why does a jointer blade face joint a board but thickness planer does not?

[bob frost]

I have read this explained but still do not understand it. If you have a combination jointer/planer , the same cutter is used, the jointer cuts from the bottom of the board and the planner cuts from the top. So, why does cutting from the bottom face prep a board but cutting from the top will not face prep the board.? Thanks in advance

[Glen Blanchard]

Because, if you are using proper form, when you joint you don't press down so hard on your stock as to compress it. Rather, you hold it down to the table enough to nip off the high spots (or low spots depending on how you look at it). Because there is no compression, the board does not spring back when you have completed your pass. This means that you are truly flattening your board. The key here is the compression issue.

Make sense?

[Victor Robinson]

It's all about where the flat reference surface is in relation to where the blades are cutting. With the jointer, your reference surface is the jointer bed, on the same side as the jointer's blades (bottom of your board). With the planer, the blades are cutting the top of your board but the reference surface (planer's infeed/outfeed table) is at the bottom of the board, meaning the planer's blades will make a cut that telegraphs what the reference surface is seeing. If the bottom of the board isn't flat, you'll end up with an equally unflat surface after running it through the planer...possibly a wedge.

[John Coloccia]

Think of it this way: You could take some make believe, hypothetical wood that is super flexible and, say, maybe 30 feet long. Pass it under the planer until it's about 1/2 way through, and then grab the two ends and make a giant loop around the cutter head. This loop will go round and round the planer all day long and it'd never be straight. As you reduce the thickness, the loop would get thinner and thinner but would always be a loop.

The jointer flattens because the outfeed is exactly the same height as the cutter head. As the wood comes off the cutter, the outfeed is supporting the freshly cut wood, constantly indexing on the newly cut flat part. That's what flattens the board. The planer is supporting on the opposite side, i.e. if the back of the board's not already flat, the face of the board will simply move up and down depending on what the back of the board in contact with the bed is doing. It'll make the board the same thickness everywhere, but not necessarily straight. The jointer makes everything straight, but not necessarily the same thickness.

[michael case]

Hi Bob,

The question is about blades, but it should be about tables. The blades are the same, indeed identical in a jointer/planer. Think of a jointer as a an inverted plane. We are truing the wood on the jointer just as with a plane, but with two major differences. Obviously we are running the wood over the jointer unlike a plane that we run over the wood. The other diffierence is that the tables on the jointer are set at two different levels. The rear table (outfeed) remains fixed exactly in line with the rotating knives. The front (infeed) adjusts down. As you plane the wood over the jointer (whether the face or the edge) the irregularities are planed off where it does not rest against the flat jointer tables. Successive passes remove the iregularites until the board rests completely against the flat jointer tables. It has now been planed flat and smooth. The longer the infeed table the longer board you can true. The idea is to create two flat surfaces at 90 degrees to each other: one flat true face and one flat true edge. These trued edges are then replicated by machines that create parallel edges i.e either your table saw for the edge or the planer for the face. In the planer (or planer mode) the relationship between the tables and the knives is simpler than the jointer. The knives merely chip off anything above a certain height from the bottom table, thus creating surface in parallel to the one that runs over the bottom table. This will not create a wedge. Rather it will flatten the top surface to a set continuous thickness even with the bottom face. The danger lies in the fact that you can create wedges while jointing. On the jointer, one face is flattened to the jointer tables irregardless of the iregularities of the opposite face of the board. So in truing a board on a jointer you often have one end that is thicker than the other. Be careful how you plane it because too great of a wedge can jamb the planer. If you set the planer thickness to the low end and the increase along the taper is too great you can have real problems. So always check for the thicker end and set your first planer passes to the thicker end. :)

[Peter Aeschliman]

I'll try another angle on this.

A planer copies the side of the board that is not being cut (the side resting on the planer table surface). So if you put a twisted board through a planer (not cupped, but twisted), the side that gets cut (the top) will eventually come out as a close copy of the other side. If the other side is flat, the planer will copy it and make the other side perfectly parallel to it. In other words, a planer will never make a twisted board flat.

