There's been some discussion of BS tension recently, so I finally took the time to snap some pics of this. I stole this general design off a Woodnet discussion some time ago. The idea is to measure BS tension by measuring the stretch of the blade over a known distance.
The dial is held in place by friction fit to the upper portion. This portion is clamped to the blade exactly 6" above the clamp of the lower portion.
The two pieces are guided by two 1/4" rods, so that movement is only allowed in the vertical direction.
The upper "holes" are close-fitting 1/4" grooves.
The key to this technique is knowing how much a given bandsaw material stretches as a function of applied tension AND how much tension is desired for a given blade and cutting task.
To be honest, I haven't put a whole lot of research into gathering that info, but have heard numbers on the order of 12,000 to 18,000 PSI for the desired tension of "typical" BS blades. For such blades, a stretch of 0.001" measured over a 6" length is equivalent to ~5000 psi.
Note that tension measured in PSI is independent of blade width, while the spring force required to tension blades will be proportional to the width.
I've only begun to play around with this, after having replaced my worn out BS spring with something that can properly tension a resaw blade. I think the jury is still out on such tension gages, but it's kind of nice to get you in the ballpark, and to help diagnose worn out springs, etc.
Regards,
Tom
The dial is held in place by friction fit to the upper portion. This portion is clamped to the blade exactly 6" above the clamp of the lower portion.
The two pieces are guided by two 1/4" rods, so that movement is only allowed in the vertical direction.
The upper "holes" are close-fitting 1/4" grooves.
The key to this technique is knowing how much a given bandsaw material stretches as a function of applied tension AND how much tension is desired for a given blade and cutting task.
To be honest, I haven't put a whole lot of research into gathering that info, but have heard numbers on the order of 12,000 to 18,000 PSI for the desired tension of "typical" BS blades. For such blades, a stretch of 0.001" measured over a 6" length is equivalent to ~5000 psi.
Note that tension measured in PSI is independent of blade width, while the spring force required to tension blades will be proportional to the width.
I've only begun to play around with this, after having replaced my worn out BS spring with something that can properly tension a resaw blade. I think the jury is still out on such tension gages, but it's kind of nice to get you in the ballpark, and to help diagnose worn out springs, etc.
Regards,
Tom
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