Saw filing 101: Saw tension
By Trevor Shpeley
Feb. 23, 2016 - “The tension fell out of those bandsaws!”
By Trevor Shpeley
“We took out the trimsaw and it was flopping all over, it’s no wonder it wouldn’t run!”
“Did you leave the 2×4 tension in? We are running 2×6 now!”
If you are a sawfiler you have heard all those statements before – and when you did, you likely just shook your head and changed the saws. To a non-filer, there are few concepts more misunderstood and mysterious than that of saw tension. Here is a little taste of what tension is and why your saws wouldn’t run without it.
Picture a flat disc of steel lying on a tabletop. It’s not vibrating, it’s not wobbling and it is very stable. Now picture that disc spinning with a rim speed of 26,000 FPM. As it’s doing that, heavy slabs of wood are being slammed into it at 500 feet a minute, and did I mention that the disc is only 1/8 of an inch thick or less? Unless you have tension in the proper amount with correct placement it would do all those things and more until it finally blew up and you could start again.
Put simply, tension is stress. We pre-stress the saws so that when a saw is at full operating speed and under load, it will be as flat as a piece of untreated steel would be while lying on a table. Saws are subject to many strains while cutting wood. Two of the strains that tension is designed to deal with are heat and centrifugal force. Since the saw does not expand evenly in all areas, different parts of the saw body must be stretched to different lengths so that when the saw is operating, the effect will be that of a stable disc or band that cuts straight and does not vibrate itself into pieces when it gets up to speed.
Visualize a saw as a series of narrow side-connected strips, where each strip is a little longer or shorter than the one next to it. In a bandsaw, starting from top to bottom, they will be stretched a little bit just under the gullet, then there will be a band that hasn’t been stretched at all (this is called the top tire-line). Below that, each section will gradually get longer until you reach the centre of the saw body where the process will reverse. The strips will get progressively shorter until just above the bottom where there will be another neutral band – this is your bottom tire. Together, all these strips make up your tension profile and serve to keep the saw on the wheel while cutting true. Consider also that you have to stretch the back of the saw 1/64 for every five feet of length to counteract heat expansion on the front edge and you begin to see what a complicated procedure tensioning a saw really is!
Roundsaw tension works in much the same way except that the bands are in concentric circles and there won’t be any tire-lines. The main focus of tension in a circular saw is to counteract centrifugal force and heat on the cutting surfaces.
The usual way to install tension involves beating or rolling the saw between two surfaces that are harder than the steel itself. The plate will stretch in a pattern determined by the shape of the hammer face or roll, the force involved and the direction of the blow. Steel can only be stretched by these methods, never shrunk. If you wish to remove tension in part of the saw you must add it to adjacent sections until the proper balance is achieved.
Heat is used less often, yet it remains a valuable tool in the filer’s arsenal. The saw is heated in a narrow band to a very specific temperature, which causes the steel to shrink where it turns blue and results in a strip with no tension. Done properly, this technique provides a hard tire-line that is long lasting and easily maintained.
Of course, there is far more to saw tension than can be explored in such a short article. The variations and possibilities are an endless test of a sawfiler’s skill but taking the time to determine the best tension profile for your applications can mean the difference between a mill running at a profit or not running at all.
Trevor Shpeley is the head filer for Tolko’s Kelowna division and is currently the financial secretary for the BC Saw Filers Association.