Shop made bandsaw tension meter.

[WellsWood]

There seems to be a lot of bandsaw discussions going on around here just recently so hopefully someone out there can help me track down the info I need to make a reliable bandsaw tension meter.

A while back I read an article about setting bandsaw tension using (I think) a digital caliper similar to those Lidl ones some of us, myself included, are so fond of. The process involved clamping the two arms of the caliper to the blade with small g-clamps, tensioning the blade, and using the caliper to measure the stretch in the blade. The article detailed the maths involved in working out how this stretch equated to p.s.i. applied to the band in the form of tension, and gave examples of required stretch for at least some common bandsaw sizes.

The trouble is (as is so often the case these days :roll: ) that I cannot for the life of remember where I read the article. Can anyone else out there point me in the right direction, or tell me how much bandsaw blades should stretch for a given tension?

 

[Jake]

If it was on here, it was Ivan: https://www.ukworkshop.co.uk/forums/view ... eter+metre

 

[Sawdust]

I think this ws it:

https://www.ukworkshop.co.uk/forums/view ... eter+metre

Mike

 

[WellsWood]

Jeeez that was quick :shock: 2 answers in 3 minutes!

Thanks guys that's brilliant. Ivans reference to this article on FWW.com started ringing bells and I think what I recall is a combination of bits from both sources.

I had to read Ivan's OP and susequent posts several times before I realised why it didn't make sense - there are typos in there in the form of missing (and later extra) zeros. Got it straight now though so I'm off to start hacking bits off my digital calipers :wink:

 

[WellsWood]

Well, thanks to Jake and Mike ( and Ivan for the post with the required figures) I now have a functional, if somewhat lashed-up, tension meter for my bandsaw.

After hacking off the protruding depth gauge "tail" to make it more managable, and grinding the points off the arms to allow clamping to the blade without damaging the tooth set we have this

Once clamped to the untensioned blade, zero the reading and crank up the pressure until...

..hey presto, 2.5 thou stretch = approx 15,000 psi.

Removing he top wheel to expose the workings of the built in "tension meter" reveals the reason all this was necessary

the indicator arm pivots on the frame of the machine, so it isn't just inaccurate, it doesn't actually measure tension at all, only the position of the top wheel :roll: , as can be seen here where the indicator cleverly says there's lots of tension despite the absence of a top wheel

Surely it wouldn't have taken much effort to design this mechanism to at least read the compression on the spring so it would have been of some value - may have to look into what I can do to correct this :-k

 

[wizer]

fab Mark.... now er, build me one

 

[devonwoody]

How many psi can a run of the mill modern bandsaw take?

Not referring to the blade.

 

[Sawdust]

Nice one Mark,

I quite fancy doing the same thing on my saw as I have never had a satisfactory was of knowing what the tension is.

One question, how do you get from 2.5 thou to 15,000 psi? Presumably it depends on the blade thickness/width and some characteristic of the steel.

Thanks
Mike

 

[WellsWood]

How many psi can a run of the mill modern bandsaw take?

Not referring to the blade.

Not sure really, I suppose it's the fundamental thing that separates the "men from the boys", so to speak, where midrange machines like this are concerned. It's also why most advice in these hallowed pages from experts like Scrit for example is to assume the machine cannot adequately tension the widest blade that'll fit. This machine did 15,000psi on a 1/2" blade and there was plenty left over. I didn't measure the distortion in the frame of course (I suspect there wasn't any at this level - at least I would hope not) but there was no noticable impact on tracking which IMO is an early sign that you're overdoing things. I don't envisage having any problems running 3/4" blades at 15,000psi in this and I'd be surprised if I ever need anything bigger - time will tell.

One question, how do you get from 2.5 thou to 15,000 psi? Presumably it depends on the blade thickness/width and some characteristic of the steel.

Mike, I got my figures from the 2 sources mentioned earlier - both reckoned .001" equates to roughly 6,000 psi. I may be misunderstanding the maths involved but it seems to be a constant, the width of the blade only affecting the amount of force needed to apply a given tension. This is why tension meters that read spring compression are so inaccurate - they only read the force applied to a blade as opposed to the resulting tension.

I should point for those that can't see the FWW article (I think you have to be a suscriber) that 15000psi is the maximum recommended tension, only required for deep resawing, for the majority of the time 6000or so psi seems more than adequate and puts a lot less strain on bearings, tyres etc. The reason I wanted to measure mine in this way is really to get a "feel" for the machine since I was having problems tuning the blade tension by ear like I used to on the EB.

 

[Sawdust]

How many psi can a run of the mill modern bandsaw take?