On a jointer, the cut has nothing to do with the other side of the board. You can flatten the side being cut even if the other side is warped.

Spend a little time thinking about it, or draw it out on paper, and it will click.

[Richard Andersen]

I have a newbe question, what are people referring to when they mention a sled when talking about planners? I get the impression you can use the sled in a planner to accomplish what a jointer does???

[Karl Card]

this is good info. because as a newbie i have often wondered how and why about planer jointer.

I have also seen some where a jointer supposedly makes rabetts (spelling). HOW?

[Danny Burns]

this is good info. because as a newbie i have often wondered how and why about planer jointer.

I have also seen some where a jointer supposedly makes rabetts (spelling). HOW?
You need to remove the blade guard, and on the outfeed table needs to be a rabbiting ledge.

You adjust your fence for the depth of the rabbit, and have part of your board not cut, which then rides in the outfeed ledge.

[Bruce Wrenn]

Double sided planers do both operations in one pass. First, bottom cutter head face joints board, and the then top cutter head planes second face parallel to first.

[Danny Burns]

I have a newbe question, what are people referring to when they mention a sled when talking about planners? I get the impression you can use the sled in a planner to accomplish what a jointer does???
Since a planer will have a downward force on your bord, a sled is used so that the bent, warped, cupped, twisted etc. etc. piece of wood is supported on the underside, so that the downward pressure will not flatten it out.
Usually sled designs have wedges that are mounted on the underside to counteract this downward pressure.

[glenn bradley]

The jointer's reference surface is after the cutterhead and on the same plane.


That is; supporting the already machined material surface.


The planers reference surface is before, after and opposite the cutterhead and on a parallel plane.


That is; supporting the already flat surface opposite the material surface to be machined.

[bob frost]

I am thinking or visualizing a badly "cupped board" with the crown facing down. As the planer has an upward pressure on the board and as the cutter cuts the top off it seem like the cutter would cut the outside edges and not the center as the upward pressure would not be strong enough to flatten the board from the bottom, thus this top part of the board would cut the edges off enough after a few passes to make it flat... ?
I can see how the joiner is flatning the board but am still nor clear I guess as to why the planer is not also flatning one side of the board.

[Joe Wiliams]

I am thinking or visualizing a badly "cupped board" with the crown facing down. As the planer has an upward pressure on the board and as the cutter cuts the top off it seem like the cutter would cut the outside edges and not the center as the upward pressure would not be strong enough to flatten the board from the bottom, thus this top part of the board would cut the edges off enough after a few passes to make it flat... ?
I can see how the joiner is flatning the board but am still nor clear I guess as to why the planer is not also flatning one side of the board.
It took me awhile to understand why they work differently too and this is how I see it...

With the planer the wood is on rollers and will follow the contours and irregularities. While the jointer is on a flat surface at the same height as the blades.

[Mike Cruz]

Bob, not to be nasty, but I can't believe that with all these "in other words" posts, the picture hasn't gotten clear. It actually gave me a chuckle that everyone had to put their spin on it...as if they needed to...or so I thought. I saw it as beating a dead horse, or belaboring the point, or to put it another way, saying the same thing over and over with different angles. So, since you still are having trouble, let me try...

Have you ever gotten your finger caught between the board and the infeed (or outfeed for that matter) table of your planer? Of course you haven't. Do you know how I know that? Because you seem to not have a grasp of just how much pressure your planer is applying to the board when it is being pressed up against those blades. By the way, DON'T GET YOUR FINGERS CAUGHT IN BETWEEN THE BOARD AND THE TABLE!!!!!!!!!!!!!!!!!

The reason that the planer doesn't take off just the high spots, like you would visualize it, is because for the planer to properly grab the wood, and keep it moving, it has to apply enough pressure that is usually flattens (or very nearly flattens) a 4/4 board without much effort.

The very first posted answer to your question was perfect. With a jointer, there is little to no downward pressure on the board during cutting...only afterwards....on the outfeed table do you apply any pressure. A planer puts pressure on the board before, during and after the cut.

