desired tension (PSI)
Want to give
it a test-spin? Enter 15000, the PSI suggested by the
manufacturer for my Blade Runner blade.
Enter the values above, then
click on the "calculate" button to
determine the tuning frequency...
were following the test-spin, you should have received an answer of 78.95-Hz.
we've computed the frequency, you can use the files below to hear
what a blade tensioned to that frequency should sound like.
These are MIDI files, which should open in MS MediaPlayer (or any
other suitable MIDI player).
NOTE: As I'm
still in the testing phase, I encourage you to proceed with
it seems like you are overtensioning or undertensioning, please
contact me and let's work through it. You assume all
responsibility for using this site
and my method .
The topic of
bandsaw tension is perhaps second only to dust collection in its
ability to spark heated discussions among woodworkers.
current bandsaw (a Delta BS150LS) has no tension gauge. It is
a direct-drive 10" bandsaw with a 1/2-HP motor, and 7" of
resaw capability (and I've resawed 7" stock with it, too).
But like I said, there is absolutely no tension gauge to guide the
user in setting blade tension. The manual instructs the user to
turn the tension knob until all the slack is removed from the
blade. Then, depending on the size of the blade, the user is
instructed to turn the knob from 3 to 7 more times (3 turns for a
1/4" blade, 7 turns for a 3/8" blade).
has worked sorta alright. The results were very repeatable,
but I had no idea if I was achieving the 15k to 25k-PSI tension that
the band manufacturers typically recommend. Thicker stock
(resawing) tended to give me more problems than cutting thinner
stock. I often assumed the tension had something to do with
that, but I had no way of knowing for sure.
had even greater difficulty when I decided to try a 1/2" Blade
Runner band on the saw. The Blade Runner (from Iturra Design)
is made from the same Atlanta Sharptech stock as the Wood Slicer
blades from Highland Hardware. These blades have done very well
in the reviews (winning the Fine Woodworking blade shoot-out, for
example). But, the manual for my saw offered no guidance in
tensioning a 1/2" blade. While the band fit, and I seemed
to have no problem achieving lots of tension with it in place, I had
no idea how many turns of the tension knob were required to properly
I considered purchasing an aftermarket tension gauge.
Typically selling for $100 to $300, these clamp to the blade and
measure how much a band stretches as it is tensioned. Young's
modulus is used in order to convert the measured stretch to tension
(in PSI). However, Mark Duginske, a noted bandsaw expert and
author of at least a couple of bandsaw books, has argued that these
aftermarket gauges fall short. Testing performed by Mike Cutler
(a materials engineer) on Mr. Duginske's behalf indicated that the
gauges are neither accurate, nor repeatable. Mr. Duginske
announced these findings in one of the woodworking forums and both
Mr. Duginske and Mr. Cutler had to immediately begin defending their
findings. Several users of the gauges were unwilling to accept
the results of the tests. However, it is clear that the tests
were exhaustive, with standards traced back to NIST (National
Institute of Standards and Testing). Mr. Cutler spent hours
speaking with engineers at Delta and blade manufacturers. The
results were accurate, the aftermarket gauges were not.
was aware of shop-made gauges. These pretty much emulate the
spendy aftermarket gauges I mention above (clamping to the blade to
measure stretch). While they're certainly much less expensive
to make than to buy ($20 instead of $100 and up), if the $100 to $300
gauges are neither accurate not repeatable, I didn't see the sense of
making a copy of one. Yet on one rainy Saturday afternoon I did
experiment with shop-made solutions. I had less than
reason I imagine these gauges don't work well (in case you were
wondering) is that they rely on the user's ability to get them on an
untensioned blade such that the blade is perfectly straight and true
between the clamps (any curvature in the blade will result in the
user overtensioning the blade).]
Mr. Duginske does give hope. During his tests, Mr. Cutler
found the electronic gauge from Carter Products to be both accurate
and repeatable. Still not inexpensive (at $200), it does get
the job done. One significant limitation: It works only
with Delta and Jet 14" cast-iron saws.
Cutler also found the gauges built into saw to be usuable.
More accurate, and more repeatable than any aftermarket gauge save
for the Carter electronic gauge. Mr. Duginske argues that there
is no need to go to an aftermarket gauge if you already have one
built into your saw. There are some caveats, however. For
example, some saws have inadequate tension springs. The springs
will not allow the user of the saw to achieve the amount of tension
necessary to accommodate wider (1/2" and larger) bands. In
those cases, the springs can be replaced by much stronger aftermarket
springs. But the gotcha is that the scale on the built-in gauge
may no longer be accurate. Users upgrading to those aftermarket
springs are kinda on their own when it comes to tensioning their saws.
users lacking a gauge or distrustful of the one present, Mr.
Duginske offers one more glimmer of hope: Blade plucking.
