Machining FAQs for solid wood

Below is a compilation of questions and answers pertaining to lumber and best practices for solid wood machining operations, as submitted to Dr. Gene Wengert, the Wood Doctor.

Q. We are seeing some end checking, and rarely some face checking, in the solid oak lumber that we manufacture. The checking is right after finishing. We are thinking that our ovens might be too hot, or maybe the residence time is too long. Any guidance?

A. The checks you are seeing are because (a) either the wood is weaker than normal or (b) if the end checks are at the glue joint, the glue joint is weaker than normal. The most common reason for weak wood is that there is a preexisting check, oftentimes an end check from drying that was not totally eliminated during the cutting of the blanks. The very end of an end check is oftentimes easy to see in rough lumber, especially on a dry day, but the lengthwise extent of the end check is often hard to see precisely, especially in humid weather.

Now, what happens in the dry oven, is that there is a small amount of drying and shrinkage, but the strength of the wood right at the end where there is a closed end check is zero, so the stress opens this preexisting check. In my experience when analyzing rough mills, this is the reason for end checks in wood 99 percent of the time.
Another reason for an end check opening, especially in the wintertime, is that the glue line is weak. A properly made joint will be 1-1/2 times stronger than wood, but if the ends are not properly glued, then the strength of the joint is much lower. A little drying in the oven means the ends dry and shrink a little bit, creating stress that exceeds the strength of the weak end joint.

The #1 reason for poor end joints is that after the individual staves for a glued panel were prepared, the ends shrank a small amount so there was a small gap (maybe only 1/100 inch) at the ends; most adhesives do not bridge this size gap and maintain their strength.

Q. We have some lumber that we planed on our two-headed planer. We are certain that the lumber was flat going in and coming out. We then stacked the lumber and now about four weeks later, we are seeing a lot of pieces that are bowed in the same direction…ends down and center up. Where should we look?

A. Because all are bowing the same direction the issue is caused by the planer. I suspect that your machine is set-up incorrectly and is taking more wood off the top face than the bottom (or vice versa). This creates a moisture gradient; when the moisture balances out after a few days, the small amount of lengthwise shrinkage on wetter side causes this uniform bow.

Q. What is the difference between No.1 Common and No. 2 Common lumber?

A. The difference is the amount of large clear area. No. 2 Common has less clear area than No.1 Common. But if you took a saw and cut out the defects in No.1 Common and compared it to the amount of defects in No.2, the difference will be only a few percentage points. That is, you will get nearly the same yield, but the pieces will be smaller on average with No. 2 Common.

Q. When edge gluing, what will provide the better edge prep, a moulder or a rip sawblade? We rip now.

A: Appreciate that the strongest glue joint is made when the edge surface is made no more than about 15 minutes before gluing. We also need a gluing surface has a clean cut. A sharp moulder blade or cutter creates an ideal surface. However, you do need to pay attention to keeping the cutter sharp.
Concerning rip saws, you need a ripsaw blade that has the correct tooth design and then has sharp sides on the tooth.
One quick test for a surface to be glued is to put a water droplet on the surface. The droplet should be absorbed within a minute if the surface is ready for gluing. This droplet test does not tell you, however, if the fibers have been damaged.

Q. We have some white oak that has a dark outside color, but the inside is whiter in color. Do you know what is going on?

A. This color difference is caused by partial air drying followed by normal kiln drying. In slow air drying, the shell develops the darker colors on the outside regions.
The wet core, however, is not yet drying much, so the color is not yet established. When the lumber goes into the kiln, the wet core is dried quickly at higher temperatures, which gives a lighter color, compared to air drying colors. It is important, and a basic drying principle, to either kiln dry green from the saw, or air dry more thoroughly in air drying so that the incoming MC into the kiln is in the 20-30 percent MC range, rather than over 35 percent MC.

Q. We manufacture moulding and buy FAS grade lumber that sometimes is cupped edge to edge so much that it gives us problems. Can you please clarify the NHLA Rules, especially paragraphs 14 and 61? Our supplier hides behind these rules. I think we might have improperly graded lumber.

A. It is my experience that the seller of dried hardwood lumber almost always tries to deliver the quality that the customer needs, rather than use the rules as an excuse for lumber that the customer cannot use efficiently. So, I do suggest considering another lumber supplier that cares more about your needs.

When grading FAS lumber, the rule concerning cup is that the grader is allowed to consider that the lumber is going to be planed down to "standard surfaced thickness."
Using an imaginary, perfect planer, if the cup will surface out for the entire piece of lumber, then cup is not a grading defect. For 4/4 lumber, after kiln drying, standard surfaced thickness is 13/16 inch, which means that FAS lumber can be somewhat cupped.

