Dealing with tearout in hard maple and other FAQs
September 21, 2021 | 12:45 pm CDT
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Q: I have some hard maple that is giving me a lot of small spot tearout or maybe you call it chipped grain. What is the cause and cure?

A: There are likely two issues that together combine to create this tear-out issue. Let’s do a little detective work.

Appreciate that hard maple has a lot of small pockets of swirly grain. When it gets really bad, we call it bird’s eye. With these swirls, we are planing with the grain for a moment and then against the grain for another moment. That increases risk of tear out. Machining must be close to perfect, and wood properties affected by drying must be under control. We can minimize the risk, working on both fine-tuning our machines and man-affected wood properties. Remember to look for two possible items that are out of “normal.”

Machine issues
When a knife comes in contact with the wood, if the blade itself is too slender (large rake angle), the knife will behave like a chisel or wedge with the wood failure traveling ahead of the knife, following the grain.

Eventually, the knife will begin to exit the wood and a big hunk of wood will be broken or chipped out. This chipped piece will leave a hole or void in the wood if the grain at this point was diving into the wood; that is, if we were planing against the grain. A second machine concern is that the knife needs to be really sharp. HHS is probably a better knife for obtaining a smooth surface, compared to carbide.

A small depth of cut and a slower feed speed will also provide a better surface. Deep cuts and fast feed will encourage chip out. Feeding too slowly and taking too light a cut will create heating, fast dulling, and compressed fibers that create a rough surface, including grain raising, during finishing.

Wood issues
There are two things we can do in drying of lumber that make the wood quite brittle and subject to chip out. First, we can over-dry the wood in the kiln. Even though we can equalize and try to bring the moisture content up to a higher value, the brittleness is still there. Never run the kiln under 5.0% EMC at any point in the cycle, Kiln operators know the terminology, but what it means is that no piece will dry under 5.0% moisture content. The other contributor to brittleness in drying is to use a kiln temperature over 160 F.

Q: We are getting a lot of grade hardwood lumber with sideband. I know that sideband is affecting our yields. How much sideband can lumber have? How about cup?

A: Sidebend, also called crook or lengthwise warp, is a serious factor affecting yield of long pieces. Cup affects the yield of wider pieces.

SIDEBEND: The NHLA Rules for hardwood lumber are based on the volume of rectangular, clear areas, called clear cuttings. All these cuttings, for a piece of lumber of any grade, must be on the same axis.  This means that they cannot curl around and along the lumber piece with sidebend. (See drawing.). With sidebend lumber, just an inch or two of sidebend can make the clear area smaller and unable to make the best grade.

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CUP: The NHLA Rules for cup require, for FAS, FAS One Face, and Select grade lumber under 12-inch-wide that the entire piece must be able to be planed or surfaced to a standard uniform thickness. This rule means that the amount of cup in the best grade of lumber must be very slight, especially if the lumber is not overly thick.

For No.1 Common and lower grades, the Rules require that only the clear cutting areas used to establish the grade must be able to be surfaced to standard thickness. In this case, each clear-cutting is considered separately. In effect, some cupped lumber can be tolerated in lower grades without changing the grade of the piece.

Standard thickness for 2/4 to 7/4 lumber is 1/16 inch thinner than the nominal; for instance 4/4 (or 1-inch thick) lumber has a standard kiln-dried thickness of 15/16 inch.

Q: What are some concerns about using urban lumber?

A: One big concern with urban lumber is that there can be small bits of metal or other foreign objects in the wood. This can certainly damage knives and saws and tear sandpaper.

Another concern is that it is common to have damaged roots, which allow bacteria and maybe insects and fungi into the tree. Fungi stain the wood and can weaken the wood. Insects leave holes and tunnels, but are usually dead, as the kiln typically heats the wood to over 133 F. Avoid green or non-kiln dried lumber. Make sure the kiln goes above at least 145 F. 

Bacteria also weaken the wood and present special issues in drying that the kiln operator may not understand. Bacteria often create fatty acids that turn rancid and give the wood an obnoxious, undesirable odor.

Urban wood often has many branches, so the highest grades of lumber are not abundant. Stains from pruning, metal screws, cutting the wood and bark, etc., are common. Stains might add character. Some urban producers do not grade their lumber, which can be a disadvantage.

In addition to the drying temperature, there are expectations for the final MC average and final moisture variation, stress relief, and flatness.

Visits to some operations already using this wood are priceless.

Q: We buy soft maple, 4/4, kiln-dried. It is usually fairly light in color, not super white, but light beige. This last order had many pieces that were light gray throughout the lumber. What caused this and will it cause discoloration of the paint we use?

A: This coloration is almost always caused by the long storage of the logs before sawing.  The starches and sugars in the wood, during storage, oxidize and turn gray, sometimes light gray and sometimes quite dark gray.

The sugars and starches are mainly in the white-colored sapwood. In pine, sometimes the color is darker brown.

I doubt that these chemicals are mobile, so darkening of paint is not likely, at least in my years of working with companies, no one has ever mentioned this.

However, when I was a lot younger, when this graying occurred, it could be removed fairly well using oxalic acid, also called wood bleach, as an initial wood finishing step.

As this oxidation is not what we would call normal for soft maple, it would make sense that the supplier should give you a credit.

In the future, you might include “no oxidation gray stain” in your purchase order.such a statement would also include sticker stain, as this is an oxidation stain.

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About the author
Gene Wengert

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.