Tuning up a Jointer to Acceptable Tolerances

      There's art as well as science in getting a jointer adjusted. January 8, 2010

I have a 6” Delta jointer that has never provided a truly straight jointed board on any board longer than 24”. My intent for this post is to lay out how my jointer is setup; illustrates the results of a jointed 24” board, and ask for your advice on what might be causing the problem (me or the machine.)

Below is a picture of my Delta jointer with an extremely accurate straight-edge spanning the in-feed and out-feed tables. Using the straight-edge and some automotive feeler gauges, I’ve measured the tables for flatness. Both tables have areas that are .006 out of flatness.

Question 1: Is .006 an acceptable tolerance for a jointer bed? Looking at the image, surely you’ve noticed the 2x4 and paper wedges propping the out-feed table up. Using this board is the only way I can get the two tables in a co-plane. Without that board, no amount of gib-screw adjusting or shimming will prop up the end of the out-feed table to be in plane with the in-feed table. I’ve had to resort to this method to keep the table accurate; however, the tables are close to a co-plane, but are out be .002 at the in-feed tables end. So even using this method does not make the jointer accurate.

Question 2: Is this a common problem with “dovetail style jointer”? The second image shows how accurate the jointed is. When I take two pieces of wood, approximately 3’ each, and joint their edges back-to-back, the results are illustrated in the image. The middle 12” of the joint are perfect, but the outer 12” at both ends are not accurate. I’m confident that the jointer blade height is not causing the problem because I use a Starrett dial indicator to get them within .001 of the out-feed table.

So my overall question: Am I being too picky for accurate tolerances with this jointer or is the equipments listed issues causing the problem? Are the tolerances poor enough the Delta would consider replacing the machine, or am I crazy to think they would ever consider it? I’ve grown tired of dealing with it, as it makes building inset door almost impossible. Please let me know your advice.

Click here for full size image

Forum Responses
(Solid Wood Machining Forum)
From contributor U:
If you can't adjust the infeed table to match the outfeed, then you might try to get it replaced (or just sell it and get another one). Rather than trying to measure the outfeed table to cutterhead relationship, I have found that if you start with the outfeed too low, then raise it until the snipe just disappears on test cuts, that is where it needs to be.
Jointing long boards is very technique dependent. Make sure that most of your pressure is on the outfeed side after the cut is started.

From contributor J:
Your outfeed table is adjusted just a tad too high. I know you quadruple-checked it, but the dial indicator is not a great tool for measuring the dimension that really counts here, and even if it were, having the top of the arc cut by the blade exactly flush with the top of the outfeed table is incorrect. Instead, the table needs to be in line with the very lowest points on the not-quite-flat surface cut by the blades.

Even when everything is working perfectly, the blades are cutting in little arcs, creating a (however finely) rippled surface with valleys (the top of the arc of the blade) and peaks (the ridges between cuts). It's the ridges that ride on the outfeed table, not the valleys.

Your feed rate also matters; if you run the workpiece over the jointer quickly, the cut arcs are further apart, and the distance between peaks and valleys is larger; faster feed rates demand (at least theoretically) that the outfeed table be lower than for slow feed rates. If your blades are at all dull and cause the workpiece to vibrate and jump up a hair as it moves past the cutterhead then, again, the surface produced is not quite where you'd expect it to be, and the outfeed table needs to be a hair lower. You've run over a knot, and now a chip in the blade produces a raised line on the cut surface? Again, the outfeed table needs to be lower to compensate.

What all this boils down to is that a dial indicator is not very useful in setting the height of the outfeed table because it relies on a simplistic understanding of where that table really needs to be. Instead, do as contributor U suggested. Set the outfeed intentionally low, take test cuts and raise it in small increments until the snipe at the end of the cut just barely disappears, and you're done.

From contributor Z:
Lower your outfeed and run a board then slowly raise it until snipe is gone. As far as getting a curve in your work piece - if your outfeed is too high and you’re applying pressure on the infeed side of the table you'll get a curve every time. Any board longer that your jointer bed will always be hard to get straight and it takes practice.

From contributor V:
These guys are right. No matter how well you set the knife height you will probably need to tinker with the outfeed table height to get it perfect. Also, after the knives begin to dull you can re-adjust again for straighter cuts. I am also pretty sure you can adjust the outfeed and infeed gibs to get the tables co-planar and dispense with the 2x4 prop. You don’t need to take it all from the out feed table - adjust the gibs on the infeed table for half the overall sag. If the gibs alone won’t adjust it out then you can use some machinist shim stock in the dovetailed ways.

Finally, these guys are also correct that a perfect setup can be thwarted by operator error in running the boards over the jointer. Look first at the shape of the edge and decide if you want to joint the hollow edge or the crowned edge. Either shape can be jointed but require different techniques. Feed speed and where you push and apply pressure affect the outcome as well as board length.

From contributor K:
While it's true that poor technique can overwhelm a good setup, good technique can only go so far with a badly machined jointer. This is one of the simplest machines in concept, and yet the most frustrating to adjust correctly - kind of like miter joints. I would consider .002" the maximum acceptable deviation from straight for a jointer this size. I don't know what Delta would say. If it is a new machine, you ought to be able to ask the dealer to set it up so that it produces an acceptable joint, and replace it if he can't.

To me, an acceptable joint is gap free - no light shining through between two boards as long as the outfeed table. If the outfeed table is concave, you will never achieve that result; if it's convex, you may be able to get close to it. It is probably not worth the time/money to repair the tables on this machine, but it is possible to re-machine or hand scrape them to an acceptable tolerance, and adjust the gibs or shim them into coplanarity. The other posters are correct about adjusting outfeed table height and using good technique, but if the tables, particularly the outfeed table, are really that far out of whack you may be beating a dead horse.

From the original questioner:
Thanks for the tips, I'll give it try. Funny thing is, I went to try everyone's tips, but when I tried to turn the machine on it wouldn’t start. So now I have to figure out of the motor is gone, or just a problem with the switch.

From contributor L:
We bought an old 8" jointer, it was a deal. The tables were worn and could not be tinkered into position. I am in MI, the motor city. We took it to a grinding shop and had the tables ground while it was together. It is the best jointer I have ever used. There is no snipe at all and the setup is easy. It’s the best $300 I ever spend (the grinding that is).

From contributor F:
Check your fence for a twist. I have an 8" Delta and I needed two replacement fences before I could get a board straight. It works great now. I hear if the casting is removed from the mold too soon they may be prone to movement.

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