Saw Blade Hot Spots and Dishing

What's the right thing to do when a circular sawmill blade heats up in one spot and dishes out of shape? April 20, 2011

Question
I have a saw blade that has been running fine for several years on my farm/woodlot mill. Just this week it got hot, dished away from the log, very unstable. It developed a very hot spot - blue hot - about one foot out from the center. The bluing is very concentrated , about 2" in diameter. It seems as though there developed a lump, although I cannot feel the lump with my fingers. The tension seems to be gone, as I can no longer saw without it heating up right away, with that same 2" spot getting very, very hot in just a few feet into the cut. Also, I noticed that at standstill, it is slightly dished with the rim area away from the log.

I had been sawing oak before, but just before this occurred, I replaced 4 of the 38 carbide tips chipped tips), and was sawing pine that had some knots. Before I get it tensioned again, I would like to know what caused this lump to happen, and how it can be avoided in the future.

Saw data:
48 " Dia
B style Bits
No of Bits/ Shanks 38
Carbide bits
6/8 Gauge
2" dia Hole
2 ea 5/8" pin holes 3" bolt circle
Hammered 650 RPM Right Hand.

Question


(Sawing and Drying Forum)
From contributor A:
When you heat up metal it expands. On a saw blade you heated up the middle of the blade but not the hub or the rim. So the middle of the blade had to push out of line somewhere in the area that got hot. Since it could not move the rim or hub you got a spot in the middle. It is this that makes the whole blade cup to one side.

It will have to be hammered and re-tensioned to your specs that you already have. Sawing with dull teeth will cause the rim to get hot and wobble but most of the time you can run the blade cool and then sharpen and it will be fine. When you rub the middle and get it hot you are out of luck. The blue spot may remain after the hammer job but will not be a problem.

It’s just the cost of doing business. Also if you replace a few teeth you should sharpen the others. You can set your blade out of balance or the blade will cut harder in some areas then the rest and set up a harmonic wobble in the blade that can cause problems. All the teeth should be sharp or dull but not a mixture.


From the original questioner:
Since the heating at the hub occurs so suddenly, is there any way to tell of impending heating so I can back the log out before the extreme heating occur?

From Gene Wengert, forum technical advisor:
When you replaced the bits, did you make sure that the shanks were returned to the same hole they were in before replacement? How did you determine that B teeth were correct? I had a saw like this and we used F in one and 2-1/2 in the other.

Unless you have a piece of wood get stuck on the side of the saw or other obvious event that heats the saw, a saw will lose tension slowly enough that you will notice it and can change it before this catastrophic failure happens. It is possible that when you changed the few teeth that these teeth had to work much harder because they were longer. Do not replace a few damaged teeth if all of the rest are fairly worn.

Maybe these logs were partly dried out? It is best to send this saw to a shop that can re-tension it. Before sending it, change all the teeth.


From the original questioner:
Yes, I changed the four bits one at a time, coated bits, shanks and grooves with oil. I purchased this blade new in 2005 from a very reputable manufacture in Ga. For the initial quote, I gave them all the data on my mill and requested F style. They came back and recommended B style, based on marginal horsepower. I have 75-hp diesel, and their standard F style blade had 44 teeth. The 38 tooth B style blade required less HP. I initially used Lunstrum’s HP calculation (from Forestry Products Utilization, Technical Bulletin No. 10). The blade seems to work fine for my application, until this hot spot problem occurred.

It is possible that the new teeth were a little longer. I’ll have to check that. However, the teeth are carbide tipped, and all others seemed to be sharp and square, showing little if any wear. I changed the 4 bits because they were chipped. The pine logs I was sawing were dry, as they had fallen in a windstorm several years ago. How does partial dryness come into play re: this problem?

I do plan to send the saw back to the manufacturer for re hammering, but need to think more about replacing all of these relatively expensive carbide teeth. On the other hand, compared to shipping cost and re-hammering cost, this may be advisable. Any more thoughts on this would be appreciated.


From Gene Wengert, forum technical advisor:
If the non-replaced bits are in great shape, no need to replace them. Dryness can be an issue, as dry wood has more energy requirements, so you are likely to saw more slowly making fine dust. The fine dust also is more likely to have dust spillage from the gullet which means heat. You might check the bearings for the saw shaft. You could run the mill without a blade and then, when you shut done after ten minutes and everything is stopped, check for heat.