Homemade sawmill questions

Ljohnsaw · January 12, 2022, 11:54:43 AM

Quote from: Joe Hillmann on January 12, 2022, 11:45:38 AMOn the down side, the blue pulley on the top left of the picture is always under strain of 2x the weight of the head of the mill, so it has to be pretty well built.

NO, that is not true. Physics 101. You have your mill head supported by two cables as shown on each end. The mill head weighs W. How much weight (tension) is on each cable? Exactly 1/2 W. If it was more, then the head would rise, less and it would fall, right? So the cable on the left has 1/2W. Going over the pulley just changes the direction of the force so the tension on the left side of the pulley (going down to the winch) is still 1/2 W. Now, the winch has to take up twice the cable to move the head a given distance. So it runs longer, with less effort. It is only pulling with half the force that would normally be required to lift the head directly.

Joe Hillmann · January 12, 2022, 12:01:46 PM

Quote from: ljohnsaw on January 12, 2022, 11:54:43 AM
Quote from: Joe Hillmann on January 12, 2022, 11:45:38 AMOn the down side, the blue pulley on the top left of the picture is always under strain of 2x the weight of the head of the mill, so it has to be pretty well built.
NO, that is not true. Physics 101. You have your mill head supported by two cables as shown on each end. The mill head weighs W. How much weight (tension) is on each cable? Exactly 1/2 W. If it was more, then the head would rise, less and it would fall, right? So the cable on the left has 1/2W. Going over the pulley just changes the direction of the force so the tension on the left side of the pulley (going down to the winch) is still 1/2 W. Now, the winch has to take up twice the cable to move the head a given distance. So it runs longer, with less effort. It is only pulling with half the force that would normally be required to lift the head directly.

After thinking about it, I think we are both wrong. The upper left pulley has the full weight of the head on it, everything else has 1/2 the weight. Either way that pulley has twice the force on it as anywhere else.

Ljohnsaw · January 12, 2022, 12:23:00 PM

Ops, yes. The weight on the left blue pulley is the sum of the two forces acting on it - so 'W' is pulling down, the full weight of the head. But like stated, if you mount your electric winch up there, then it only is supporting 1/2 W on the mounts and the cable tension remains the same at either location of 1/2 W.

ktm250rider · January 12, 2022, 02:00:54 PM

Easy solution would be to just cut the cable so that there is only enough to wrap around the drum once. That is what i did with my hand winch. I have 2 individual cables that attach to the power head. Im using a worm gear hand winch and there is plenty of room on the drum for this. Both cables lay flat on the drum and has worked flawlessly. I had my cables setup like the free body diagrams a above, but i didnt like how the head raised. I got nice flat cuts but it seemed to be "jerky". With the current setup, everything is smooth.

JoshNZ · January 12, 2022, 04:58:14 PM

Even the angle at which the cable lays on the drum is going to change its length per revolution slightly, I think rolling both cables onto one drum is inherently a bad design, period.

I also think your diagrams assume the mill heads CoG is exactly in the centre too, which is unlikely. The lighter side would want to rise higher. I don't like that design as much as the Y cable

rusticretreater · January 13, 2022, 12:44:00 AM

Here is another method that is used on my Woodland Mills HM130Max.

They constructed an arm that extends. Instead of cranking the cables, you push against them. The system uses an acme screw for accuracy and smooth movement. But you see both cables are pushed on equally, maintaining a level cutting head that moves easily.

fluidpowerpro · January 13, 2022, 01:01:49 AM

Thats a really slick set up. Pushing against the cables the way they do means that for every inch of travel on the screw, the cable only shortens 1/2" at the saw head. That helps accuracy. It wouldnt be hard to use this same concept using an electric linear actuator instead of a crank.

JoshNZ · January 13, 2022, 01:19:50 AM

You might have that backwards, I think 1" of extension on the screw is 2" of cable? Should still be a lot more accurate than a drum.

My mill uses two acme rods (called trap rod over here) to lift the head directly and they're chained together, works great.