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Discussion Starter · #1 ·
Hello,

Another fairly novice question that’s driving me insane. I bought my first jointer, a 6” JET JJ 6CSDX long bed. I can’t tell if my issue is technique or adjustments needed to the tables but when I face plane or joint the edges I get a slight concave shape in the board but only in the middle. Closer to the ends, the board is flush to the table. Any advice as to what I can adjust either with the tables or my technique it would be much appreciated. The photos are of the whole board, the concave middle, and the flush ends.


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Do you know how to adjust your outfeed table?

And has the amount of bow been consistent no matter where the fence is positioned?
 

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I believe you either need to lower your outfeed, or align your knives

It's also possible to push down so hard you induce or prevent the jointer from removing a bow, but that's a large piece for that to be happening
 

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Discussion Starter · #4 ·
Do you know how to adjust your outfeed table?

And has the amount of bow been consistent no matter where the fence is positioned?
I do based on the manuals instructions but I haven’t done it yet as I just got it and was hoping it was still setup correctly from the factory. I did a test with scrap to ensure the outfeed aligned with the blades and it looks good so I’m hesitant to lower the outfeed table. The bow has happened with the fence in different positions.
 

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Discussion Starter · #5 ·
I believe you either need to lower your outfeed, or align your knives

It's also possible to push down so hard you induce or prevent the jointer from removing a bow, but that's a large piece for that to be happening
When you say align knives do you mean with the outfeed table?
 

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Discussion Starter · #9 ·
I made adjustments and same issue but I think my problem might be the table in the photo. It’s an oak plywood top that I installed. I’m thinking there has to be a dip in the table. When I lay the board along the outfeed table it’s flat. How do you check your work from a jointer and is there any tolerance for gaps? I presume most use straight edges? I put it on my Kreg router table and that showed a bow as well so now I’m wondering what outside of cast iron is flat enough to check against.
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Say you have jointed the edges of 2 boards about the same length, lay them down with the edges together, as if you are gluing them. Are there gaps? Yes one could be concave and the other convex in an exactly matching way, but that'd be freaky lol.
 

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Even if the factory sets it up perfectly it gets jerked around during shipping.
That jointer might have come halfway around the world, and freight shipping isn't like flying first class.
Jointers cut concave if the ends of the tables have settled down so the faces of the 2 tables are not parallel. Your jointer looks like it has long tables cantilevered off a relatively short base. This alignment is less robust than a machine with a longer base. I wouldn't be surprised if the gibs needed to be tighten to level up the tables.
 

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Discussion Starter · #12 ·
That jointer might have come halfway around the world, and freight shipping isn't like flying first class.
Jointers cut concave if the ends of the tables have settled down so the faces of the 2 tables are not parallel. Your jointer looks like it has long tables cantilevered off a relatively short base. This alignment is less robust than a machine with a longer base. I wouldn't be surprised if the gibs needed to be tighten to level up the tables.
I could be wrong but don’t the gibs help hold the tables in place? Tightening them needs to happen together with the adjustment to ensure the tables are level, right? I don’t understand why just tightening them would fix it and I don’t know what “level up means”?
 

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Disclaimer: I don't own a jointer.

@JohnGi is saying that even if the tables left the factory perfectly adjusted that now it is likely the far ends are low. The tables do not need to be level to the earth. They need to be parallel to each other, the out feed slightly higher and the same height as the blades. So if you imagine extending the surface of the out table over the in table they'd be 1/32" (for example) apart for the length and width of the in feed table.

As I said I don't have experience adjusting a jointer, but as an outside observer it seems like getting a jointer perfectly adjusted is one of the more difficult of wood working machine set ups. So be patient with your Jet and yourself. Experiment, adjust, repeat.
 

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Concave cuts are symptomatic of sagging beds. This is pretty common with most jointers with dovetail ways because they are limited in the amount of adjutment. This applies to both infeed and outfeed beds, but because the infeed bed is frequently moved, you need to make the corrections at the outfeed bed to match the infeed bed.

