Power hacksaw

[Chriske]

Hi
Planning to build a power hacksaw.
I would use a windscreen wiper motor to drive this hacksaw. Just bought this motor in UK but problem is after after some time this motor gets very hot, and I mean HOT. Only way to keep it cool during a long (free)run is to install a 80 or 120 mm fan against it. It gets warm still but not as hot as before.
Is there a reason these motors getting so hot. I have an older windscreen wiper motor and it's just the same, it gets very hot to.
Idea's...?
Thanks,
Chris

 

[mcostello]

They are usually used in the rain!

 

[GLCarlson]

How much load are you putting on it? (Hint: if it's hot, it's overloaded.) These are very light duty motors- doesn't take much to push a little water off a bit of glass.

Cutting metal is a lot different. Typical metal cutting saw is at least a quarter horse, usually more like a half to 2 or 3.

 

[Chriske]

How much load are you putting on it? (Hint: if it's hot, it's overloaded.) These are very light duty motors- doesn't take much to push a little water off a bit of glass.

Cutting metal is a lot different. Typical metal cutting saw is at least a quarter horse, usually more like a half to 2 or 3.

While I was testing it there was no load at all.

 

[GLCarlson]

While I was testing it there was no load at all.

Hmmm. I'm not sure I understand. No load, as in "motor not connected to anything" or no load, as in "motor hooked up to the saw but not cutting metal"?

If the former, I suppose the motor just expects to be cooled by airflow from a moving vehicle. In that case add a fan and hope for the best. If the latter, there's your load: just moving the frame is a bit much.

All of that avoids what I think the real question will be: is a wiper motor adequate to run a small saw? Depends on what "small" means, but I'll take that to be a typical hobby 4x6 bandsaw equivalent. That is capable of cutting not just thin stock, but 1-3" bar, heavy angle, and at least 1" plate. A 4x6 is a nice hobby size; price is not irrational ($300, often on sale for less than $200, can be found used for half that with a bit of luck). So an equivalent frame-type saw might use 10" blades and be able to go 3" (better 6") deep. I owned one of each type, years ago.

Just poking around, a heavy duty 24vDC wiper motor appears to have a 6 amp stall current and delivers about 110 inch-pounds. A more typical 12v motor stalls at around an amp -some less. The bigger motor is about 150 watts; a quarter horse motor, allowing some windage for efficiency, is around 200+ watts and is underpowered for a 4x6 bandsaw type saw (mostly, those use 1/3 or 1/2 hp motors (Chinese, so probably over-claimed, but anyway 300 watts or so).

Since you're using a reciprocating drive, I presume you're planning a power hacksaw, not a bandsaw. Building a reciprocating drive is pretty easy. A disk with an eccentric crank to the sawframe, and a double reduction pulley (a couple 5:1 ratios) will get you down from 1740 rpm to 70 strokes/min. I'd be tempted to use a chain drive; higher ratio is easier. Or a worm type reducer (there are worm gears in old antenna rotators that will handle the power, for example).

In any case, I recommend you reconsider your drive. I predict it will be underpowered and frustrating to use for material of any size above an inch or so- maybe smaller.

(I understand you've got a UK motor; my comps are all US, but probably close enough).

 

[Chriske]

It is indeed a reciprocating saw I'm thinking of. I will use the classical 300mm(12") long blade.
I don't need to add a reduction because this unit has an output of about 105rpm. That's about the speed I was aiming for. But to be on the save side I'm planning on reducing it's speed electronically.

 

[MRA]

I use a 1/4 hp induction motor to drive my small recip power hacksaw - it gets pretty hot chewing through 2" bar, which is the biggest it will take. I just drive via a belt, and dealt with speed via pulley sizes - it does about 120rpm or 2 cuts per second.

Not having a 'lift' on the return stroke wears blades out, no doubt - perhaps I'll think about adding such a refinement one day, though students at work keep me in free broken blade fragments. I also have to squeeze across the saw back in a g-clamp to shrink things whilst I clamp the blade in - so it springs back into tension when I release the g-clamp. You might want to think about blade tension in your design.

 

[Wizard69]

In response to your earlier question about heat, it really depends on the motors design as to how hot it will get. Realistically you cant tell by touch if the motor is too hot. This especially on Steppers, Brushless DC and other motors with the wiring near the case. The motors can run hot enough to boil water. Some have a temperature tise spec of 70 to 100 degrees C. You need to measure the case

It is indeed a reciprocating saw I'm thinking of. I will use the classical 300mm(12") long blade.

Now too much load can cause your motor to overheat. The problem here is that wiper motors vary significantly from model to model as does user expectation. It is a wild guess to say any one wiper motor will meet your needs.

I don't need to add a reduction because this unit has an output of about 105rpm. That's about the speed I was aiming for. But to be on the save side I'm planning on reducing it's speed electronically.

Im not sure speed control is worth it. Id keep things simple here.