Maybe I'm just not as sharp as I ought to be, but something I discovered last night surprised me. But I'll pass this on in case others could not figure out why their jointers did not work as well as expected even after precisely setting the knife height with a dial indicator.
I used to use one of those magnetic knife setting jigs, but last time I set my knives I used a dial indicator for greater accuracy. I was only building a porch at the time, so accuracy to furniture standards was not necessary, but I sensed something was amiss. Since the porch project thoroughly dulled my knives, last night I set out to install a newly sharpened set. I had built a magnetic jig to firmly hold the indicator out over the knives.
They key to accuracy, of course, is making measurements from the highest point in the arc of travel of the knife, or "top dead center." I placed the indicator over the first knife and slowly moved the knife back and forth until the needle on the dial would move in one direction, then stop and move back the other direction. This I always interpreted as having found the top of the arc or the highest point in the knife travel - i.e., the knife has reached the top of the arc and is beginning to move back own. So I made the settings at the point where the dial paused between moving one direction and another, assuming I had found the top of the arc. WRONG.
After setting one end of the knife to .002 above the outfeed, I moved to the other end. After setting that, I moved the jig back to the first end to check and it was off by .006 or so. Hmmm, I thought maybe tightening the bolts caused the shifting. I reset it all and got a totally different set of readings. So I sat back and pondered - then it dawned on me. The end of the dial indicator pointer is a little tiny ball bearing. The only way you can measure actual top dead center is if the knife is truly at its highest point AND the knife is contacting the very tip of the ball bearing. This seems nearly impossible to achieve to me. Almost no matter where you place the jig you will see the needle move one way, stop, then move the other way - not because the knife has reached the true top of it's arc, but because it has moved past the tip of the ball bearing and is moving down the side of the ball. It gives the appearance of the top of an arc, but it is not. So what did I do? I moved the jig to the middle of my outfeed and took a steel plate diamond honing "stone" and lifted the pointer and put the steel plate under it and slide it back and forth until I has accurately made the ball bearing into a flat spot. Having the ball bearing flat cancels out the false reading of the top of the arc caused by the round ball. Now the only time I see the needle move one way, pause and move the other way is when the knife is at true dead center.
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In addition to the little ball bearing issue, I also had problems because for some reason, my base had this little joint in the rod. Everytime I moves the indicator, that rod would flex a little and there was nothing I could do to stop it. 
Originally Posted by Kevin Herber