One plucks a tensioned blade on the side of the saw opposite the
guides. Mr. Duginske provides some instruction on what to
listen for in the text of his book. While this method helped me
return a blade to a tension that had previously worked well for me,
it didn't offer enough guidance in absolute terms. That is, I
still had no idea if I was setting the band to 10k-PSI, 20k-PSI, or
there have been some attempts on woodworking forums to compare notes
(sorry for the pun) on what plucked blades sound like on members'
saws. I read a poll on the Family Woodworking site, and started
a poll on SMC. I found the results and ensuing discussion enlightening.
then I had a eureka moment. Back in high school physics, we
did a simple lab where we tensioned a steel wire between two
posts. With very simple math, we were able to determine what
fundamental frequency a plucked string would generate. The only
variables were the amount of tension, along with the length and mass
of the string. There is absolutely no reason this lesson
couldn't be applied to bandsaw blades!
was able to refresh my memory of the math involved, and create the
above worksheet to help me in the process. And of course,
everyone benefits as it is on the www.
interesting aside: While experimenting with my BS150LS in the
days and weeks after purchasing it, I did break a few blades (new,
expensive ones). Now that I've been able to compute the
frequency my plucked blades should generate at various PSI's, I have
come to an alarming conclusion: Those broken blades were WAY
overtensioned. My best guess (it has been months) based upon my
aural memory of those plucked blades is that I was tensioning them to
60k-PSI or even more. I was able to derive this using this
worksheet and specifying higher and higher PSI's until the sample
file for the suggested frequency sounded similar to what I heard when
I plucked those now broken blades on that fateful day. Now I know.
still in the process of testing my method. I've spoken to
numerous engineers and even bothered one of my old physics professors
about some of the more subtle aspects. But the consensus of
these experts is that I'm onto something. I will hopefully have
either one of the Carter electronic gauges or another load-cell based
system soon and will be able to directly correlate my results.
tuned (this subject lends itself to puns. Sorry)...
I have used your worksheet to compute the frequency for some of my
blades. The suggested frequencies seem quite low (approx.
40-Hz) compared to your example. I was anticipating a low E
(82.4-Hz) that was suggested in some of the recent polls. Am I
doing something wrong?
Not necessarily. The parameters you provided me (185-grams,
131.25" long, .024" thick, .408" wide, 39" wheel
separation) indicate that this is a BIG saw. As the distance
between the wheels increases, the frequency of a plucked blade drops
quickly (assuming a constant PSI). Using your numbers and a
target PSI of 20000, my worksheet suggests a frequency of
44.7-Hz. If I keep increasing the target PSI by 10000 until I
reach a frequency of approx. 82.4-Hz, we find that your blade would
be tensioned to almost 70000-PSI (yikes) to get you to 82.4-Hz.
Clearly WAY too much.
Have any other examples of how wheel span influences the derived frequency?
Let's take the example of my little BS150LS. A 72.5"
Blade Runner weighs 84-grams. It is .022" thick, and
.390" wide (remember, we're excluding the teeth). My wheel
separation is 19.75". The manufacturer suggests a tension
range of 15000 to 17000-PSI. So if my target PSI is 15000, then
I should tune the blade to 78.95-Hz.
let's assume that they made a 6" riser block for my saw (they
don't, but this is strictly theoretical). The blade length
would increase to 84.5" (twice the length of the riser block),
and the weight of the new blade would be 97.90-grams ((old grams/old
length)*new length). My new wheel separation is 25.75 (adding
the 6" riser to the original 19.75"). Plugging these
new numbers into the worksheet and targeting the same 15000-PSI, we
find we have now reduced our tuning frequency to 60.55-Hz.
A couple of years ago I purchased a used 14"
Jet bandsaw. It has a riser block installed, and included two
blades. I haven't had much luck with it. It works fine
for cutting thin stock. But resawing stock even 3" high
often results in the blade curving in the cut. It gets noisy
and starts to fight me. I've adjusted the tension as high as I
can without fully compressing the spring. So tonight I went
through the process of weighing the blade and measuring
everything. I computed the frequency using your worksheet
(approx. 60-Hz) and listened to the sample file. Even when I
have the tension cranked nearly all the way to the max, my blade
sounds considerably lower when I pluck it (about 39-Hz). Is it
possible that my saw can only do 6500 PSI?
a word, yes. Both Delta and Jet have apparently had problems
with some of their springs. I'd suggest purchasing a
replacement high-performance spring from one of the woodworking
catalogs. Once you replace your current spring, reinstall your
blade and try tensioning it again. With the new spring
installed, I'd shoot for a tension of 15000-PSI and try resawing some
6-8" high material. If your blade still curves, try taking
it up to 20000-PSI. Let me know how that works!
respect my time and effort by not taking credit for my work.
Please don't submit my methods as a tip to a magazine, or attempt to
make a commercial product based on what you see here. I reserve
any and all rights possible to the concept of an audible tension gauge.
First release. Waiting for input from testers...
Added some MIDI files which users can compare to their plucked
blade. It was a lot of fun learning about MIDI (sarcasm), but
at least this should largely negate the need for a chromatic tuner.
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