I can tune a bandsaw blade

[sunnybob]

Quite literally.
I like reference points and its been driving me bonkers every time I change blades as to how tight they should be.
So, working on the principle that its just a very big instrument string, I borrowed a digital guitar tuner from a friend and experimented.
This kind of thing...
http://www.ebay.co.uk/itm/SNARK-Sn5-CLI ... xyUylTSEGf

clipped it to the blade so the body is resting on the table, and dinged the blade with a spanner.

Guess what? A solid "F".
Three different thicknesses of blade, all adjust to cut nicely at "F".
Any one else got one of those gizmo's and prepared to test my theory?

 

[NazNomad]

Is this you?

:-D

I do have a digital tuner, I'll check mine later.

 

[Alexam]

I doubt it, but I have heard of looking for a specific 'note'. You have so many ways to 'tune' and balance your bandsaw and should be 'there' by now. Good idea to backtrack on all the video's you have seen on tuning bandsaws and try and get comfortable with the results. No good to keep on looking for more and more when you have the answers already and just have to work them.

I don't know, but I would have thought that the tension of a bandsaw blade related to a specific note would have to vary due to thickness of the steel and depth of the blade and it's the tension of each blade that is important, so I can't see that one note could achieve that.
Malcolm

 

[Wildman]

the note you achieve depends on the length of the free portion of blade so every machine will be different. Also like a guitar string different cross sectional areas would produce a different note. The deflection method is the only true test without expencive strain gauges, mind you using a luggage spring balance to a given poundage for a given deflection should work. If using a set note works for you you can get a guitar tuner app for your android phone.

 

[bigbob1]

I have a few guitar tuners about the house along with guitars but I am sticking with give the blade a gentle tap and if it deflects 1/8" thats fine by me although I do tension my scroll saw up by pitch but don't use a tuner for this just by ear. The other day I tuned my BS350 up with a new tuffsaws 5/8" blade and it now cuts as straight as my benchsaw so I am not touching it.

 

[NazNomad]

Ok, mine is just sharp of G.

 

[Alexam]

Not sure what mine is but whichever note it is, it's singing well.
Malcolm

 

[thetyreman]

machines also have harmonic resonances, to hear the F note, it's often a comination of several odd and even harmonics. I often sing the note out loud when using machines, it creates a cleaner and more accurate cut on the wood.

 

[sunnybob]

Oh well, I cant be right ALL the time.
(g)

 

[worn thumbs]

I'm glad my bandsaw has a tension gauge.A few years ago I was using another,quite old,machine and had to change the blade.The replacement was a Starret and came in a box that recommended a blade tension of 12,000 psi.Since the saw had a tension adjuster that worked by moving a weight to the most suitable notch on an arm,I had to do a bit of calculating which proved that the previous blade had been correctly tensioned too.

 

[sunnybob]

I'm not ready to completely give up on this idea yet.
3 blades, from 3/16 to 1/2" and with different teeth, all produced the same note. The third one i left loose and gradually tightened untill it reached F, and only then did I cut with it, and it went fine through 16 cm of laminated hardwood.
I understand the free length of the blade could be an influence, but what if every bandsaw had its own resonance?

If any one else is so inclined to test different blades on their machines, that would be an interesting experiment.

 

[jpa]

I'm very interested to try this experiment once I get back to my workshop next week. I have a Lidl tuner as well as a couple of different instrument tuning apps on my phone.

My bandsawing experience is very limited as I've only just recently bought one. I've had excellent results cutting dovetails using a well used 15mm blade, and then disappointing results trying to cut curves with a brand new 8mm one. Prime suspect is that the latter was due to lack of tension (as well as lack of experience, obviously).

 

[MusicMan]

Sunnybob is certainly approximately right for a given bandsaw. The maths is as follows. The frequency (pitch) of a transversely vibrating round string is

frequency = (1/2L)* sqrt(tension/mass per unit length)

where L is the vibrating length of the blade.

The equation for a rectangular blade (vibrating sideways) will be similar apart from a multiplier, which might change a bit from narrow to wide blades - need to look that up.

If you double the width of the blade, by going say from a 1/4" to a 1/2" blade, you will want to double the tension to keep the stress in the blade constant. This will keep the sideways defection under load also constant. If you double the width of the blade, you double the mass per unit length. Therefore you need to double the tension to keep the quantity under the square root constant, and hence keep the frequency constant.

If your bandsaw has a longer blade then the note will be lower at a given tension. If it is twice as long it will be an octave lower. But the note will be constant for a given bandsaw.

I do tension mine by listening to the note. Incidentally, one can get a tuner app on any smartphone.

We might be able to get a formula which is applicable to any bandsaw, but I'd need more research for that.

Keith

 

[sunnybob]

Hah! I knew I was right! (lmao).
But interesting stuff.
as a point though, I have been tapping the blade on the back edge, with a spanner, pushing towards the front of the table. i tried side on, but the movement was so great it threw the gizmo off the blade.

Maths loses me very quickly, but if someone can work it out, we can all benefit.

 

[NazNomad]

... if someone can work it out, we can all benefit.

Very accurate measurement of the length of the free-standing part of the blade.
Same with width & thickness.
Metallurgical make-up would doubtless have an effect.
Workshop temperature would affect it.

Pretty much the same variables as with musical instrument strings & tuning, I suppose?

Is it really worth it when the flutter test is available? :-D

 

[sunnybob]

"Is it really worth it when the flutter test is available?"

Good grief man, invention is what made Britain great.
(and its a good bit of fun)

 

[NazNomad]

Yeah, it's so ''Great'', some of us even stay here. :-D :-D

 

[sunnybob]

Just remember, if this works out, the patent is MINE, ALL MINE!

 

[NazNomad]

www.youtube.com/watch?v=JPyc2iDQnOA&vl

 

[MusicMan]

That video is excellent, NazNomad, and shows how to calculate the tension quite accurately enough from the note. I think it is correct. I'm afraid it beats sunnybob's patent though!

On further reading, the equation I put up earlier is correct for the first mode of vibration (which is the one we hear). The mass per unit length accounts for the differences in blade thickness and width, and one works this out simply from the cross sectional area and the density. But these factors are all linear, and so it is true that for a given bandsaw the vibration note uniquely indicates the tension. Say to about 10% as we don't take the effect of the teeth into account.

The effect of temperature and metallurgical variables is small for elastic properties (a percent or so). What can make some difference is the amount and type of prestressing in the blade, but this mainly applies to huge blades for industrial bandsaw mills, which is where most of the heavyweight research has been done. This does get mathematically quite nasty, especially as they really want to know all the vibration modes at all operating speeds.

But as a simple workshop setting test we can use it with confidence, say to about 10% accuracy. Especially since most of us carry a frequency meter around in our pockets (when installed with a suitable app).

If you don't want to work out the sums shown in my previous post or in the video, set your tension with any blade by any means until you are satisfied with the cut. Measure the frequency (pitch of the note), then when changing blades, tune to this note again.

Nice experimental observation, sunnybob!

Keith