A note on bandsaw tension.

[Robin GrahamParticipant @robingraham42208]

I was interested in Gary Wooding's article 'A Bandsaw Blade Tension Meter' in MEW 319, detailing the construction of a device for measuring strain (or 'tension' ) in a bandsaw blade. This is a topic which frequently comes up in woodworking forums because blade tension is one of the many variables involved in the black art of setting up a woodworking saw to cut true.

One of the standard recommendations is to set the tension so that 'twanging' the free part of the blade gives a musical note – but no one gives the frequency!

I set my own woodworking bandsaw 'tension' by intuition, and it works OK, but after reading Gary's article I was moved to make a measurement. A carbon steel blade set at my normal tension has a dominant resonance at about 180Hz over a free length of ~750mm. That translates to a stress of 144MPa or ~ 20,900 psi – towards the top of the range 14500-21750 psi Gary gives for CS blades, but it fits with my intuition that ' if I tighten it any harder something might break'.

I'm actually surprised that my crude measurements have led to a plausible result, and it makes me wonder if the 'twanging' method of setting tension might be set on a firmer footing given that tuning apps are readily available for smartphones. If Gary (or anyone else who can measure strain) can correlate measurements with frequency it might be a good way for people without a shed load of tools or money to set up their saws.

Nice work btw Gary – I may have to make one!

Robin.

Edited By Robin Graham on 25/09/2022 02:17:08

Edited By Robin Graham on 25/09/2022 02:18:14

Edited By Robin Graham on 25/09/2022 02:24:54

[Robin GrahamParticipant @robingraham42208]

[Keith MathesonParticipant @keithmatheson47708]

As an aside Ducati’s desmodromic engines have a rubber toothed belt and for a few years the factory method of setting the tension was to pluck it and adjust to a particular frequency. Clearly their deign engineers considered it a good reliable method.

[not done it yetParticipant @notdoneityet]

One has to remember that the note will not only depend on the tension, so different notes will emanate from different lengths of free band. Certainly once you know the note for your particular band, it should be straight forward – unless you change its width or thickness, I suppose.🙂

[duncan webster 1Participant @duncanwebster1]

Google Mersenne's law, it tells you the relationship twixt frequency, mass per unit length and tension. I suspect it will be a bit off as a saw blade has inherent stiffness unlike a string. Some bright person might work it out if having trouble sleeping.

[Robin GrahamParticipant @robingraham42208]

Posted by not done it yet on 25/09/2022 11:03:18:

One has to remember that the note will not only depend on the tension, so different notes will emanate from different lengths of free band. Certainly once you know the note for your particular band, it should be straight forward – unless you change its width or thickness, I suppose.🙂

 

Yes, the frequency is inversely proportional to the free length, but it turns out (if – and it's getting to be a bigger and bigger if as I get older!) my calculation is right the the stress/fundamental frequency relationship is independent of the width and thickness of the blade. I get:

sigma = 4 x L^2 x f^2 x rho

where sigma is the stress, L the free length, f the frequency and rho the density of the blade material.

 

Posted by duncan webster on 26/09/2022 00:22:46:

Google Mersenne's law, it tells you the relationship twixt frequency, mass per unit length and tension. I suspect it will be a bit off as a saw blade has inherent stiffness unlike a string. Some bright person might work it out if having trouble sleeping.

Good point – the result above was based on the Mersenne's law, though I didn't know the name for it. My gut feeling is that the zero tension beam strength of a bandsaw blade is negligible compared to the tensioned strength, but I shall investigate that when next I have a sleepless night and a coincident fit of brightness. Don't hold your breath! I shan't.

After starting this thread I remembered that I had splashed out a fiver or so on something called a Lindley extensometer at a junk shop some years ago. It looked interesting, but has since been gathering dust:

I think that this is functionally equivalent to the device in Gary's article, so I should be able to make measurements myself. However, having a look around I see that the Science Museum has something similar but with an extra part:

Can anyone suggest what that part does? There is no indication of anything missing (eg redundant threaded hole in the casting) in my example.

Robin

PS the second image was downloaded from the Science Museum and has been altered to the extent of adding a red arrow to indicate a particular part.

R.

 

 

Edited By Robin Graham on 27/09/2022 02:26:35

[Robin GrahamParticipant @robingraham42208]

Ah, cripes, I've just noticed that my one has a part that the Science Museum one doesn't – a sort of a cone on a spring which when pressed in 'zero's' the gauge. So maybe that's what the indicated part on the Science Museum one does, but I'd be interested to know which was the earlier development.

Robin.

Edited By Robin Graham on 27/09/2022 02:40:15

Edited By Robin Graham on 27/09/2022 02:49:18

[Michael GilliganParticipant @michaelgilligan61133]

Robin

Did you notice that the Science museum has another example ?
**LINK**

https://collection.sciencemuseumgroup.org.uk/objects/co509965/lindley-type-extensometer-for-measuring-the-extension-of-samples-extensometer

…it’s more complete and and the provenance is more detailed

[‘though I might question the veracity of it being made by Mercer with a Baty dial gauge]

MichaelG.

