Typical Lumber Thickness Variation

      Rough sawing produces lumber of variable thickness that has to be planed to finish dimensions. But how much variation is normal and what causes it? May 13, 2013

Question
I had some oak and pine logs sawn into one inch boards and some of the boards vary from as much as 1/16 to 1/8 of an inch from end to end. I was told that every mill makes lumber the same way and if I wanted one inch boards I would need to put it through a planer. If that is what everyone else does, oh well, but what do I do with the 15/16 inch boards?

Forum Responses
(Sawing and Drying Forum)
From contributor D:
You should have had them cut to 1-1/8 and then planed them down to 1 inch.



From Professor Gene Wengert, forum technical advisor:
For hardwoods, we need to specify that this is for green, never-dried lumber. It is common to make the thinnest piece 1.00". (Lumber that is being sold and that is thinner than 1.00" in the area used to establish the grade, is called 3/4 and not 4/4.)

Because of variation, the average is larger than the minimum (often 1/16" or so) and the thickest piece is even larger. Many accurate mills will have an average of 1-1/16" or maybe even 1-3/32" average. Less accurate mills will target 1-1/8". The variation along the length does exist, but if it exceeds the standard (for example, 1/4" difference between thickest and thinnest in 4/4 in the grading area), then it is measured and graded normally, but the word "miscut" is added to the description.

Once the lumber is dried, 4/4 can be 1/16" thinner than above for the minimum.

Of course, if the customer wants thicker lumber, they can specify it. However, each 1/32" thicker is 2 to 3% yield loss.

For softwoods, the requirement is that 4/4 lumber must be 0.75" thick at the time of grading. You could grade it green, air-dried, or kiln-dried. For each case, you would have a different target thickness at the sawmill. However, with about 4% shrinkage in drying maximum and with a planer allowance of 0.09" or less, the green minimum sawn size for 4/4 lumber that will be kiln dried and then planed, is somewhere close to 15/16".

So, without specific instructions, it sounds like the pine is fine and perhaps the oak is a bit thin... at least a few pieces.



From the original questioner:
Thank you for the information. The dimensions of my lumber are not real important for what I plan to do with the boards. I was just interested to know what is and is not acceptable. Had I known beforehand, I would have had the lumber sawn a little thicker.


From contributor H:
Well, not every mill makes lumber the same way, and after reading this it makes me feel pretty good about our lumber variation. We run a Timber Champ 3 blade circular saw. If I have 1/16" variation anywhere down the run of an 18' board, I'm looking for a problem. I must be very lucky to have a saw that cuts so tight to the numbers. I frequently spot check the dimensions in 3 places down the run of whatever we mill and there is no variation. I might detect 1/32" variation once in a while, but rarely. It is possible to cut very tight to the numbers with a well built, properly set up and sharp saw. Circular that is - they make more sawdust but do cut true.


From Professor Gene Wengert, forum technical advisor:
I have done many mill studies where I measure thickness. We measure to the 1/1000". We measure in six places on the board... three on each edge. The best mills will have plus or minus 0.045" variation total between the thickest and the thinnest, about 3/32". This variation is caused by both variations within a single piece (caused by saw wobble, for example) and also variations from piece to piece (caused by set works repeatability, or by log stress, for example). As mentioned by Harlan, when the number gets much bigger than this, there is likely something wrong. However, lots of mills cannot afford to fix things until they get a lot worse.

In the many mills I measured in Virginia, the best was a band mill, but there were several circle mills that were very close, as well. There were some band mills that were poor, as well as some circle mills. It is incorrect to assume that circle mills are always poor or that band mills are always good.

Many states had a utilization specialist in their forestry department that conducted thickness studies for free... Maybe your state has this service.



From contributor D:
"We measure to the 1/1000"." Wow! I wouldn't think that would be possible. Did you use calipers, micrometers or what?


From Professor Gene Wengert, forum technical advisor:
Digital micrometer. Of course, with surface roughness, you do the best that you can. We take 30 pieces and get 6 readings per piece. We take the readings in a specified order, so that #1 and #6 are the thicknesses (one on each edge) of the beginning of the piece of lumber as it enters the saw, #2 and #5 are in the middle, and #3 and #4 are the tail end. It is somewhat amazing what trends and problems that are developing we can see.

Note that the customer is almost always concerned about the thinnest piece of lumber, not the average or thicker pieces. In my studies in VA, it was surprising to see the variability from mill to mill on the actual size of the invest piece.

Note that statistics are used to determine the thinnest piece, so the thinnest piece can be thinner than any of the 30 that we measured. Also, we are able to suggest an adjustment of the target average lumber size to assure happy customers.



From contributor H:
My world is not perfect. I do get some variation now and then and I did use calipers until I got tired of running back to the house for them. Now I just use a quality tape measure as I can quickly put the tape on the board and read dead on the line numbers. Most generally good enough for what I am doing. Still get the calipers out now and then to cross check though. This saw is fairly tight as I have spent the time to dial it in within less than 1/32" in 21' span, front to back, and this "tight to the numbers" is with 4/4 and 8/4 material, clean wood and sharp teeth... and paying attention.

I have noticed some variations in larger cuts and thought it could be from saw wobble and running dull teeth. I believe Gene has hit the nail on the head. Possible saw wobble and/or stresses within the wood. I have seen boards (now firewood) lift off the log and peel to the side before the mill was 2/3 down the log. It makes me wonder with the variances in MC within the log at various times (just fell or been in the yard for a while) that with the right conditions (internally dryer and compressed or stressed) it might be possible for the wood to expand almost instantly, up to 1/32", after getting milled, depending on the size and thickness of material. Gene, could releasing a dryer compressed wood cause a slight expansion?



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