Bandsaw Blade Tensioner

Had a bit of spare time and so I thought I’d make myself a bandsaw tensioner. I claim no originality here, this is a very well-trodden path.

Unless you have one, and they seem to cost an arm and a leg for something you'll use once in a Preston Guild, I have no idea how you get anywhere near the right value for blade tension.

The design is simple and so I milled these dial indicator clamps up in a few hours. There are grub screws in the threaded holes to clamp the blocks to the blade and the calliper to the blocks.

And this is it how they fit on the bandsaw blade. No need for super accuracy here placing the blocks on the blade. I've opened up the caliper to 6 inches; the more the better.

As for the calculation that is simple too, but I like to do it in old money. I am no Luddite, I fully embrace the SI system of units, but for the life of me I have no concept of what a GPa or a Newton is in any physical sense. However, I do understand what lbs/sq. inch are and I can visualize lbs, 56 of them are a sack of potatoes and 112 are a sack of coal, and that’s very heavy.

So, avoiding any tricky sums, Young’s modulus for AISI 1075 steel; the sort they make common or garden carbon steel bandsaw blades from, is 30 million lbs/sq. in.

The load per unit area for bandsaw blades should be in the range 15,000 to 20,000 lbs per square inch. Note that you don’t need to worry about the cross-sectional area of the blade, ie, its width and thickness are already factored in.

Dividing the load per unit area by Young’s modulus gives you one half to two thirds of a thousandth of an inch per inch that the two blocks are nominally apart. In my case it’s 6 inches, as indicated by the dial gauge in the picture above.

So, I need to stretch the blade by between 3 and 4 thousandths of an inch to be at the right tension.

Quite simple really.

As an aside, I measured the cross-sectional area of my blade, and it turns out that the load applied to my half inch blade was between about 3 and 4 sacks of spuds! Or if you prefer that in sacks of coal, it's 1.5 to 2.