Digital bandsaw tension gauge for under a tenner (Lidl!)

[ivan]

Tensioning bandsaw blades on smaller saws can be a bit hit and miss, as apparently the saw's own indicator is often way out, or not helpful (mine has a scale of 1 to 10). Precision tension gauges can be bought , but at a 3 figure cost. Hopefully what follows may be useful to others...

It suddenly ocurred to me to use my Lidl 150mm digital calipers to measure the stretch in the blade, as 'real' gauges do. Set them at 150mm, and clip to the untensioned but straight blade, (I used a couple of 2" G cramps). Zero the display, and then wind on the tension. ). 0.01mm of extension equates to 2000 psi. A reading of beween 0.07mm and 0.08mm will equate to 15,000 psi, usually reccomended for deep cuts on smaller machines. Half that is probably Ok for shallow cuts, and twice (~30,000psi) for bimetal blades.

 

[johnnyb]

what a superb tip. i wish i had thought of it. the only gauge i have seen made was very cumbersome with nails and feeler gauges. many many thanks.

 

[ivan]

Thanks, and me too! I'd been ferreting for a better idea since that wood and nails job in FW. Overcome at the low price I got 2 of the Lidl calipers, though I only use 1 for the intended purpose. I think I'll cut off the depth gauge from one so it's a bit more compact and comvenient on the bandsaw. The reading does wobble about a bit by +/- a digit, but it's a heap better than a guess!

 

[Anonymous]

you've got a tension guage on your bandsaw :shock:

The stuff dreams are made of!

Nice tip and I might try it out with my digital vernier later. Thanks

One quesion, how did you come across the figure

0.01mm of extension equates to 2000 psi

and how accurate is it?

 

[Chris Knight]

Ivan,
The yield stress of ordinary steels is around 30,000 psi - I wonder if that isn't rather high for a bandsaw blade?

 

[ivan]

My old saw had marks against the spring length 1/8, 1/4 etc which worked whatever the exact blade length, until the spring began to fatigue and shortened...My new one a scale from 1 to 10, less than helpful, especially as the reading is blade length dependent...

Most of the literature reccomends around 15,000psi for carbon blades, and up to 30,000 psi for bi metal and carbide tipped. Try The Bandsaw Book, Bird, Taunton Press who quote Lenox. Waterhead37, I think your fig is for mild steel, unlike the Young's modulus, the break point or tensile strength of steels varies quite a bit, carbon steel anealed, and hard, and alloys, 2 or 3 times higher. I must admit I've not knowingly exceeded 23,000 psi, (experiment, saw on max tension, old blade)

I worked my figs. out with a bit of school physics and a standard reference text book (Kaye and Laby) Young's modulus for various steels varies by around 5%, somewhat less than the error of measurement itself. Blades stretch according to Hooke's law:

Young's Modulus (Y) = Stress(S)/Strain(E)

for steel Y = 30,000,000 psi S = load per unit area of X section (psi), and E = extension per unit length

Extension/unit length = 0.01mm/150mm, so 30,000,000psi x (0.01extension/150) = 2000 psi blade load

FW magazine also published a wood and nails device that relied on a feeler gauge for measurement. A three thou extension of a 5" blade sample corresponded to a blade load of 18,000psi. This also approximately equates to the figs. in my original post. They said their feeler gauge system agreed to within 10% of a Starrett / Lenox tension meter (about 250 quid) which is good enough for me.

If you want to work out the load on the spring you have to take into account the cross sectioinal area of the blade, and remember to double the figure, as the blade is pulling down on both sides of the upper wheel.

 

[Jake]

Do you think these cheap gauges are accurate to hundreths of millimetres?

And do you think your clamping mechanism is stable to those levels too?

It sounds like a way to get very precise sounding numbers, but I wonder how accurate they are.

 

[ivan]

Jake, in an engineering sense, probably not. However we're not trying to measure the size of an object accurately to 0.01mm. Here, we're measuring a stretch from ~150mm to 150 and a little bit, and it's only the little bit, the extension due to tensioning the blade, that we're interested in. The error here is probably less than 0.01mm. 15,000 psi stretches 150mm of blade by 0.075mm, and I'm happy to stretch till the caliper reads 0.07 or 0.08. (you set the reading to zero before you wind on the tension) In practice the reading fluctuates between 7 and 8, so I'm not pretending this is great engineering science, just a helpful guide. I have comfidence in the results, for the reading is repeatable.

The expensive pro gauges clamp onto the blade 125 - 150 mm apart with thumbscrews, (just like a hand fretsaw). As the blade stretches a little arm pivots, pressing up the plunger of a dial gauge. There must be some slack in the pivot for it to move, and the dial gauge is more sensitive than usual at one tenth thou per division. And this with little brass gear wheels...