Also, pieces 12 inches and wider can be ripped with the grader's imaginary rip saw, and then the two pieces measured for cup separately? This ripping will automatically reduce cupping to one-quarter of the original value. Again, FAS wide lumber can be somewhat cupped.
Note that the piece of lumber must be flat and not bowed. Also, with side bend warp, the clear cuttings must all be in the same axis, and cannot curve around with the warp. Even a small amount of side bend will lower the grade to No.1 or lower; likewise, a small amount of side bend will greatly impact your yield and cost. Twist warp is also a big loser.
For No.1 Common grade and lower, flatness applies to the clear areas used for grading and not the entire piece of lumber.

Q. What is the difference between the hardwood lumber grades of No.1 Common and Select or FAS-1 Face (also called “Face”)?

A. The lumber grades were developed and are overseen by the NHLA. The difference between Select grade lumber and No.1 Common lumber is that when grading both, we look at the poorer of the two faces to determine the grade. For both No.1 Common and Select, the poorer face is graded as No.1 Common; that is, the poorer faces are the same.
The difference between the two grades is the better face: When grading No.1 Common, the better face is the same or better than the poorer face. When grading Select, the better face must grade FAS, which means that the better face of Select must be clearer than the better face of No.1 Common. There are also some edge wane restrictions in Select.
Overall, this means that if you are making clear-2 face cuttings, it is the poorer face that determines the yield, so (except for the wane improvement in Select), you will get nearly identical yields when cutting either grade.

With FAS-1 Face grade, the lumber grading rules are identical to Select with one difference. FAS-1 Face lumber must be at least 6 inches wide (Select pieces can be 4 inches). Cut-up yields should be slightly higher than for No.1 Common.
For the best grade, FAS, the lumber on the poorer face must be somewhat clearer than for No.1 Common, so you do get perhaps 25 percent higher yields when cutting this grade.

Q. We bought some No.1 Common kiln-dried hardwood lumber and it is warped up and down along the length. This cannot be used to make our larger doors, as the pieces we cut will not be flat. The supplier says that warp is not covered in the grades. Is this correct?

A. It is incorrect. The NHLA rule states that for Common grades, like No. 1 Common, the clear areas, called cuttings, used to establish the grade, "shall be flat enough to surface two sides to standard surfaced thickness after [the cutting] has been removed from the board.” So, in practice, you will find that the clear cuttings will often be 4 or 5 feet long or longer in the No.1 Common grade, and so that region must be flat.
The rule goes on to state that "In the grades of Selects and Better the entire board must be flat enough to surface two sides to standard surfaced thickness." which is quite restrictive for flatness.

Q: We see some claims that automated crosscut saws have the potential to improve yields and lower costs. Do you agree?

A. Yes indeed, such claims are valid. They are valid because the computer and scanner work early on Monday morning as on Friday afternoon, but they also work for 10 hours a day without a break. Further, the computer can make a bunch of imaginary cuts and test solutions before arriving at the best solution. Plus, we can teach the computer that a 22 inch long cutting is worth a lot more than two 12 inch pieces — lower yield but longer cuttings is worthwhile to some extent. Note that these last two items are looking at value and not just yield alone.

Where we really see an improvement in both yield and value is when we have a variety of lengths. You can probably imagine that with just one length, the yield decisions would be pretty easy and a human could do about the same as a computer. A lot of lengths and varying values, the human would need lots of luck to be competitive.
When we look at the cost of furniture or cabinets, we might find that 75 percent of the cost is the wood itself. So, this means that managing the cut-up costs (higher yield and more value with a computer) will be a huge benefit.

The actual numbers or benefits will vary from one operation to the next. I would easily expect to see many operations looking at 5 percent improvement and some doing even more when they fine-tune the overall operation.

Another improvement will come when instant yield or cost data is related back to the raw material sources and back to lumber sawing, grading, drying procedures. Some suppliers, for good and bad reasons, will affect yields and costs quite substantially, yet subtly. With this instant yield info, you can find out why and where you are potentially losing money and do something about it. Just one example, you can easily see the effect of proper end coating of the lumber right after sawn, boosting yields several percent, as well as reducing rejects later in manufacturing.

Q.  We are noticing that our plant yield seems to be gradually decreasing, year after year.  What comments or direction can you give, please?

A. The best “answer” to this issue is to have an experienced person (like the Wood Dr., but there are others) spend a day at your plant analyzing your production.  Without an inspection, here some ideas that you can possibly use to direct your own investigation.

