When it’s time to adjust the knives in my jointer I usually use a device called the Magna-Set. The idea behind this gizmo is two parallel magnetic bars that attach to the outfeed table and protrude over the jointer knives. With a knife positioned at the top of its cutting arc, that is, top dead center (TDC), the magnets on the jig hold the knife in place so it can be tightened without slipping down.
The manual for the Magna-Set describes a setup procedure for orienting the jig that involves scribing two vertical lines in the jointer fence as well as one horizontal line across the outfeed table. When matching alignment marks on the jig are aligned with the scribe lines, the forward most alignment mark is directly above the knife at the TDC position.
Unfortunately, when I first set up the device, I scribed the two vertical lines in the wrong position but somehow managed to get the horizontal line in the outfeed table in the right position. Not wanting to further scratch my jointer fence, I decided to try the jig using the single horizontal line and discovered that it actually worked pretty well – the key is making sure the forward alignment mark on the jig is positioned correctly in relation to TDC.
To use the jig, the cutter head is rotated until the edge of the knife is at TDC and lines up with the forward alignment mark on the jig. With the locking screws slightly loose, the powerful magnets on the Magna-Set will grab the knife and hold it in position level with the outfeed table. The locking screws are tightened and the process is repeated for the remaining knives.
Sometimes jointer knives have a tendency to rise up as they are tightened, which could push the magnetic bars of the Magna-Set slightly above the surface of the outfeed table. If I observe this happening, I press down on the body of the jig with my left hand while tightening the locking screws with my right hand. I’ve seen articles where people will actually place heavy weights on top of the jig to hold it in place but this seems like overkill.
I purchased the Magna-Set back in the early 1990s for between $30-$40 (don’t remember the exact amount). Delta currently sells a tool called the Jointer Pal that looks identical to the Magna-Set and retails for $70-$80. Rockler also sells an identical-looking jig that only costs about $30 and gets decent reviews on their site. As I see it, the Rockler jig is clearly the better choice especially given how inherently simple the device is. In fact, if I was granted a do-over, I would actually give serious thought to making a homemade jointer knife adjustment jig – there are several designs floating around on the web.