I think the bottom line is that the planer applies a LOT more pressure than you think.

Again, this post was not mean in a nasty tone at all. I hope it helped. If not, go back and really read all the answers you got to your question. I read most of them, and they really are saying the same things (accurately)...just differently.

[bob frost]

Ok , I think I have it now.( I visualize the planer roller to be one continues roller ) one in front and one in back.

The planer on an extremly cupped board, would trim the edges of the board as I described above but as the board then become less cupped the rollers would be applying enough pressure( or compression) to make the board flat on the top as it passes thru the planner, but would spring back to the less cupped form when it exited the planner. Resulting in a still, cupped board.
I get how the jointer flattens the board due to the outside table feed being slightly higher and the lack of downward pressure ( or compression) is put on the board by the worker.

It seems to me that you could then actually use the jointer to then continue to thin the board. However , if you needed to thin more than, say a 1/16" , you would need to defintly have a thickness planer due to the amount of time it would take to thin a 1/4" or more from the board.

Am , I getting close here?

[bob frost]

Bob, not to be nasty, but I can't believe that with all these "in other words" posts, the picture hasn't gotten clear. It actually gave me a chuckle that everyone had to put their spin on it...as if they needed to...or so I thought. I saw it as beating a dead horse, or belaboring the point, or to put it another way, saying the same thing over and over with different angles. So, since you still are having trouble, let me try...

Have you ever gotten your finger caught between the board and the infeed (or outfeed for that matter) table of your planer? Of course you haven't. Do you know how I know that? Because you seem to not have a grasp of just how much pressure your planer is applying to the board when it is being pressed up against those blades. By the way, DON'T GET YOUR FINGERS CAUGHT IN BETWEEN THE BOARD AND THE TABLE!!!!!!!!!!!!!!!!!

The reason that the planer doesn't take off just the high spots, like you would visualize it, is because for the planer to properly grab the wood, and keep it moving, it has to apply enough pressure that is usually flattens (or very nearly flattens) a 4/4 board without much effort.

The very first posted answer to your question was perfect. With a jointer, there is little to no downward pressure on the board during cutting...only afterwards....on the outfeed table do you apply any pressure. A planer puts pressure on the board before, during and after the cut.

I think the bottom line is that the planer applies a LOT more pressure than you think.

Again, this post was not mean in a nasty tone at all. I hope it helped. If not, go back and really read all the answers you got to your question. I read most of them, and they really are saying the same things (accurately)...just differently.

Thanks Mike, I just was not realizing the amount of pressure that the rollers put on the board. That was the part I was missing. I was thinking of them as used just to hold the board steady and feed the board through. Also read my last remark above yours for my most resent understanding.
Thanks again

[Glen Butler]

It seems to me that you could then actually use the jointer to then continue to thin the board. However , if you needed to thin more than, say a 1/16" , you would need to defintly have a thickness planer due to the amount of time it would take to thin a 1/4" or more from the board.

Am , I getting close here?

Yes the jointer will continue to make the board thinner as you take more passes, but once you get a board straight on the jointer only one side is straight. So you will thin a board that does not have parellel edges. The thickness planer will use that straight side as a reference and make the whole board the same thickness.

[Glen Butler]

Well I am wide awake and bored so bear with me while I put pictures with the words.

Image 1 - shows the manner in which material comes off a board through the jointer.

Image 2 - shows the finish product off the jointer. If you flip the board over and "thickness plane" it, you are starting over without reference. The whole point of the jointer is to begin a reference that will allow you to do all the other work that needs to be done to the board.

Image 3 - shows how the planer uses the first side to reference the second side, ending up with perfectly parallel sides.

[Mike Cruz]

Yup, Bob, you are on the right track with respects to the planer.

As for the jointer, as Glen points out, yes, you can keep making the board thinner on the jointer, but a jointer will only make one side flat. If you made one side flat, then flipped the board over to try to then flatten the other side, you would succeed! However, what you would end up with is two perfectly flat sides that are NOT parallel to one another.

