Problem jointing plywood

[jim gossage]

I am having a problem jointing the edge of a 3/4 x 2 x 24" piece of oak plywood. The edge started out pretty flat (within 0.01") but I wanted to make it perfect. I have an 8" jointer w a spiral cutterhead. During the first pass the jointer removed wood from the first 14-16" but not the back. With each successive pass the back edge got further away from the outfeed table, i.e. the edge was getting less flat and developing a bow. I tried sending the back end first but the same thing happened. I tried the opposite edge, but again, it got less flat with each pass. It didn't matter whether I took off 1/64 or 3/64" per pass. I pulled out a piece of 1 x 4 x 36" cherry and sent it over and it was dead flat after the first pass. Sent it over a few more times and it remained flat. Seems like my basic jointer technique is good since I can get the hardwood flat w/o a problem (also had no problems milling a bunch of rough walnut last week). I have had the same problem with MDF in the past. I can't imagine why jointing manmade materials would pose a problem, but it seems like it does - in my hands at least. Anyone have any thoughts?

[Michael Peet]

I'm not sure that's the right tool. Half the thickness of the plywood is endgrain, not to mention all the glue. I would use a tablesaw to true up plywood.

Mike

[Todd Burch]

Either your jointer tables need adjusting, which you seem to have ruled out, or perhaps you are putting a lot of pressure on the ply/MDF, and maybe it is flexing.

[Leo Graywacz]

It is possible that the blades are loosing their edge while jointing the plywood. The glues can cause havoc on steel blades. It they are carbide it probably isn't happening. If you push a pc of solid stock over the jointer, does it work properly?

Myself I'd use the table saw too. But if the pc has a curve on it the TS isn't going to remove it without jigs.

[Jamie Buxton]

The general wisdom is that plywood dulls planer knives. Once when I had a set of knives that were knicked up enough to need sharpening, I tried jointing plywood to see how many feet I could do before the knives dulled. It was astonishingly quick. In less than five feet of jointing I could hear the sound of the knives change as they dulled into useless. Then I got curious about what had dulled. I'd heard that it is the glue that dulls the knives. I pulled out my loupe to examine the knives, and found that it was not the glue. The dull areas were where the end grain laminates were. It was the end grain that was dulling the knives, not the glue.

When I do need to edge-joint plywood, I use a low-angle handplane -- one intended to cut endgrain. It survives jointing plywood perfectly well.

[jim gossage]

Thanks for the thoughts guys. I don't think the problem is dulling of the knives because they are carbide in a spiral cutterhead. The surface of the plywood was perfectly smooth, but bowed. The piece of cherry that I ran through after the plywood came out perfectly flat and smooth. My feed pressure is medium and I was jointing the edge of the plywood, so I doubt that I flexed it much. Still a mystery.