Woodworking Talk banner
1 - 8 of 8 Posts

·
Registered
Joined
·
54 Posts
Discussion Starter · #1 ·
I recently started using a jointer and noticed a test board go from being flat one day to being bowed the next day. This is a board thats been sitting in my garage shop for over a year. I didn’t plane the opposite face parallel because I was testing the jointer to see if it was cutting flat.

My guess is that it bowed because I didn’t plane that opposite face flat/parallel, but I don’t want to assume so I have a few questions. I’m trying to understand what caused this, and what I can do to avoid this from happening on future projects. Also just wanting to improve my basic understanding of wood movement and how it relates to milling.

1. What causes wood to move after jointing and planing?

2. I’ve read that removing an equal amount of material from both sides of the board will reduce wood movement after planing. I can see how this could cause the problem with the test board I face jointed. How precise does this have to be? What’s the best approach to achieving consistent material removal from both faces when planing? Just flipping back and forth?

3. Before jointing and planing lumber, what am I looking for in terms of moisture content. Is it a standard target like 6-8% or does it have more to do with the wood moisture content acclimating to the shop space it’s sitting in? If it has more to do with acclimating how do I measure this so I know it’s ready to use?

4. I’ve also read that people do two millings to deal with wood movement. A prep and final. Example 4/4 board is milled to 7/8 at prep and than a week later milled to 3/4 at final. My question about this process is how does milling in two stages help reduce movement? If milling releases tension in the wood that leads movement wouldn’t this still happen with two-stage milling?


Sent from my iPhone using Tapatalk
 

·
Premium Member
Joined
·
4,015 Posts
You've got the basic idea.

Wood can move due to 1) unequal moisture and 2) internal stress. Kiln dried boards generally have more internal stress than air dried, and becomes apparent when a board is ripped to width and the tension is released. Case hardening can be an issue with KD boards, especially thicker boards.

The time between millings its often where people go wrong. Very important to have equal air flow - the use of stickers, quiet airy location (not under a fan or heater, up off the floor, etc). Allowing enough time to acclimate is somewhat of a guessing game. Some people use a moisture meter I've never found it necessary, usually allow 2-3 days the wood will be equilibrated enough. How many millings or how aggressive kind of depends on the lumber and how stable it is. I'm usually looking at 2 rough millings and one final which more often that not is a drum sander to remove planer marks and bring into final dimension.

Important to rough cut material to length and width (or multiples) prior to milling, rather than milling long boards and cutting them up in parts - that's a recipe for disaster. For example if I have 4 parts 10" long, I will cut two 21" long boards, mill them and cut to length.

But - welcome to our world, meaning you can do every thing right and the wood will do what it wants. o_O
 

·
Registered
Property mgmt
Joined
·
448 Posts
2. I’ve read that removing an equal amount of material from both sides of the board will reduce wood movement after planing.
It’s not so much of taking equal amounts off each face, the issue is more of exposing the board to equal moisture from both sides. If you were to create a nice flat table top, and then paint or varnish only the top surface, wouldn’t be too long before top would cup or warp from moisture getting in from the unfinished bottom surface.
 

·
Registered
Joined
·
1,331 Posts
Adding a few thoughts...

Flat boards tend to be flat after milling too.
Warped/curved boards did that for a reason, and will always be much less stable.

I wait after milling, then take a lighter pass. It does reduce the amount of new wood that gets exposed, but I also look at it in terms of I would rather a stile or leg decide to potato chip before I turn it into a door or table than after.

I also like to think that making a light final pass is much faster, and I can do it on only the lumber I will use that day...so I can get important joinery cut the same day it is final milled.
 

·
Registered
Joined
·
1,360 Posts
I recently started using a jointer and noticed a test board go from being flat one day to being bowed the next day. This is a board thats been sitting in my garage shop for over a year. I didn’t plane the opposite face parallel because I was testing the jointer to see if it was cutting flat.

My guess is that it bowed because I didn’t plane that opposite face flat/parallel, but I don’t want to assume so I have a few questions. I’m trying to understand what caused this, and what I can do to avoid this from happening on future projects. Also just wanting to improve my basic understanding of wood movement and how it relates to milling.

1. What causes wood to move after jointing and planing?

2. I’ve read that removing an equal amount of material from both sides of the board will reduce wood movement after planing. I can see how this could cause the problem with the test board I face jointed. How precise does this have to be? What’s the best approach to achieving consistent material removal from both faces when planing? Just flipping back and forth?

3. Before jointing and planing lumber, what am I looking for in terms of moisture content. Is it a standard target like 6-8% or does it have more to do with the wood moisture content acclimating to the shop space it’s sitting in? If it has more to do with acclimating how do I measure this so I know it’s ready to use?

4. I’ve also read that people do two millings to deal with wood movement. A prep and final. Example 4/4 board is milled to 7/8 at prep and than a week later milled to 3/4 at final. My question about this process is how does milling in two stages help reduce movement? If milling releases tension in the wood that leads movement wouldn’t this still happen with two-stage milling?


Sent from my iPhone using Tapatalk
There are a number of reasons wood can move as has been stated below. Species can also be a factor. I find hard maple seems to move on me a bit no matter what I do (in the 3/4" range). Here is a process I would recommend to minimize these issues. Assuming you are using kiln dried lumber.
1. Sticker boards in shop and allow to acclimate
2. Joint one face and square an edge.
3. Plane opposite surface parallel and rip opposing edge parallel (S4S)
4. This procedure should be done oversize of final dimensions
5. Sticker boards over night
6. Repeat 1-2 to final dimensions.
I have been following this procedure for years and it has generally served me well.
 

·
Registered
Joined
·
28,789 Posts
Some wood can bow for no apparent reason. Facing the board on the jointer shouldn't affect it. Most of the time it's caused by an imbalance in moisture content from one side to the other. This can easily be caused by laying the board on a solid surface such as a work bench. Laying on a work bench moisture can get to the side facing up and not for the side laying down. If that happens and you have time you could put sticks or dowels under the board for a few days where air can get to both sides it may flatten itself out.
 

·
Premium Member
Joined
·
4,015 Posts
A common misconception is moisture but boards you can’t keep jointed are caused by tension issues, not moisture. Similar to a twist. It’s inherent in the board and there’s nothing you can do about it.

The solution is to use for shorter parts, twisted boards go in the burn pile.
 

·
Registered
Retired engineer
Joined
·
447 Posts
You ever rip a board and have it pinch the blade? Or it spreads out the other way? DrRobert said it: tension. Here's another way to look at it: Wood is an organic material. It's gonna do what it's gonna do, no matter how hard we try to stop it.

Not too long ago I bought a 2x4 at HD. It had a lot of knots in it, which I thought was odd, but I bought it because it was straight. Two weeks later it had bent itself into a beautiful C. Seriously. I looked at it again and realized this board was not cut from the trunk of the tree, it was a branch. It had grown out from a tree horizontally (that is what branches do) and it was big enough they could bet a few 2-bys out of it.

The top of this branch grew under tension while the bottom grew under compression. Take all that stress away and that branch/board is going to relax, and as it gives off its extended "aahhh" the side that was under compression gets longer as the other side, no longer under tension, gets shorter.
Don't waste your time on a board that won't stay straight. Every board is good for something, unfortunately sometimes its only use is firewood.
 
1 - 8 of 8 Posts
Top