Analyzing Broken Blades

Analyzing Broken Blades

by Tim Cook

Our motto here at Cook’s Saw is “Our goal is to help you make more money”.  We get the opportunity to apply this principle daily as we do our best to help people who neither have our sawmill nor have our blades, but they call us because their sawmill mfg. or their band blade supplier can’t solve their problem.

One such issue I would like to help you better diagnose is dealing with blade breakage.  This problem is best answered by asking questions as there are many variables to a sawmill and sawmill blades which varies with each individual case.  So it is necessary to gather certain information in order to properly identify the cause of the breakage.

If you call and talk with any of our staff about blade breakage here is what we will ask and here are some things you need to be familiar with in order for you to diagnose the issue as well:

 

1)  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 band wheels 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; it’s just that simple. Let me explain:

Example: A 19 inch wheel is 4.97 ft around the wheel (circumference). With s nominal band speed of 4970 ft per minute that would put the wheel turning 1000 RPM. This equals out to be 16.66 rounds per second.

Multiply16.66 RPS x 4.97(cir) = 82.80 feet per second of band being pulled through the 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 for long.

Many V-belt sheaves (pulleys) are out of round by at least .015” thousands. However, they do have a v-belt tucked in the v-grove and 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 loses its ability to absorb shock.  At this point the blade is suffering greatly and blades begin to break, seemingly without cause.

 

Were the blades rising or diving before they broke?

If they were, that can account for the breakage since there is a great amount of stress and heat being administered to the band.  This is normally caused by a blade body flatness issue. We have talked extensively about this subject so there is a wealth of information on our site in the articles section for further reading.

 

What kind of blade guides do you have?

I believe that roller guides are the best for cutting 8 inches or wider as I have proved this hundreds of times. I believe that sandwich guides are the fine 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.

 

What kind of tensioner do you have?

This is important because a blade is continually expanding (while in the cut) and contracting (while out of the cut) because it is continually heating and cooling.  If a tensioner does not have the ability to expand and contract with the blade then it is considered ‘rigid’ and it will cause premature blade breakage.  A hydraulic tensioner is rigid 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. Ideally a spring type or air tensioner are more ideal as they will expand and contract with the blade.

 

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 a blade 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, rubbing a bolt, or rubbing a guard.  All of these things create instant heat that crystalizes the surface of the back of the blade and causes 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.

 

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 of a blade or from grinding too hard in the gullet.  If you’re grinding so hard and fast that it leaves 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.

 

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