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ViKARLL
08-26-2011, 01:25 AM
Hi folks,
I am brand new to this forum and six weeks old to building a homemade power hacksaw. It is meant to take work-pieces up to 4" X 4"

I loosely follow this design http://www.mooseforge.com/library/powerhacksaw.pdf with a few mods.

I have surfed the web fruitlessly for nearly a month to figure out a way to implement the blade lift mechanism on the return stroke.

Many people elsewhere have touched this topic but are either vague about it or sounds that it is so trivial. Unfortunately, I have never got the opportunity to see the anatomy of a gravity operated power hacksaw closely, so I am lost.

Could someone please educate me on where this so called "cam mechanism" is mounted, how does it work uniformly for all work-piece hights etc ?
If you can kindly offer some sketch / close-up picture, that would be great.

Thanking you in advance,
ViKARLL

rohart
08-26-2011, 04:51 AM
I had the dubious pleasure of owning a Qualter and Smith power hacksaw.

The cam was on the pulley that also had the crankpin on it. That's the only place it can be, because the cam and the crankpin have to work on the same rhythm.

There was a cylinder whose main piston lifted the arm, and the actuating rod has to be offset from the centreline, so the main pivot bearing must have zero play or the arm will twist. This vertical cylinder had a small sub-cylinder at about 60 degrees at its base, and in this subcylinder was the piston that was driven by the cam.

It was not a great design. To get the cylinder out you had to let the piston attached to the cam get pulled out, so half the oil hit the deck before it was half out. A lever from the cam, and a vertical subcylinder would have been more sensible.

If you mount the cam with bolts so it can be repositioned, you'll be able to adjust it later.

The clever stuff was all incorporated in the design of the main piston. As the cam pressurised the cylinder nothing would happen at first as a washer moved up to seal a hole in the piston. Then the arm was lifted when this hole was blocked, and finally there was a mechanism (rod to the top) for adjusting the distance of this washer from the hole in the piston, and thus the bleed off, to adjust the speed with which the arm came down - changing the effective weight of the arm during the cutting stoke.

After describing it all above, I have o say I got rid of the machine because I could never adjust the drop speed properly. It would either drop too fast, and teeth would fly off the blade or it would come to a grinding halt, or it would drop too slow so the teeeth would get polished smooth.

I still have the manual, so I could put up a page or two if it would help.

Good luck - I hope this helps.

spongerich
08-26-2011, 09:26 AM
I have a Miller Knuth that I could snap some pics of... unfortunately cleaning it up is still on my extensive to-do list (There's 40 years of grease in it) but I can probably get some passable pictures for you. I took a look at the parts diagrams in the manual, but they're not of much use.

email me if you want some pics. I'm outta town until Monday, but should be able to get some for you early next week. my email is the same as my username at gmail dot com.

Dunc
08-26-2011, 09:28 AM
Many years ago I built an enlarged version of this Pop Mech, Feb 76 design.
http://books.google.com/books?id=B-IDAAAAMBAJ&printsec=frontcover&source=gbs_ge_summary_r&cad=0#v=onepage&q&f=false

It lacked any dedicated lift mechanism for the saw on the back stroke. Did a lot of cutting & can't say that dragging the teeth backwards over the material negatively affected blade life. I was using power hacksaw blades, not hand hacksaw ones.

strokersix
08-26-2011, 09:46 AM
A simple crank driven hacksaw tends to pull the blade into the cut on the cutting stroke and lift it on the return due to crank and rod geometry.

Don't run it backward.

dalee100
08-26-2011, 10:25 AM
Hi,

I have a home-made power hacksaw too, (though I didn't build it). It doesn't have any lift or decent control. And I will agree with Strokersix, the crank will unweight the blade just a bit on the backstroke. The teeth probably still drag just a tiny bit on the backstroke. But it does not appear to damage the teeth at all or prematurely wear out the blade any noticeable amount.

I'm still using a 14T bi-metal blade that has been in the saw for well over 10 years. It is a power hacksaw blade though.

I was going to try building the saw you are. But I got my saw for helping to clean out an old building. So I would be very interested in hearing your opinion when you get it done.

dalee

Magicniner
08-26-2011, 05:56 PM
One of the simplest ways to do this is to have an adjustable oil filled damper controlling drop speed and have the blade set in the frame at a very small angle to the stroke, lifting the frame slightly on the cut.
On the back stroke (with the damper is set to lower the blade just fast enough) this provides a little clearance so the blade doesn't drag .
My Rapidor Manchester works this way and has been cutting various grades of stainless, ally and titanium all this year on the same blade ;-)
Regards,
Nick

Chester
08-27-2011, 03:53 PM
Power saws don't get much simplier than John Dean's "Metal Slicer" In the April 1995 Issue of Projects in Metal.............by our host.

http://img831.imageshack.us/img831/7886/hacksawpowerjohndeanclo.jpg (http://imageshack.us/photo/my-images/831/hacksawpowerjohndeanclo.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

rohart
08-27-2011, 05:24 PM
Looking at the image Chester posted, it's clear that by running the shaft in the right direction, the crank can do a pretty good job of unloading the blade on the back stroke. The only requirement is that the crank arm is short compared to the stroke.

