Jointer Tolerances and Setup Techniques
A furnituremaker who wants super-fine tolerances gets advice on tuning and operating a jointer. May 2, 2011
Question
One year ago I purchased a brand new 8" Powermatic jointer. I feel like I'm constantly making adjustments and I am seldom satisfied with the results I'm getting. I've abandoned trying to use the blade setting jig that comes with the jointer. It's not nearly accurate enough. I've just finished setting the blades to the outfeed table with a dial indicator. Once they were even all the way across on all three, I lowered the outfeed until I started getting some snipe. I then raised it a hair. After jointing the face of a board, I still can't get it to lay perfectly flat. I probably have about .002 between the middle of the board and the table. It doesn't matter if the board is 4' long or 8' feet long. Does anybody know what I'm doing wrong?
Forum Responses
(Furniture Making Forum)
From contributor A:
You could be pressing down too hard on the outfeed side of the table. (You can't bend the board at all throughout its entire pass over the cutterhead.)
From the original questioner:
I'm definitely applying pressure on the outfeed table, but not directly over the blades. Thanks for the suggestion.
From contributor I:
0.002" in the middle is not bad at all. Some folks purposely set up to cut 0.007-8" hollow so the ends clamp together tightly. Cracks start at the ends, not in the middle, and this prevents the problem.
That said, you have to be very careful about balancing the board, where you apply pressure, etc. To get truly flat boards, you need to be within 0.001" from knife to knife (yes, it's possible, and yes, it makes a difference). Your tables must be machined flat and set up parallel. The outfeed table must be in the right relationship with the cutting circle (sounds like you did that the best way).
Setting up a jointer and keeping it set up are two different things.
From the original questioner:
Thanks for the sound advice. Maybe I should put a gauge on them once more. The gauge I'm using isn't really made for this application. I'm thinking of purchasing the multi gauge from One Way. Have you ever heard of it?
From contributor J:
0.002" is practically nothing. It also may have nothing to do with the jointer setup, and everything to do with your technique while operating the machine.
Say you place the board on the infeed table crown up and start pushing it towards the blade. As you do this, you are pressing down on the board. The downward pressure deforms the board a bit, temporarily removing a bit of its natural bow while you are cutting the bottom flat. When you remove pressure after you finish cutting, the board springs back to its original shape, replacing the bow you thought you'd just removed.
Other factors may be in play too. For example, removing the surface of the board may change the balance of tensions within the material and cause it to change shape as it is being cut. Bottom line is, 0.002" is flirting with the limits of what's possible with wood, and generally nothing to worry about.
From contributor I:
What contributor J says. You can get flatter, but for almost all applications (read: all reasonable applications) you really don't need to. I've always used a plain ol' dial indicator. A friend gets extremely accurate results with a flat stick, but he can't quantify them. He puts a mark on a 6-12" stick, cranks the outfeed down a hair, and lines up the mark with a knife. Then he rotates the cutterhead and sees how much the stick travels as the knife carries it forward. If the next knife pulls it the same amount, they are extremely even.
From the original questioner:
Thanks. I'm trying very hard to apply pressure only on the outfeed table and not directly over the blades. I just don't think it's normal to have a bow, especially in a 24" board. I would understand if it was flat straight out of the jointer and then the next day find it bowed the .002".
From contributor B:
Yes, a slight hollow in the middle is exactly what you want while any snipe off the ends is totally unacceptable. I always feed in and out lightly while pushing down in the middle. This will create the slight hollow that you're looking for. I believe that's the way we're supposed to use a jointer?
I've never had any luck with any of those setup tools (not even the one that was included with the machine). After positioning everything just right, you tighten the set screws or nuts and then discover everything is now wrong!
Your approach using the dial indicator is probably the best you can do. If you want more accuracy than that, you might consider a knife grinding setup on the machine itself.
Another option which sounds good to me might be a replacement spiral insert cutterhead. Having said all of that though, I think you're splitting hairs (your .002 while a human hair is .004).
From contributor J:
It's not even possible to apply pressure only on the outfeed table, because the wood starts out on the infeed table. By the time it gets to the outfeed side of the blades so you can move your hands there, it's already too late.
The movement that occurs as internal tensions are shifted when material is removed is pretty much instantaneous. Further movement might occur over the next day or two, but this is more likely due to changes in moisture content because you've exposed material that's either wetter or drier than EMC for your shop's current climate.
If you're managing flat within 0.002" over 24", you're already doing a great job. You should find something else to obsess over.
From the original questioner:
I must respectfully disagree. I should be able to run a 24" long x 6" board through my jointer and have it lay dead flat on the outfeed table.
From contributor M:
I have to disagree somewhat with contributor B on always wanting a hollow on your edge. While you may want a little for edge gluing, someone with an 8" jointer is probably doing a lot of surfacing. A proper jointer should be able to render a perfectly flat surface, end to end!
