Lumber Thickness and Sawing Tolerances

      Here's a thoughtful discussion of the risks and benefits of trying to rough-saw to fine tolerances in order to get more board feet per log for a given finish lumber thickness. February 14, 2010

Question
When sawing cherry, what are the dimensions I want to cut my boards? If someone asks for 4/4, 6/4, or 8/4, what do I actually need to mill them to? If someone asks for 4/4 (1"), should I cut them to 1 1/8"? When selling 4/4, and milling them to 1 1/8" thick, do I charge for the 1" or the 1 1/8" thickness?

Forum Responses
(Sawing and Drying Forum)
From contributor S:
I contract cut for a local hardwood mill and they request that 4/4 be cut 1 1/8" for a finished size of 15/16". I believe that 4/4 (and under) is sold as 1" when calculating board feet, whereas 5/4 is 20% more. Not real sure on that, as hardwood is not a big thing where I live and work on the left coast, where everybody's thinking is stuck in softwoods.

Some of the big western mills that are into big box store sales are providing hardwoods 1" nominal (3/4" actual) that are selling as if they were 4/4. Widths are also nominal, e.g. 6" is actually 5 1/2". Within the rule book but kind of a rip-off to the unknowing. And much of the oak boards are glue-ups to provide wider boards - rip-off times two.



From contributor A:
You should saw them 1 1/8 unless they request something different. 5/4 is 1 3/8 and 6/4 is 1 5/8 and 8/4 is 2 1/8. This way if you get excess and have to sell on the wholesale market, you will not be rejected.

Sawing thinner may make an extra board, but if you lose a board for being too thin, it is not worth the money or time. If you are sawing and drying for your own wood, then take the chance because you can sell the thinner stock.



From Professor Gene Wengert, forum technical advisor:
Very many mills today saw hardwoods for an average of 1-1/16" thickness; 1-1/8" was the most popular years ago when mills did not have good thickness control. The ultimate requirement for 4/4 is that it cannot be under 1.00" in the section of the lumber used to establish the grade; waney areas, for example, can of course be thinner. As all mills have a little plus and minus around the thickness, an average of 1-1/16" assures (for most modern, good mills) that the minimum of 1.00" will be met.

However, if you are air drying the lumber first and then selling, the minimum thickness requirement is still 1.00", but you may have 1/16" thickness shrinkage in air drying on some pieces (many will be closer to 1/32"). So, then you need to cut 1-1/8" to assure 1.00" after air drying on all pieces.

Note that each 1/32" of added thickness is roughly 3% in yield, so cutting 1-1/8" when 1-1/16" is acceptable for the customer, will cost you 6% in yield, or 6% in value. Stated another way, someone that wants 1-1/8" should be paying about 6% more for that lumber compared to 1-1/16" thickness average. If they do not pay more, then you are losing 6%, as your yield will be 6% less.



From contributor W:
The standard around here is 1 1/8". I drop my saw down 1 1/4" for each cut and with the kerf, I end up with between 1 1/8" and 1 3/16".

If you are selling wholesale and the buyers are real picky, then you run the risk of having your whole load rejected because of a few thin boards. You need to run your lumber business with enough margin that you don't have to worry about 1/16" of thickness.

If I saw pine siding, I'll drop down 1" and the boards will be about 7/8"+. If the customer wants to plane these boards smooth, though, he'll have to go below 3/4" when it's all dressed.



From contributor H:
Always saw 1/8 over nominal, unless the customer is leaving it rough. From a planing mill standpoint, thin wood is junk!


From contributor L:
You cut what the customer asks for at a price you agree on, usually set by the customer.


From contributor A:
1. If I saw all day at 1 1/16th, how much more lumber will I make if I am sawing 5 mbdft a day?

2. How many more cuts do I need to make on a face to gain a board if my blade is 0.055 thick and set at 0.022?

3. I buy hardwood logs down to 10 inch tops, and say the average log size would be 16 inches in a daily average (large butt logs over 20" are quartersawn). So what is your average log size?

4. If with a good 16" log I make a 6x8 tie, how many boards will I make at 1 1/8th? At 1 1/16th?

I ask this not to be a smart aleck, but to let you know that when the blade hits the wood out here at the mill, things do not figure the same as they do on paper. I get 13 to 14 boards either way, just slabs 3/16th thicker and a bit wider face on the boards of the 1 1/16th cut.