.

Edit. ___ there is a nice concise descripion of the Lindley here:

http://scienceuniverse101.blogspot.com/2015/03/measurement-of-strain.html

… I would say your £5 was very well spent 

Edited By Michael Gilligan on 27/09/2022 07:14:33

[Martin Johnson 1Participant @martinjohnson1]

Uses of the Lindley, for example, is verifying elongation on metal tensile test samples. Yhings like boiler plates have to satisfy such tests to prove ductility as opposed to brittleness. Anyone up to standing next to a brittle boiler? Thought not.

Martin

[jacques maurelParticipant @jacquesmaurel42310]

The tensiometer described in MEW319 issue is inspired from the one I described in EIM (engineering in miniature) june 2016 issue. Some more info given in "scribe a line" of MEW 320 issue.

In the march 2017 issue of EIM I've described a spring tensionner (using belleville washers) the advantage is that you can see the actual tension, inconvenient, it must be dedicated for a blade dimension.

In the dec 2014 issue of EIM a "magic stick" is described for bansaw aligment.

A plain extensometer can be seen in my video about "tension test": http://youtu.be/JU0LbZbTMd

JM

[Robin GrahamParticipant @robingraham42208]

Posted by Michael Gilligan on 27/09/2022 07:07:21

[…]

Edit. ___ there is a nice concise descripion of the Lindley here:

**LINK**

… I would say your £5 was very well spent

Edited By Michael Gilligan on 27/09/2022 07:14:33

Well it was one of my better buys perhaps – like many small towns I suppose my own is seeing a decline in traditional high street shops and the emergence of short-lived 'antique' shops. The owner of the the one I bought it from had no idea what it was, and because there was no obvious upcycling potential (you can't make a retro industrial chic table lamp from an extensometer) let it go cheap.

I did see the other Science Museum example, but thanks for the link. I didn't pick up on the dubious Mercer claim.

My own example is Baty for sure:

My guess, based on the faux leather and the Dymo label is that it is 1960's -70's.

Posted by jacques maurel on 27/09/2022 15:09:16:

The tensiometer described in MEW319 issue is inspired from the one I described in EIM (engineering in miniature) june 2016 issue. Some more info given in "scribe a line" of MEW 320 issue.

In the march 2017 issue of EIM I've described a spring tensionner (using belleville washers) the advantage is that you can see the actual tension, inconvenient, it must be dedicated for a blade dimension.

In the dec 2014 issue of EIM a "magic stick" is described for bansaw aligment.

A plain extensometer can be seen in my video about "tension test": http://youtu.be/JU0LbZbTMd

JM

Jacques – thanks for your input. I would be interested to see your designs but it seems that back issues of EIM don't go further than September 2016 on the Pocketmags site. Is the magazine archived elsewhere?

Robin.

 

 

 

 

Edited By Robin Graham on 28/09/2022 00:31:31

[John HaineParticipant @johnhaine32865]

How about something like this?

**LINK**

Even has a handy exploded view to show how it's made.

I've wondered about something similar as a more scientific version of the "thumb press" test to judge the inflation of a bicycle tyre. Ordinary tyre pressure gauges don't fit Presta valves and anyway the amount of air they waste as you press the gauge onto the valve affects the tyre pressure you're trying to measure. And they usually don't go up to 100+ psi which some bike tyres do.

[egaParticipant @ega]

Posted by John Haine on 28/09/2022 10:29:25:

…Ordinary tyre pressure gauges don't fit Presta valves and anyway the amount of air they waste as you press the gauge onto the valve affects the tyre pressure you're trying to measure. And they usually don't go up to 100+ psi which some bike tyres do.

It sounds as though you need to invest in a track/floor pump equipped with a gauge. If you get one with a Presta/Schrader connector you can also use it for topping up the car tires!

[John HaineParticipant @johnhaine32865]

Posted by ega on 28/09/2022 11:13:51:

Posted by John Haine on 28/09/2022 10:29:25:

…Ordinary tyre pressure gauges don't fit Presta valves and anyway the amount of air they waste as you press the gauge onto the valve affects the tyre pressure you're trying to measure. And they usually don't go up to 100+ psi which some bike tyres do.

It sounds as though you need to invest in a track/floor pump equipped with a gauge. If you get one with a Presta/Schrader connector you can also use it for topping up the car tires!

Yes, I have one of those. When it's attached to a valve, in order to pressurise the tube and gauge a certain volume of air has to be taken out of the tyre which reduces its pressure. Also you can't carry a track pump out riding. I have a hand pump with a small gauge but pumping to 100psi is a real fag, so I also carry a CO2 inflator. A little gadget one can carry in the toolkit would be very handy.

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