• Grade: It should be appreciated that rough mill yield is greatly affected by lumber grade.  For example, when cutting clear 2-face strips, we could see around 70 percent yield for FAS, 59 percent for No.1 Common, 43 percent for No.2 Common and 30 percent for No.3A Common.  Therefore, changes in yield over time might reflect changes in the incoming lumber grades. (Note that the basic grade rules have not changed substantially for decades.)

Sometimes a mixed grade can be an issue; with No.1 Common and Better, we do not see much “better.”  Another possibility is that the lumber within one grade is being sorted into two groups and you are getting the poorer group.

End Splits:  Although they do not affect the lumber grade very often, end splits can realist in a loss of yield of around 4% or so.  It will be more with shorter lumber — about twice as much for 8-ft versus 16-ft lumber.

Lumber Width:  Lumber width also affects yield with narrower pieces having lower yield.  It is really an issue when the sawmill makes certain width pieces and the rough mill has only a few widths at the rip saw.

Part Size:  Yield is also affected by part width and length; wider and longer greatly affects overall yield.

• Drying Quality: Drying can also affect yield, especially with respect to surface checks for oak and stain is some other species.  The climate that last few years has been drier and warmer than we are used to, so modification of air yards may be required in order to maintain quality drying. Note that warp is not a controllable factor in drying, except for some cup.  Smaller trees generally mean more warp.

• Shake & Pith: Shake (splits running parallel to the rings) is a grading factor, as well as pith (the bullseye center of the growth rings).  Both drop yield very badly, even though the lumber might have a reasonable grade. That is, a piece that is No.2 Common because of the location of the knots will have a higher yield than a No.2 Common piece with pith or shake. You might consider discounting pieces with pith or shake, as such pieces are expensive for you as their yield is low.

Q. We have some white oak lumber that has a dark outside color, but the inside is whiter in color, which creates a big issue. Do you know what is going on?

A. I am familiar with this defect where the core is much lighter in color than the shell. It happens with other species as well as with white oak. This color difference is caused by partial air drying (rather than complete air drying) followed by normal kiln drying. In slow air drying, the shell develops the darker colors on the outside regions that are drying. The wet core however is not yet drying much, so the color is not yet established. Then, when at rather high MCs, when the lumber goes into the kiln, the wet core is dried quickly at higher temperatures, which gives a lighter color, compared to air drying colors.

It is important, and a basic drying principle, to either kiln dry green from the saw, or air dry more thoroughly in air drying so that the incoming MC into the kiln is in the 20-30 percent MC range, rather than over 35% MC.

Q. We are having problems with our door panels cupping and the glue joints showing. Any insight?

A. Wood moves, including cupping, and glue lines are seen due to excess pressure when the wood warps, only when the wood's moisture changes, so the wood is too wet for your plant's environment. You should check the incoming lumber's MC everyday...several times a day if a lot of lumber is being most cases, nothing wetter than 7.8% MC is a good target. Chances are that your shop and your customer, during the heating season have humidity that averages around 35% RH which means the wood will get to 7.0% MC. Your shop should be no drier or wetter than this...within 3% RH. These conditions are typical for much of North America, but a minor amount of fine-tuning may be needed in a few cases.

Many people check the MC of incoming lumber with a pin meter using insulated needles...a good choice. Drive the needles half the lumber's thickness to get the core MC. But, it probably would be best to drive them 1/4 of the thickness, which will give you the average MC for the piece. This is a critical and useful number.

In most cases, the core MC is the moisture level that the lumber achieved in the kiln. Poor shipping or poor storage may change the outside MC, but it takes a big moisture difference between the air and the lumber and many months to change the core MC. True unless the lumber is exposed to liquid water.

Q. What’s the optimum number of knife marks per inch when planing?

A. To answer your question, I need to know, “How much sanding do you want to do and how smooth does the surface need to be?” The best for many cases is between 12 to 20 marks per inch with the higher number meaning a smooth planed surface with less sanding to be really smooth. The lower number meaning a bit rougher and perhaps an experienced eye will see knife marks if sanding is not extensive. (There is variation in species when assessing the best number of knife marks.) When sanding, remember that moisture content and sharp sandpaper are also important factors.

Source: Gene Wengert, the “Wood Doctor,” has been training people in efficient use of wood for 35 years. He is extension specialist emeritus at the University of Wisconsin-Madison. He can be reached at [email protected]. Read his columns at


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Gene Wengert

Gene Wengert, “The Wood Doctor” has been training people in efficient use of wood for 45 years. He is extension specialist emeritus at the University of Wisconsin-Madison.