Saw filing 101: Washboarding
By Trevor Shpeley
March 28, 2016 - Named for its resemblance to the pattern of ridges on an old-style washing board, washboarding in a sawmill refers to the visible end result of conditions that combine to produce a self-perpetuating wave in a saw that causes the teeth to weave back and forth in the cut, leaving a textured-defect that can be difficult to plane out. The presence of washboard forces a sawmill to run larger target sizes and to feed the wood through the saws at a slower speed than desired, which has a direct and detrimental relationship with the mill’s production and recovery statistics.
To eliminate or reduce the washboard effect on your lumber you must first determine what is causing it. The conditions that produce washboard are numerous and what works in one mill isn’t necessarily going to work in another, making the problem very difficult to diagnose and cure. Recognize also that the experts in the field are not in full agreement with each other on the specific causes and cures of washboarding, and you have all the ingredients of a difficult problem that will take careful experimentation and observation to solve.
The elevated diagonal ridges on an affected piece of lumber are often thought to be produced by a vibration of the saw teeth, perhaps as a result of skinny or “weak” tooth design. In fact, the travelling wave that causes the teeth to deflect from one side to the other in a serpentine pattern occurs in the saw body itself, although it is usually generated at the saw tips by the impact of the tooth into the cut. If you picture the contributors to washboarding occurring in an imaginary band with the least damaging forces occurring on the outside edges and the most damaging occurring in the middle, you can see why some washboard is easy to fix while other examples can be tenacious and difficult to solve. If the condition, or combination of conditions, that are causing your washboarding occur on the outside edges of the imaginary band, reducing the resonance sometimes requires a relatively small adjustment such as reducing your hook face or gullet depth by as little as 1/16 of an inch, or changing your wheel speed by five per cent. (Keep in mind that reducing your gullet capacity is possibly going to require other adjustments to your feeds and speeds to keep the saws cutting properly).
Other times you will need to be more aggressive. Reducing your hook face and gullet depth by 1/8 of an inch or more, or flattening out the bottom of your gullet will sometimes increase blade stiffness enough to help with your problem (for every 1/16 of an inch your hook angle is increased, your chance of washboard goes up by 50 per cent). Increasing the radial and tangential side-angles of your tooth a couple of degrees has been known to help as well. In extreme cases you may want to increase your plate thickness to increase the stiffness of the saw as thinner plate has a natural tendency towards washboarding, but since the next step in increasing plate thickness is usually to increase feed speeds, you can quickly lose any advantage you gain.
Another method that has long been known to dampen resonating forces in band and round saws is to switch to a variable pitch pattern. The principle being that the uneven spacing of the saw teeth, sometimes coupled with a corresponding change in gullet depth, will prevent the impact forces from multiplying and creating the damaging resonance. It’s simple in theory but in real life it can be a little more complicated. The causes of washboard are many, and in some cases your problem may be caused or exacerbated by something else entirely, such as vibration in your guide assembly or a bad bearing. The only way to really know if variable pitch will solve your problem is to try it, but since that is not without its own significant costs, it makes sense to reach for the low hanging fruit first and fix the small issues before laying out hard to come by capital on a large project.
The forces that produce washboarding are a complicated puzzle for even the most experienced filers. In some mills, a tiny change can make a huge difference while in others the path to smooth lumber can be long, frustrating and expensive. It’s always worth experimenting no matter how much trial and error it takes because washboarding costs your mill money and in most cases, improvements can be made, it just might take a while.
Trevor Shpeley is the head filer for Tolko’s Kelowna division and is currently the financial secretary for the BC Saw Filers Association.