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.