Damaged Band Saws from Overtensioning

[Brett Baldwin]

My informed layman's reading of all this and particularly Chris F. and Glenn C. posts leads me to these two understandings about overtensioning:

1. If the spring material/manufacture is inferior, the spring could be too weak in spring rate or too short from the start, leading to the potential for coil-bind and from there a quick possibilty for overtensioning.
2. If the spring is only "adequate" for the "normal" tensioning and not over engineered to handle the tension required for larger blades and thicker materials and it is constantly used beyond its fatigue point because of this, it will weaken and probably coil-bind leading to the overtensioning again.

So that leads me back to the gist of Mark's orginal position that the guage on the saw is good enough for average use. What I am getting out of all the disussions is that this is probably true as long as the spring in the tensioner is up to design specifications and you are using a blade whose tensioning needs don't exceed the capacity of the tensioning system on the saw.

Does that seem like an accurate summarization of all this?

[Greg Mann]

My informed layman's reading of all this and particularly Chris F. and Glenn C. posts leads me to these two understandings about overtensioning:

1. If the spring material/manufacture is inferior, the spring could be too weak in spring rate or too short from the start, leading to the potential for coil-bind and from there a quick possibilty for overtensioning.
2. If the spring is only "adequate" for the "normal" tensioning and not over engineered to handle the tension required for larger blades and thicker materials and it is constantly used beyond its fatigue point because of this, it will weaken and probably coil-bind leading to the overtensioning again.

So that leads me back to the gist of Mark's orginal position that the guage on the saw is good enough for average use. What I am getting out of all the disussions is that this is probably true as long as the spring in the tensioner is up to design specifications and you are using a blade whose tensioning needs don't exceed the capacity of the tensioning system on the saw.

Does that seem like an accurate summarization of all this?

Seems pretty close from my point of view, but, I might question the term "up to design specifications". We tend to think that there are some real brilliant and wise designers who know exactly what "design specifications" are needed. One of the reasons we all tend to rhapsodize over 'old iron' is that, quite often, those guys had no clue what size spring, bolt, axle, motor, etc. was needed so the overkilled their choice. Subsequently, most components never got overstressed and the tools still work today. (At least the ones that had the excesses built in. All the mediocre versions have long been discarded. We don't hear much about them and our ignorance of them feeds the notion that "They don't build 'em like they used to.") I think Glenn's chart is very useful. If the design of the tool keeps the spring in the safe zone we will probably never need to worry about it failing. OTOH, even well engineered and crafted springs can fail (in muscle cars for example) if taken across the line in his chart. And, of course, that is exactly where they are taken 'in just a couple runs'. In the end, I believe that if a spring is engineered correctly for the application, the saw is buttressed for the tension by good design and so on then we should be able break the largest blade designated as proper for any given saw EVERY TIME without damage to any other saw componentry. Someone asked if they should junk their 3/4 blades for use on a Delta 14, IIRC. Well I have that saw (older American made) and it allows for 3/4 blades but, if I find it really isn't engineered for that big a blade, I won't use one.

My summary would be: Try a blade at tensions you are comfortable with. If it works to your satisfaction, make note of the tension. If not, use a smaller blade and repeat. If none of this is to your liking get a bigger saw. I am reasonably sure I will find applications for which my 14 inch Delta will be insufficient. If it is with blades for which it was 'designed' to work then I will conclude it was under-designed.

[Phil Pritchard]

Wouldn't springs tend to work-harden over time? Any other time you have a metal part which is flexed repeatedly you get work hardening?

To an extent, yes, but the rate of work hardening varies with the composition of the steel used.

Also, Phil didn't say how old his bandsaws were, but wouldn't the metalurgy on an older spring tend to be poor compared to modern metalurgy--meaning those old springs might be more like round wire rolled into a spring-like shape than today's springs?

All the machines I've rebuilt were post-1930. I don't believe that modern steel is that much better. Part of the skill in tempering a spiral spring is in the art of the springmaker

Phil, can you explain what you've done to your springs?

In two cases I've had to replace them as the old springs had, indeed cracked. These were machines made in the early to mid 1940s, so maybe the war was the reason (poorer steel). Mostly the springs have been annealed/detempered, pulled slowly out to length then retempered by the springmaker.

Phil

[Mark Duginske]

That saw was sold by Trendlines and they are no longer in business. It is one of the generic Taiwanese saws. I'm glad to hear that you are happy with it. Most of these saws are very similar and often take the same replacement parts. For example, I get involved with a lot of saw repairs or restorations.
I recently fixed an "Elephant" brand top wheel hinge (the broken one in the original photo in this post) with a Jet part.

