Adjusting a Jointer

Here are tips on truing a jointer to prevent snipe at the beginning and end of the cut. October 26, 2011

Question
I have a Griggio 12" jointer that is taking more off the start of the cut and the back 2-3". I tried adjusting the tables some, but can't seem to get it perfect. Does anyone have any suggestions on how to get the tables in perfect alignment?

Forum Responses
(Solid Wood Machining Forum)
From contributor L:
I do this for a living going to schools and commercial shops. You didn't give much info, but typically I use a 6' machined straight edge and a set of feeler gauges and a dial indicator to indicate off the head in relation to the edge of the tables. That machine, like most European jointers, is a parallelogram design so it is adjustable. One should never try adjusting a jointer without a machined straight edge or one will make things worse!

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From the original questioner:
Where can you get a 6' straight edge? I have a 3'. Can you tell me how you go about doing this? If you raise the infeed up all the way then should it be level end per end and side to side?

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From contributor K:
Start with sharp knives set in an even cutting circle. I shoot for +/-.0015". There are numerous ways to do this. I use a 2' straightedge set on the outfeed table overhanging the cutterhead, and adjust the knives so that when I rotate the cutterhead by hand each knife picks up the straightedge and moves it about 1/16". A very minor change in height produces an obvious change in knife travel. Once the knives are set, adjust the outfeed table even with or slightly below (~.0015") the knife cutting circle. Check by jointing two pieces of material the same thickness and at least as long as your outfeed table and mating the two jointed edges. Typically, setting the outfeed table too low will produce concave edges and snipe at the trailing ends, too high gives a convex edge. Adjust the outfeed table height accordingly in .0005" increments. A dial indicator is useful here. The results are very sensitive to small adjustments. You may need to drop the outfeed table slightly as the knives wear.

If you can't achieve good results at this point, you need to look at deeper issues. As mentioned you need an accurate straightedge, ideally as long as your jointer or as close as you can get. Used with feeler gauges, you should be able to get both tables level with the cutterhead and parallel to each other within a few thousands of an inch. Start by checking each table for flatness and twist- check along the length from side to side and across the diagonals. If they are more than .002"-.003" out, especially the outfeed table, and especially if it is concave over its length, you may need to get them flatter by machining or hand scraping. If you have to go that route, find a machine shop you can trust or resign yourself to hours of meticulous elbow greasing. If and when the tables are flat, adjust the table supports at each corner to first get the inboard table lips parallel to the cutterhead, then get the out board table ends aligned end to end. Check the diagonals across the entire length as well. Plan on spending at least half a day on alignment if things are out of whack and you are unfamiliar with the process. Take your time and get it as perfect as your patience allows. Jointers are simple machines, but they require careful initial setup to achieve accurate results.

Check machinist supply houses for long straightedges. Starrett is a reliable name, though expensive. I had my 6' Starrett checked, ground and certified to .0005" a couple years ago at a cost of $300 or so.

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From contributor L:
Most likely your problem initially was in your knife height. A common problem - but you probably made it worse by throwing your tables off. A 3' straight edge isn't long enough. I'd start with a 6' level if you have one.