I've had a lot of trouble with my longbed Powermatic 6" with the infeed table drooping over time (in other words, going out of being coplanar). Still working with the distributor for a fix.
From Professor Gene Wengert, Sawing and Drying Forum technical advisor:
Hollow ground edges have been common in the past to account for the higher MC of the lumber than the air. The surface would dry and shrink and give a perfectly flat surface. If the surface was jointed perfectly flat and then the lumber surface dried, there would be a gap which would give a thick glue line appearance. Most folks today joint and edge glue promptly, so the hollow ground surface is not needed. Also, today we control MC much better than in the past and control the air's humidity better. Nevertheless, some folks do not check the MC and have lumber that is too wet for the air and would benefit with a hollow edge joint. I suggest that a hollow joint is a poor fix for incorrect MC, however.
Regarding the initial question here, it is also possible that there is a small amount of casehardening (more accurately called drying stress, as nothing is harder). This stress condition will lead to immediate hollow ground joints or cup while jointing. If this is indeed happening, then the opposite face that has cup before jointing will have less after jointing, or if flat initially will have crowning after jointing. This is an easy check to make. In other words, the crowning you see is caused because of stresses in the wood.
The normal way to check for stresses in lumber is to take a small piece, cut at least 12" from the end of the lumber, that for 4/4 lumber would be 1" thick x 1" along the grain x width of the lumber. Then take this piece and resaw it into two 1/2" pieces (that is, make the thickness half of the thickness of the lumber; each piece will be 1/2" thick x 1" along the grain x width of lumber). If there is stress, these two halves will not fit back together tightly without pressure.
It is possible that when the shaft is spinning and is unbalanced, it will bend or curve.
From contributor D:
You can eliminate the deformation while passing over the cutterhead from your inquiry by using a chunky piece of 8/4 on edge, the same length as your infeed table. Start the cut with some weight on the board with your left hand - just enough to keep the stock from bouncing as it is being cut. Push the stock with your right hand, and let the machine do the cutting - you just push it through, without pushing down, over, here or there. Then do the same with another identical board. Place the two edges together and that is twice the gap - if there is any.
For what it is worth, most every joiner I have ever worked with had to have the outfeed table a fuzz lower than the knives to make a completely straight cut. A mini snipe - maybe .002 - was almost inevitable, but the stock is dead straight. If I raise it to eliminate snipe, then I get a slight hollow.
From the original questioner:
I must say these are all great suggestions! I will take them all into consideration. My blade settings still have me wondering. I'm going to check them once again and see what happens. What a great forum this is for exchanging ideas!
From contributor I:
A couple things to consider. If your tables rise out of parallel, your board will be convex. If they sag, depending on where you press, hollow with possibly a snipe. If the outfeed is low, a snipe. If high, a snipe on the leading edge and convex. If your tables are themselves hollow, your results depend on many factors, from how hollow to how long your board is to how hard and where you press, but you'll probably get snipes and a hollow cut both.
To test for parallel, put a straightedge on a table and line it up with the other table, and use a feeler gauge. You need a good straightedge, the longer the better. Since none of mine are this good, I use a piece of fine piano wire with weights on both ends hung over the tables. Tap it to see where it touches, and see if you can work a feeler gauge under it without scraping.
Better jointers have beds that can be shimmed. I could never get my old Delta 6" (adjustable gibs) shimmed so that it held parallel. Now I have a Northfield - even when the adjusting wedges wear, the bed floats on top of them and can be shimmed as needed.
If your knives are freshly sharpened, the cutting circle is where you want the outfeed to be. If they are dull, the wood will bounce and you'll get a wavy cut, plus your outfeed needs to be a tad low of the knives because of the slapping. The cut will be less than desirable for flatness and straightness, depending on how dull the knives are and how hard the wood is.
There's a wonderful feeling when you joint a board and it sucks down to the outfeed table because it mates so perfectly, like Johanssen blocks at a machine shop. When you are that flat, you don't need clamps for small joints, you can spread a thin layer of glue, rub the pieces together and let them set. Luthiers sometimes use this trick, and it also works for putting in corner blocks or gluing small novelties. Of course, if you take the time to tune your jointer this well, don't be climbing on it to change the light bulbs.
From the original questioner:
Great advice! I haven't considered the possibility of the knives possibly being dull. And now that you mention it, I thought one of my boards could have had a wave in it. I've had these knives sharpened once and I'm wondering if they're not true. Maybe I'll start with a new set of blades and start from scratch.
From contributor I:
Now I design and carve, but I used to build a lot of solid wood furniture. My used jointers were never right, and once when I was building a 10' top, my best cabinetmaker, a really precise guy, showed me a few things. Opened my eyes about "close enough." What I've posted here I learned from days of butting my head against a wall on many different but related issues over the course of many years.
