Reverse rainbow jointing?

[Phil Thien]

Conventional wisdom is to joint boards with bowed boards presented like a rainbow.

This guy does the opposite, "the reverse rainbow:"

https://wunderwoods.wordpress.com/2012/0...ow-method/

Anyone care to discuss?

[jteneyck]

If Ferg were here he would feel vindicated.  He got beat up pretty bad a couple of years ago showing that way of doing it.  And there was a discussion over on SMC a day or two ago where a couple of guys advocated belly up as better.  Personally, I'll stick with the conventionally approach. 

John

[Alan S]

The challenge in flattening a face on a jointer is to choose the plane you want to cut through the board. Generally you want it as close to the average plane as possible, so as to diminish board thickness the least. Concave down, you are more limited in the amount you can deviate from the ideal angles: it comes closer to taking care of itself.

The problem with concave up, or "reverse rainbow" is that if you start cutting at the end of the board, as seems to be suggested in the linked document, you will need to take off much more thickness than needed to make the board flat. If, on the other hand, you press down nearer the center of the board, you can select a spot than makes the cut parallel to your chosen plane. Then if you keep pressing down over that spot as you make multiple passes over the cutter, you can flatten the board in a controlled manner: the flat grows with each pass, remaining roughly parallel to the desired plane.

So my contention is that if you do it right, there is no problem with "reverse rainbow". If you do it as I think the link is suggesting, there is a problem.

[JGrout]

Conventional wisdom is to joint boards with bowed boards presented like a rainbow.

This guy does the opposite, "the reverse rainbow:"

https://wunderwoods.wordpress.com/2012/0...ow-method/

Anyone care to discuss?

 I have far better luck losing stock in the ends where I can control the removal  and easily measure loss 

I prefer my stock to end up a thick as possible and taking the middle out does not always make that happen 

Besides the fact that working on a rocker is fraught with failure 

I can do it but is is much harder to control 

Joe

[Phil Thien]

The problem with concave up, or "reverse rainbow" is that if you start cutting at the end of the board, as seems to be suggested in the linked document, you will need to take off much more thickness than needed to make the board flat.

His solution seems to be placing the board past the cutterhead and onto the outfeed table.
I gave his method a try and the jointer didn't start to remove any stock until it got into approx. the center 1/3 or so of my boards (about ideal for the stock I was jointing).

After a few passes I could tell the jointer was cutting closer and closer to the ends, so I finally took a full pass.

While some dexterity is required to get past the guard, but jump the cutterhead, it wasn't terrible (although I wasn't trying with particularly long boards, maybe 4-1/2' or so).

I still use a benchtop jointer and it is very difficult to completely remove a bow from longer boards using the conventional approach.

This does, indeed, seem to work a little better for me, although I've only tried it a little so far.

[JGrout]

so now that you have done it that way turn your stock over and try it the center up way.

Three reasons to try 

1 you can set the stock on the jointer in the middle and if the head is turning it does not contact the stock  then just push forward from the end 

2 once you are close cutting (at least the last 40% of the length)  rotate the stock end for end and work on the second end. 

3 once you are close figure out which way created the least tearout put that end in the jointer first and make a final pass 

It is IME also about twice as fast and you do not have to think to make cuts providing your jointer is tuned properly .

It also doubles in essence the length of the stock you can flatten successfully.

but you have to try to understand; it is not impossible to do it your way, just inefficient

[Phil Thien]

so now that you have done it that way turn your stock over and try it the center up way.

Three reasons to try 

1 you can set the stock on the jointer in the middle and if the head is turning it does not contact the stock  then just push forward from the end 

2 once you are close cutting (at least the last 40% of the length)  rotate the stock end for end and work on the second end. 

3 once you are close figure out which way created the least tearout put that end in the jointer first and make a final pass 

It is IME also about twice as fast and you do not have to think to make cuts providing your jointer is tuned properly .

It also doubles in essence the length of the stock you can flatten successfully.

but you have to try to understand; it is not impossible to do it your way, just inefficient

Yeah that is the way I've removed bow until now.  It just isn't the best on a jointer that is only about 30" long.

For example, if you take a 5' long bowed board and put the center down on the cutterhead, a lot of the curve of the board is off the tables.  So while a board with a lot of bow won't hit the cutterhead, a board with a smaller amount might.

And it is difficult to push the board from the end, it can pivot easily, because the tables are so darn short.  So you are temped to put more downward pressure near the cutterhead, which doesn't help.

I wouldn't necessarily encourage anyone with a conventional jointer to use this technique, but it seems to work well for this one.

When I build my jointer, I'm going to use longer tables.

[Steve N]

A different perspective. I wouldn't have bought the board shown in the article. I've never needed wood so bad I would settle for junk where I was going to lose a large % of it making it right. If I bought a board and it became that way cutting it in half and using it for shorter piece which takes a lot of the curve out. If I was forced to work that entire length, it would be done crown up, but I would be disgusted with myself for having to.

[JGrout]

Yeah that is the way I've removed bow until now.  It just isn't the best on a jointer that is only about 30" long.

For example, if you take a 5' long bowed board and put the center down on the cutterhead, a lot of the curve of the board is off the tables.  So while a board with a lot of bow won't hit the cutterhead, a board with a smaller amount might.

And it is difficult to push the board from the end, it can pivot easily, because the tables are so darn short.  So you are temped to put more downward pressure near the cutterhead, which doesn't help.

I wouldn't necessarily encourage anyone with a conventional jointer to use this technique, but it seems to work well for this one.

When I build my jointer, I'm going to use longer tables.

you may find it odd then that when I was taught the process above the entire jointer was a mere 42" long  the infeed was 28" of that. I was tasked with flattening stock 8' long for door stiles.  The two things I did not mention was that I could choose the stock for length out of full bunks of lumber and two I bolted the jointer to the floor because the usual hardwoods in 8/4 would tip the jointer at the end of the cuts.  


I am capable of putting straight edges on 10/ stock and flatten 8' + on that short jointer and on a 8" jointer 14' stock and flatten 12' stock if need be I cannot even begin to fathom how to balance stock that long in the middle and end up with workable materials in the required thickness 

Additionally,   Table lengths have little to do with technique if it did you see hand planes in lengths way exceeding the length of a jointer plane   

Joe

[alnandy]

We teach our apprentices to joint both ways, concave up or down.   Some boards are easier to flatten one way, some the other; there is no "best" way.
As for the comment about not buying warped boards, I wish it were that easy.    We routinely work with olive, ebony, rosewood, etc.   We usually buy the entire log and have it custom cut (these are not usually large logs) and dried.   Tossing a very expensive, but warped, board isn't usually an option, so we have to figure out how so salvage the wood.   We've found that the "best" method for a particular board will vary.
One other issue is grain direction:  Figured woods usually have strong grain preference, which further complicates the choice of jointing method.