Metal cutting bandsaw experiment
I wanted to experiment with cutting metal on my bandsaw, so I bought some metal cutting bandsaw blades.
The problem is that metal cutting bandsaw blades need to run at less than a tenth of the speed of wood cutting blades. I figured I'd experiment by making an auxiliary pulley to go on the back of the motor to get an extra reduction stage. This wooden pulley is from when I experimented with motorizing the apple grinder.
The flanges have 1" holes in them, but the motor has a 3/4" shaft, so I cut them off on the bandsaw. The pulley was too high to fit between the table and upper blade guide, so I removed the table to make it fit.
The second motor is a 1/3 hp 1750 RPM motor with a 5 cm pulley, connected to the 35 cm wooden pulley, making for a 7:1 reduction. This passively turns the main motor at 250 RPM instead of 3500 RPM. The reduction from this motor to the main pulley is a further 8:1 reduction, for about 31 RPM on the bandsaw wheels. I measured the actual RPM and worked out the blade speed at 153 fpm (46 meters per minute)
Metal cutting is at a much lower speed, but also needs a lot more torque. The drive belt to the lower wheel was slipping. I forced the motor a little further from the lower wheel to add more belt tension. I had this belt at a relatively low tension before this.
When drilling, it helps to oil the drill. But small metal cutting bandsaws often don't use oil or cutting fluid, but I figured oil couldn't hurt, as long as I'm not using motor oil (the detergents in motor oil can damage rubber).
I then researched blade speeds for portable metal bandsaws, and, given that the blade speed was lower than a lot of these saws, my cutting speed worked out to be proportionally slower.
Next I wanted to try the 25 mm wide 3 TPI metal cutting blade that I got from sawblade.com. But for my saw, this saw blade was welded together "backwards". that is, if I were to put it on the saw, the teeth would be facing up. This type of blade is meant for a large horizontal metal cutting bandsaw, and these usually run counter-clockwise (when looking at the teeth), as opposed to clockwise for most upright bandsaws. I flipped the blade inside out to remedy this (it took me a few tries to remember how to do this). There is a plastic protector on the teeth of the saw, which is why it looks like the blade has a white edge.
Large metal cutting saws run with some sort of cutting fluid. I figured I should add a bit of oil to the teeth to sort of make up for that. But with the coarse sharp teeth, I couldn't just hold a rag against them. So I bunched up some paper towel, put oil on it, skewered it on a screwdriver, and let it spin against the teeth.
But with the teeth as coarse as they are, I don't think this sort of blade is designed to cut sheet metal or hollow steel. It's designed to cut large solid chunks of steel into smaller pieces so that they can then be put on a metal lathe or milling machine.
With the teeth as coarse as they are, I figured the blade should also work fine cutting wood. But pushing a block of wood against it at the slow (153 fpm) speed, as soon as the teeth grabbed the wood, it would bind and stall the saw (causing the belt to slip). But once I changed the speed back to the regular wood cutting speed (2000 fpm), it cut through the wood just fine.
At 1 mm thick, it's a fairly thick blade, almost as thick as the sawmill blades I tried earlier. So the blade may die from metal fatigue if I use it for wood cutting. Metal fatigue is much more of a problem with wood cutting saws because, at the higher speed, the blade has to bend and unbend many more times. And with metal cutting, the blade only lasts so long, so between the shorter life and lower speed, the metal cutting blade doesn't have to bend and un-bend as much in its lifetime as wood cutting blade does.
So this bandsaw does work for metal cutting. But I'm not sure if I'll use it for that much in the future. I would need a lot of pieces to cut, and in a way that I couldn't already do with a dry cut metal chop saw, plasma cutter or an angle grinder. But if, for some reason, I needed to cut a 2"x2" solid chunk of metal in half, this saw would be my best option.
If I were to build this saw optimized for metal cutting, I would change a few things however:
1) I'd make the V-belt pulley on the lower wheel almost as large as the wheel itself to get more reduction and more torque.
2) I wouldn't put rubber tires on the wheels
3) I would reduce the maximum cutting height of the saw to maybe 15 cm. This would make the upper guide column shorter, which would make it stiffer
4) I'd change the orientation of the upper thrust bearing to be the same as the lower one, so that the blade runs along the outside edge of the bearing instead of against the face of it. There's a lot of thrust when cutting metal, and the bearing would last longer if it rolled against the back edge (more like what the bearing is designed to do).
It would be a fun challenge to make something equivalent to a large horizontal metal cutting bandsaw out of wood. I'm quite sure it's possible, though the use of water based cutting fluid could be a challenge.
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