Apparently the factory settings weren't as good as they might have been. My jointer cut tapers

After replacing the cutterblock, I got to setting my tables and was being fussy with them.  After obtaining a coplanar condition  with a TIR of 0.002:" over 6"  of table ( no small feat on a J/P because the tables  have pitch roll and yaw), - well after that -  I  took  my dial indicator to the relationship between the outfeed table and the cutterblock.

I set it to within 0.001" allowing the cutterblock to be proud by that one thousandth of an inch.

Then I took a length of pine board and jointed it keeping the same end pointed in the same direction till I'd taken better than an inch off it. Took a whole huge mess of passes. 

The normal wisdom is that this should have produced a  noticeable taper . 

It did not produce any measurable taper in the piece.

For years now I'd succumbed to the  notion, which I now believe is pure mythology, that jointers cut tapers as a natural consequence of how they work.

But I never really though too much about it.

There's a guy with a U tube video where he tries to explain this phenomena concluding that  "jointers cut tapers so get over it~!"  Except he doesn't touch on why the machine cuts tapers.  He says that the side opposite surfaces won't be parallel  because unlike the planer, they aren't registering off the surface opposite of the cutters.  This is a great explanation for why the jointer won't give you parallel  results, but it says not one thing about why  any given  jointer machine cuts more heavily into the work as it progresses creating a taper where none existed before.

I've concluded that the reason jointers cut what appears to be  a taper is all about how they are traditionally set up.

It's a bit of a misnomer I think to call it a taper even though the resultant  shape of the work piece ends up tapered.  What is happening is that the depth of cut increases incrementally as the piece is fed through the cut.

The reason the cut depth increases is because of how jointers are traditionally set up.

We all know the drill: Lay a nice flat piece of  wood on the outfeed table and bring the jointer knife to it and adjust the table so that the knife drags the wood toward the infeed table just a scosh - - the usual wisdom is between 1/8" and 1/4"

This absolutely means that the cutter is standing proud of the outfeed table by a few thousandths.  The further it drags the wood, the  prouder it stands and the larger diameter the cutterblock, the  great the arc path of the blade and thus  larger  cutter blocks drag the wood further than small ones for the same amount of proudness  (yes, I just made that word up: proudness is not really a word - forgive me please).

Well with my brain churning on this I decided to go to my trusty CAD and draw this out , but because  I don't have animation (it was another $grand$ to get animation) I can't  get it to move through the process but with a couple of still pics maybe I can say it:

This below pic is the configuration where the  out feed table and  the TDC of the cutterblock are in the same plane

Please observe that  if everything proceeds as the  above picture shows that there is not a  snowball's  chance  in that hot place below of a taper being generated.

But for the picture below I have exaggerated the  proudness of the cutterblock  to show what is happening when it stands proud.  And how much this odd cut path  produces this effect is all a factor of how proud the  cutter block is of the out feed  table.

With the Cutter block standing proud the work has to tip  by whatever small amount in order to make contact with the out feed table.  Granted the operator is holding the piece on the in feed table  which will prevent this tipping  - - right up till the work has progresses far enough that the operator brings that  down pressure  to the piece on the out feed side.  And then the tapering begins.

So I have concluded that jointers DO NOT cut tapers.  Not unless the cutterblock is a little proud of the outfeed they don't. 

I always thought they cut tapers cliff because of human feeding of the material. The material will go in lightly at the leading front edge and most of the weight on the material is applied at the rear or tail of the stock or even mid way, but the the front edge typically does not get the same pressure that the mid section and tail does, in my humble opine!

---

John Morris
The Patriot Woodworker

John Morris said:

I always thought they cut tapers cliff because of human feeding of the material. The material will go in lightly at the leading front edge and most of the weight on the material is applied at the rear or tail of the stock or even mid way, but the the front edge typically does not get the same pressure that the mid section and tail does, in my humble opine!

The table saw would my number one choice Cliff.

Cliff said:

John Morris said:

Well  now that's a stylistic difference if  ever there was one.  I transfer the pressure  to the board as it enters the out feed end but pressure is hardly the best word.  More of a guiding slight down force.   I can get my wood so flat that it  literally wrings to the cast iron table and doesn't want to be lifted off.

I used to use other settings to try to  get things like the slight bow in the middle of the board to create a joint that acts like a caul across the boards being joined, but that's fallen out of favor with me. I like 'em dead straight so that when I hold 'em up to the light there ain't none of the light peeking through.

But as to the tapering effect:

Next time you are changing blades set the out feed table lower on the cutter's arc for an exaggerated effect and you'll get tapers  in spades.  I'm sure there's some kind of math you can do to predict the exact amount of progressive cut so that if one wanted a taper of X in so many of work then one could just set the table height to the cutter just so and take X number of passes.   The math is more than I'd like to do so I'll just do  my tapered legs on the TS thanks anyway.

---

John Morris
The Patriot Woodworker