Blade Tension Essentials
Understanding band saw blade tension is essential to executing an accurate cut. Bandsaw blade tension is the term used to describe the minimum amount of stretch applied to the blade in order to make it stable. It’s a technical description for how tight the bandsaw blade ought to be while in use. Bandsaw blades have to be stretched tight to work properly. The level of tension a blade can withstand is gauged by measuring the amount of force per cross sectional area of the blade, expressed as PSI (pounds per square inch).
How to Set Blade Tension
Blade tension is a key factor for making sure you can successfully operate a band saw. Properly tensioned bandsaw blades cut straighter and longer, while improperly tensioned blades can cause a variety of operational obstacles. Most of these are problems that, on their own, might not be a reason to halt your progress. But, these issues also may cause serious issues to the operator--someone who is critical to ensure the quality of the cut and accuracy of the operation. It's essential to understand blade tensioning before you begin operating a saw blade. Setting proper blade tension can be done in several ways, depending on the type of blade:
- Carbon blades: The typical tension applied is around 15,000 to 20,000 PSI.
- Bi metal blades: Require a greater amount of tension to be applied but should not be more than 35,000 PSI.
Two blades of different sizes will require different forces to achieve the same PSI. For example, a ½” x .025 blade requires 312.5 lbs of force to achieve 25,000 PSI. A 1” x .025 blade requires 625 lbs of force to achieve the same 25,000 PSI, twice as much force. That's because the blade has twice the cross sectional area. Improper tensioning of band saw blades will cause multiple problems. If the blade is tensioned too tightly, it will most likely cause the blade to develop a twist and/or will cause the blade to break prematurely. It may also cause the weld to break and cause gullet cracks. If the applied tension goes below its minimum requirement, it can cause the blade to vibrate and even to lead in the cut. Loosened tensions on blades can lead to belly shaped cuts and might also cause the blade not to cut in the desired square shape.
How to Measure Blade Tension
Some band saw machines available in the market are already equipped with a gauge or mark to set the tension of a saw blade. These marks generally assume a certain size of blade. The correct force for one blade as indicated by the gauge will be half that needed for a blade twice as large. Some shops also invest in blade tension meters in order to read the tension in the blade. It is also possible to develop "rules of thumb" for tensioning a blade based on number of turns of the crank. After using a blade tensioning gauge on a saw a few times, it's possible to get a feel for the correct blade tension. After a “calibration period” using a tension gauge, a skilled operator will not need to use the blade tension gauge every time. Other machines use Air Pressure Blade Tensioning to set the needed tension for the blade. Most machines have a screw sitting on top of a spring to set the blade tension. The tighter you turn the spring the more tension on the blade. An air pressure tension system uses an air cylinder to tighten the blade. The operator sets the air pressure using a machine mounted pressure regulator. The machine manufacturer will provide a chart showing how much blade force is generated by any air pressure, and the blade tension in typical blade sizes. For large blades or cutting applications that require a very specific blade tension this is the best way to set the tension.
Blade Tensioning Techniques
It is preferable to set the blade to its lowest tension without sacrificing accuracy or speed. Operators should always de-tension saw blades after every use. It can damage the blade to keep it under strain, especially when the saw is not in use. A saw blade normally warms up while cutting, producing stretch. Always DE-TENSION the band immediately after use.