Thread: Changing Blade Size

I was doing a search on Camo's Stenner saw and came across an older thread about changing blade sizes and comparing cuts which I found interesting. Unfortunately the forum doesn't allow you to post to threads more than a couple of years old so i couldn't add to it.
The thread is here https://www.woodworkforums.com/f153/increasing-blade-size-teeth-162329
A bit of thread necromancy by proxy.

The conclusion reached in the thread was that comparing newer smaller saws, in this case 12" diam. with faster Rpm rates to older saws which with their bigger blades commonly run at lower RPM's all came down to getting the same number of "teeth per unit time" which sounds logical enough.

So in the case that Camo was talking about it was suggested that he needed to increase the number of teeth on the larger slower blade to get the same "teeth per unit time".

There is another aspect to this that wasn't mentioned.
As you increase the radius of a circle you are changing several characteristics of the section of blade that is in contact with the material being cut.

For the same thickness of material ( the sagitta of both blade arcs is the same) the portion of blade in contact with the material ( 1/2 of "arc length") will be greater for a larger radius blade. The difference between a 12" and an 18" blade is approximately 20%.

So if you simply increase the number of teeth on a larger blade to get the same "teeth per unit time" as a smaller blade you are in fact going to have an increase in the number of teeth in contact with the material at any given time because the section of the blade in contact is greater on the larger diameter blade, and as you feed the material through you will have a different cut rate. The feed rate is also going to have an affect on the quality of the cut.
Trying to compare cuts between blades of different diameters, which is something we do as we try to restore old saws and choose appropriate blades and tooth configurations to those we are used to with modern machines, is a bit more complex than just "teeth per unit time".

Have fun,
Alli