Bandsaw Blade Tension

[Scott Rychnovsky]

I have been setting my bandsaw blade tension with the flutter method and am reasonably happy with it. I was curious about tension meters but have not been willing to put up the money for one. I realized that I already have a tension meter in my garage. It just is not calibrated for bandsaw blades:

http://www.amazon.com/Park-Tool-Spok...=spoke+tension

Has anyone tried to use a spoke tension meter on a bandsaw blade? It seem alike it should work if I had a way of calibrating it. It has the advantage of being cheap and I already own one.

Will it work? Any advice for calibrating it for a band saw blade?

Scott

[John TenEyck]

I have no idea how that tension meter works. But I do know how you can easily measure the tension on a bandsaw blade with nothing more than a digital vernier caliper, a couple of small C-clamps, and some math. Remove the upper blade guides, and lower the tension on the blade to almost nothing. Open the vernier calipers to 4 or 5 inches and clamp it onto the side of your blade with the small C-clamps. Write down the exact opening on the vernier calipers, then reset it to zero. Now add whatever tension you want to the blade and then note the reading on the vernier caliper.

Now a little math. The Young's Modulus (E) of steel is about 30 million psi. Also, E = Stress/Strain, so Stress = E x Strain. OK, Strain = the reading on your vernier after you cranked up the tension (It might be 0.003" or so.) / the initial gap on the calipers (say 4.505"). Plugging in the numbers: Stress = 30 x 10^6 x 0.003/4.505 = 19978 psi. If you want to know what the corresponding load was on your saw and spring: Stress = Load/Cross sectional area of the blade. Let's say you have a 1/2" blade made with 0.025" thick band. Measure the width of the blade from the bottom of the gullet to the back edge; let's say it's 0.4". So Load = 19978 psi x 0.025" x 0.4" = 199.8 lb. If you have the spring constant you can check to see if the calculated load corresponds with how much it compressed. Of course, if you have the spring constant you can work the equations backwards to determine what the blade tension was w/o having to use the vernier calipers.

Hope that makes sense.

John

[Alan Schaffter]

PSI, schmee si!! Who cares!

Do some experimenting with various settings, blades, species of wood to find out when each blade cuts the "best for you," not some manufacturer's recommended, mathematical, or engineering-based tension setting. Put the spoke gauge on after you find each "best for you" setting and record them.

[John TenEyck]

The engineers who figure out how things work and how to make them work better care. Thankfully.

John

[Alan Schaffter]

But does an end user need to know the math and engineering, or just does he just need to how to set it so cuts right- he doesn't need a number on a gauge or a scale- letters will work, so will smilley faces, since it is a relative setting, as long as it is repeatable.

[Scott Rychnovsky]

Hi John,


Thank you for the explanation. I was surprised when I followed up and looked at the tension meters that were available for sale. All I found were meters that measures the stretch, so they were doing exactly what you described. They also cost a crazy amount of money given how simple they are! I will try your method with a digital caliper. The saw has a tension gauge on the spring, but I would like to bench mark it against a real tension number once just to see where I am.

The spoke tension meter is a deflection meter. I put it on my blade and can easily read tension differences. I am not sure it will read the tension of a fully tightened blade, however. (I am using a 1" blade.) The spoke tension meters are much cheaper than the blade tension meters, and they do respond to changes in tension. I will try it out on a fully tensioned blade to see if it works.

Scott

[Alan Schaffter]

Scott, one question for you or anyone- where does a "real" tension number come from? How does a blade manufacturer determine the correct tension number? When you think about that answer, you'll see where I'm coming from when I say all you need is some way to measure relative tension like your spoke meter.

[Alan Schwabacher]

Scott:

The problem with measuring tension by blade stretch is that at the tension you want, the blade doesn't stretch much, so the precision of your measurement will be poor. At full tension it will vary by something like 0.003", which can be measured but...

You will see much more change in your measurement with a small change in tension using the pitch of a plucked blade. Most phones today can measure such frequencies quite accurately (Pitchlab on Android is a good free program), and Phil Thien has put up a calculator to let you convert to blade tension: http://www.jpthien.com/tg.htm

Once you have calibrated your saw, the built in tension gauge is probably the best choice for getting back to a given setting, even though its units probably are arbitrary.

I'd have to agree with the other Alan that there's no real need to know the tension in order for your bandsaw to work well, but I understand the need to measure things.

Have fun,

Alan

Are you making more telescopes, or something else now?

[Ole Anderson]

This is why I like the concept of tensioning using an air cylinder and air pressure gauge where you can quickly get repeatable results, whether you set the tension with the flutter method or do the calcs to determine actual blade tension. Ok, so I am willing to spend time coming up with a solution that probably takes more time than it saves. I guess that is the engineer in me.

[Gus Dundon]

I'm good with flutter method for putting tension on the blade. I have never tried this one.

[Scott Rychnovsky]

Hi Alan,

I am starting out small with boxes and moving on to tables. I owe Terry some tables. I am done with telescopes--I made more than I know what to do with.

I agree that I will normally use the bandsaw tension meter to set up a blade, but I am curious about the actual tension value. I do like measuring things.

Scott

[Scott Rychnovsky]

I put together a tension meter for my bandsaw this morning. It was based on John's suggestions and the commercial ones. The basic idea it that the two ends clamp to the slack blade in slots to position the blade on the jig. Most of the wood is cut away from the bottom part to create a pivot. A micrometer is positioned to read the flex on the other side of the pivot. The geometry of the pivot point give the micrometer a 3X motion compared to the motion of the blade. I use that correction plus the math John described to calculate the strain on the blade and the stress on the blade.

The reading of 5.0 (thousands of an inch) corresponds to a stress of about 7800 psi. I have been using this blade at a setting of 7 (bandsaw gauge) which gives me a micrometer reading of 7.5. That corresponds to a stress of about 12000 psi. That is also about where the flutter test puts me--just above 6 on the bandsaw gauge. This is not 20,000 psi that I have seen recommended, but it does seem to work OK.

In the future I need to be sure that the clamp is not resting on the table--that may mess up the readings. I will play with the gauge a bit to get a sense of how much pressure I am putting on my blades. I will probably retire it soon and just use the gauge on the bandsaw. It was an interesting project, and it did not cost me $150-300 for a commercial gauge.

Scott

[Alan Schaffter]

You can also build your own electronic gauge- put a load cell under the foot of the tension rod and either connect it to a A/D (analog to digital converter) and display, or just connect it to an inexpensive digital milli-ammeter. You'll need a reference table (should be supplied with the load cell) and remember to divide the result by 2.

[Guy Belleman]

Well, Scott. Seems like this is as good a reason as any to look for a bandsaw with a tension meter. I do like the one on my Grizzly 0513X, and although it probably isn't nearly as precise as your device above, it is built in, easy to operate and easy to see through the view window.