Now on stock that you are going to dry and plane, you may look at it like this. There will be one less pass in the planer and any board that is too thin may be sold as thinner stock. The slightly wider faces may look better, but the board will edge out the same after planing since you are just removing the wood at a later date. Wood chips are selling good right now. Just something to think about while you are eating sawdust.



From Professor Gene Wengert, forum technical advisor:
Good questions. With a thinner average target size, there will be a few logs that allow you to get an extra piece of lumber. There will be a few logs that allow you to get a wider piece, wide enough so that the footage increases. There will be a few logs that allow you to get an extra foot or two of length. There will be a few logs that produce the same footage at either target size.

Note that to cut 1-1/16", a mill needs to have very little variation in thickness. Indeed, many of today's mills do have much less variation in thickness than the older mills had. In fact, it is indeed possible that the 10% thinnest pieces with a modern mill cutting 1-1/16" will actually be thicker than a mill cutting 1-1/8" with wider variation. So, at 1-1/16" from a modern mill, the customer will find that the thinnest pieces are actually thicker than in the past or from a poorer mill.

The shrinkage in drying to 12% MC (softwoods) is on the order of 4% or just over 1/132". For hardwoods, drying to 7% MC, the shrinkage is on the order of 1/16". These numbers are for quartersawn lumber which shrinks nearly twice as much as flatsawn. So for a flatsawn mill, the thickness will be even greater after drying.

Bottom line: On a modern mill, it would be worthwhile exploring a thinner average target size due to the potential increased yield... 3% increase for each 1/32" thinner.



From Professor Gene Wengert, forum technical advisor:
The numbers above on shrinkage are for 1" stock. For nominal 2" softwoods, add 50%. For 8/4 hardwood, double the values. (Of course, the percentages would not change.)


From contributor H:
Gene, you are part of the problem, telling people it's okay to saw thinner. It's not okay. I have never seen lumber that is sawed that consistent. If your blade dips just a little, that spot might not clean up planing. Then what? Your 3% is out the window. Some boards will shrink more than others. We custom plane and cut from dozens of small mills. I fight this problem all the time. Also each wood shrinks in width differently. That needs to be figured in. We custom plane about 750,000 bf per year.


From Professor Gene Wengert, forum technical advisor:
You did not read my message accurately. I stated a fact and that fact is that many mills are cutting 1-1/16" for hardwood lumber. I am not sure what problem I am part of, but reporting a fact about what mills are doing and also what the requirement for lumber is (unless modified by a customer) is certainly not an issue.

I work frequently with one mill that is nearby and they cut over 3 million BF a year at 1-1/16", and their lumber is highly regarded. In fact, in this economic downturn, they are still doing very well.



From contributor A:
The good Doc said that the saw must be set up and sawing well. And even in a small operation like mine cutting 5mbdft a day, if I lose 10 boards for small dips but make an extra 150 bdft, I will gain the 100 extra bdft.

It is people like contributor H that end up eating that 50 bdft because it did not plane out. Now is where the problem comes. After a while he figures out I am the reason he is eating 50+ bdft a day (250 a week) and no longer takes my lumber. Now I have lost a market for my gain of just $250 a week and have to figure out where to go with that 25mbdft of lumber. Can not afford to lose a market right now, and contributor S can not eat that much fiber.

For in-house market I may take the gamble, but for my wholesale markets I do not. I like having wholesalers calling me wanting more lumber every week.



From Professor Gene Wengert, forum technical advisor:
Again, please note that a 3% increase in yield is gained by cutting 1/32" thinner. Even if one would cut 1/64" thinner, that would be 1.5% increase. The challenge is to see how much over-thickness a mill is cutting. If someone says they are cutting 1-1/8" average thickness, how accurate are they? Are they measuring the thickness to the closest 0.001" or to the closest 1/32"? I have measured the average thickness (by measuring 6 places in a 30 piece of lumber coming from the headrig, a resaw, or a gang edger) at over 50 mills. I can tell you that the actual average is seldom what the owner claims it to be and variations are often more than 1/32". Thickness is not measured with a steal tape; a micrometer is used. Many state forestry offices do a thickness analysis for free.

Finally, in most, if not all cases, the customer is worried about the thinnest pieces and not the average thickness. It is therefore impossible to come up with an average thickness if one does not know the thinnest piece that can be used by the customer.



