These two questions are very good questions and are asked very often. Everyone who saws should consider how fast their saw is turning, no matter who manufactured the sawmill.
I will reveal from experience some facts that have been tried and proven. I will also give a few myths that are not true and have very little, if any, relevance to the formula of figuring the band speed.
First you should consider the horsepower of your sawmill. I will give some guidelines. The reason we consider the horsepower first is the fact that we need the torque to maintain a constant band sawmill speed.
Sawing logs is like a heavy loaded truck pulling up a long tall hill. The truck cannot pull up the hill in the highest gear that it has or that it could use to run flat out. It has to down shift to a lower (slower) ratio to get the torque level up to climb the hill. With a band sawmill it is wise to make the ratio for the hard pull of sawing as if we are pulling the long hill all the time.
While considering the horsepower, note that if the band sawmill has hydraulics and the pump is mounted to receive power from the saw engine, that you will loose 5 to 10 horsepower. For example a 25 hp manuel sawmill will cut faster and stronger than a 25 hp with a hydraulic pump robbing horsepower from the blade.
If I am making a judgment as to how fast to make a band sawmill I would rather stay in a safe zone and turn the blade slower and have the higher torque, than to turn too fast and have low torque.
Another consideration on band sawmills with 30 or more hp, is friction. When the blade exceeds 5,500 fpm it has to deal with a higher level of friction in the body of the blade. In the past we thought if we had the hp and torque that we could rev up the blade and saw faster. We in time realized this is a mistake because of friction.
On some sawmills we would run as high as 7,000 fpm. This caused some high friction and when the bandsaw blade ran into some adverse knots or hard places the blade would cut badly. On the same sawmill we slowed the band to 5,500 and the problem with sawing disappeared.
You might think it would slow production down. It actually picked up production and the blades lasted longer, all because we increased our torque and dropped friction. When friction is dropped the bandsaw blade is more stable.
The horsepower of your sawmill is the greatest factor in deciding how fast to turn a sawmill. There are other factors such as wheel diameter and wheel roundness. In the guidelines you will notice the progression of the more hp the faster the bandsaw blade can go around the band wheels to a certain point and I believe you should be careful when exceeding the maximum point.
Note: All speeds are given in Feet Per Minute (fpm) rated for Gas or Diesel engines. When considering electric motors note that they have about 25 to 30 % more torque than gas/diesel motors.
Here are some guidelines: 8 hp max speed 3,500 fpm, 12 hp max speed 4,000 fpm, 16 to 18 hp max speed 4,500 fpm, 25 hp max speed 5,000 fpm, 30 hp and above max speed 5,500 fpm.
One thing that does not have relevance to the speed is the bandsaw wheel size, but remember that wheel roundness does. If wheels are true in roundness they can run at maximum speeds. I will add that larger wheel have benefits only when they are true round. Note this: an 18 inch wheels that is true is better than a 30 inch wheel that is not true.
Does the speed need to change for frozen Logs?
Yes, we have proved that it will help when a band is turning over 4,800 fpm. It will give best results in sawing frozen logs to slow the band down to 4,800 fpm or less.
I believe what happens in frozen logs when the tooth impacts the cut area the fiber is cut along with the moisture partially thawing under impact and friction and the dust that passes by the blade gullet will freeze back tight against the blade body and it acts like a brake pad pressing against the body of the bandsaw blade.
This in turn causes heat and makes the band stretch and cut wrong. By slowing the bandsaw blade down under these conditions allows the tooth to get a little larger dust bite and the larger dust will not pass by the gullet as easy as before.
Every little help we give ourselves here is positive. Also at the slower speed the band does not have the friction level to deal with and will not heat up as much, and this will keep the band stronger, as heated metal become weak.
A formula for figuring bandsaw wheel circumference (length around the wheel per revolution).
Diameter of bandsaw wheel X 3.14 = _____ then divide by 12 the total is in ft and inches.
Example 26 inch bandsaw wheel: 26 x 3.14 = 81.64 then divide by 12 is 6.80 feet around the wheel. So if I want my band to run at 5,500 then I divide 5,500 by 6.80 = 808. 808 rpm is the speed I want to turn the shaft of the drive bandsaw wheel.
In Conclusion: The bandsaw blade needs constant speed with torque more than it needs speed. If you put the speed to match the horsepower and torque you will have a great cutting band sawmill.
Until Next Time,
aka... the "Saw Doctor"
Leaders in Bandsaw Technology!