What the jointer's purpose is, is to flatten one side so that you have a flat frame of reference to make the board a certain thickness on the planer...and be left with a flat AND consistantly thick board.

So, to recap: If you use ONLY a jointer, and worked both faces of a board, you will get two flat faces (in length and width), but not parallel.

If you use ONLY a planer, and worked both faces of a board, you will get a consistently thick board, with neither face flat (either in its length or in its width).

That is why we use both machines. To get ONE true flat face on the jointer, then move to the planer to get the other face parallel to the first one...at a consistant thickness.

NOW, to add a little bit of a lesson here: To KEEP the board as flat as possible, once you have thrown the board through the jointer, and then, through the planer to get a consistant thickness (supposedly still thicker than what you want as your finished thickness) be sure to keep flipping the board over with each subsequent pass through the planer. Why? So that you are removing the same amount of wood on both sides of the board in order to keep the board as stable AND FLAT as possible...after all that work you did to get it flat, you want to do all you can to keep it that way.

One more note, Bob. Do you HAVE to have a thickness planer to do a glue up? No. If you get the face of a board flat, put that flat face up against the fence, and joint one edge, take the board to your TS (with the just flattened FACE side DOWN, and the just jointed EDGE against your rip fence), cut the board to width, then repeat the process with other boards, you will be able to glue the boards together successfully with ONE nice flat face. The other side will be a mess, however. So you either leave that the way it is, or hand plane it flat. This can work on glue ups meant as table tops etc. But if it is something that would need to fit into a dado, well, have fun!

Hope that helps.

[Mike Cruz]

Bob, I just thought of one more thing to add.

A good rule of thumb is to always joint with the cupped side (concave)down. This applies to width and length. And also to jointing the edges. You will see that in Glen's drawings, the board's orientation is correct.

[John Coloccia]

Ok , I think I have it now.( I visualize the planer roller to be one continues roller ) one in front and one in back.

The planer on an extremly cupped board, would trim the edges of the board as I described above but as the board then become less cupped the rollers would be applying enough pressure( or compression) to make the board flat on the top as it passes thru the planner, but would spring back to the less cupped form when it exited the planner. Resulting in a still, cupped board.
I get how the jointer flattens the board due to the outside table feed being slightly higher and the lack of downward pressure ( or compression) is put on the board by the worker.

It seems to me that you could then actually use the jointer to then continue to thin the board. However , if you needed to thin more than, say a 1/16" , you would need to defintly have a thickness planer due to the amount of time it would take to thin a 1/4" or more from the board.

Am , I getting close here?

The board bending does have a little to do with it, but the predominant reason is the planer bed is usually much shorter than the board you're planing. If the bed were very long, and the board didn't wobble, the planer would do an excellent job of flattening. This is the whole idea behind a planer sled. In order for a planer to work, you need to be able to snap an imaginary line through the board somewhere, lengthwise, and that line has to be exactly the same height above the planer bed the entire length of the board. In other words, the board has to pass through straight in order for the cutter to cut straight. You can see that with any sort of warp or cupping, as you slide a long board over the relatively short planer bed, it will NOT go through straight at all. And then to complicate it, the board will also warp due to pressure from the rollers.

You can't really reliably thickness on a jointer because it only can reliably make a surface flat. It does not make the opposite surface parallel. So if you tried to thickness my jointing both sides, you will most likely end up with two flat sides that aren't parallel to each other, i.e. a trapezoid. To get side A parallel to side B, you have to cut on side A while indexing on side B (like a planer). To flatten side A, you have to cut on side A and index on side A. This is why you have to joint before going to the table saw. First you flatten side A with the jointer. Then you index with side A against the fence, and cut on side B to make it parallel. The operations seem very similar, but they are completely different because of the side of the board you're indexing off of.

To be straight, you must always index from a flat surface. The jointer cleverly does this by indexing the freshly cut surface. This gets you flat at the expense of parallel. Every other tool in your shop indexes on an already flat surface to give you flat AND parallel.

