Power Hacksaw

[baggo]

I bought a power hacksaw off Ebay earlier this year but have only just got around to finding it a permanent home. It's very well made but doesn't have any makers name on it anywhere which makes me think it may be a home brewed job. It doesn't seem to match any of the designs which have been published in ME. The motor/gearbox is a complete assembly which is definitely a commercial item. I've included a couple of pics in case anyone may recognise it as I'd like to get some info on it. The one thing I'm not sure of is which way round the blade should be fitted - should it cut on the forward stroke or the return stroke?

John

[dscott]

All the power hacksaws I have ever used and seen work with the blade cutting on the return stroke. obvious but it is easier to pull than push even on larger machines.
David.

[Tel]

Likewise - they work on the pull stroke. Definitely a 'home brew' I'd say, but a cute lil bugger - unlike the one I just bought (also off ebay)

[JohnP]

Tel,

Did you tie that to the loadbed yourself?

They look like real knots, like lorry drivers used to use until they went for those quick release straps!

JohnP

[baggo]

Thanks for the advice Dave/Tel. I'll set it up to cut on the return. I suppose there is also less chance of the cut wandering if the blade is being pulled rather than pushed, especially if there is any play in the slides.

That's a nice piece of machinery Tel - you should be able to do some serious damage with that ;D

John

[gwrfan]

johnp said:

Tel,



"They look like real knots, like lorry drivers used to use until they went for those quick release straps!"

Oh yes, sailors' knots too - Sheepshanks, used to shorten ropes. And very nicely tied too, Tel.

[Tel]

Somebody gave me a set of those nylon strap things but I never use 'em except on furniture. You can tie & clinch up a double sheepshank quicker than you can fool around with the buckle on them things.

It doesn't show in the pic - but that saw has a variable stroke - the crankpin works in a slot in the disk. Lot of work to do on it, but for $60 ............

[SteveW]

All,

Just a point on the direction of cut.

I'm sure the one a school cut on the forward stroke. The geometry was such that as the blade moved through the metal is rose slightly lifting the piston up in the oil damper. On the way back the damper held the blade off the material on the return stroke.

Another point I remember was that the thrust line wasn't in line with the crank thus giving a different forward/reverse cycle.

Yep! I spent far too long on the thing and have one of those trick memories for this sort of thing.

[baggo]

I did wonder if there was something in the cutting arm geometry on mine which relieved the pressure on the blade when on the non cutting stroke. That's why I wasn't sure which way round to fit the blade. There's no damper or similar fitted but it's got a very ingenius feed mechanism:

The handle at the bottom turns a leadscrew which raises and lowers the whole of the cutting arm. To use the saw you wind the handle to raise the saw blade above the material to cut and then set the machine going. A microswitch on the arm operates a solenoid which engages a push rod attached to the arm which drives the leadscrew through a ratchet. This gradually lowers the blade until it starts cutting. The solenoid holds in for a short time after the cutting starts and drives the leadscrew about another turn. The microswitch then releases the solenoid removing the drive to the leadscrew. The blade carries on cutting until it 'catches up' and then the microswitch operates the solenoid again and lowers the blade a bit more. This action repeats until the blade has gone right through the material and the saw switches itself off. A bit difficult to describe but very simple in operation.

Steve, you were lucky - we were never allowed to use the power hacksaw at school I was let loose on the shaping machine for a time though - great fun (wish I'd got one now!)

John