From contributor L:
I think these are all good suggestions, especially those about not being too obsessive! I can add only a few. I'm not clear how much you're concerned with face jointing vs edge jointing. I borrowed a really heavy 7ft straight edge from a machine shop to check the beds for parallel and twist. The best way to get the knives aligned is to joint them after they are in the machine. For most work this is not necessary but if you want them really sharp, it helps. Old timers would do this with the machine running - no way am I going to do that again. I turn it by hand - much slower but safer. Another way to get a perfect (so light can get through) edge joint is with really good hand planes and either lots of skill or patience.
From Professor Gene Wengert, forum technical advisor:
If you try to joint the edge of a 1x4, it may develop a hollow ground joint because of longitudinal stress in the wood. As an example of this stress, consider what happens when a 1x4 is ripped into two pieces to make two 1x2s. It is not uncommon for the two pieces to bow, touching at the ends, but having a gap in the middle.
Why does this happen? There is stress on each edge that wants to make the wood at or near the edge longer. Technically, the edge is being compressed and wants to release this compression by getting longer. This compression is balanced on both edges, so the unprocessed piece looks fine... straight and flat. The large amount of wood in the center has no or little stress. This stress is sometimes called casehardening or drying stress; it sometimes is called growth stress when it originates in the tree. Casehardening can be removed in drying and is almost always removed with hardwoods; it is sometimes removed with softwoods, but not often.
Now when the one edge is jointed, some of the compression stress is removed on one edge. Now the stress is unbalanced, edge to edge. Overall, there is more stress on the non-jointed edge and this unbalance will slightly cause the piece to bend lengthwise. As mentioned previously, you will notice the bend on both faces, so jointing one edge will cause the other edge to become un-flat too. Note that with a wider piece, the center section will be able to hold the wood straight better and so this effect will be less noticed on large, wide pieces when edge jointing.
The point is that when using wood to check the setup of a jointer or planer, there can be misleading results. The hardware setup mentioned above is often the best approach.
From contributor R:
To the original questioner: do you have an engineering background? Just curious. It's my advice that you are trying to do precision machining on a material that is not inert. It will move. In the business of high precision metal machining, temp of the material can even influence the measurement. Holding the part in your hand can even make a change. I have already read that you don't accept .002 as an acceptable tolerance, but what would be acceptable? What is dead flat? .0005 off? .0001 off? You can't get dead flat in anything... It can always be measured by something more accurate.
From contributor E:
I've seen the coplanar issue with Powermatics. Here's a way to see - set up a really shallow cut, 1/32. Joint about a third of the board and power off with the board in place. Drag the board over the outside of the bed (away from the fence). Is the gap the same? It either the bed or knives, but if you're using a dial indicator on the outfeed, it will just transfer the error to the knives. I used to push rather hard on the outfeed, thinking that would guarantee flat. When I would turn to go square, I'd notice the gap against the fence. I couldn't figure it out. I recently learned to lay off it and in most cases let the board's own weight hold it to the outfeed. Now I get square and flat every time. Use your bandsaw for hogging and take light cuts on the jointer +/- 1/32. Not the fastest method, but it really seems to work.
From contributor E:
Sorry - in my test method I should have said, is there a gap now on the outfeed or infeed, (the outfeed might have a bump)? You should not have one to begin with. Your dial indicator can do this too, but a board is longer and can show you issues further into the tables.
From the original questioner:
I'm not an engineer but I come from a long line of them. I also work on Heidelberg offset printing presses, which have extremely tight tolerances and specs. (I am also a bit of a fanatic.) Above, contributor I talks about a surfaced board being sucked down onto the outfeed table and what a great feeling it is. I've experienced this and I know the jointer is capable of achieving it. I understand nothing is perfect, but I also know when my machine is not performing consistently. As I stated earlier in this post, if I surface a 24" board and I see daylight under it, something is not right. Once again gentlemen, I thank you for all of your suggestions.
From contributor G:
All good replies. Here is how I set knives. Raise the infeed and outfeed tables so they are parallel, use a good straightedge (the piano wire was innovative). Lower infeed and remove knives if to sharpen, loosen if just want to set. I purchased two 6 inch long magnets which I set onto the outfeed and directly over the knives. This pulls them up. Rotate one knife and find the high spot where it just hits the magnet, make reference mark on your jointer, set them all to this mark. They are now level with the outfeed. The infeed doesn't matter. Back the outfeed down a smidge, just a smidge. Raise your infeed and run a board on edge. Watch as the edge goes from infeed to outfeed. It should go over without any gap or hitting outfeed. Check for snipe, adjust as necessary. Of course, if you aren't coplanar or have warped, cupped or convex beds, this won't be the cure.
