Power hacksaw

[Dunc]

A neighbor is interested in building one using the Pop Mech plans from Feb '76. He wants to get into it as inexpensively as possible hence the hacksaw vs one of the 4x6 bandsaws. I assured him that this plan works since I had built it and used it extensively myself some 25 years ago.

While the plans work as presented (what I built) we wonder about modifying it to use longer blades that are readily available. Is there any formula to suggest the length of the stroke to use for a given length of blade. Thinking that the longer the stroke, the more cut/stroke & better use of all (at least, most) the teeth on the blade instead of concentrating the wear on a small portion. Not really considering the "surface feet per minute" or equivalent here although if this is a serious factor then the belt drive pulley ratios could be adjusted.

Speed is not the issue. Even if it is a bit of a plodder as long as it cuts he'll be satisfied.

[wierdscience]

Well since a longer stroke means large cross sections can be cut more efficiently lengthening the stroke would be a good idea assuming-
The plans will allow for a longer set of guides.
And the connecting rod angle doesn't become to great.

IIRC the optimum conrod angle is 60* total arc,more than that the torque requirement raises exponentially.

[Jaakko Fagerlund]

The problem starts when the swarf becomes too big to fit in the space between the teeth in the blade (in the "flute"). Then it won't cut, heats up and ruins the blade.

If you are worried that you only use a small portion of the blade, put a spacer block in the vise once some part of the blade won't cut properly anymore. That way you can use the whole blade length.

[SteveF]

True, but if you are cutting a 3" solid you should not be using the same blade (tooth count) as you would if cutting a 3" piece of tubing.

Steve

[Alistair Hosie]

In the uk we can buy these very reasonably I would therefore never contenplate a shop made on myself.If the hold good used prices and are very expensive second hand i.e used then by all means if you enjoy the challenge make your self one .It's not rocket science, but is not as straight forward as you think . The cutting stroke being lifted on the backstroke and dropped on the forward cutting stroke and oil pot are complicated affairs. I have seen some really nice home made ones but never the weight of a good bought one. have fun and good luck Alistair

[Duffy]

Allistair, one CANNOT buy them "reasonably" or "unreasonably" in North America, so building is really the only option. Dashpots and upstroke relief are really great, but they dont really matter a damn when the blades only cost a dollar each.
I built one using a Boston Gear 10/1 drive and a 1/6 hp motor. I used a 6" (more or less,) stroke at about 90 strokes/min. and it works a TREAT. None of the fancy extras and the blades STILL last, (Starret Green Stripe 14 tpi.)

[Duffy]

I forgot to mention that I used that drive system simply because I had it.

[spongerich]

Allistair, one CANNOT buy them "reasonably" or "unreasonably" in North America, so building is really the only option.

Power Hacksaws show up on Craigslist reasonably frequently. I picked up my Miller Knuth at a machine shop auction for $25. If he wants the satisfaction of building his own, that's one thing, but unless he needs one right away, it's probably cheaper to find an old Miller or Craftsman.

[alanganes]

Around here (NE in Massachusetts) they seem to turn up with some regularity on Craig's list and similar places. There is one for sale near here now, but the person wants somewhat crazy money for it. Not sure you can get new ones anymore, but it's not likely anyone would want to anyhow. Bandsaws are the tool for the job now.

[Duffy]

There you go! I stand corrected.
In my neck of the woods, I have not seen one for sale in the last ten years, but then i have not really looked hard.

[alanganes]

There you go! I stand corrected.
In my neck of the woods, I have not seen one for sale in the last ten years, but then i have not really looked hard.

The availability of used machines does seem to be a pretty regional thing. A great deal in one area could be rip-off someplace else. Funny how that supply-and-demand stuff works...