Can’t Cut a Flat Joint

No matter what I do, I keep cutting curves on the jointer and not the perfectly flat surface I expected. I’ve checked that the tables are perfectly level with a borrowed machinist’s straightedge. I’ve carefully aligned the knives to the outfeed table. I’ve tried every variation of applying pressure (and not). I’ve even ripped a piece flat on the table saw and then managed to put a concave curve on with the first pass through the jointer! I’ve noticed that halfway through the cut, the leading edge is not on the outfeed table or the trailing edge is lifting off the infeed table! Help!

Ellis Wallentine: A couple of things in your description don’t add up for me, namely the “concave” edge you say you’re producing and the tendency of the leading or trailing edge to leave the table. Regardless, if your tables are properly aligned with each other — dead flat and straight from end to end — the problem is either in the height adjustment of your knives or in your technique.

To check whether the problem is in the height of your knives, try raising or lowering your outfeed table very slightly and playing with this adjustment until it produces straight edges. If you’re getting concave edges, the knives are too high, and vice versa for convex edges.

Technique may also be the culprit here. It’s important to sight down the edge of your board before you start jointing it, so you can see where the high and low spots are. You need to reduce those first before you can make a clean, end-to-end pass for that final straight edge. That means that if you have a convex spot in the middle of your edge, begin your first cut near the center of the board and take ever-lengthening passes until you’ve flattened the hump. If your edge is concave, you may need to take a couple of partial passes at each end of the board before you can go for the long, straight pass.

Michael Dresdner: What you have described comes from the two tables being non-coplanar. That means that although the tables may be flat, they are not in the same plane as one another. Specifically, to make the cut you describe, the two far ends of the infeed and outfeed tables are lower than the ends nearest to the cutterhead. In other words, they are like the two top sides of a very obtuse triangle.

To test for a coplanar surface, raise the infeed table to the same height as the outfeed table, turn the cutterhead so no blade is at the top (the head will then be below the two tables), and lay a straightedge that goes all the way from the far end of one table to the far end of the other. Yep, that’s a darned long straightedge. You can also test this with a laser level, if you or one of your guild members has one. The bottom line is that somehow you must shim or adjust these two tables so that they are on the same plane.

Another way to do this is to stretch a piece of monofilament (fishing line) from end to end, very tight. Add a .010″ shim under each end, and use a feeler gauge to measure the height of the monofilament near the cutterhead. I’ll warrant that it will be much closer — maybe even touching the tables — at the center, and that is the cause of your problems. By the way, the cards in any standard deck of playing cards are exactly .010″ thick — a cheap way to buy 52 very accurate shims.

Lee Grindinger: I would really like to see you to get the list of your troubles in person. It seems impossible! My guess is that it’s a technique problem judging from the variety. Slow your feed rate and concentrate on keeping the stock in firm contact with the outfeed table. The outfeed table has to be the same height as the knives. If the wood does not clear the knives and ride exactly onto the outfeed table, you need to make the adjustment. The depth of cut is determined by the amount the infeed table is set below the knives and outfeed table.

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