‘Swingmill’ Blades – Proper Blade Tension a Key to Good Sawing
(Editor’ Note: This is the third in a series of articles about saw blades for swing blade portable sawmills, also known as ‘swingmills.’ The articles are provided by Peterson Portable Sawmills.)
This article in the series will focus on the saw plate, gullets and tensioning. It is an area not very well understood, even by owners of swing blade mills.
You don’t need to know much about tension when you are first learning to run a swing blade mill. A good blade can easily last through two sets of teeth or around 34,000 board feet of sawn lumber before tensioning is required.
Often, a good saw doctor will automatically check the tension of the saw plate each time he puts new teeth on for you. However, the more sawing you do or if you work your blade too hard and ‘smoke it up,’ the blade may begin to lose tension.
Know what to look out for when the blade begins to lose tension. Taking good care of the blade will make it last three to eight years or more.
About Blade Tension
Tension is put into a blade when it is manufactured so that it will hold its shape in both the vertical and horizontal position and won’t ‘cup’ or wobble with speed.
Think of using a rolling pin to roll 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.
Excessive heat during sawing will release tension, and make your blade floppy and vibrate in the cuts.
Loss of tension is caused by fatigue and hard work. Some mill owners will know more about tension than others — if they have blades that need more frequent servicing. If you lose blade tension too frequently, you either have poor quality blades, blades made to incorrect specs, or your adjustments are out, causing stress on the blade. Or, you are just a very tough sawmiller!
Premature loss of tension is caused by forcing the blade through the cut. It also occurs when the blade is out of adjustment, dull or blunt, or doesn’t meet the correct specifications. In these cases the blade will heat up, which causes the metal to soften and lose tension.
When the blade loses tension, it will vibrate and cut rough, and you will see a ‘washboard effect’ on the lumber as the blade dips and rises. This is visible by three or four saw marks, then none for several inches, then another three or four marks. The blade will more frequently jam in the cut, smoke up, and be left with purple or blue burn spots on the surface.
There is a fine line between plate thickness, the number of tips, kerf, saw rpm and collar size. All these factors affect saw blade performance — production speed, yield, finish, and how long the blade will hold tension.
If you are struggling to cut, something is not right with the blade specs or adjustments. You will need to stop and correct them or you will heat the blade and lose tension.
A good saw doctor will re-tension the blade during re-tipping service for very little extra cost, if any. You will need to tell your saw doctor what rpm the blade is running at and the tooth specifications (a spec sheet usually is supplied by the manufacturer).
Modern saw shops will roll-tension a blade; the blade is rotated while it is pressed by two bearings. You will see rings on your blade if it has been roll tensioned. This method distributes tension more effectively. The old fashioned method of hammering a blade to tension it is sufficient if done by a skilled saw doctor. A tensioning service can cost anywhere from $10 to $40.
Caring for the Blade
Keep your blade sharp! This is the top priority. It takes only five minutes to sharpen the blade, and it saves your blade as well as your legs. A dull blade works twice as hard. It tears at the timber rather than slicing it, and it bogs down easily. If you run a dull blade, you can lose 20-40% production, and you will work a lot harder.
Make sure your blade adjustments are spot-on. If your blade dives or taxis for takeoff, it is fighting the timber, stressing the blade and heating up. Think of running the 100-yard dash sideways or with your nose to the ground! Learn the correct blade adjustments – the next topic in this series of articles.
Lubricate the blade with water for very hard or sappy wood. Sap build-up can cause drag and side friction, which will heat the blade prematurely. Scrape off any sap build-up. Add a little dishwashing liquid to the water supply; this will give more lubrication and also prevent algae build-up in the water line.
The gullet is the rounded-out area in front of the tooth. Once the tooth slices off a piece of sawdust, it moves down into the gullet, which acts as a sort of holding scoop and throws the sawdust out when the gullet exits the wood. The steel edges of the gullet will become rounded after time (around 500-1,000 hours) and make it inefficient at removing sawdust. A saw doctor should know how to service the gullets as needed by grinding the steel back to a 90-degree edge again.
Inspect the blade to look for the beginnings of stress fractures. In the second year and every year thereafter, inspect the mounting holes and the center bore for any tiny fractures lines. These can be caused by blades that are too thin, sloppy mounting screw holes, or overworking your blade when it is out of tension or out of adjustment. Any of these issues will cause the blade to flex more than normal around the mounting holes.
If you see any fracture lines, discard the blade. Do not continue to use a blade with body fractures.