Part 3 of a 5 part series covering all you need to know about swingmill blades – by Kerris Browne
This is an area that is not very well known, even among owners of swingblade mills. In the early learning period, you don’t need to know much about blade tensioning. A good blade can easily last through 2 sets of teeth, or around 34,000 board feet or 80 cubic meters of sawn timber before tensioning is required. And often, a good sawdoctor will automatically check the tension each time he puts new teeth on for you. But once the blade gets on in life, or if you work your blade too hard and ‘smoke it up’, you may experience loss-of-tension problems. So it’s a good idea to know what to look out for. Good care of your blade will see it last you for 3-8 years or more.
Tension is put into a blade so it holds its shape in both vertical and horizontal position; so it doesn’t ‘cup’ or wobble with speed. Think of a rolling pin rolling out a piece of dough from the center to the edge. Tensioning presses the blade around the middle area, forcing tension into the main body of the blade, so that it holds its shape. But excessive heat during sawing will release that tension, and make your blade floppy and vibrate in the cuts.
Premature loss of tension is caused by forcing your blade through the cut; when it’s out of adjustment, when it’s blunt/dull, or if it is incorrectly spec’d. The blade then heats up, causing the metal to soften and lose its tension. The blade will then vibrate about, be noisy, and cut rough, and you will see a ‘washboard effect’ on the log as the blade dips and rises. This is visible by 3-4 saw marks, then none for several inches, then another 3-4 marks. Your blade will more frequently jam in the cut, smoke up, and be left with purple or blue burn spots on the surface.
A good sawdoctor will retension your blade during a retip service, for very little extra, if any. You will need to tell your sawdoctor what rpm your blade is running at, and the tooth specifications (spec sheet usually supplied by the manufacturer). Modern sawdoctors will roll-tension your blade, where the blade is rotated while it’s being pressed by two bearings. You will see rings on your blade if it has been roll tensioned. This method distributes tension more effectively, but the old fashioned hammer-tension is still sufficient if carried out by a skilled sawdoctor. A tension service on its own can cost anywhere from $10 to $40.
Sap buildup can cause drag and side friction, and heat the blade prematurely. Scrape off any sap buildup. You can even put a little dishwashing liquid in your water supply; this will give more lubrication but also prevent algae buildup in your water line.