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  1. #1
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    Default Quantitative measurement of bandsaw tension

    While I was upgrading my 19" BS it wondered what the actual forces/tensions were on BS blades.
    One could of course purchase an expensive blade tensometer but then it dawned on me that there might be a relatively easy way to measure the tension on the blade using a known weight and digital angle finder.

    Method
    Construct a small pulley to ride on the end of a piece of flatcar steel and clamp this to the BS table
    Construct a simple blade clamp (a square block of steel with a slot in it that could attached to the blade with an eye bolt)
    Attach Eye-bolt to a known weight using a piece of string and allow string to run over pulley so that the weight hung vertically over the side of the BS table (like this)
    Quantitative measurement of bandsaw tension-tension1-jpg

    The measure angle of deflection of the blade from the vertical using a digital angle finder
    I found I had to remove the throat plate, and not use too large a weigh otherwise the blade touched the pre guide structures under the table

    Quantitative measurement of bandsaw tension-angle-jpg

    The Weight used (3kg) divided by the tangent of the angle should then give the blade tension?
    Maybe someone could check that that is right?

    In the case above picture 3/tan(1.1º) comes out to be 156 kg of tension

    Method
    I systematically varied the tension setting on the BS and measured the angle so I was able to plot the tension as a function of angle/
    Note this is for a carbon steel, 18 x 0.65 mm blade.

    The graph shows the actual tension is linear from about setting 3 to about setting 8 and does not depart far from linearity for the other settings.

    Quantitative measurement of bandsaw tension-tensionvsettingcurve-jpg

    The curve looks quite smooth but is a result of multiple iterations of data collection ,
    For example, just increasing or decreasing the tension and taking an angle reading gave very inconsistent results, sometimes just changing the tension did not change the angle at al.
    To get around this I adjusted the tension and then manually spun the wheels by hand for at least 5 revolutions to redistribute the change in tension around the wheels. At high tension I could hear faint creaking sounds as the tensions redistributed.
    Despite this there were still slight systematic differences between when increasing and decreasing the tension - the points shown on the graph are an average of going up and then down the tension scale and they seemed to have evened themselves out in the process.

    Is it good for anything? I don't know, I usually just the blade flutter method to set the tension - I was just curious as to what the actual tensioning forces were.
    One could go on from here to work out the MPa involved but I will leave that to those that are interested to do for themselves,
    Attached Images Attached Images

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  3. #2
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    Not sure what use it would be Bob as it will vary for every blade as it is dependant on the lateral stiffness of the blade. Better to make a better measurement on the current spring system the bandsaw already has which is direct tension in the blade itself.
    Neil
    ____________________________________________
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  4. #3
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    Quote Originally Posted by dai sensei View Post
    Not sure what use it would be Bob as it will vary for every blade as it is dependant on the lateral stiffness of the blade. Better to make a better measurement on the current spring system the bandsaw already has which is direct tension in the blade itself.
    Yes I realised it was blade specific.

    If you are referring to the spring tensioner system inside the BS then that is also blade specific because different length blades will have different tensions at the same BS spring position. If the blade is longer than usual then the spring will need to be tensioned well before it starts to stretch the blade.

    What I was hoping for when changing blades was to forget about the tension scale on the BS and hook up my rig onto the blade and dial the tension up for a specific angle (specific to each blade) as that would give me the same force/tension every time.

    Lenox and others make tension meters
    These typically use a 0.0001" dial indicator which kind of makes them expensive.
    A couple of guys on the practical machinist forum have also made their own using 0.0001" gauges.
    Others commercial uses high resolution digital vernier scales clamped directly to the blade.

    What I was aiming for was to use gear I already had in the shed
    I have played around clamping a standard (0.01mm) digital vernier direct to the blade which works but the resolution is poor at low tension and not that good at higher tension.
    The change in length for my 18 x 0,55mm blade is about 0.15 mm across the range which means at lower tensions where the change in length is small e.g. 0.1 mm that it is impossible to measure. Mid range the resolution is about 1 in 7. There is a way that a jig can be setup using a lever to magnify the change in length of a vernier which will improve the resolution by a factor of about 3 but it is still not as good as my method for low tension.

    With my method, using a 3kg weight, the resolution is about 1 in 40 at low tension and 1 in 25 at mid range which is similar to the vernier and lever method. However with my method the weight can be increased at high tension to further improve the resolution. My method gives the actual force/weight after one simple calculation and does not require knowing the Young's modulus of the steel the band is made from.

  5. #4
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    I think Neil might have been referring to the spring that is compressed when the handwheel is tightened, not so much the lever indicator, which is next to useless!
    For a comparison I'd measure the deflection in that spring for each blade deflection test, then the spring rate needs to be determined, the spring rate is, for e.g. 50kg/mm, an engine rebuild place should have the suitable test rig, they use them for testing valve springs, or knowing Bob he might rig up his own
    So if the spring rate is known and a deflection measured this should give a comparison with the blade deflection test to see how each compares.



