The Real Scoop on Gullet Grinding

The Real Scoop on Gullet Grinding

by Tim Cook

For more than 50yrs. the theory that blade gullets must be ground or the blade will get cracks has been perpetuated to the point that it spoken at fact. The question today is:  Is it True or False?  I will tell you that it is either true or false and only one answer is the real truth. Through my experience and looking at the real facts at hand, I believe that I have learned the truth and I will share with you my experience and hard facts.

I first heard the theory when I was about 14 years of age when my dad was repairing 4 to 8 inch wide bands (the type where you had to swage the teeth to get the desired kerf).  As I helped him in his Shop handling the blades and listening to his instruction he would teach me the things that he had been taught. One of these teachings was if you fail to grind the gullets every four hours the gullets would crack.  So, quickly I assumed that all of the blades that we were repairing had at some point been run more than four hour because many of the gullets had cracks that had been welded up, and new cracks were forming in gullets where cracks had not been before.  

My dad was a hard worker and he was the type that was determined to solve problems for his customers. I witnessed him work all day and through the night until daylight the next day trying to solve gullet cracking problems. This went on for years, and seeing this had me determined to help solve this problem.

Something that is helpful to understand is, the saw filer was always blamed if the blades did not cut well or if the blade cracked or broke.  It was amazing how many things were blamed on the saw filer.  I witnessed sawmills that would purchase new blades and they would crack in the gullets and of course, even that must be the saw filler’s fault (little sarcasm).

It was also frustrating because my dad was doing everything that he had been taught and was studying books and calling people to solve the problem. Some told him to make sure that he ground the gullets to stop the cracks...but it never stopped the cracks.  So he got an air die grinder coupled with a carbide debur tool to grind every gullet…it made no difference.  So then he took round files and hand filed every gullet and even that never stopped the cracks.

During this time I started studying steel composition and trying to understand steel fatigue. I wanted to know what was causing these cracks during the sawing process.

Here is another piece of the puzzle you will find interesting:  One engineer from a large blade manufacturer told me that he knew what was causing the gullets to crack, so I was all ears ready to hear the solution to the problem.  He said that the sawdust going around the gullet during the sawing process caused friction and that friction crystallized the steel in the microstructure surface and thus you had to grind the gullets every 4 hours or they would crack.  Well I knew something would cause them to crack because I had seen the cracks develop in the 1st and 2nd runs. But what he said did not make sense!  So I questioned him:  “Is it friction that is the enemy that causes cracks?” He answered “YES!”  So I said: “Do you know at what temperature steel starts to harden?” The question puzzled him. The answer is that steel has to be heated to around 1400F or more and then quickly cooled for crystallization.  So my next question was “Do you know at what temperature sawdust ignites to flame?”  He said “No” and that he had not considered it.  I told him it is around 450 F when sawdust ignites to flames.  I then asked if he had ever seen a saw blade ignite sawdust on fire. He said no, and I agreed that I had not either. So my conclusion was that sawdust moving around the gullet could not cause crystallization because it could not make enough friction to catch on fire.

 So I ask:  If friction is the enemy, which of these two things would you say causes more heat?

 #1 Sawdust going around the gullet? Or #2 a grind rock grinding the surface of the gullet causing sparks that are red hot (1200F +)? Well we all know that answer.  The engineer wanted to stick with his story but I became a non-believer in the theory.  In fact, the sawdust polishes the gullet smoother and improves the gullet surface and never causes crystallization or cracks.

So what can cause crystallization and cracks in the gullet? – Grinding the gullets too hard.  I would suggest staying out of the gullet as much as possible and getting more blade life.  Let the sawdust polish the gullet smooth. Metal experts know that a smooth surface is less likely to crack than a rough surface.  However, when sharpening, the gullet has to be ground to keep the gullet depth as the gullet helps to remove the sawdust.

As I pondered all these facts I thought about the many sawyers I had been around, and when they had a blade that was cutting straight they deemed it sharp, and when it started cutting bad then they deemed it dull. This was simple enough.  So obviously the blade is dulling perpetually with every cut until it reaches the point that the body can no longer hold straight due to the dullness of the tooth.

We already know the friction in the gullet is not getting hot enough to crystallize and we know that the dullness of the tooth stresses the blade body to the point it cannot hold true in the cut.

So what is causing the cracks in the gullet? Blade stress!  So the real question is “What causes the stress?”

Several things:

1. Blade Tension - Blade tension should be as low as possible to maintain straight cuts along with achieving your desired production. On any machine, the higher the tension the less life the blade will have before cracks form.  All steel has a point at which when stretched enough, it will begin to pull apart.

Cracks form because the band saw body was pulled apart.  Remember the blade is being pulled through the cut by a powered band wheel.

2. **Dull Teeth - As we have noted, a dull tooth is going to stress and pull on the body of the blade and keeping a sharp blade is in turn going to reduce this stress and keep the blade body cool by reducing friction.

**Dull teeth are probably the biggest culprit to gullet cracks.

3. Blade Body - Maintaining a flat blade is going to stop diving or rising blades and reduce stress. We have written many articles on the subject which can be found online.

4. Untrue Band Wheels - People with true band wheels will always have longer blade life than those with out of round band wheels as vibration is a blade killer.

The whole theory of grinding gullets to prevent cracking goes back to the saw filer who was at the receiving end of blame if gullets cracked, so their answer became ‘grind the gullets every 4 hrs. of run time’.  In turn, the blades would also be sharpened at the same time and it was the sharpening of the teeth that was the key to preventing the cracks, not the grinding of the gullet.

This same fact holds true with most sawmill blades – they need to be sharpened every 3-5 hrs.  Can they run longer? Sure, but again, as the tooth becomes duller the blade body stress increases in the gullets.

I have witnessed people who get 10 to 12 sharpenings out of their blades. I will quickly tell you they are running true wheels, low tension, and they are keeping their blades sharp, set, and flat. However, a lot of sharpenings may not mean high production.  When I saw for high production the number of sharpenings is not the greatest concern. I want the most lumber per hour for the amount of money I spend on a blade. This reduces the amount of labor cost per board foot.  Labor is usually much more expensive on a monthly basis than the cost of blades. And since I usually want to saw fast and straight this means I will run my blade with a higher tension to meet my goals while keeping the blades sharp, set, and flat.

 

So having said all that, the answer to the theory is that it is not true that grinding the gullets improves blade life!  It is true that grinding the gullet does not cause great harm either unless you over heat the gullets by grinding to hard.

Bottom line: Keep your blades Sharp, Set, and Flat and this will improve your blade life and production.

 

Until next time

 

Tim Cook