I frequently see threads that mention the proper adjustment of band saw blade. Proper adjustment of the tension is important for a satisfactory cut. There is quite a range of acceptable tensions and usually a recommended value is just a starting point and adjusted for the work planned. Values in the range of 14,000 to 25,000 are commonly mentioned. Commercially available tension gauges are often several hundred dollars. There is often a spring gauge with a pointer on the saws themselves that will measure relative tensions, but don't deliver actual numerical tensions. Realistically, I don't plan on buying many band saws and perhaps maybe owning just a few blades, so buying a tension gauge isn't a practical purchase. Having calibrated the built-in pointer once, I think it's possible to use the scale to estimate what is a good adjustment for new blades. My best guess is that the built-in scale measures a relative force, and knowing a new blade's dimensions, I can calculate the setting needed to get a desired tension. I will describe a technique to use an inexpensive dial indicator and magnetic holder to measure blade tension.
DIY devices on the internet range from beautifully machined equivalents of commercial units to simple devices made of pieces of wood. Calipers can be clamped to the blade and used to measure the displacement of the points while the blade is tensioned. In studying these different methods of approaching the problem, it became clear to me that all I needed was a way to measure the displacement of two points on the blade. For my measurement, I used a cheap machinist's dial indicator on a magnetic mount (HF $16.99). Mine has two surfaces that are magnetically switched (bottom and back, at right angles). Miscellaneous bits include a metal bar (1x1/4", 7 inches long), two small "C" clamps, 2" piece of aluminum angle, a rubber band and two small blocks of wood. If your base is only active magnetically on the bottom, you need one more clamp to hold the bar to the base.
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The clamps are used to clamp the blade, about 5" apart, to the metal bar and the aluminum angle. The wooden blocks just keep the clamps from marring the blade (avoid clamping over the teeth to avoid altering the set of the teeth). The metal bar rests on the band saw table top and is held to the magnetic base on the side magnet surface and the magnetic base is fastened to the band saw surface by activating the magnet. The combined weight of the bar and the magnetic base serves to hold the blade in position at the table. Since tensioning the blade doesn't rotate the wheels significantly, the lower part of the blade, where clamped to the bar, stays in position. The dial indicator's point is set on the aluminum angle and will register any displacement away from the table (and the blade that is fixed at the bottom to the table). The rubber band is attached to the clamp/angle at the top. It's purpose is to prevent the weight of the clamp and angle from causing the blade to sag when obtaining your zero. Since it will only generate a small force, it's contribution to the final measurements is very small and negligible. Set it so that the blade is taut when there is no tension on the blade. One end is on the stuff attached to the blade and the other end is attached to the band saw blade guide holder.
The way to take a measurement follows.
- Disconnect the power to the band saw.
- Preload the dial indicator so that there is no slack. Lock the clamps. Make sure the dial indicator is parallel to the blade. It's good to be sure that the blade isn't rubbing on the indicator or the arm of the magnetic base.
- Measure the distance between the two clamps. You probably can use the center of the clamping points as a guess.
- Ease off the tension adjusting wheel. I found that even when the dial indicator wasn't moving, the built-in saw tension indicator was still registering a value. Adjust the tension wheel so that the built-in indicator reads zero tension.
- On the dial indicator, note the setting: this is your zero.
- Start adjusting the tension wheel until you get a 0.001 inch displacement. I found on my saw (Grizzly) that I was getting about a "3" on the built-in scale.
- My built-in scale is numbered and I was able to get a set of displacements and numbers.
- Ease off the tension wheel and see if your dial returns to the previous zero.
Measure the blade that you have installed to get the cross sectional area behind the gullets. Realistically, you are mostly stretching this area, but I guess the error in this assumption is that you will get a value that is on the low side for the effective area. (This is probably where a commercial gauge would give you a numerical value for the tension. You could use a blade with a fine pitch to calibrate the saw.) I followed Wandel's calculations at Woodgears to get the tension. The cross sectional area is used to calculate the force applied to the blade. This force is the value that you can follow on the built-in gauge, so use it to correlate the pointer position to calculate the tension when you change blades. Anyway, it's not my intention to give a tutorial on calculating tensions. Excuse me if this is too wordy, but I thought it was important to allow others a chance to measure the elusive band saw tension for $17.
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Originally Posted by Wayne Lomman
This seemed to give a pretty functional tension at least, but I had no means of confirming. As I recall the bandsaw blade does not give a very true "note"- there are lots of weird harmonics and overtones that try to disguise the fundamental pitch. I'd be curious if a cheap electronic tuner would be capable of reading the pitch.