Another thought about power hacksaw

Andy Wakefield wrote: (clip) Yes, I also realize that it would be far cheaper, if I value my time at all, to buy one of the HF or other cheap import bandsaws than to build this saw ... but metal-working is a hobby for me, a way to relax and do something different (clip) ^^^^^^^^^^^^ The idea is intriguing--it could turn out to be a very useful tool. I assume you will design it to use standard 12" hacksaw blades, so blade changes would be easy and inexpensive. I would approach it as an experiment, rather than a project destined for completion and success.

One of the problems you will have to deal with is vibration. To hold good tension on the blade, the beams will have to be pretty heavy. To get a useful throat, they will have to be pretty long. So you will have a lot of mass shaking. In order to work, you have to make the saw table steady. The only solution I can see is to make it very heavy. Possibly you could add rotating counterweights.

Keep an open mind, and keep us posted.

Yup, four things I see:

- Light saw frame

- Slow rate

- Heavy base

- Counterweights

You have to expect some sort of deflection when clamping down that blade. I'd use aluminum in the shape a scroll saw uses: a U frame with tapered, I-beam-shaped aluminum supports. The taper allows less mass out at the end where it's moving the most, and thus contributes the most inertia. It's also efficient use of material, since the leverage of the tension has the most effect where there's the least motion, by the fulcrum.

A light frame isn't stiff, and certainly one made of aluminum isn't. I wouldn't worry about it - just be sure to have enough clamping range so you can get the tension you need. A nice long bolt (as on a hacksaw) with locking nuts (to hold against the vibration) would probably work great.

You can figure frequency for whatever conditions you want. But generally, SFM seems to be the limiting factor with blades, so going extra fast holds no advantage unless your goal is tooth wear. It just so happens that around

60SPM (strokes) for an 8 to 12 inch stroke is about right, IIRC, so going for this project instead of the original power hacksaw design is rather foolish anyway.

Last option, counterweights: you could have an opposed weight on the crankshaft, for instance. The goal being to balance the "up" reaction with a "down" reaction. Entirely a balancing act between phase, angle and mass. Unlikely to be a concern at 1Hz!

Tim (how do you like the new sig?)

-- "I have misplaced my pants." - Homer Simpson | Electronics, ______________________________________________| Metalcasting and Games:

In woodcutting, those things often have their geometry worked out so the teeth of the blade progress into the work, cutting agressively on the stroke. In metal, particularly in steel, this would be very tricky. You'd have to get the bite exactly right or you'd break blades -- or you wouldn't cut.

Fine Woodworking published an article years ago about making one of these things out of wooden beams, for cutting really thick stuff, if you wanted to. The example was around 6 or 7 feet tall IIRC but you could make it any size.

Ed Huntress

If you can find a copy of a book called "Making and Modifying Machines" published about 20 years ago by Fine Woodworking Magazine, there is an article about building a similar saw. As I recall it was built mainly from wood and probably ran faster than you would want for metal cutting, but it might be a place to start. The book I looked at has a photo of the saw on the cover. Mike

Yeah, that's a reprint of the article I mentioned in an earlier post. I have the book.

Ed Huntress

Sounds like a fun project. Maybe this will give you some ideas?

Best Regards, Keith Marshall snipped-for-privacy@progressivelogic.com

"I'm not grown up enough to be so old!"

A few years ago (1965 ? ) I needed an hacksaw that would give me more precision than I could achieve by hand. Left behind by the previous owners in the basement was an old defective wringer washer. I removed the super structure and the tub. Kept the four legs with the wheels and built around the motor and gearbox a frame from angle iron (old bed frame material) a support structure for a vice and support for the actual saw blade holding U frame. Two car tie rod ends on the ends of an half inch pipe translate the go around motion on one end at the gearbox to a back and forth motion on the other end which moves the before mentioned U frame. This frame accepts saw blades from 10" thru 14" inch lengths. The stroke is adjustable. Looks ugly but it works: I can slice off a

1/32 thick sliver from a 1" by 2" steel bar easily. HTH

I saw plans for one in a Lee Valley "Shop Notes" reprint book (either 1915 or 1925, I'll look up the reference if you're really interested). They also had plans for a bench-top power file, which is the same basic idea. Now, if you could make the functions interchangeable, saw or file, then you would have a truly versatile and functional machine.

Though, one problem with plans from old books is that they seem to assume that your average home machinist can pour iron castings, which is not all that common these days. You might have to redesign based on building up certain parts out of available stock.

David...

Wait - here's an idea: the "agitate" function of a washing machine requires a back and forth that the _transmission_ supplies. Well, it's a rotary back and forth, but that's easy to translate.

Anybody ever use a washer transmission for this? I gotta remember this

- there are functional washers at the dump all the time.

Bob

Yes, that article is part of what led to this idea! As you said, the saw in the article was made almost entirely of wood (except for bolts & bushings). I don't recall the RPM, but it used a surprisingly small motor, IIRC.

Thanks for the encouragement and suggestions. I don't know how soon, or if, I will go ahead and build it, but I'll let you know how it works out if & when I do!

Andy