[glenn bradley]

Here you go:

[Mike Cruz]

John, I have to disagree with you with respects to planer bed being the cause of not flattening a board. Even if you have a 12 foot infeed table and a 12 foot outfeed table, you wouldn't get rid of a bow in a board. Besides, a cup alone the width of a board has nothing to do with the length of the beds.

Also, my jointer beds are WAY shorter than a 10 foot board, yet I can get a 10 board perfectly flat on a jointer. The reason is applied pressure...

[John Coloccia]

John, I have to disagree with you with respects to planer bed being the cause of not flattening a board. Even if you have a 12 foot infeed table and a 12 foot outfeed table, you wouldn't get rid of a bow in a board. Besides, a cup alone the width of a board has nothing to do with the length of the beds.

Also, my jointer beds are WAY shorter than a 10 foot board, yet I can get a 10 board perfectly flat on a jointer. The reason is applied pressure...

If the planer bed were longer, and you had a board that didn't wobble, the planer would flatten it. That's exactly how a planer sled works. While it's true that the board can also warp due to roller pressure, and I mention that, the primary problem is the board doest go through straight because it's warped. If you had a 5" thick board that couldn't possibly bend via pressure rollers, the planer still wouldn't flatten it unless it went through straight and without wobbling. The jointer is different. The outfeed doesn't have to be the length if the board because you're always indexing on a flat. On the planer, if the back of the board isn't flat, you'd better have a bed the size of the board or the board height will change as one end falls off and other end gets picked up...or make a sled :D

[Mike Cruz]

When you ust a sled, do you use shims under the gaps (or high spots)? If so, the reason for the sled working isn't because of its length, it is because of its ability to keep the board from succumming to the pressure that the planer puts on it. You could build a 2 foot sled and it would work (with shims). Without the shims it wouldn't work. The reason for the sled is to make sure the shims stay in place. If you put shims under the board without a sled, the friction of the table would move or dislodge the shims. By using a sled, there is no longer friction between the the shims and the table, all friction is taken by the sled.

Now, incorporating sliding table saw technology, if the bed of your planer "slid" under the cutters, then you could shim the board, in place, on the sliding table, and pass your board through. Thus jointing the one side of the board, then with that side being flat and now your reference point, flip the board over and thickness plane it.

Hey, what a great idea. Anyone make something like this? It would be simple and save so much space, I mean, you wouldn't need two machines. Oh yeah, except that you would probably need about a 12 foot table to accomodate a 12 foot board... I'm sure some manufacturer makes 'em.

[glenn bradley]

Interesting that this thread has taken on a life of its own. The tools work as designed, despite what we may want to believe they do. If a planer could joint or a jointer could plane, I would only own one ;-)

[David Epperson]

The reason that the planer doesn't take off just the high spots, like you would visualize it, is because for the planer to properly grab the wood, and keep it moving, it has to apply enough pressure that is usually flattens (or very nearly flattens) a 4/4 board without much effort.
I must have been taking much too light of a cut when I planed the boards I'd milled - because just taking the high spots was all it did...course that's all I wanted it to do at the time.
And on a cheap 10" HF lunchbox planer to boot. I think I'll still look into building a sled. Or better yet a router bridge, since I'm going to be working with small already cut to near size pieces. I haven't heard a good way to remove twist from a board short of cutting it smaller before flattening and thicknessing.

[John Coloccia]

When you ust a sled, do you use shims under the gaps (or high spots)? If so, the reason for the sled working isn't because of its length, it is because of its ability to keep the board from succumming to the pressure that the planer puts on it. You could build a 2 foot sled and it would work (with shims). Without the shims it wouldn't work. The reason for the sled is to make sure the shims stay in place. If you put shims under the board without a sled, the friction of the table would move or dislodge the shims. By using a sled, there is no longer friction between the the shims and the table, all friction is taken by the sled.

Now, incorporating sliding table saw technology, if the bed of your planer "slid" under the cutters, then you could shim the board, in place, on the sliding table, and pass your board through. Thus jointing the one side of the board, then with that side being flat and now your reference point, flip the board over and thickness plane it.

