The two most common problems with planer jointers are sniping and bowing. Both are attributable to less than perfect setup. It's not uncommon for a poorly adjusted machine to exhibit both problems, and often one thorough going over of the jointer will cure both, as well as a few other less aggravating conditions.
Snipe is defined as a cut that becomes slightly deeper at the end of the pass, and is very easy to see, since it occurs rather abruptly, usually over the last couple of inches of the workpiece. Bowing is what happens when the board is cut deeper in the center than it is at the ends, and ironically enough, the need to correct that shape is what often brings the board to the jointer in the first place.
The idea of the jointer is quite simple: Two separately adjustable tables separated by a cylindrical cutter-head. The first, and most important specification for the jointer is that the tables must be linear -- in other words, you must be able to adjust them so that their top surfaces lie in a perfect plane.
The first step is to check to make certain that the two tables are planar. Over time and years of wear and abuse, no matter how incidental, jointer tables often begin to sag at the ends. Check this with a long, reliable, accurate straight edge. You need something more precise than a garden variety yard stick to do this. An accurate level is often a pretty good straight edge, or you can buy a precision ground one from machine shop suppliers. A short, 12" or so rule is fine for adjusting the height of the tables in relation to the cutter-head, but to check and adjust them for sagging, you really need one that is the full length of the machine.
Zero both tables, lay the straight edge across them, and look for gaps at the ends of the tables. If you see any deviation from flat, you need to adjust your tables until they are truly linear. You must also check this adjustment across the full width of the tables to make sure that they will be planar regardless of where you have the fence positioned. Different machines handle this adjustment differently, so check your manuals to see how to do this on your machine. Often it's done by adjusting Gib screws along the sides of the table ways.
I prefer to make this adjustment on the out-feed table first and then move to the in-feed table, and it's normally done by driving the Gib adjusting screws all the way home and then backing off slightly to allow for sliding clearance. If you reach the limits of adjustment and your tables are still not aligned, you may have to shim the out-feed table with brass or paper shim stock. I recommend shimming the out-feed table only, since it's generally not moved except in tiny increments, and the shims will probably not accidentally move out of place once you've installed them. The in-feed table is a different matter, since it's moved often.
It's important to note, however, that some smaller and low-end jointer planners have out-feed tables which are actually part of the base casting, and cannot be adjusted. In this situation, your only recourse is to adjust and /or shim the in-feed table.
Almost all jointers can be adjusted. If you try again and again and just can't seem to get things right, your machine may simply be worn beyond the limits of what human patience and good sense dictates. In this case, consider adjusting the tables to remove as much play from the sliding table ways as you can, zeroing them, and then having a qualified machine shop surface grind the tables flat.
This may sound drastic, but it's the way Delta machines their jointer tables flat before they leave the factory. Assuming that the sliding ways are not severely worn, which would render any adjustment you can make rather pointless, re-grinding the tables will make your jointer pretty much as good as new. I wouldn't rush into this option, though -- it really should be the last resort. You may find, after having the tables re-ground, that your fence is no longer square to the tables at all points, which will probably necessitate having it ground or milled as well. And if your jointer has a fixed out-feed table, you must make certain that the grinding process does not remove so much material that a new set of knives will be too tall to make the following adjustment.
Once you are certain that your tables are flat and planar, you need to adjust the knives and the cutter-head/out-feed table relationship. In order to do accurate work, you must make certain that the cutter-head is in good condition, and that the knives protrude equally, creating an accurate cutting arc. This is key to a really smooth cut. If one knife is set too high, you'll see a washboard effect on the board, since that one knife is doing all the work, and the other knives are just along for the ride. So check the cutter head, and make any necessary adjustments to the height of the knives.
The easiest way to do this is to lay a reliable steel rule on the out-feed table, letting it extend over the cutter-head. Then rotate the cutter-head by hand and check that each knife touches the rule the same way. Adjust the out-feed table so that the highest knife just barely touches the rule and then tighten down the out-feed table. Some people find it easier to use a very straight stick rather than a steel rule, since the knife edges will 'catch' it more easily.
Either way is perfectly acceptable, but if you use a wood stick, be sure to use the same face of the stick as your reference for all adjustments, and make all of your knife adjustments in one session. If any of the other knives vary even slightly, they should be loosened and their exposure adjusted. This check needs to be made at both ends of each knife to assure that they are all in perfect alignment with the out-feed table.
This can be a tedious process, but it is extremely important. If the out-feed table is set above the cutting arc, your jointer will cut tapers. If it's too low, you'll get hollows and snipe. Believe it or not, assuming that your jointer is in good condition, you're practically home free. All you need to do now is to adjust the in-feed table so that it's exactly the same height as the out-feed table, adjust the table stops if your machine has them, and set the depth indicator to zero.
Remember that the key is to have both tables and the arc of the cutter-head in a perfect plane. It's a very simple idea, but often difficult to achieve, especially on older machines that have seen a lot of use and wear. But believe me, the effort will be justified the first time you make a couple of passes on a well-tuned jointer, and see two board edges that mate absolutely perfectly.
written by Paul Koch
solving jointer problems with the correct set-up
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