by Professor Gene Wengert
Editors note: The following article is an excerpt from the book "The Wood Doctor's Rx," by Gene Wengert, Professor and Extension Specialist in Wood Processing Department of Forestry, University of Wisconsin-Madison.
It is quite agonizing to see lumber, purchased at prices exceeding $600/MBF and then painstakingly dried at $150/MBF, run through the roughmill planer and end up with splits running several feet along the board (and even, in the worst cases, end up splitting the board into two pieces). There are five reasons why planer splits can be a splitting headache. Many times two of these reasons can individually be minor or "slight problems", but they can combine to make a severe problem. Therefore, don't be too quick to discount any of these conditions as being too simple or obvious.
Too Much Cup
As described in the preceding chapter, this failure occurs when cupped lumber fed into a planer is pushed flat by the feed mechanism and the knives. The feed mechanism and the knives can push so hard upon the edges of the board that it splits -- often the entire length. The cure is to control the cup during the drying process and not to push too hard during the planing (see "Excessive Pressure," below). Cup develops in drying as a result of the difference between radial and tangential shrinkage. It is accentuated by slow drying at high humidities. Cup also develops as a result of having thick and thin lumber in the same layer within the lumber pile during drying. Slow drying is a characteristic of predrying (compared to air drying), so more cupping is seen in predryers. As a cure, avoid overly conservative drying procedures in predryers. Also, the stickers help hold the lumber flat, but they must be in contact with the boards to do this. Encourage sawmills with thick and thin to tighten up their operation -- usually the problem is in the set works.
The degree of warping is a function of final moisture content. The drier the lumber, the more warp that can be expected. Over-dried lumber is also less bendable, so it is more prone to splitting. On the other hand, however, wetter final MC's can result in panel splits, open glue joints, and warping of furniture and cabinets. Avoid excessive over-drying, but at the same time avoid, at all costs, under-drying, especially in winter.
During drying, all lumber develops stresses that result in a stress pattern called casehardening, although the "case" is no harder than anything else. The result of casehardening is instantaneous warp when machining -- bow when ripping and cup when planing. The cup is toward the planed side. Consider the top head to be the first head in a planer and a slightly cupped board. If the board is fed cup-side-up into the planer, the first head will actually increase the cupping forces as it removes wood from the top face. The rule (when splitting is a problem) is feed the cup-side down into a top-head-first planer (or vice versa for bottom-head-first).
When a slightly cupped board is fed into a planer, the feed rolls or the knives themselves, when running at high feed rates or heavy stock removal rates, can exert excessive pressure that will split lumber. Use as light a pressure as needed on the feed rolls (perhaps even having an occasional thin board feed poorly) to avoid planer splits. Also, try slowing feed rates slightly when splitting is encountered and see if the situation improves. Also check the knives: the jointed surface (or land) can be too wide (over 1/32 in. is too wide) and will contribute to splitting (and grain raising too) by pounding the boards. Reduce jointing to reduce splitting.
In my younger days (!), I can remember that many roughmills had a "busting saw" that was used to rip wide boards before planing. Today, I see very few of these saws (and few wide boards for that matter). Wide boards will usually be sawn from areas near the center of the tree and their grain pattern makes warping likely and planer splits can result. Consider ripping wider boards before planing to reduce splitting.
Professor Gene Wengert is Extension Specialist in Wood Processing at the Department of Forestry, University of Wisconsin-Madison