Hey, what a great idea. Anyone make something like this? It would be simple and save so much space, I mean, you wouldn't need two machines. Oh yeah, except that you would probably need about a 12 foot table to accomodate a 12 foot board... I'm sure some manufacturer makes 'em.

I've only had to use a sled a couple of times, and not for many years, but I just got the board to the point that it didn't wobble. That does mean shimming the high spots (or low spots, however you see it) but I didn't go out of my way to keep the board from flexing. If you didn't have the sled, the height of the board would change as the various bits of shims and boards fall on and off the planer bed. You could hot glue the shims to the board and it still wouldn't work properly without the board being fully supported.

Your sliding panel idea is actually exactly what the sled does. I know of a guy that made an arrangement where a small jointer (the whole jointer!) could slide upside down over a piece of wood. He used it to make radiuses and other shapes. I believe he got the idea from Bob Benedetto, a luthier, that I think also used that same concept to radius fingerboards. :eek:

[bob frost]

The board bending does have a little to do with it, but the predominant reason is the planer bed is usually much shorter than the board you're planing. If the bed were very long, and the board didn't wobble, the planer would do an excellent job of flattening. This is the whole idea behind a planer sled. In order for a planer to work, you need to be able to snap an imaginary line through the board somewhere, lengthwise, and that line has to be exactly the same height above the planer bed the entire length of the board. In other words, the board has to pass through straight in order for the cutter to cut straight. You can see that with any sort of warp or cupping, as you slide a long board over the relatively short planer bed, it will NOT go through straight at all. And then to complicate it, the board will also warp due to pressure from the rollers.

You can't really reliably thickness on a jointer because it only can reliably make a surface flat. It does not make the opposite surface parallel. So if you tried to thickness my jointing both sides, you will most likely end up with two flat sides that aren't parallel to each other, i.e. a trapezoid. To get side A parallel to side B, you have to cut on side A while indexing on side B (like a planer). To flatten side A, you have to cut on side A and index on side A. This is why you have to joint before going to the table saw. First you flatten side A with the jointer. Then you index with side A against the fence, and cut on side B to make it parallel. The operations seem very similar, but they aren't completely different because of the side of the board you're indexing off of.

To be straight, you must always index from a flat surface. The jointer cleverly does this by indexing the freshly cut surface. This gets you flat at the expense of parallel. Every other tool in your shop indexes on an already flat surface to give you flat AND parallel.


Thanks John, you make some good and understandable points

[Greg Mann]

When you ust a sled, do you use shims under the gaps (or high spots)? If so, the reason for the sled working isn't because of its length, it is because of its ability to keep the board from succumming to the pressure that the planer puts on it. You could build a 2 foot sled and it would work (with shims). Without the shims it wouldn't work. The reason for the sled is to make sure the shims stay in place. If you put shims under the board without a sled, the friction of the table would move or dislodge the shims. By using a sled, there is no longer friction between the the shims and the table, all friction is taken by the sled.

Now, incorporating sliding table saw technology, if the bed of your planer "slid" under the cutters, then you could shim the board, in place, on the sliding table, and pass your board through. Thus jointing the one side of the board, then with that side being flat and now your reference point, flip the board over and thickness plane it.

Hey, what a great idea. Anyone make something like this? It would be simple and save so much space, I mean, you wouldn't need two machines. Oh yeah, except that you would probably need about a 12 foot table to accomodate a 12 foot board... I'm sure some manufacturer makes 'em.

