Can anyone tell me why joint a board (generally over 36 inches) I end up with a concave surface? I have checked and recheced alignments and set ups and find nothing wrong. I have a 42 inch Rockwell. Yes it is small. Is that largely my problem? I have changed my technique every way I can think of or read about.
Thanks.
Ed
Replies
Typically your infeed and outfeeds tables aren't aligned
Ed:
You might want to post this under Power Tools and give the model of the Rockwell jointer you are using. there are several folks who know a lot more about power tools and how to adjust them than I do.
What little I do know is that a concave edge can be caused by the infeed and outfeed tables not being in the same plane. One or the other slopes away from the cutterhead. The only way I can think of to check for this is to put a good long straight edge on the outfeed table with it overhanging the infeed table. Measure the gap between the outfeed table and the straightedge at both ends of the outfeed table. If the gap on the end away from the cutterhead is larger, either your infeed table or outfeed table has a slope to it that must be corrected by available adjustments or shimming. I have also heard of a slightly warped outfeed table causing this.
gdblake
Parallel, not on same plane
Ed, I think what gdblake might have meant to say was that the top surface of both the infeed and outfeed tables shold be perfectly prarallel, not in the same plane because they need to be at different hieghts. The infeed table is set to the depth of your cut. The outfeed should be exactly the same as top dead center of the knife edge. Check your tables for parallel by placing a straight edge on the outfeed and running out over the infeed. The gap over the infeed should be the same on both ends.
Sounds to me like you might have the outfeed table set lower than the TDC of the cutter edge.
Make sure there is no sawdust or chips under your workpiece when you start your cut. When you have passed the front of the piece over the cutterhead it should align perfectly with the out feed table while holding it firmly to the infeed surface. Once you have enough wood over the outfeed table you need to transfer the down pressure to the outfeed. If the workpiece drops down at all when you do this then the outfeed table is probably set too low.
Your jointer should produce a perfectly flat surface for you. That's what it's for. It does get more difficult as your workpiece gets longer and the jointer gets shorter. Eight foot boards on a 42" jointer is a challenge.
That being said, if you are just edge joining two boards together, it might be preferable to have a very slight concavity to the adjoining surfaces.
Good luck, Bret
Bret, thanks for clarifying the remedy, good post
Bret:
Thanks for helping out. Your explanation was much more thorough and clearer than mine. Yes, the tables need to be exactly parallel to each other which is something a lot of people overlook when making adjustments to their jointer. Bones and forrestb touched on a common problem when using a jointer. I've seen lots of guys feed the stock through by putting all downward pressure directly over the cutting head rather than the tables. This causes the board to flex in the middle and will cause an irregular or concave cut as well. At the start of the cut, the downward pressure should be on the infeed table and then transferred to the outfeed table as the stock moves through the jointer.
gdblake
Milling madness
gdblake is right that you might get better advice under power tools. Depending on what your purpose is, a slight concave is good when joining boards. It tends to squeeze the ends more so that when they dry out more quickly than in the middle they will resist showing a gap.
Also you might review your jointing technique. If you only hold the boards in the middle you will get a light cut on the two ends but a deeper cut in the middle - otherwise known as a concave cut. You need to apply constant pressure to whatever part of the board is over the blades.
forrestb
on edge or face
From your description I'm not quite sure but I'll take a stab that you mean an edge to joint, I would questin is the board possible concave to begin with and are you possibly applying downward pressure as you pass the board through. My technique is to pass over the head till it gets to the outfeed table then only apply pressure to the outfeed side a complete the pass. If you press down you are artificially removing the curve and upon exiting the jointer it is simply springing back. . Now if you are describing a concave face I would still aks the same question but it would be less likely.
Have you checked the height of the blades relative to the outfeed table? I like to set the blades so that they are just proud of the outfeed. With the machine off, take a straight board, place it on the outfeed table over the blades, if they just barley graze the board you should be okay. Also I disagree about wanting a slight curve to the pieces you are gluing up. You want them as straight as possible, so that you have even clamping pressure down the whole length. Your boards might not dry out and shrink anyway, it depends on the conditions in which you are working in, and the conditions in which the piece will live. It is possible they will want to expand instead.
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