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Make Your Own Banjo Head Tension Gauge
Here is a simple head tension gauge that you can assemble from a few inexpensive components. I used a dial indicator from Harbor Freight ($15), and attached it to a simple base made from scrap wood. You could also use a base created from a hockey puck purchased from your local sporting goods store, but that would increase the cost by a couple of bucks...
Making the head tension gauge:
The gauge was made by turning a scrap of walnut to 1-7/8" high by 2-5/8" diameter. A 3/8" diameter hole was bored 7/8" deep to house the dial indicator barrel. A hole was drilled from the side and tapped for a #10 by 32 by 3/4" setscrew to lock the indicator barrel in place. A 1/4" hole was drilled through to the bottom to allow the indicator stem to protrude. The area on the bottom was recessed slightly so only 1/8" remained on the bottom edge. The base had a quick application of wipe-on polyurethane and was then assembled. Approximately 1/8" protrudes below the bottom lip to allow the indicator pointer to rotate a full revolution when it is placed on a flat surface. None of these measurements are critical, and I selected them based on what materials I had on hand.
Using the head tension gauge:
The banjo head tension gauge shown here is used by placing the indicator on a level and flat solid surface, turning the dial so the needle is at the "0" position, and then placing the base upon the banjo head at several reference points to indicate the relative level of tension as well as the consistency between measured points. "Relative" is used here, because of the variables involved in this measurement. These variables include differences in head materials or size, point of measurement, and differences in the amount of dial indicator return spring force from unit to unit or between different manufacturers. Different brands of indicators may exhibit an even greater variance between indicated readings. The gauge is still useful to repeat setup results using the same size and type of head when the measurements are taken in the same manner. Since the readings can vary so much based on the previously mentioned factors, the gauge is best used by setting up a banjo to your personal liking using "traditional" methods and then taking the readings to establish an "acceptable" baseline reading.
My readings on a few different banjos that I have were generally in the 90-97 range, based on a fairly tight head tensions. NOTE: The actual head deflection is only .003" to .006" (three to 6 thousandths of an inch). The indication is in the 90's because the reading is actually increasing to "0" as the head experiences less deflection.
How the gauge works:
The measurement of banjo head tension is indicated on this device because the shaft return spring used in the dial indicator has enough force to overcome the resistance of the banjo head, causing it to deflect slightly below the surface of the circular base that the dial indicator is mounted on. The tension gauge dial indicates the distance in thousandths of an inch that the banjo head deflects below the surface of the base, thereby reflecting a corresponding amount of actual head tension.
Increasing the tension of the banjo head also increases the amount of pressure countering the dial indicator return spring force, therefore the deflected distance decreases and is shown as an increased reading on the dial indicator scale. Please note that the commercially available version of this gauge uses a stronger return spring, so readings will not be the same on this home-brew version!