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ironhoarder
05-08-2006, 04:00 PM
...I don't know what else to call it.

I am rebuilding a little Covell power hacksaw. I have it in good shape now, but I am having trouble with the mechanism that lets it drop a wee bit only on each stroke.

They have a curved rack gear (except the teeth are sawtooth shaped) that gets engaged by 2 little pawls that are on levers. These levers get bumped by a cam on each stroke and it looks like the idea is that it should let it drop by one tooth only on each stroke.

The problem I have is that I cannot manage to get it to do that. It either doesn't drop at all, or the whole thing comes down at once. I have carefully sharpened the pawls (matching the edge angle). The rack looks ok and there is no oil on it.

This just seems like a sensitive, failure-prone mechanism to me. I think I'd rather come up with a hydraulic or pneumatic damper, like my Atlas/Craftsman horizontal bandsaw has.

Any suggestions on a source for an adjustable damper, or good ideas on making one?

I'll try to post pics tonight.

Cheers

firbikrhd1
05-08-2006, 07:45 PM
From your description of the problem it seems that you have a set up similar to that on my racine Power hacksaw. If so, the Racine has a spring that applies pressure to the pawls causing them to engage securely. Could your spring be missing?

Scatterplot
05-09-2006, 03:39 AM
Hmmm, pics might help give me a slightly better understanding of the problem. I've never actually seen a power hacksaw in person. I do think it would be kinda hard though to make a hydraulic mechanism to do what you're talking about. It could be done, but it would seem to need a bunch of carefully tuned valves, and probably specially made ones.

bpsbtoolman
05-09-2006, 08:42 AM
I have both a small Covel power hacksaw and the Craftsman horizontal band saw. The blade lifting system has me puzzled on how it works. It seems to work most of the time, but sometimes doesn't. The nice thing I like about it is I can use Morse bimetal blades I get at Menards 10 for $ 9.00.
I saw in one of the yahoo posts or HMS a homemade hacksaw that had a hydraulic cylinder that lifted the blade on the backstroke and quickly droped it on cutting. Maybe search www.metalworking.com index for this.
Walt

ironhoarder
05-09-2006, 10:26 AM
firbikrhd1 - Instead of springs that force the pawls into the escapement, they have weights. Alas, these seem to not apply enough force. Thanks for the idea tho- I may try some experiments with additional springs, weights, whatever.

Scatterplot- what I am proposing is no more complicated that a screen door closer. Basically a little shock absorber. The one on my craftsman saw is very simple and effective. In fact, I'm going to see if I can by one from clausing...

bpsbtoolman - mine takes 12" blades, but I think it really wants 1" wide blades rather than the 1/2" ones for a hand hacksaw. And on my saw, the blade lift works fine- the saw cuts on the backstroke and a cam raises the blade slightly on the front stroke. The mechanism I'm having trouble with is the escapement that lowers the blade slightly on each stroke. I think I just need to look at this thing really carefully and get it adjusted right. I'd love to see pics of your covel. Does it have the curved rack and double pawl system that I describe?

Bill Cook
05-09-2006, 12:14 PM
OOPS, Should've read more before posting!

On the cutting stroke the blade, bow, arm, and any added weight rest on the work.

The mechanism is for lifting the blade on the return stroke to keep from rubbing the teeth and dulling them prematurely. Then it sets it down nice'n easy for another cutting stroke.

If the saw is on the backstroke it will stay up when picked up. If it's on the cutting stroke it won't.

http://img.photobucket.com/albums/v50/skytop19/09280r.jpg

http://img.photobucket.com/albums/v50/skytop19/09278r.jpg

What doesn't show in the pix, is a cam that causes the arm the pawls are on to go up and down.

The saw is on the return stroke in the first pic - one of the pawls is engaged and has lifted the arm. As the cutting stroke comes up, the cam will lower it onto the work. Then as the cam drops farther, the tail on the pawl assy will contact the square head setscrew below it and completely disengage. This allows the blade to advance into the work at it's own pace.

The pic below shows why there are two pawls.

http://img.photobucket.com/albums/v50/skytop19/09285cr.jpg

This is an old 6" by 6" capacity Racine.

http://img.photobucket.com/albums/v50/skytop19/Racinerr.jpg

There are quite a few different lift mechanisms out there.

BC

LastOldDog
05-09-2006, 03:17 PM
Tried to post with Photobucket, a failure. Sorry