I would see if a local dealer, Home Depot, Woodcraft or Rockler has a riser in stock. If it is not too far a drive I would take the top casting of the saw off and take it to the store and see if it fits. Chances are that it will fit.
Good luck.

[Bart Leetch]

Does anyone know where I can purchase 5/16" round cool blocks. I have an 1950's Sears 12" band-saw that came with round 5/16" steel blocks & I would like to replace them with manufactured cool blocks. I have been using pieces of hardwood dowel.

[Alan Schaffter]

Mark can you or someone else address the below issues-

I read this thread and the one a few months back, and no where do I recall seeing a discussion about how desired blade tension figures (for sawing wood) are determined and how do we know they are correct, assuming there is or will be a bandsaw gauge to measure them.

Are these numbers theoretical or were they determined by testing and experimentation by the blade manufacturers? To me, an accurate tension gauge is only half the problem.

I realize, it would be nice to have a tension gauge to help prevent bandsaw damage from over-tensioning, but can we trust the tension numbers given for these blades? Do all blade manufactures provide recommended tension figures ? I thought some like Timber Wolf recommend the "flutter test" As a blade is used won't the tension need to be adjusted to account for blade elongation and dullness? And what about adjusting blade tension for differences in material being cut? If so, by how much?

So what is the answer to proper blade tensioning? Might there be another whole methodology, simple or complex, we have not even touched on?

[Ralph Lindberg]

Mark,I have an old Rellant 14"..

Herb, I put a Grizzly block on my old one, it did take a little work, as they were not 100% the same. But it was doable

I also upgraded the guide system, it helped (but not enough), or why I own a Grizzly G0457 now.

[Matt Meiser]

Does anyone know where I can purchase 5/16" round cool blocks. I have an 1950's Sears 12" band-saw that came with round 5/16" steel blocks & I would like to replace them with manufactured cool blocks. I have been using pieces of hardwood dowel.

Bart, Olson lists Cool Blocks "For Gilliom and old Sears Band Saws using round guides" but it doesn't say the size. Here's their list of part numbers. You might try giving them a call and seeing if they are what you need. The phone number is on their contact us page.

Re: Riser blocks for generic saws--I had a no-name 14" for my first saw which I put a Ridgid riser block on. I ran into a couple problems. First, the alignment pins didn't line up, so I just took them off and used a long straight edge to align everything. Second, the replacement bar for the guide was the wrong size. My dad is an amateur machinist so he just turned it down for me. Alternately, I could have bought some bar stock in the right diameter, though it would probably have been metric and thus harder to find. That saw worked well once I replaced the wimpy stock motor with a 2HP motor from Harbor Freight, but when I found a great deal on an old Rockwell 14" I bought it and sold the generic saw.

[John Thompson]

Mark can you or someone else address the below issues-

I read this thread and the one a few months back, and no where do I recall seeing a discussion about how desired blade tension figures (for sawing wood) are determined and how do we know they are correct, assuming there is or will be a bandsaw gauge to measure them.

Are these numbers theoretical or were they determined by testing and experimentation by the blade manufacturers? To me, an accurate tension gauge is only half the problem.

I realize, it would be nice to have a tension gauge to help prevent bandsaw damage from over-tensioning, but can we trust the tension numbers given for these blades? Do all blade manufactures provide recommended tension figures ? I thought some like Timber Wolf recommend the "flutter test" As a blade is used won't the tension need to be adjusted to account for blade elongation and dullness? And what about adjusting blade tension for differences in material being cut? If so, by how much?

So what is the answer to proper blade tensioning? Might there be another whole methodology, simple or complex, we have not even touched on?

Alan, you have "ask" the most pertinent questions I have seen ask in this thread. I'm curious if Mark will answer and what his answers will be!?

I personally don't have an a tension guage other than what are on my 2 BS's. I buy BS's that I know have over-sufficient springs and all components (upper case.. column.. hub and pulley mounts,, etc.) are re-enforced well enough to support them throught years of use.

I just crank it to BS tension guage reading for a given blade, then make a test cut. It gets cranked up or down a notch to find the tension that will deliver properly on a given blade with a given type stock. Only takes a few minutes. Never had a problem using this "Kentucky windage" approach.

Again... great questions and anxious to see a reply from Mark!

Sarge..

[Jeff Clow]

My take on the discussions here are:

Aftermarket tension gauges are not accurate/repeatable
The clamped on dial caliper method is not accurate
The flutter method is not accurate
The Built in gauge on the bandsaw is accurate enough

My old (late 1940's) bandsaw does not have a built in gauge. What should I use to tension my saw? The saw also does not have a de-tension lever either, so I must manually de-tension when not in use. This requires re-tensioning every time I use it.
Is there an aftermarket gauge, or some other method that I can use to properly tension blades?

