Why is the accuracy of a saw guide so important? If you look deeper into its importance, what it actually comes down to is ‘Gap Control’! You are probably wondering what this is. The gap that I’m referring to is between the babbitt pad and the saw blade.
In most Sawmills this gap is approximately 0.0015”. To put this in perspective, a human hair is 0.010” thick. This gap is set by the operator that runs the babbitt grinder. The operator starts by taking a saw guide and mounting the newly poured babbitt pad to the guide. Then the saw guide is mounted on the precision face to the babbitt grinder. The operator then sets the grinder so that the amount milled off will allow for a 0.0015”gap between the babbitt pad and the saw blade. Once the operator has established this gap on the first saw guide they proceed to mill all remaining saw guides to that setting. It sounds like a straight forward procedure, but when you dig below the surface you start to see some problems. What if the tolerance from guide to guide is +/- 0.0005” or more? That means between the two saw guides, the gap can vary by 0.001” or more.
Typically saw blades are not completely flat, so they oscillate between the babbitt pads on either side of the saw. Different problems occur depending on whether the gap is set too small or too large.
If the gap is too small the blades will hit the babbitt pads more often, causing increased friction. This increased friction causes the saw blade to heat up and, in many cases, lose its tension and fold over. When this happens the edger goes down and causes production problems. If you work in a sawmill the Mill Manager or Owner will let you know exactly how much that costs per minute—and it adds up pretty quick.
If the gap is too large the saw blades will oscillate back and forth and cause within board variation (there will be variation in the thickness of lumber). This will lead to quality issues of the lumber, and therefore decrease the value of the lumber sawn. To adjust for this, mills will often increase the target size and utilize the planer to eliminate variation, however this causes unnecessary wood waste.
Whether it’s smaller or larger, having to adjust your gap to accommodate inaccurate saw guide thickness is not ideal. Over the years, sawmills have discovered that saw guides delivering +/- 0.0002” surface tolerance, greatly decrease the production issues listed above—it really does come down to ‘Gap Control’.
Author: Udo Jahn