From Professor Gene Wengert, forum technical advisor:
Lumber thickness is measured (in most cases) about 1" from an edge at three locations (one near the end, one near the center length, and one near the other end). Then switch to the other edge and measure three more (end, center, end). It is important to measure in the same order on each piece. This allows a more thorough analysis of thickness patterns. Thirty pieces should be measured.

When we measure the thickness of lumber coming off of a particular machine, we get a variety of thicknesses. If we plot the thicknesses on a histogram (or frequency graph), we see that the thicknesses form a bell-shaped curve (called a normal distribution). The average (or actual target thickness) will be the peak of the curve. If, in addition to calculating the average thickness, we calculate the standard deviation (SD, done with the push of one button on a $20 calculator), then we can determine the spread of the data. The average thickness plus one SD, minus one SD, will make an interval that will include roughly 2/3 of the thicknesses. Plus or minus two SD will include 95%. A typical SD on a good mill is 0.015. So, if the average thickness is 1.063", then two-thirds of the thicknesses will be between 1.048 to 1.078". Ninety-five percent will between 1.033 to 1.093". And virtually all will be between 3 SD, or 1.018 to 1.108".

From the calculations above, we can determine if the thinnest or thinner pieces are too thin. For example, in the data above, 2-1/2% of the pieces will be between 1.018" and 1.033" (roughly 1-1/64" to 1-1/32"). Also, we can ask ourselves if we can afford to be 100% perfect. Sometimes it is better to make a few thin pieces (thin by a few 1/1000") and give the customer a refund than to make every piece overly thick.

Note that by measuring the thicknesses in a particular order, we can also determine if the leading end of the lumber, the middle, or the tailing edge is often thin. We can also determine if the lumber has wedging or bevel, if the saw itself is causing the variation in thickness, or if it's the log that is moving (or a combination of both).

Also, note that many of today's band mills (such as the Wood-Mizer LT-40HD) produce lumber with a very smooth surface. Such lumber requires less planing after drying, as the wood is already quite smooth. This means that the target thickness at the sawmill can be reduced slightly. Also, we are seeing more knife planers with opposite heads that remove equal amounts off of both top and bottom surfaces, so a thinner piece will have a good chance of cleaning up on both faces.

As a final comment, one might think that cutting lumber 1/32" too thick just costs 3% in profits. But it is well to remember that it is also costing 3% more timber harvesting (do we really want to waste our resource?), as well as the fuel and energy used for manufacturing. Our forest resource is just too valuable to waste (and not just money value).



From contributor H:
This is all good on paper. All I'm saying is this: why take the chance on lumber that might not clean up? The most accurate mill will still cut thin boards. There is human error, dirt, log stress, bowed logs, dull blades, etc. So if 1/32 is 3%, what is a 1x12 scrap in percentage? If your motor will run at 8500rpm, isn't 8000rpm safer?

Bottom line is, why take the chance? So you cut at or under 1-1/16 for your customer, and he brings the wood to me for kiln drying and planing. We can't get all the boards to clean up at 3/4. Whose reputation is at risk, yours or mine? It's me, because I handled it last. Did we do anything wrong? No, but the customer thinks we did. So I'll say it till I die, thinner cut wood sucks!

Gene, I do respect you and the work you do for the industry, so don't take it personal.



From Professor Gene Wengert, forum technical advisor:
Thanks. But I do wonder if you can reduce the average thickness by 1/64". As I was trying to say, when someone says 1-1/8", do they really mean 1.125" or do they mean somewhere between 1-7/64 to 1-9/64" (or even larger if they use a steel tape). Second, by doing a thickness study, we are often able to determine the source of thickness variation and subsequently tighten up the mill and reduce the variation, which means the thinnest piece of lumber is now thicker than it was before. This then means we can reduce the target size.

This is more than "on paper," as the US Forest Service has done hundreds of sawmill improvement studies and has documented the results and the improvements in target size. It is certainly possible that your mill cannot cut a smaller average thickness, but many mills can be improved so their variation is reduced and target is safely decreased.

A stumbling block to accomplishing this is that sometimes a customer says they want 1-1/8" lumber; however, a check with them shows that it is the thinnest pieces they are concerned about and not the average (which is also what you indicated in your last post). So, the real key is not to indicate an average size but rather to indicate the minimum thickness (or maybe the minimum thickness for 99% or 98% of the pieces).

Not only have I done many sawmill thickness studies and helped many of the mills improve, I also managed a mill for several years, so I appreciate the reality of sawmilling too.



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