a jointer?

I have lowered the outfeed table until the piece of wood has snipe, then raised it until the highest blade on the head (it has 3 blades) just barely touches the straight edge. The other three do not. Do you think that the outfeed is still too low? And will the infeed and outfeed being out of parallel cause this problem? I may have to borrow a long straight edge to check for this.

From contributor T:
I'd jack it up to check it anyway. I personally don't mind a bit of snipe if the board is square otherwise. I'll burn the waste ends that I added in as extra to get through this anyway, and sometimes they'll come out clean after planing so it gives me some longer pieces of scrap. I still think your outfeed is too low.

From contributor J:
First thing to do is check your tables with a good straightedge. My guess is you'll find they are not perfectly aligned. If this happens on more than a couple boards, it is not the wood. Also, you only put pressure on the infeed table at the beginning of the pass. Once the board is established on the outfeed side, your pressure should be concentrated there. Think of an inverted hand plane. Go get that straightedge and check. The tables should be parallel to within a couple thousands.

From contributor D:
The jointer is no magic machine that fixes all your crooked boards with no effort involved. Little short pieces maybe, but if you have 6 feet of jointer table and a crooked 10 ft board with a hump 5 ft back (in the middle), when that high spot comes in contact with the table, it's gonna pick the front of the board up, unless maybe you're cutting off more than the belly.

Same for a bow. If the arc (for lack of better word) of that bow fits on your table length, you'll never get a straight board by simply running it through the machine full length every time and nothing else. Sight down a board before during and after. Straightening wood isn't as easy as it sounds - it took me a long time to really learn how to straighten a board, and I still have a lot to learn as I’ve only been doing this for 10 years professionally.

From Professor Gene Wengert:
I would sure like to have you rip an 8" wide piece into two 4" pieces and see if they fit back together perfectly. Unless you know there is no stress, you will be unable to fix the jointer.

From the original questioner:
Gene, a ripped board does appear to be straighter than the jointed board. As I stated before, not every single board that I get (for the past three months) will have internal stress. I'm just not going to believe it. Eventually I'll run into one that doesn't. I might be exaggerating a bit, but I still don't think that's what is going on here. If it was, I think the amount of belly would vary a bit.

Contributor D, I ran into a guy yesterday who does just what you recommend when he's got a board that has a belly. He put it this way. "You have to first make it a concave surface by placing the center into the head first." In other words, you take a bit out of the center so that then ends will touch instead of the middle. Otherwise you'll either wind up with a belly still, or a severely tapered board by the time you get the edge straight. So it looks to me like I still need to do some checking/adjusting to see what's going on. I'll try raising the head a little more first, then go across the street to borrow that straight edge.

From contributor G:
Sounds like to me your infeed and outfeed tables are not machined properly. The only other thing I can think of is that the infeed is running uphill and the outfeed is running downhill. Although they are flat and offset, they may not be level with one another. Raise the infeed up to zero and put a 6 foot level across them. You may have to remove the knives for this or rotate them out of the way. I think you will find one is going uphill and the other downhill or vice versa. I would return it and get a new one. Although the tables are offset, they must be perpendicular to one another or you will get the exact problem you are having. Without precision aligned tables, a jointer is worthless. I have a straight line rip saw, but I still use my jointer a lot.

Dr. Wengert is right about growth stress in lumber. I have had it bust right down the middle when trying to line it on the saw. Try to line it again and it will bust again. No cure for this, not even in a kiln when drying. Use it to heat your shop. I don't think, from what you say, that this is your problem. Sounds like the tables are misaligned. You may just have a lemon. Never heard of anyone having this problem with a jointer. Let us know what you find out.

From contributor K:
a) the tables are not parallel
b) operator error

If the board is a couple inches wide, one pass at the joiner isn't going to wreak havoc on internal stress.

From contributor V:
Stop thinking so much - sell the jointer, spend a few bucks on a new, better quality machine, and then post photos of your work in the Project Gallery.

From contributor G:
Check the cutterhead out. I just got to really thinking about this problem. Check out the bearings. See if the parts holding the bearings are tight, if they are bolted in. The more I think about it, I can't see how the infeed and outfeed tables could cause a belly unless they were moving while feeding a board. How much belly are we talking about? Is it concave or convex with respect to the side of the board being jointed?

From contributor S:
This post has really gotten my interest. I have a Grizzly 6" jointer I rarely use and was planning on selling. I decided to give it the once over and run a board through it and low and behold, a belly. With all these posts in mind, I started checking it out and found that the infeed table was running uphill. I loosened the set screws that adjust the drag on the dovetail slider and placed a thin piece of steel in the bottom of the slider, rechecked with the straight edge and the tables are now parallel and the board is dead flat after two passes through the jointer. I have sold it and made sure to inform the buyer (hobbyist) of what I did. I gave him a heck of a deal and he was happy I was honest with him.

From the original questioner:
This is a Delta DJ-20 (good heavy cast iron) fairly good shape jointer used in a cabinet shop. The belly is convex on the jointed side. The belly is at 1/32" maximum on pretty much any board that is run through it. Interesting that you should ask about the bearings. I've noticed a noise when this thing is spooling down. I still need to check the tables for being parallel.

From contributor O:
I'd be willing to be that your tables are off. I ran into this problem with a hardwood floor I was making. After much frustration, I discovered that dust had worked its way down into the adjustment tracks, and threw it off parallel just enough. If the tool gets used this way long enough, you will actually need to shim here. I spent a good day on the project, but now have a 75" long (8" width) table that has less than a thousandth in variation from either end, both on a diagonal, and parallel to the fence. Meticulous setup is key.

