Sawmill horsepower assessment

Figuring the horsepower and speed needed to run a circular mill. April 20, 2001

I have a small circular mill (Foley-Belsaw m-14). In sawing some hardwood, I've found that the mill is a little under-powered. I'm using a slant 6 dodge motor for power. My saw size is 48". Does anyone know what the equation is to find out the horsepower needed to run this size of saw? How can I find out if I'm running the saw fast enough?

Forum Responses
Here is a standard HP formula:
Gullet area x teeth per minute x .003

Use .003 for a kerf of 11/32. For thinner or thicker kerf, use the percent it is of 11/32.

A lot of mills have saws hammered for 550 RPM. You need to know what RPM your saw is hammered for. That is what you need to run it at. Also, an F pattern tooth has a gullet area of 2 square inches. A B style is 2.5 square inches.

I would say that a slant 6 with a good governor should have enough power unless you are cutting deep cuts all the time. I would highly recommend finding a diesel powerplant of some type. Your fuel costs will go way down.

Also make sure you have enough hook in your teeth. If you don't, it will take a lot of power to feed the log through the saw. You will push like heck on the feed lever and she just won't go.

Gasoline engines are usually designed to produce their peak horsepower at a high RPM, so you have to run at a higher RPM than you think to get your full power. A good estimate on an engine like that would be around 4000-4500 RPM, but check a Haynes book and see if they list the specs for your engine. Also, remember automobile engines are not normally designed to run at full power for long periods of time. If an old Farmall tractor with less than 50rwhp can run a mill, I can't see where a slant six with over 100hp should have a problem.

I have the same mill, I think. Bought as a Timberking m-14. I brought home a Dodge w/318 and have been planning on trying cruise control from JC Whitney. Figuring on using tap down to idle it and resume to bring RPM back to operating speed. Anyone tried this? My blade was from IKS hammered for 560 RPM. I have seen hand held tachs in Grainger's catalog. Should have one too. I think I'm throwing the tension out of the blade, if this is possible. Have been running on TO35 fergie, way underpowered. My neighbor's Ford 800 seems to be the bottom good limit.

From the original questioner:
I have never heard of throwing the tension out of a saw. I have an ext. governor on mine and it works well. I think my problem is not enough hold on the horsepower. I have tried everything that I can think of and still I'm not getting enough horses out of my slant 6. Does the 318 seem to be hard on gas? The slant 6 uses approximately 10 gallons in 10 hours with very little production. Working alone I'm lucky to get 600 feet a day. I think this is due to the fact that the motors are not able to keep up the horses all the way through the cut. I did get the RPMs checked and it is running at the right speed. I'm not having much luck with this type of setup.

Do you notice the saw slowing down in the cut? Are you getting enough throttle when the governor pulls in when you start into the cut? You have to maintain a constant RPM in the cut. Not slow down, not speed up.

As for the fuel consumption, all I can tell you is look for a diesel engine. Problem is, if your mill is set up for PTO you can not direct drive the saw. It will turn the wrong way. I have seen fellows run a tire up against a pulley to change direction of rotation, but never tried it myself. I have taken a 3/4-ton pickup with a 6.2 in it and welded the rear end solid, put a PTO stub on the right axle and had more power than I would ever need.

The way I see it, a gas engine with a carb on it at a given RPM will mix a fixed amount of fuel with the fixed amount of air the engine sucks through it at that RPM. It just gets worse under load. A diesel will only feed enough fuel to keep you at governed RPM with no load. Makes a hell of a difference at the end of the day. I have sawed for 10 hours on 5 gallons of off road diesel. My brother has a Chevy 6 and could burn 25 to 30 gallons a day.

If you look around you can find the older Rockwell PTO units on some of the old gas powerplants. Back around WWII and later with step down unit that also changed direction of rotation. Every time I find one I pick it up and I always find someone who wants it. Last one was on a LeRoi 4 cyl. for 25 bucks.

Over the years I have rigged gas motors to sawmills. Of course, there is a formula for HP using feed speed, tooth spacing, depth cut, etc. A good saw man will know this.

Here are some things I learned:
1) HP of car/truck engines: take the advertised brake HP and cut it in half (this will equal PTO or electric motor HP).

2) You really need to know at what RPM is maximum torque (very important). This is the RPM the engine must not go below. The saw must not fall below hammered RPM so the speed reduction, pulleys must be right.

To run the saw with too small an engine: You must have the saw hammered for very slow RPM (say 350). Keep the engine running just over maximum torque and change the drive (pulleys) to lower saw speed. This increases the power.

I once changed the diameter of a drive pulley by 1/2" and dropped the engine 200 RPM and hit maximum torque and started sawing much better.

The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

From contributor A:
I have sawmilled quite a lot and would have to say that of all the power units I've used, GM has to be the best. Here in NC that's about all you see pulling mills. I've used the 3 and 4 cly. units and have had good luck with both. The 3 would be plenty of power for a farm mill or even for production. I could keep an edger man and two offbearers busy with the 3 - you can saw hard all day on 7-8 gallons of fuel.

From contributor B:
I have an 18hp Kohler Magnum on my mill. I don't think I would go less. While the smaller horsepower will work, it means you must go slower. Also, I find that sawing hardwood like oak doesn't mean much. In fact sawing gummy softwood of the same diameter is actually harder on the motor. The real work comes from sawing large diameter logs, so if you have a smaller motor like a 13 hp, saw smaller trees, say 10 inch trees, and you'll do fine.

From contributor C:
I also own/operate a M-14 Belsaw.I have found from my experience with power sources that more is better! I have run a 35hp gas and it just isnít enough. If you are going to go with a fuel motor then use a diesel. It will run 10 times as long and will keep up in the cut. I now run mine
all electric. I can say one thing; you can put a 75hp behind a 36"carbide and saw hard all day soft or hardwoods.

My advice is if electric is available then use it. It is expensive, but in the long run you are better off if you want to get serious. If you stay with a fuel engine then you would be better off using a smaller saw with higher mandrel rpm. Smaller saw equals thinner plate and kerf which in turn increases hp.