Mind the gap
By Udo Jahn
April 18, 2016 - Gap control is the one of the most critical items to control in a sawmill. It determines how accurate your cuts are, and the quality of your lumber. Having gap control means maintaining that perfect 0.0015” gap between the saw blade and the babbitt pad of the sawguide. I have always looked at sawguide accuracy as one of the top five ways of maintaining a perfect gap.
Too tight of a gap and the sawguide begins to act as a disc brake, which causes your saws to heat up quickly. Having heat buildup means there’s a high probability that your saw blade will lose tension and fold over. If the gap is larger than the ideal amount, you will get a lot of within board deviation (saw wandering) while the saw is cutting, which will result in poor quality lumber.
I thought I had written about almost all the causes that affect the gap size between saw blade and sawguide until recently, when I ran into someone who reminded me of yet another cause, which brings me to this blog today.
“What about the tolerance of the saw plate?” this person said to me. I was silent for a good 15 seconds, with a confused look on my face the whole time. “What about it?” I replied. “It can vary too,” he replied. This caused me to stop and think, because I had never taken saw plate thickness into consideration before. I had always assumed that all saw plates were very accurate and I never considered it as part of the equation. Everyone knows what happens when we assume!
Not wanting to look worse than I felt after realizing I totally missed saw plate tolerance as an issue, I decided to ask this person a question. “What kind of tolerances are people getting?” I asked. “Well, I have seen +/- 0.002 inches,” was the answer back. I was stunned by the answer as it was far more than I was expecting. I became gravely concerned about the ramifications of this piece of information.
Sawmills are trying to maintain a 0.0015” gap between their saw plate and their babbitt pad, but some mills have a saw plate that varies +/- 0.002”. This could cause some pretty big problems. As I mentioned in the first paragraph, a sawmill will have issues if the gap begins to vary too much in either direction.
It’s now time to add one more dimension to this gap control issue. It is critical that you measure your saw plate, and not just your kerf, when a new batch of saws come in. It is equally important that you compare these new blades to the thickness of your existing saws and saw plate. You need to make sure that there is no significant variation between your existing batch and your new batch. If there is a variation, you will have trouble maintaining your gap, and therefore your sawmill will have both lumber quality and uptime issues.
I don’t know why there is so much variation in some of these saw plates, but it may, and I am only speculating here, be due to the need to reduce cost on saw blades. As I have mentioned in my blogs, “You get what you pay for!” When you opt for the cheapest option, the vendor who needs to make money, may compromise on the quality. A precision-made saw plate is more expensive than a less precise plate. This is logical. There is more work involved in making a precision plate. The money you save upfront on the less precise plate will cost your sawmill more money in downtime and poor quality lumber. The overall cost is exponentially higher than the initial savings of the cheap saw plate.
It’s amazing how much I am always learning about the sawmill industry, and how surprises like this seem to come my way every day. I hope that those of you who are receiving saws make sure that you are specifying, and getting, the best quality saw plate possible. A variation of +/- 0.002” is going to cause the mill a lot of problems. These days where lumber prices are not the best, productivity and quality are paramount to your mill’s success. It can easily be the difference between a mill that is running and a mill that is shut down.
Udo Jahn is a general manager at Modern Engineering.