The PHYSICS of a larger diameter blade
It is very important to recognize that the diameter of the blade and the power of the motor are two different things entirely.
A saw with a larger diameter blade, but having the same motor power as a saw with a smaller diameter blade, has LESS cutting torque at the teeth! Let me explain.
The purpose of a more powerful motor is to provide more power. The purpose of a larger blade is NOT to provide more power, but rather to provide more cutting depth.
And "power" is an interesting thing. A "more powerful" motor cannot necessarily cut more wood per second than a less powerful one.
The formula relating power and torque is:
Power = Torque at blade x blade rpm / (numerical constant factor = 5250 if power is measured in "horsepower")
So Torque = Power x 5250/rpm
Let's say our first saw has a motor that turns at 3500 rpm and is "rated" at 2hp.
Then Torque at blade = 2 x 5250/3500 = 3 ft lb
Let's say our second saw has a motor that turns at 5250 and is also "rated" at 2hp.
Its blade torque = 2 x 5250/5250 = 2 ft lb.
It's got only 2/3 the torque at the blade that the first saw has, even though the "power" rating is the same.
Which saw do you think is going to cut through harder woods better?
Now, let's imagine we have two saws, each with a 2hp 3500 rpm motor. They each then produce 3 ft lb of torque at the blade.
But one has an 8" blade and the other has a 12" blade.
Torque = force x (distance from center of rotation that the force is being applied)
So, force at the blade teeth = Torque / (distance from center of rotation that the force is being applied)
On the 8" bladed saw, force at the blade teeth = 3 / (8/12 feet) = 4.5 lb force.
On the 12" bladed saw, force at the blade teeth = 3 / (12/12 feet) = 3 lb force.
So, the saw with the SMALLER blade applies more cutting force at the teeth, meaning it can cut harder wood before it stalls.
To get the SAME cutting force at the teeth, the saw with the larger 12" blade must have a motor that produces 12" / 8" = 1.5 times the power of the smaller bladed saw.
So, when buying a saw, look at:
- Does a saw with a larger diameter blade have a correspondingly more powerful motor?
- Does the motor have more power via more torque or via more rpm? If via more rpm, it will NOT cut through harder or thicker wood. It must have more TORQE at the teeth to cut through harder wood or thicker wood.
Also, "worm drive" circular saws have a reputation as being "powerful" than a saw of same rated power but direct drive. That is not strictly true. The worm drive saw actually has more TORQUE at the blade teeth because it uses internal worm gears to "reduce" the rpm of the blade below the rpm of the motor, thus increasing the torque at the blade teeth. This means it can cut through harder and/or thicker wood than a direct drive saw OF EQUAL POWER.
However, the SPEED at which the saw can be pushed through a cut is still controlled by the POWER available (That's basic physics). The worm drive saw won't cut any FASTER than the direct drive saw. But, it WILL be less likely to stall.
One DISadvantage of a worm drive saw is it is notably heavier than a direct drive saw, because it needs that "gear transmission". This can be tiring if you are doing more than just single or few cuts at a time.