I saw exactly this machine in the Henry Ford Museum, except that the cutting head was a vertical spindle with a big rotary facemill. Another issue: A sliding table will take up 24 feet to flatten a 12 foot board. When you make this machine use a traversing cutting head and leave the table stationary. You will need only slightly more than 12 feet.:D

Also, a facemill will give you rotary marks on your workpiece while a planer style cutter will not. The facemill will not generate nearly as much force upon the wood as a planer style cutter would so the board would not need to be suuported as much. The problem is the finish (and dust control), which is probably why I saw it in the museum and not in the Grizzly catalog. :rolleyes:

[bob frost]

OK, Thanks to all for fellow wood workers at SMC for your input!
I am really pleased and somewhat surprised at the length of thread this has created. I think I may even become a contributor now.
I understand this much better now and I will print out this thread because some points of view have been expressed in ways that are clearer to me than what I had previously been exposed to . To Glen's point, yes, I think we all understand that there is good reason for the manufacturing and usage of both machines and could just stop there. However, I am one who likes to have more in depth knowledge about how and why things actually work, rather than just being satisfied that they do work. At least in many cases such as this one. It,s a little like turning on a light switch; most people will go thru life and never understand how it actually works and just turn it on. And thats Ok. But for many of us we will study the issue , as I did many, many moons ago and know why it does or does not light up when one throws the switch.
Again, Thanks to all
Bob :)

[bob frost]

OK, Thanks to all for fellow wood workers at SMC for your input!
I am really pleased and somewhat surprised at the length of thread this has created. I think I may even become a contributor now.
I understand this much better now and I will print out this thread because some points of view have been expressed in ways that are clearer to me than what I had previously been exposed to . To Glen's point, yes, I think we all understand that there is good reason for the manufacturing and usage of both machines and could just stop there. However, I am one who likes to have more in depth knowledge about how and why things actually work, rather than just being satisfied that they do work. At least in many cases such as this one. It,s a little like turning on a light switch; most people will go thru life and never understand how it actually works and just turn it on. And thats Ok. But for many of us we will study the issue , as I did many, many moons ago and know why it does or does not light up when one throws the switch.
Again, Thanks to all
Bob

[Glen Butler]

This thread makes me chuckle, and Glenn, where did you get that wonderful graphic showing the jointer and planer?

[Greg Mann]

However, I am one who likes to have more in depth knowledge about how and why things actually work, rather than just being satisfied that they do work. :)

If there is a single woodworking machine where it is important to know the how and why of how it works in order to get good results it's the jointer.

[Eddie Darby]

Ran into this in another thread.

http://www.newwoodworker.com/usejntr.html

What A Jointer Can and Cannot Do

A jointer can machine a face or edge of a board perfectly flat and straight. It can also make adjoining surfaces square to each other. What it can’t do is make any surfaces parallel to each other. That is the job of the thickness planer or in the case of edges, the table saw.
The jointer “sees” one surface of the wood at a time. The knives are set parallel to the outfeed table and the machined portion of the wood, sliding on that table is the only registration a jointer has. When squaring an edge, the fence comes into play but the outfeed table surface remains the only point of registration that keeps the edge straight.
The jointer has no way of keying of another surface to make a cut parallel to that surface. Consequently, while the jointer can make adjacent surfaces smooth, straight and square to each other, it has no way to make those cuts parallel to each other.
When working with edges, we often joint one edge straight and then go to the table saw to cut the opposing edge parallel to it. With the wide faces, after jointing one face flat, the planer is used to cut the opposing wide face flat and parallel to the jointed one. It this combination of machines that allows us to produce perfectly straight wood with square, parallel edges.

[Michael Weber]

Well I gotta jump in here and add my 2 cents worth. It's easier in my mind to consider bowed boards instead of cupped boards. A bowed board (bent along the boards length) will be flattened by the roller pressure underneath the cutter but after exiting will resume it's bowed shape. I can see how, and I have milled cupped boards on my planner by putting the concave side down and taking light cuts until I have a flat reference surface and then flip the board over and start taking the high edges off. Going back and forth gives me good results. YMMV. Of course it only works if the board is otherwise straight.

[glenn bradley]

Glenn, where did you get that wonderful graphic showing the jointer and planer?

That one came off of FWW online. The article, oddly enough, is entitled: "The Jointer and Planer are a Team (http://www.finewoodworking.com/ToolGuide/ToolGuideArticle.aspx?id=29511)" :D.