rookie mistake with jointer

[charlese]

Yes, I have jointed end grain on a table top. This was before I made a sled. I'll probably never joint end grain again. Not because it doesn't work, but because sawing, if you can, is easier. One thing about jointing end grain - you need to have a little chamfer on the trailing edge. The same as you would have when hand planing. Of course you could use a push block flush on the in-feed table, but that's too much for me to handle!

I think the error rdewinter made was in having part of the end grain edge sticking out further than the part already jointed. The protruding piece caught in the blades at a severe depth. End grain has to be jointed in very small increments.

[Nick]

Okay, folks, here's why you set you jointer knives .002" to .003" above the outfeed table: Springback.

At a microscopic level "cutting" is actually a stress-failure process, the same as breaking a twig. The edge of the cutter contacts the wood fibers and compresses them. Thus stressed, the fibers eventually fail (or break) and part. The reason that a "cut" surface is so much smoother than the surface of a broken twig is that the keen edge of the cutter causes the fibers to fail along a line defined by the motion of the cutter. On a jointer, this would be an arc the same diameter as the cutterhead.

The word I want you the sieze upon in this explanation is "compressed." The edge of the cutter compresses the fibers to the point of failure. As the cutter passes, the compressed wood decompresses and "springs back." How much is this springback? Would you believe .002" to .003" on the average? So, in order for the jointer to make a cut perfectly parallel to the surface of the outfee table, the knives have to be .002" to .003" above the surface at the highest point of the arc.

The springback when jointing end grain is somewhat less than when jointing long grain -- quite possibly less than .001". However, there is no sense in resetting your knives to joint a surface that is probably less than 12" wide. The slight deviation from "perfectly parallel" caused by the knives being too high won't be visible on such a short cut. But I do caution the novice to check and make sure the knives are properly set for ordinary jointing. If the knives are extremely high or set at different levels, the possibility of a kickback goes up dangerously.

It's the nitty-gritty stuff like this we teach at the Academy, you know. It's not just recipe-style instruction where we walk you through a project. You leave with a much better understanding of the material that you work with and are better able to solve you own woodworking problems as you face them.

With all good wishes,

[Ed in Tampa]

Okay, folks, here's why you set you jointer knives .002" to .003" above the outfeed table: Springback.

At a microscopic level "cutting" is actually a stress-failure process, the same as breaking a twig. The edge of the cutter contacts the wood fibers and compresses them. Thus stressed, the fibers eventually fail (or break) and part. The reason that a "cut" surface is so much smoother than the surface of a broken twig is that the keen edge of the cutter causes the fibers to fail along a line defined by the motion of the cutter. On a jointer, this would be an arc the same diameter as the cutterhead.

The word I want you the sieze upon in this explanation is "compressed." The edge of the cutter compresses the fibers to the point of failure. As the cutter passes, the compressed wood decompresses and "springs back." How much is this springback? Would you believe .002" to .003" on the average? So, in order for the jointer to make a cut perfectly parallel to the surface of the outfee table, the knives have to be .002" to .003" above the surface at the highest point of the arc.

The springback when jointing end grain is somewhat less than when jointing long grain -- quite possibly less than .001". However, there is no sense in resetting your knives to joint a surface that is probably less than 12" wide. The slight deviation from "perfectly parallel" caused by the knives being too high won't be visible on such a short cut. But I do caution the novice to check and make sure the knives are properly set for ordinary jointing. If the knives are extremely high or set at different levels, the possibility of a kickback goes up dangerously.

It's the nitty-gritty stuff like this we teach at the Academy, you know. It's not just recipe-style instruction where we walk you through a project. You leave with a much better understanding of the material that you work with and are better able to solve you own woodworking problems as you face them.

With all good wishes,

Thanks Nick! That excellent explanation answered my question. Springback! The .002 merely compensates for the springback that is going to take place in the normal cutting process.

Like I said I was trained on a jointer with adjustable infeed and outfeed we always set the outfeed parallel to the blades and made a cut we then tweaked the adjustment as needed. I never realized what we were doing was adjusting for "springback" or compensating for the compression that occurs when making the cut.

It all makes perfect sense to me now, thank you!!!!!
Ed