Ripping hardwood straight

      Why do ripped pieces come out banana-shaped? July 9, 2002

Question
How do you rip solid hardwood lumber down to the width you need and wind up with straight pieces? For instance, you've got a board 4" wide and 96" long and you need a piece 2 1/2" wide and full length. Do you rip it way oversized, then straight-line one edge on the joiner and finally rip it to size?

Forum Responses
From contributor G:
It depends on how crooked the board is to start with. If it is close, a joiner will work. But if it is quite crooked, snap a chalk line and free hand the board through your table saw on the line. Flip the board over and rip oversize with your last cut to the fence. Flip and rip again. Usually you have a pretty straight board by now. If you want clean edges, don't be afraid to run it through the thickness planer on edge for each side (depending on the size of your board, of course). I have also used a router and straight edge on long, heavy boards.



From the original questioner:
What I meant to ask was this: how do you keep from getting bananas when ripping lumber? Often I'll start with a wide board that I've already got a straight edge on. Then I rip it into narrower pieces and end up with a bunch of banana shaped pieces.


If the lumber is properly conditioned at the end of the drying cycle, you shouldn't have much problem. Try a different lumber supplier to see if there is a difference. I dry my own lumber in a dehumidification kiln and was having the same problem. My kiln manufacturer taught me how to condition (stress relieve) the lumber and the problem was solved.


I cut a straight edge on all my lumber before I ever use it. Then I just rip my pieces to size and go to work. Once in a while I get a banana, but it is rare when you can't use that piece somewhere else in shorter lengths.

Contributor G, you are either braver than I or have a smaller motor on your saw, because I have seen what happens when the saw decides to throw something and it is not pretty (as evident by the hole in the steel door into my shop).



I've struggled with pieces that move after you have jointed and ripped them. Every time it goes though the saw, it relieves stress and moves again. The stress on this wood can be dealt with by cutting slightly oversize if the movement is not too bad. You can re-joint and turn on edge to run through the planer. As mentioned earlier, the problem could be solved at the kiln. Ever have a plank of pine pop apart in front of the saw blade because of the stress?


One method I use when bowing is likely is to lower my saw blade to a little over half the thickness of the board, then rip and flip and rip again. I know this sounds hokey, but the first cut will relieve the stress while the top holds the board straight. I still oversize a little and dress to size with the planer. Sometimes this can make the difference between finishing a project or having to stop and get more material.


I think your problem comes from heat buildup. Have you ever noticed that sometimes when you rip a board, it gets little black spots where you paused? Your board is getting hot enough to catch fire. There is a tremendous amount of heat being imparted exactly where and when you are simultaneously releasing stress in the board from the cut.

We have been using a whole lot less material since we started ripping our lumber on a bandsaw. If you feed in a board that is straight on one edge, it usually stays straight.



I rip lumber on a 16" table saw with a 1 HP power feeder. From time to time I will get a batch of lumber with a lot of internal stresses, but not often. But there are always a few boards that have a mind of their own and have to be straight line ripped again. Once I was ripping 16/4 maple and it closed right up on the blade. Popped the breaker. I had to split the board to remove the blade. It's the nature of the beast, I guess.


Please never, ever - for any reason - free hand any material, especially solid lumber, through a table saw. Don't try it. It won't work. You may kill yourself or someone else unfortunate enough to be present when you attempt this absolutely foolish stunt.


Wood will change size or shape immediately when you rip or machine it if it has drying stresses (also called casehardening) that were not relieved in the dry kiln by a process called stress relief (or conditioning). You have wood that has not been properly dried; that is, the drying operation did not complete the job.

People who can rip straight pieces have stress-free lumber. The stress that we are concerned about when ripping is called lengthwise stress or longitudinal stresses (or casehardening). In the text 'Drying Hardwood Lumber', there is a special section about longitudinal stress relief and measurement. Many operations only check for across the grain stress and miss this. Change suppliers of lumber and your problem will likely disappear, as most people do relieve stresses in drying.

Any changes in size or shape that occur after machining are related to moisture changes!

Incidentally, heat does not affect wood's size or shape. Only moisture changes affect size or shape.

Gene Wengert, forum technical advisor



Perhaps some of the stress has to do with drying, but some also comes from the way the tree grows, or so it seems to me. Having sawn logs on a band mill, I notice that sometimes the kerf remains strait and other times the end of the board is a couple inches in the air before I finish the cut. Likewise, some boards stay straight and some take a bow immediately.


I agree - don't ever free hand on your table saw! I never would - I love being alive and having all my limbs.

Fence alignment can be an issue here, too. You need a little clearance at the back of the cut, but a few thousandths too much and you get bananas.



The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor H:
Just a couple of things I would like to interject after reading...
1. Heat does change wood. When steam bending, only the outside perimiter gets wet; the rest of the board moves beacause of the heat and molecules.

2. The only way to cut a scribe that has been applied to, let's say, a piece of wainscot for an outside corner that needs to be mitered would be to freehand on the table saw. I definetly agree it's not the best thing to do and none of us want to get hurt. I've been doing this work for 26 years and freehand through my saw frequently, but I make sure I have control of the material and am careful not to get things in a bind.

3. I agree with everyone else - it sounds that the mill is not cutting the logs the correct way to insure the strength of the boards when used in the fininshed stages, and of course the moisture content is a biggie. I run air conditioning in my shop to keep the moisture content down, but I also have a small shop which is insulated and kept shut up as much as possible. Wood is like a sponge when it comes to moisture. If you aren't doing something about it in your shop, all of your projects will suffer in more ways than one.



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