From contributor G:
Are you jointing the edge or face of the board? Does width seem to matter? Delta stuff is usually pretty good. You know, you could have a screwed up head. Sounds like the knives are sagging down or something. Are the gibs tight? Check knife alignment. Sometimes jointer knives are very thin and seem to be flimsy. Did you put the knives in or were they in when you got it? Put a straight edge on the top of each knife. I bet they are not properly aligned. Probably sag in the middle. I don't think your problem can be caused by the tables after I thought about it. Got to be head or knives. Probably is the knives.

From contributor J:
Just to clarify, although it may not seem like tables being out of alignment could cause this problem, the reason I suggested checking them is because I had the same problem with my jointer a couple years ago. I also had it happen while using another woodworker’s jointer in the past. If the tables are perfectly parallel and the cutterhead is off, I do not believe that would cause a belly. The boards would be going over the cutterhead correctly but the cut would be rough or perhaps impossible.

Also, it is not only the cheaper machines that will have this problem. With the exception of parallelogram style machines, jointer beds slide by metal to metal contact. Over time metal wears away and can leave the bed out of parallel. Whether it's a $400 or $4000 machine, it will still wear eventually. It's just probably more likely to happen sooner on the less expensive machines.

Check those tables out with a good straightedge first and see if that's the problem. From what was described, I think it will be. Then spend time doing a tuneup and a little shimming and you should be back to making flat stock.

One more tip - if you do need to shim your tables back into alignment, it may be better to shim the outfeed. That side is usually pretty static. If you shim the infeed, which can get adjusted more frequently, those shims may work their way out.

From contributor G:
At first I thought it was bed alignment, but after I thought about it, I don't see how bed alignment could cause a convex belly. Sounds like the knives aren't aligned to the bed. They should be aligned to the bed and not the head.

From contributor J:
Contributor G, I know exactly what you mean. When it happened to me I thought the same thing. I spent a couple hours really fine tuning the knives to the head and then the outfeed table, no change. I then went out and bought a 8'- 1"x4" aluminum tube. I know it's not exactly a precision instrument, but I figured it was dead straight and that's what I wanted my boards to be. Turned out my tables were off by roughly 5-6 thousandths of an inch over 6'. Not much, but it definitely showed up in the boards. I used a couple shims on the outfeed table to get the tables within the recommended 2-3 thousands of an inch and boards came out straight as you could want.

I think your previous post was dead on - if your infeed table is low, you will get a concave board. However, if it is just a little high, you will get a convex and tapered board. As the outfeed table starts to lift the board, that's where your convex surface comes from. And by the time the end of the board goes through, it's barely hitting the knives, so you have a slight taper. Basically, if your beds are off by more than a couple thousands either direction, you’re not getting a straight board.

I'm not sure if this is the questioner’s problem, but it is where I would start first. It's one of the easiest things to check once you know what to look for, and it's a fairly simple fix as long as the tables are flat. I would compare it to aligning your t.s. top to the blade - just something you need to know how to do. Yeah, it would be nice if every machine came perfectly aligned, but that's just not the real world. Warped tables are a reason to return a jointer. Tables a little out of alignment - well, that's just a little tuneup.

From contributor G:
The only jointer we have in our shop is an 8" Grizzly. I really never checked it for accuracy because the board goes to the edge sander after we joint it, or else to the rip saw, and sometimes to the tablesaw. This convex belly really has me stumped. I can't see how this problem could be caused by anything other than knives. The knives in a jointer are usually set with a mic to the infeed bed. If the infeed bed had a dip in the center of a few thousands, then the end result would be a convex belly. I really can't see how bed alignment can cause this problem as long as the beds are true and flat. Whether one is up or down would only affect the depth of cut.

From contributor J:
I think we may be talking about a different type of belly. Your last post makes me think what you are describing is a belly across the width of the board. At least that's how it reads when you describe a dip in the center of the infeed table. What the questioner is describing is a belly in the length of the board. So when you edge joint the board you get a sort of rocking effect along its length.

Picture an outfeed table which is 1/32nd of an inch higher at its tip than at the cutterhead. As the board passes over the cutterhead and starts along the outfeed, it is gradually being raised off the cutterhead. If you are only cutting 1/64", by the time the board reaches the end, you are barely skimming that part of the board. And since the board was gradually changing pitch along the way, the result is a belly.

Also, did you mean outfeed table when you described setting the knives? I always set my knives to the outfeed table. Hopefully this clarifies the situation. With luck the questioner has got his machine back in alignment by now and is back to work.

From contributor G:
I think you are right about aligning knives to the outfeed. Sometimes I get mixed up. I also think you are right about table alignment. Jointing long boards, when one table is up, and one is down, would cause a rocking action that would create a belly. I guess I was thinking about a wide board that was flat having a belly in the middle.

From the original questioner:
I haven't got to this yet. I will try to get that straight edge real soon and check the table alignment out. It shouldn't be too difficult. When I do get to it, I'll post what I find. Thank you for all your input.

From the original questioner:
I finally was able to work on this problem last week. I borrowed a four foot straight edge from the machine shop across the street and checked it out. The beds were not coplanar. The infeed and outfeed beds were tilted up away from the cutterhead so that the cutterhead was the lowest point in a board’s travel. When a board travels through this, the leading and trailing edge will hit the most, and in the worst case scenario, the center of the edge will not hit at all, leaving a belly on the edge when looking down the length of the board.

I finally had to call Delta and get their procedure for lining up the beds (not in the original manuals) and fussed with it for a couple of hours before getting in lined back up. There are 8 eccentric bushings to adjust and 8 measurements to get right before you can call it done. So yeah... it was a pain in the butt. Thanks for your input - maybe this will help someone in the future.

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