jointer alingment issue

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Forum topic by rexm posted 03-02-2018 06:24 PM 982 views 0 times favorited 10 replies Add to Favorites Watch
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2 posts in 858 days

03-02-2018 06:24 PM

Hello all,
I bought a used Delta DJ20 about 5 years ago. The machine is in excellent condition but has always left a slightly CONVEX cut along the length of the cut. In other words edged joint a board, lay a 3 foot straight edge on said board and I can easily slide a .0015” feeler gauge between them for the first 8 inches or so at each end. Not severe but when edge gluing two boards that is unacceptable, the discrepancy doubles at this point. Thankfully I can overcome that particular issue with a hand plane to produce a properly sprung joint.
I believe I am versed in jointer alignment. The tables are each acceptably flat and coplanar to each other. Knives are sharp and properly set for projection from cutter head and height against outfeed table.
In theory a CONVEX cut would likely be caused by the tables being CONCAVE to each other, in a “V” pattern so to speak. Right?
I have intentionally set the out feed table to droop at the out feed end, still leaves a CONVEX edge.
With the tables coplanar I have experimented with the height of out feed table, lowering it until the machine snipes the trailing end of the cut, still leaves a CONVEX edge
Has anyone actually dealt with this problem? Not just in theory but actually had this issue?
What am I missing here.


10 replies so far

View Loren's profile


10477 posts in 4420 days

#1 posted 03-02-2018 06:48 PM

If you run a concave board over the jointer
does it still create a convex edge?

When jointing convex edges I usually make
a partial pass or too in the middle of the board
to remove the bump. There’s a knack to it
I suppose.

View bbasiaga's profile


1243 posts in 2767 days

#2 posted 03-02-2018 06:55 PM

Can you blacked the edge of the board and take a couple of partial passes and see what is happening?

Loren is correct, in that if the board starts out with a belly it can’t be taken out with the jointer unless you take a few passes out of the middle first to create a tiny hollow.

To leave a board that started straight with a protruding belly in the middle, something would have to be lifting the board up slightly in the middle of the pass., and then letting it back down again at the end. Sounds like maybe a bit spot on the infeed table somewhere?


-- Part of engineering is to know when to put your calculator down and pick up your tools.

View rexm's profile


2 posts in 858 days

#3 posted 03-02-2018 07:14 PM

Loren,When I take a pass out of the middle I have essentialy created a concave edge. The next pass or two taken in an attempt to make a straight edge result in a convex edge.
If I start with a perfectly straight edge one pass instantly creates a convex.
I am an accomplished woodworker with over 40 years expeireance, Ive owned and tuned jointers all along.
This one refuses to follow known knowledge, it is simply posessed!

View Loren's profile


10477 posts in 4420 days

#4 posted 03-02-2018 07:31 PM

That’s a vexing situation I agree.

I use a 78” level to check machines and long
edges. I bent one once and it threw me off
for a bit before I noticed.

I’ve had difficulty with jointers on long edges
due to insufficient support but not being able
to joint a 3 foot edge easily is really strange.

I saw a video or article somewhere where the
guy had set the knives higher on one side than
the other and by moving the fence could get
different amounts of concavity in the edge.

View Lazyman's profile (online now)


5408 posts in 2159 days

#5 posted 03-02-2018 07:39 PM

Maybe I am thinking wrong, but I think you adjusted the outfeed the wrong direction? I think having the outfeed too low would make it more likely to be convex. Seems to me like you should have adjusted the outfeed table so that the blade wasn’t touching when you hold a straight edge on the outfeed table and then lowered it a smidge so that if you hold a straight edge on the outfeed and turn the blade by hand, the blade will just barely move the straight edge as it rubs against it. If you think about it, snipe is an extreme form of convex.

Also, a convex edge can also be caused by technique. Once there is enough of the board on the outfeed table you need to only maintain the downward pressure on the outfeed side of the blade as close to the blade as is safely possible.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Loren's profile


10477 posts in 4420 days

#6 posted 03-02-2018 08:16 PM

View BoardButcherer's profile


144 posts in 866 days

#7 posted 03-02-2018 09:19 PM

Same jointer, same problem:

- Loren

Priceless addition to any shop.

Not because of the spirit levels, but because a well made construction level is hard to bend or mar and will be a great way to check all of your working surfaces for planar characteristics.

View pintodeluxe's profile


6153 posts in 3585 days

#8 posted 03-02-2018 09:33 PM

I have the same jointer, and compared to a little 6” model you need quite a good straightedge to set this jointer correctly. A 48” level from the big box stores might be out of flat .005” or more. You need to set the jointer beds so they are coplanar within .002”. It needs to be checked straight along the fence, straight along the outside edge, and diagonally both ways. Test your level by flipping it upside down… do you get the same readings with a feeler gauge?

If you are within .002” each way, and assuming the beds themselves are flat within .002, you should be able to tweak it with the cam screws.

Did you really mean to say that the defect in the board is only .0015”? One and a half thou in woodworking is called “perfect.” If you meant 15 thou, then that’s a problem.

-- Willie, Washington "If You Choose Not To Decide, You Still Have Made a Choice" - Rush

View Lazyman's profile (online now)


5408 posts in 2159 days

#9 posted 03-02-2018 10:22 PM

Here's a link that confirms what I suggested (outfeed lower than knives) could be the problem:

Whenever you change or sharpen the jointer knives, check their height relative to the outfeed table. At the top of their rotation, they should be almost dead even with the table surface, maybe just 0.001 to 0.003 inch above it. If they’re too high or too low, you can’t make a straight cut. Too high, and the jointer will make a slightly concave cut; too low and the cut will be convex.

Edit: I forgot to mention in either of my posts that I have used this technique for making convex clamping cauls (lowering the outfeed very slightly). I think that I read this in a manual for a 60 year old jointer that I no longer have.

-- Nathan, TX -- Hire the lazy man. He may not do as much work but that's because he will find a better way.

View Knockonit's profile


675 posts in 974 days

#10 posted 03-03-2018 02:42 AM

holy smokes dang good info here, have a pal that has this issue, we have drank ourselves silly after spending a lotta time trying to finger out issue, i’m a gonna go over on sunday and see if we can finger this out with the new found info.
thanks to all, another avenue to finger the solution to a problems. way to go LJS

-- Living the dream

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