Not referring to the blade.

Not sure really, I suppose it's the fundamental thing that separates the "men from the boys", so to speak, where midrange machines like this are concerned. It's also why most advice in these hallowed pages from experts like Scrit for example is to assume the machine cannot adequately tension the widest blade that'll fit. This machine did 15,000psi on a 1/2" blade and there was plenty left over. I didn't measure the distortion in the frame of course (I suspect there wasn't any at this level - at least I would hope not) but there was no noticable impact on tracking which IMO is an early sign that you're overdoing things. I don't envisage having any problems running 3/4" blades at 15,000psi in this and I'd be surprised if I ever need anything bigger - time will tell.

One question, how do you get from 2.5 thou to 15,000 psi? Presumably it depends on the blade thickness/width and some characteristic of the steel.

Mike, I got my figures from the 2 sources mentioned earlier - both reckoned .001" equates to roughly 6,000 psi. I may be misunderstanding the maths involved but it seems to be a constant, the width of the blade only affecting the amount of force needed to apply a given tension. This is why tension meters that read spring compression are so inaccurate - they only read the force applied to a blade as opposed to the resulting tension.

Thanks Mark,

I'll give it a go. My saw is fairly chunky and will take a 1 1/4" blade which hopefully it will be able to fully tenstion:

Mike

 

[WellsWood]

My saw is fairly chunky
Mike

:shock: you're not kidding

 

[Anonymous]

I may be misunderstanding the maths involved but it seems to be a constant, the width of the blade only affecting the amount of force needed to apply a given tension. This is why tension meters that read spring compression are so inaccurate - they only read the force applied to a blade as opposed to the resulting tension.

You are measuring the extension of the blade (change in length of the blade), and strain in the blade is (change in length)/(original length).

The tension is really the stress present in the blade that causes this strain.

Young's modulus is a constant for a given material and is equal to Stress/Strain (where stress is force/area)

So for blades with the same Young's modulus (i.e. same material), the thickness of the blade will not affect your values and the stress (tension) will be the same for a given displacement of the upper wheel (extension in the blade), although the force applied through the frame will be higher for the thicker blade to achieve the same displacement.

 

[Sawdust]

I may be misunderstanding the maths involved but it seems to be a constant, the width of the blade only affecting the amount of force needed to apply a given tension. This is why tension meters that read spring compression are so inaccurate - they only read the force applied to a blade as opposed to the resulting tension.

You are measuring the extension of the blade (change in length of the blade), and strain in the blade is (change in length)/(original length).

The tension is really the stress present in the blade that causes this strain.

Young's modulus is a constant for a given material and is equal to Stress/Strain (where stress is force/area)

So for blades with the same Young's modulus (i.e. same material), the thickness of the blade will not affect your values and the stress (tension) will be the same for a given displacement of the upper wheel (extension in the blade), although the force applied through the frame will be higher for the thicker blade to achieve the same displacement.

Tony,

That makes sense but does it mean that the distance between the measurement points (original length?) is critical too?

Mike

 

[WellsWood]

Err.... yeah, what he said :wink:

Thanks Tony, I thought I had it right.

 

[WellsWood]

Mike, sorry I think I'm guilty of leaving that out (ironically in exactly the same way as the FFW article did). You are right of course it's meaningless unless the distance measured for stretch is constant - I used .001" stretch over 5" to equal 6000psi. I don't know what the exact figures are, you'd have to know the Youngs value (if that's the right term) for the particular steel, but I reckon it's near enough for our purposes

 

[Sawdust]

Thanks Mark,

It makes more sense now.

 

[Anonymous]

Tony,

That makes sense but does it mean that the distance between the measurement points (original length?) is critical too?

Mike

Not if you just look at the strain in the blade as our required measurement, then all that you are interested in is the change in length over original length of the section you are measuring. Once you have determined the strain you want to work at, you just need to do the calc each time

Strain = (change in length)/(original length)

Otherwise, yes :wink:


If you want to get a bit geeky, you could find out what material the blade is made from, look up its Young's modulus (loads of tables on WWW), and then you could calc the required extension for each blade for the desired strain...

I'll get me coat.......

 

[Bean]

Tony
If we we are getting geeky ....... you would also have to take notice of where you are clamping the gauge, is it parallel to the blade? And also if you are measuring over the welded section of the blade as this would have different properties to the rest of the blade

 

[devonwoody]

And temperature would have to come into it, because the blade would be longer if warmer. :roll:

 

[Steve Maskery]

Or you could just pluck it 'til it sounds right...
S