    Pete

  6. #5
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    Quote Originally Posted by pjt View Post
    I think Neil might have been referring to the spring that is compressed when the handwheel is tightened, not so much the lever indicator, which is next to useless!
    For a comparison I'd measure the deflection in that spring for each blade deflection test, then the spring rate needs to be determined, the spring rate is, for e.g. 50kg/mm,
    I agree that the standard indicator is not very useful because it is measuring the movement of the carriage that contains the spring rather than the length of the spring itself.
    Thus blades of different length will not have the same force applied to them at the same position. a few mm of carriage travel might be used to take up the slack, or it might already have tension on it when the tension lever is applied.

    Knowing the spring constant would indeed allow the amount of compression of the spring to be used to measure the force on the blade but measuring the spring compression in situ would require getting in behind the wheel to measure the spring length which is near impossible with my cast iron wheels. If the standard indicator could be attached to the spring that would be more useful.

  7. #6
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    Your string over the pulley introduces a small amount of friction in the system. That's why you sometimes got no change in response and drift around data points. Precision scales use knife-edge fulcrums to reduce friction.

    Nice try, though.

    Cheers,
    Joe
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  8. #7
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    Quote Originally Posted by joe greiner View Post
    Your string over the pulley introduces a small amount of friction in the system. That's why you sometimes got no change in response and drift around data points. Precision scales use knife-edge fulcrums to reduce friction.
    You are right about the pulley friction being significant.

    I measured the pulley friction using two digital balances, one before and one after the pulley and the difference was in the 500g range.
    However I was using a force of 3000 g on the side of the blade so a difference of 500g alone is not sufficient to explain the zero change in blade angle.
    The friction in the pulley was eased using lithium grease and pushing the blade back and forth sideways several times by a mm or two before each reading. Without looking at the the two digital balance scales they always ended up reading to within about 50g.

    The lack of response appeared to be due to the added tension not being evenly redistributed around the wheels because after I rotated the wheels I could easily see a more consistent response.

    This has a practical application for those setting their blade tensions by the 1/8" deflection method. i.e. if you tension the blade and the amount of deflection doesn't appear to have changed then it may pay to rotate the wheels a few times and check BEFORE adding more tension.

  9. #8
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    Quote Originally Posted by BobL View Post
    You are right about the pulley friction being significant.

    I measured the pulley friction using two digital balances, one before and one after the pulley and the difference was in the 500g range.
    However I was using a force of 3000 g on the side of the blade so a difference of 500g alone is not sufficient to explain the zero change in blade angle.
    The friction in the pulley was eased using lithium grease and pushing the blade back and forth sideways several times by a mm or two before each reading. Without looking at the the two digital balance scales they always ended up reading to within about 50g.

    The lack of response appeared to be due to the added tension not being evenly redistributed around the wheels because after I rotated the wheels I could easily see a more consistent response.

    This has a practical application for those setting their blade tensions by the 1/8" deflection method. i.e. if you tension the blade and the amount of deflection doesn't appear to have changed then it may pay to rotate the wheels a few times and check BEFORE adding more tension.
    Quite so. And also if you estimate tension by plucking the blade and listening to change in pitch.
    Of course truth is stranger than fiction.
    Fiction has to make sense. - Mark Twain

  10. #9
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    Quote Originally Posted by joe greiner View Post
    Quite so. And also if you estimate tension by plucking the blade and listening to change in pitch.
    I have experimented with using my guitar tuner to assess tension - fun but not that useful.

  11. #10
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    There's a device used to measure the tension to properly adjust an engine fan belt. Possible?
    Some sort of known force applied at right angles and the deflection of the belt measured.

    BobL: what note(s) did you find with a properly tensioned BS blade? I can do that, too.
    Could be very useful.

  12. #11
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    Quote Originally Posted by Robson Valley View Post
    There's a device used to measure the tension to properly adjust an engine fan belt. Possible?
    Some sort of known force applied at right angles and the deflection of the belt measured.
    I have seen those - they go up to about 150ls, would be OK for small blades but nowhere near enough for larger blades

    [/QUOTE]BobL: what note(s) did you find with a properly tensioned BS blade? I can do that, too.
    Could be very useful.[/QUOTE]

    The note is of course dependent on the length of the blade between the wheels and affected by whether or not the blade is touching (even very slightly) and therefore rattling on the guides.
    I remember the note only registering on the guitar tuner maybe once in every 10 or so twangs and it not changing very much between low and high tension.
    After all this all I decided it was too complicated

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