Jointer Problem: Results in boards concave up along their lengths

To make a long story short, what started out as a knife change on my G0490 yesterday resulted in a reset of the infeed and outfeed tables to be coplanar as well. I've had this jointer for five years now and I initially setup the tables to be coplanar but some minor adjustments were required yesterday. Not a very exciting afternoon as I spent about five hours total to get back up and running.

I started milling up some rough sawn material that was in bad shape to begin with, i.e. twisted and bowed, so I first thought that the odd results were from the material. Out of curiosity, I switched to some fairly flat and straight rough sawn poplar about 5' in length and the results were the same. The problem is that the boards are concave up along their lengths, meaning the boards are thicker at the mid-span than at the ends so it rocks on the midpoint. After several passes, it's very visible sighting down the face of each board that a hump exists at the middle. I've run hundreds of board feet through this jointer so I'm very aware of stock orientation, consistent pressure across the length of the board on the outfeed side, etc. so I'm puzzled as to the problem.

I did speak with Grizzly's customer service but their only suggestion was about consistent pressure so no new revelations there.

What's causing this?

Thanks in advance!

That actually sounds like convex and usually means one or both table ends are pointing up slightly rather than parallel. Dave

Not understanding .If they are concave ,don't see how they could be rocking. Are you talking about facing or jointing edges?

Dave,
I was afraid this would require yet another several hours of adjustment to the infeed and outfeed tables and then probably resetting the knives too. Is there a more accurate way to verifying coplanar tables aside from a straight edge? Obviously, any error will be magnified with longer stock but I thought I had it dead on.

Mel,
I'm referring to face jointing and by concave up, I mean the concavity of the board faces the ceiling. Also, the ends of each board are thinner than at the midpoint.

Is there a good alternative at either HD or Lowe's for a straight edge, e.g. 80/20 aluminum angle, etc.?

Most 4' levels are pretty straight and stiff enough not to sag over their length due to their own weight.

I've found that aluminum levels are pretty good. I put mine against an 8' Pinske edge and now never use the Pinske. 80/20 isn't as straight. Dave

It's possible out feed table is just too low . I would not redo what you have already spent time to fix, without knowing for sure what is wrong . I have never owned a machinists level,but I think I would trust one more than something like a substitute straight edge. Accurate straight edges can be made in home shop,but a very time consuming process.

So much for a happy new year as the frustration continues and I'm not sure now what's heads versus tails. I ended up resetting everything again and the results are no better than my first attempt. I figured a picture is worth a thousand words so I snapped a few to hopefully key in on the problem or problems. Now, I'm thinking part of the problem may still be in the setup and part of it with the knives. The reason I now think the knives may be part of the issue is based on picture 231 and the face that the board rocks side to side slightly on the face, not just along the length.

I ended up marking the entire length of a scrap board to check the knives. In picture 231, the full width of the face of the board is cut up until about halfway, then you can see that only half is cut for the last two or so feet.

Picture 247 is an exaggerated version of what I was trying to convey originally. You can see that at the mid point of the board, it touches the surface; however, the ends in each direction get increasingly thinner towards the ends.

The other photos are trying to show the coplanar tables before making the adjustment for the cut on the infeed side. There are some shadow lines from the level, which are deceiving, that make it appear like there are gaps along the the length of the level.

Man, this is frustrating and I'm not sure what to do next. The installed knives may not be sharpened perfectly square across the ~8" length but it's ever so slight, if I had to get picky. What do you think is the problem with these symptoms? Any other ideas to correct these problems?

Thanks again,
Brett

A concave surface curves inward. It’s easy to remember because a concave indentation in a wall makes a cave. A convex surface curves outward. Like many pairs of antonyms that are relatively rare and similar in sound, these two adjectives are easy to confuse.

The correct term is Convex in relation to your problem. You do not reference from the ceiling in this case. Seeing as the jointer was fine for 5 years until you changed the knives your problem is simple. Your outfeed table is a few thousandths too high. This will cause the problem you are having. You cannot check coplanar without a feeler gauge along with a straight edge. Since everything was fine until you changed the knives your tables are coplanar. If you drop the outfeed table down too far you will get snipe at the end of the board. Drop the outfeed table down a few thousandths will correct the problem. Too far down will cause the board to stop against the outfeed table. A jointer is very simple and at the same time very complex in the understanding of it's set up.

I have been adjusting machines for 30 years in commercial shops so I know what I am talking about!

I think this youtube video might help,just be patient and watch the whole thing:

http://www.youtube.com/watch?v=fvbfdzkPPSg

If you're a member of FWW's online content, this video shows some tips for jointer adjustment and addresses your issue.

http://www.finewoodworking.com/how-t...a-jointer.aspx

If you're not able to view it, the summary is your outfeed table is likely slightly too low.

If the knives are not perfectly straight, have a nick, or not parallel with tables,that would cause trouble even if everything else was perfect. So you do have to be picky. A small nick means that the out feed table is now "too high"......even though it has not moved. On table heights ,2 thousandths is a lot. If you were in an old movie ,your best friend would hit you in
the jaw and you would say "thanks,I needed that" . Putting in more time without a good straightedge and perfect knives installed perfectly is not going to help...unless you are doing this on someone else's time. I would rest and regroup. You might want to look up making a straight edge

Drop the outfeed table down a few thousandths will correct the problem. Too far down will cause the board to stop against the outfeed table. ??????

Rick, i am confused by this statement. It seems that for the board to stop against it, the outfeed table would have to be too high, not too low. I have very little experience, so i am just trying to understand, not trying to criticize. Patrick

Our lazy way to set up the outfeed table without straightedges and gauges.

Take two pieces of wood the same length, and plane them both, keeping consistent pressure. Hold the jointed edges together against the light, adjust until no gaps can be seen.

It is a fiddly little job sometimes, and if you can feel or see the table moving it is too much.

What I often do to demonstrate the difference between too high and too low is to deliberately move the outfeed table way out and show the effect each setting have.

Personally I like to start of with the outfeed table too low and move it upwards against gravity until the snipe is gone.

http://www.grizzly.com/videos/

Take a look at the video titled "How to align the tables on an 8" Par. type Jointer".

You can then re-set your knives after that if the problem still persists.