Saw blade with cracks in gullet

[flatrock58]

I was sawing some cedar a couple days ago and the blade got dull.  When I took it off I noticed a crack in the gullet  After looking more I counted 20 cracks in the gullet around the blade and two cracks in the back of the blade. It is one of my 7 degree doublehard blades that had only been sharpened maybe once with my CBN and then once at resharp.  Anyone have this kind of issue with a newer blade?  I have older 9 degree blades that came with my mill that have been sharpened 7+ times with no cracks.

[DDW_OR]

........and then once at resharp.........

maybe you did not get your blade back

[Brucer]

It's not the age of the blade that matters. How long did it run, and did you grind the gullets deep enough to get rid of any cracks that were starting. There's also a statistical factor that come into it. I've had a blade break after only 1 sharpening, and I had to retire another one after 13 sharpenings because it was getting so narrow the guide rollers were taking the set off the inner teeth. My average for a few hundred blades was around 6-7 sharpenings.

[kelLOGg]

When cracks begin to form I notice the blade "jogging" or moving forward and back when running (but not sawing). Dead giveaway that the blade is about to break. I assume all bandmills do this?

[MartyParsons]

Hello,
   Gullet cracks is a result of :
       Flex life, running the blade longer than it should be. 500 to 800 bd ft on a .045 sometimes less on a thicker blade
       Improper grinding, not getting the gullet ground deep enough.
       Hope this helps.
       Marty

[dgdrls]

One point I've noticed;
when sawyers post issues with bands breaking or cracking in the gullet they often
report the band was run until it was "dull" 

What I believe is happening is with shallower tooth angles like 7's & 4's the tooth is "standing-up" more and is subjected to more loading than a 10° or 13° tooth.  

So, IMHO with the shallower angles and probably in general, bands should be swapped out before they start to show signs of being dull.

Best
D

[flatrock58]

Thanks for the reply.  I don't think that the blade cut over about 400 bf of pine and cedar before I changed it.  It was dull but not wavy like a really dull blade.   I have cut a lot more with some of my old 9 degree blades and never had this sort of an issue.  this is the first batch of blades that I have sent to resharp.  Before I got the 7 degree blades I was sharpening my on with a cbn sharpener and single tooth setter.  Since I sold the setter and have not replaced it, I am using resharp for now.  I was told it could be the blade guide pad being too tight below the blade.  I have not checked to see if that could be the issue.

[Skipper11A]

It was dull but not wavy like a really dull blade.   I have cut a lot more with some of my old 9 degree blades and never had this sort of an issue.  this is the first batch of blades that I have sent to resharp.  Before I got the 7 degree blades I was sharpening my on with a cbn sharpener and single tooth setter.  Since I sold the setter and have not replaced it, I am using resharp for now.

I'm wondering why you're not still sharpening your own blades?  I have a hunch that sawyers who send their blades out for resharpening will try to squeeze every bit of sharp out of their blades, whereas sawyers who sharpen their own will take the thing off the mill as soon as it begins to dull because it's so easy (and cheap) to resharpen.  Have you really noticed any difference when your teeth aren't reset?  I'm betting that a sharp blade beats a dull blade with reset teeth every time.

[gmmills]

  This may not be directly related to your issue, but I thought it may need to be mentioned. Those of you that sharpen your blades with a CBN sharpener need to make sure that you run the blade around the sharpener twice each sharpening cycle. This assures that any micro fractures in the gullets are taken out and the exact, entire, tooth profile is maintained.  Remember that Re-sharp grinds each blade twice. A preliminary grind then a finish grind after being properly set. The CBN system is very capable of getting a blade sharp after first grind but the second assures all tooth tips are razor sharp.  I have always run a blade twice around the sharpener and great overall blade life.    

[Peter Drouin]

I use the 1¼x55x7 I go around one time and grind heavy with the cbn. Have no trouble.

[bandmiller2]

Flatrocks is this troublesome band the only one or are their others in the same lot that have cracks. Sounds like that band may have been hardened too much.

[flatrock58]

Skipper11a I am not sharpening blades now because I have no way to set them.  I do notice a difference when the set is gone. 
gmmills I don't always make passes.  I did not realize it made that much difference.

Peter I need to look at all the newer 7 degree blades to see if there are any other problems.  I might just be one blade, but I was just surprised to have so many cracks.  In the past when I had a blade break it might have 1 or two cracks.

[Magicman]

I have a hunch that sawyers who send their blades out for resharpening will try to squeeze every bit of sharp out of their blades, whereas sawyers who sharpen their own will take the thing off the mill as soon as it begins to dull because it's so easy (and cheap) to resharpen.

This may apply to some but is not reflective of all sawyers.  Running a dull blade is very bad economics regardless of the resharpening means or method.  Squeezing every bit of the sharp out of a blade is squeezing a blade to death.

[esteadle]

What is the radius of your bandwheels? How much tension do you put on the blade? 

I don't think newer blades with 2 sharpenings would develop cracks that early, even if run to dull. 

Multiple cracks that develop in the gullet of a blade are the result of fatigue in the metal. Fatigue develops when metal is repeatedly stressed, as happens when the band wraps around the bandwheels. Stress comes from multiple place: how much tension you put on the blade, the radius of your bandwheels (smaller bends the blade in a tighter circle as it goes around) and whether your wheels are properly aligned with each other. If your bandwheels are steel and your blade rides on it (vs. a belt), your crown may also be imprecise, causing additional forces to stress different parts of the blade as it goes around. 

Try lowering your tension a bit to see if your blades will spin longer. Time them accurately, and don't just "remember" how long they've been on... to know if your changes make a difference. Especially if your wheels are small.  

Also check the bandwheels to see if they are co-planar (in the same plane). From another posting on another popular Sawmilling site: 

I have had this problem with my mill and the solution for me was to align the bandwheels so that they are co-planar (in the same plane). To do this, lay a flat piece of steel or a good straight edge like a level, across the front of the bandwheels over the top of the axle, and again below it. Check that all four of the contact points of the wheel are all touching the flat edge. If any are not touching then that bandwheel is not in the same plane as its partner and needs to be adjusted. Out of plane bandwheels put additional tension on the blade (much like an out of round bandwheel does) and will cause stress cracks in the gullets of the blades, eventually allowing them to snap under tensions. Before I corrected this, I was snapping blades after one or two sharpenings if I was lucky. Once I corrected my bandwheels and straightened them out, I achieve five or six sharpenings consistently.

[Pabene]

IMO it is to high feed force as are the most dangerous for the blades. The hardest stress level in the steel in gullets is concentrated to the area in front of the guide rollers, when the flange presses the blade forward in the log. If you, or the saw feed motor, starts to push to hard to keep the feed speed, you can destroy a new blade. It could be so that miss aligned wheels also means one of the guide rollers pushes harder and makes the problem worse.
 Scratches across the blades in the gullets, from the grinding wheel, are an other bad thing as will trigg cracks.