Jeff

[Allan Brown]

Mike,
Just wanted to say thanks for your latest book. Just got it in from Amazon and can't put it down. Just bought my first bandsaw this weekend, although I won't pick it up for a week or so. Your book will give me a big jump on getting everything adjusted and ready to go@
Allan

[Mark Duginske]

I haven’t followed these discussions until I came across the posting of a dial caliper clamped onto a blade. What troubles me about these concepts is that you are attempting to correlate one parameter by measuring another parameter. And while that may be possible under the correct conditions, there is an unknown variable that separates the two measurements--namely the blade itself.

These methods are based on the assumption that all blades will extend the same amount with the same tension. I am not a metallurgist, but I do not believe this is true. The blade stretch will vary with the quality and type of steel. I would suspect that this may even vary over the length of the blade depending on the quality controls of the processing and tempering of the blade.

The most accurate method for measuring the tension on a blade is the hydraulic force gauge a previous poster mentioned. This reads the direct forces on the blade, and eliminates the blade itself as a variable.

Those bandsaws that use a compressive spring to set tension should actually be fairly accurate so long as the indicator of the spring’s compression is calibrated (or designed). The correlation between a spring’s compression and the amount of force the spring is applying is fairly consistent. I don't know if this concept is applicable to the design of the 14" bandsaws because I don't own one and have not examined the design. However, on the larger bandsaws where the tensioning spring is pressing upward on the axle, in-line with the blade, this method should be reasonably accurate.

I think that this is well put and a good explanation. The gauge on the saw is about as usable as the gas gauge in you car. It is accurate enough. If the blade is dulling or cutting thicker material one can set the tension for the next high setting. For a 14" saw that would mean setting a 1/2" on the 3/4" setting.

[Mark Duginske]

I have one of the last American made Delta 14" bandsaws with a riser and the documentation states that this saw will handle a 3/4" blade just as the poster stated for his Jet. I have three 3/4" blades that I have never used. Am I better off just junking the new blades and staying with 1/2" and smaller or am I safe with the 3/4" blades that require higher tension?

Bart Leetch also ask about this.

Although the owner's manuals often say that one can use a 3/4" blade it is not a good idea.

The 3/4" blades are usually .035" thick and that band is too thick to bend over that diameter wheel. I've seen these blades crack prematurely.

Also the 14" crowned wheels do not support wide blades the way the bigger flat wheel saws do.

Also some springs do not tension the bigger blades as well as the 1/2"
blade.

For a number of reasons the largest blade that is practical for the 14" saw
is the 3tpi hook tooth.

[Mark Duginske]

Mark can you or someone else address the below issues-

I read this thread and the one a few months back, and no where do I recall seeing a discussion about how desired blade tension figures (for sawing wood) are determined and how do we know they are correct, assuming there is or will be a bandsaw gauge to measure them.

Are these numbers theoretical or were they determined by testing and experimentation by the blade manufacturers? To me, an accurate tension gauge is only half the problem.

I realize, it would be nice to have a tension gauge to help prevent bandsaw damage from over-tensioning, but can we trust the tension numbers given for these blades? Do all blade manufactures provide recommended tension figures ? I thought some like Timber Wolf recommend the "flutter test" As a blade is used won't the tension need to be adjusted to account for blade elongation and dullness? And what about adjusting blade tension for differences in material being cut? If so, by how much?

So what is the answer to proper blade tensioning? Might there be another whole methodology, simple or complex, we have not even touched on?

When I talked Starrett and Lenox they said that the 15.000 psi rating for woodworking is an international standard by an industry group. Chuck Olson from Olson Saw doesn't use a psi number.

The most important factor concerning overtensioning in not to compress the spring completely. At other places in the post I state that I use the gauge and that I set it one setting higher when resawing or when the blade dulls.

[Mark Duginske]

Hello everyone. I am new to this forum and found this thread interesting. There is an inexpensive way to check tension that should work well. Plucking the blade while under tension and using a musical instrument tuner should give a frequency that relates very well to tension. The frequency would be different for different blades but calibration should be possible. Once the frequency of particular blade is known, increasing tension until it is "tuned should yeild very good results. This is the method used to tension guy wires on a tower. With guy wires, the frequency is so low that one can use a wrist watch to measure the frequency by timing the return wave.

Mark, do you know if this method has ever been investigated?

Cheers.

This is another one of these theoreticals. Since I am not musical
and don't know a musical note from a post-it-note it would not suit me.
All things are relative and for wires on a tower that makes sense.