Troubleshooting Blade Breakage

How to Troubleshoot Blade Breakage

We say it often “Our goal is to help you make more money”.

This philosophy is applied daily as we do our best to help people who neither have our sawmill nor have blades that we have supplied to them.

But sawyers call us because their sawmill manufacturing company or sawmill band blade supplier can’t solve their problem.

One such issue I would like to help sawyers better diagnose is the irritating problem of dealing with premature breaking of bandsaw blades.

This can only be answered with a series of questions to correctly troubleshoot the problem. If you call me with this problem here is what I’m going to ask:

Question... What kind of band wheels do you have?

I have found over the years that most major mfg’s have gotten away with using untrue bandwheels for the simple reason that there are so many areas for which blade breakage can be blamed.

Band wheels that are untrue will snatch the blade apart. Example: A 19 inch wheel is 4.97 ft around the wheel (circumference). A nominal band speed of 4970 ft per minute would put the wheel turning 1000 RPM.

Which is 16.66 rounds per second. Multiply 16.66 RPS x 4.97(cir) = 82.80 feet per second of band being pulled through his cut. So if you’re using a 13’2” for example, that blade is making a complete revolution 6.24 times per second. During each revolution of the band there are two wheels it must go around. So 2 x 6.24 equals a nominal 12 times per second that a band goes around a wheel. That’s a mile every minute!

When the band wheels end up being the culprit of broken blades I find they are always out of round by at least .015 of an inch or worse.

Now multiply 2 wheels x .015” out of round and you get .030” that the band wheel is snatching (stretching the blade) per revolution. There is a .030” stretch happening in the area between the guides and another .030” in the non-cutting area. This totals a stretch of the blade, or a potential stretch of the blade by 60 thousands (1/16th of an inch). This shock is happening 12 times per second!

The band simply can’t take this punishment.

Many V-belt sheaves (pulleys) are out of round by .015” thousands and have a v-belt tucked in the v-grove. This v-belt becomes a rubber shock absorber. This belt is the only reason that a v-belt sheave can work being so out of round.

The v-belt shock absorber needs to be changed on regular intervals. If the belt gets worn internally (which you cannot see or measure) it breaks down internally and looses its ability to absorb shock. At this point the blade is suffering greatly and blades begin to break, seemingly without cause.

Question... Were the blades rising or diving before they broke?

If they were that would account for the breakage, and diagnosing the reason for the rising or diving is the next step, which by the way, is normally a blade body flatness issue. There is a great amount of stress and heat being administered to the band. A good cutting sawmill blade has the least amount of stress.

Question... What kind of blade guides do you have?

I believe that roller guides are the best for cutting 8 inches or wider. I have proved this hundreds of times. I believe that sandwich guides are the best for narrow cuts such as pallet resaws cutting 6 inches or less.

When using sandwich guides it is very important that your wheels are true. If the wheels are not true with sandwich guides the blade will hammer the guides up and down as the wheel is going up and down with each revolution.

Most resaws have steel wheels, but for those that have belted wheels I would suggest using roller guides because of the wheels being out of round the blade life will always be less than your competitor who has true steel wheels.

Question... What kind of tensioner do you have on your sawmill?

I like tensioners that are able to give and take. When the blade stretches during the cut, and warms up, a spring or air tensioner will take the slack and when the band is out of the cut it will cool down and the tensioner will give back to the band.

A hydraulic tensioner is dead tension and will not take or give during the sawing process. I am not saying Hydraulic will not work, because many of you reading this article know you are sawing every day with it, but it does contribute to poor blade life.

Question... Are the blades breaking from the front or the back?

This is important to notice. The first break will have discolorations and the last break will be bright steel. A band breaking from the front is due to some type of stress. It could be from sawing stress, too much blade tension, bad wheels, or grinding stress.

When blade that breaks from the back, I have found that 95% of the time it has taken on damage from something dragging on the back of the blade. Such as a bearing that should be spinning freely but is dragging on the back of the blade, rubbing a bolt, or rubbing a guard.

All of these things create a instant heat that crystalizes the surface of the back of the blade and cause tiny cracks to form. To identify this, bend the blade sharply and look at the back of the blade and you will notice the multiple cracks. All sawmills that use back up bearings on the guide system need inspection daily.

Question... Do the blades break in the first run or after sharpening?

If they are breaking on the initial run you must first determine if you’re running your blades too dull rather than changing them at the first sign of dulling.

Secondly, some people report that after they sharpen the blade it cracks in the gullet. This could be caused by running too dull or grinding too hard in the gullet. If you’re grinding so hard and fast that it leave a heavy burr on the back side of the gullet, it is not good for the blade. Grind as lightly as possible to allow the grit of the rock to cut rather than melt the steel away. A ceramic Rock works best on the Cook’s Cat Claw sharpener.

A few last thoughts…

One thing we have found since the release of our Super Sharp Band is how forgiving they are in applications with mills that are not necessarily set up to par. With the profile of the blade coupled with the razor edge, masks a lot of flaws.

It has also shown to give extra band life especially in a Resaw application. Because of the new technology and gullet profiles the NEW Super Sharps are cutting up to double the life of any of the best competitors blades. Also on Sawmills we are seeing an increase in clean logs of 35 to 50 % cutting life between s harpenings.

I hope this has helped you with a bit more understanding on how to troubleshoot breaking blades. Please keep in mind when you make a correction you will need to use a new sawmill blade to see if you have fixed your problem. A stressed blade will never run like a new blade.

Until Next Time,

 

 

aka... the "Saw Doctor"
Co-Owner CooksSaw.com
Leaders in Bandsaw Technology!