If your gibs are loose, then you can tighten the upper gib screws to bring some of the sag out. However, if both your upper and lower gibs are already tight, then you are stuck with shimming the bottom of the dovetail way and locking the gibs down tightly.

I can pretty much guarantee that what I say next will cause an argument with some woodworkers, but you also need to stop transferring your downward pressure to the outfeed table, contrary to what so many people will tell you. The rationale for transferring your downward force to the outfeed side as soon as possible is based on the false assumption that your beds are perfect. They are rarely ever perfect.

Instead, you want to keep your downward pressure on the infeed side for as long as humanly possible. Yes, the far outfeed end will canteliver over a sagging bed, but that is what caused the concave cut in the first place.

If your beds were perfect, then it wouldn't matter where your downforce was applied, but for sagging beds (the most common of all jointer problems) your results will improve with infeed downforce.
 

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i agree with an earlier response that the tables are not parallel. there are adjustments for getting a concave cut corrected. check your manual or online for help on that.

and yes, i respectfully disagree about not transferring pressure to the outfeed side. if you consider that the board that has passed over the knives has been flattened and trued (at least to some degree), and now has become the reference surface, that maintaining that reference throughout the remainder of the cut is what you want. continuing to apply pressure on the infeed table, which that portion of the board face could be total garbage with respect to being flat or true - makes no sense to me. just my opinion, got a hunert of 'em.
 

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Say you have jointed the edges of 2 boards about the same length, lay them down with the edges together, as if you are gluing them. Are there gaps? Yes one could be concave and the other convex in an exactly matching
i agree with an earlier response that the tables are not parallel. there are adjustments for getting a concave cut corrected. check your manual or online for help on that.

and yes, i respectfully disagree about not transferring pressure to the outfeed side. if you consider that the board that has passed over the knives has been flattened and trued (at least to some degree), and now has become the reference surface, that maintaining that reference throughout the remainder of the cut is what you want. continuing to apply pressure on the infeed table, which that portion of the board face could be total garbage with respect to being flat or true - makes no sense to me. just my opinion, got a hunert of 'em.
I think I said that...I mostly agree with you

If you don't get all the bow out in one pass, then it matters more.

There's also a difference between pressure and pushing for all you're worth. I don't recommend pursuing down hard, just firmly
 

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The first thing I would check is the outfeed - to - knife height. The outfeed should be level with top dead center if cutter. Actually I like to do a thou or 2 higher.

One really easy way to check is lay a metal ruler on edge, flat on the outfeed and spanning the knife, then turn the cutter head it should drag the ruler 1/16 to 1/8”.

If you have a helical head its get a little trickier. Not sure about that, since mine was already set, when I converted to helical it was just a little trial and error. You can check by edge joints a known straight edge and stop about 1/2 way , turn machine off and check contact on outfeed.

That said, technique is very important for example, with the concave face down, if you press too much on the middle of the board, you’re making your jointer act like a planer. You’ll produce a thinner board, but ever get rid of the bow.

My method for dealing with a bow board is to keep pressure on the leading end as it contacts outfeed, and use a push block (more accurately termed a “pull block”. The key is NO pressure on the middle of the board, then more pressure as the other end passes over. This way you’re selectively removing more material from the ends, IOW you’re removing the bow.

Hope this helps!
 

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I transfer it to the outfeed table as soon as possible unless the board is that bad.

We bought a new Oliver at the furniture shop. Foreman used a guage to check it. Guy came to me and said it wasn't right. Took me three minutes to fix it.

Guages...

Love or hate em. I dont use them..
 

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The outfeed should be level with top dead center if cutter. Actually I like to do a thou or 2 higher.

One really easy way to check is lay a metal ruler on edge, flat on the outfeed and spanning the knife, then turn the cutter head it should drag the ruler 1/16 to 1/8”.
Do you mean you like the cutter a thousandth or 2 higher than the out feed? Why?
 

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Do you mean you like the cutter a thousandth or 2 higher than the out feed? Why?
Seems to work better. Maybe just my impression, but after setting outfeed level with TDC knives, when I bumped it just a skosh higher I got better results.
 
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