So, I bought a pair of magnetic jigs from Klingspor for about $30;" Mini Planer Pal".
....
But today I ran a test board through the planer, with poor results. A three inch wide board was .005 thicker on the outside edge versus the inside. That happened both on the right and left edges of the bed. When I tried an 8" board it varied .010 edge to edge. It also had significant chip out on about 5" of the left side. I turned it end for end to reverse the grain direction and got the same results.
I believe the magnetic jig you have is the cheaper one I was referring to but could not find on Woodcraft's site (though I know my store carries it).
I've generally not found the springs to be especially compatible with magnetic jigs. You may find that you get
better results once you remove the springs from the equation and allow the magnets to simply suspend the knife.
Neodynium magnets are not as solid as their chrome finish makes them appear, they are simply composed of compressed powder (and if you have some older neo magnets around the shop, you might even find you have some that are returning to their powdered state). Depending on how powerful your springs are (my Jointer springs, for example, are quite powerful) you may actually be cutting into the magnet a bit.
I also find it helpful to focus on tightening down the bolts at either end (or near the end) of the knife first, while the jig, also located an inch or so from each end, holds things securely, then tighten down the midle of the knife. After that you can remove the jig and tighten down the remaining bolts. By focusing on the two outside edges (nearest the jigs) you limit the opportunity for the blade to shift while tightening the bolts. NOTE: Because the magnet tops are flat (where they contact the knife edge) it is ideal that the knife hit the center of the magnet. Hitting the magnet towards the outer edges of the magnet will introduce a small amount of additional error.
None of this, however, really explains your severe chipping. A 0.01" error over 8", even if it was all focused on a single knife, should only result in scalloping, not severe tearout and chipping.
I don't mean any insult, but are you certain your new knives are sharp AND installed in the correct orientation? I ask because a reversed knife could exhibit such behavior. Then again, there are some boards that are just highly prone to tearout -- try running some Poplar (or other well-behaved species) through the planer and see what happens.
Ultimately, your goal will be to try and get the error to no more than 0.005" over the full width of your planer. 5 mils over 12-13" is a fair goal and the error minute enough that any difference will be quickly leveled with just a bit of sanding of the assembled project.
PS - This all, of course, assumes that your new jig was manufactured perfectly AND that the cutterhead assembly is perfectly parallel to the bed. If you have a good set of calipers (the sort that measures in mils) then it may be worth measuring to ensure that the magnets were all fully seated when their glue (usually epoxy) hardened.
If you have the means to do so, measuring the knife projection above the cutterhead is a good double-check. With this sort of jig, the knife height above the cutterhead should be constant over the width of the cutterhead and knife.
It is also quite possible that your error has nothing to do with knife setup or your new jig, but is the result of the cutterhead being out of alignment/parallel with the bed. In many portable thickness planers it is possible to adjust the cutterhead to bed alignment by loosening the threaded shafts that raise and lower the cutterhead and slightly rotating one (if there are two such bolts) or two (if there are four such shafts, one at each corner) relative to the other(s) to slightly raise or lower the cutterhead on that side. If you have a four-shaft model, loosen only the left or right side bolts (not front/back of diagonal). Don't make any changes to unthreaded shafts.
ONLY ATTEMPT THIS ADJUSTMENT if you have ruled out error within the jig (try swapping the jig left-for-right and see if you get the same error, or if the error follows the placement of the jig).
The other option, if your model allows it, is to shim the low side of the cutterhead's bearing blocks enough to neutralize the error (e.g. if the error is 0.01" over the full 12-13" of the knife, then that is roughly the amount you will want to raise the low side by). If you have to shim the actual bearing (as opposed the the block the bearing is mounted in) then you should use two shims at about the 4- and 8-o'clock positions of the bearing to prevent the bearing from rocking within its mount (which it may do if you use only a single shim).
HTH