ViKARLL
08-31-2011, 02:35 AM
Hi, I am back with some pictures of the partial build.

http://www.flickr.com/photos/66181305@N06/?saved=1

Thank you rohart, for the detailed description on the Qualter & Smith. As Nick has pointed out, I have realized that the ideal method would be the oil filled damper. But unfortunately, I am dead-meat when it comes to hydraulics. Besides, building that sort of a mechanism at home shop and fine-tuning it to avoid the difficulties rohart mentioned about adjusting drop speed etc., would be a tall task for me.

Dunc, thank you for the link and the encouraging news that drag did not tell much on your blade life. I hope to use 14T 16 Inch blades too as
dalee100, and hope it would last as long as his. I will certainly keep my progress posted.

Spongerich, it is great to see some pics of your Miller Knuth. But before troubling you to take pics, please check for me whether it’s a mechanical lift or otherwise.

You may notice from my pics that the crank-arm of mine is connected to the ‘rocker’ but not the saw frame as the original design. I did this mod to make the saw more compact (lengthwise). Due to this, I am deprived of the technique rohart and strokersix have mentioned last, about the crank itself unloading the blade.

Any suggestions, please ?


BTW, is there a way to insert pictures directly to the post itself ?

Thanks & best regards,
ViKARLL

J Tiers
08-31-2011, 08:52 AM
The Covel I gave my FIL has mechanical lift.

According to my recollection, :

There are two counterweighted "L" shaped levers on the base with a tooth on one end (top), and two vertical notched plates on the arm.

The counterweight swings the teeth into the notched plates, but an adjustable "stop" swings them away when the "lift" is down.

Every turn of the crank, a cam and arm lifts the levers and lets them engage so that they raise the plates and arm.

When they let the arm back down, they can move to the next notch if necessary, due to the counterweight being lifted swinging out the tooth.

The two plates are staggered, even though the notches are about coarse hacksaw blade pitch, so that rather fine increments of lift are possible.

I don't have a clear picture of the whole mechanism, but I can post one of the general layout, which may/should help. I'll have to do it later.

firbikrhd1
08-31-2011, 10:43 AM
How about an adaptation of "the hinge trick" that is used as a tool lifter when cutting "T' slots on a Shaper? A long "cam" could be bolted along the stroke of the hacksaw and a hinge attached to the end of the hacksaw frame that is closest to the crank. As the frame is pushed forward on the cutting stroke the hinge would simply move up over the "cam". As the frame reached the end of it's stroke the hinge would flip down, hit the cam and lift the frame up over the cut until the hinge reached the beginning of the stroke when it would flip down again and repeat the process. This system could be reversed if the cutting stroke is a draw stroke rather than a push stroke.

Below is a diagram of the mechanism on a shaper. Simple and effective.

http://i283.photobucket.com/albums/kk287/firbikrhd1/hingedclapper.png

dalee100
08-31-2011, 10:50 AM
Hi,

The build looks good so far!

My biggest problem with my saw is on the cut stroke. A 1 1/4" solid square bar was used for the cutting arm. It's pretty heavy and there is no way to lighten the down pressure on the cut stroke for thinner profiles. So I can get some chatter on the cut stroke. I have been contemplating adding an adjustable spring tensioner to ease the chatter on those thinner profiles.

dalee

Chester
08-31-2011, 05:57 PM
The Covel I gave my FIL has mechanical lift.

According to my recollection, :

There are two counterweighted "L" shaped levers on the base with a tooth on one end (top), and two vertical notched plates on the arm.

The counterweight swings the teeth into the notched plates, but an adjustable "stop" swings them away when the "lift" is down.

Every turn of the crank, a cam and arm lifts the levers and lets them engage so that they raise the plates and arm.

When they let the arm back down, they can move to the next notch if necessary, due to the counterweight being lifted swinging out the tooth.

The two plates are staggered, even though the notches are about coarse hacksaw blade pitch, so that rather fine increments of lift are possible.

I don't have a clear picture of the whole mechanism, but I can post one of the general layout, which may/should help. I'll have to do it later.


This is the best picture I have of that arrangement. Those pawls would walk down the toothed bars if you took the time to play around with the set up of them. They look rather dull in that picture, but were actually cut sharp to about a 45* angle.

http://img62.imageshack.us/img62/2122/excelpowerhacksaw9.jpg (http://imageshack.us/photo/my-images/62/excelpowerhacksaw9.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

http://img835.imageshack.us/img835/1169/excelpowerhacksaw6.jpg (http://imageshack.us/photo/my-images/835/excelpowerhacksaw6.jpg/)

Uploaded with ImageShack.us (http://imageshack.us)

Chester
08-31-2011, 06:06 PM
Hi, I am back with some pictures of the partial build.

http://www.flickr.com/photos/66181305@N06/?saved=1

BTW, is there a way to insert pictures directly to the post itself ?

Thanks & best regards,
ViKARLL

Easiest way, for pictures, is with these guys, no account required just post pictures as needed, where you want them. (copy and paste the "forum code") Might be something with Flicker also, but the imageshack is simple.

http://imageshack.us/

ViKARLL
09-01-2011, 06:55 AM
Hi,
It's pretty heavy and there is no way to lighten the down pressure on the cut stroke for thinner profiles.
dalee

How about extending the cutting arm from behind and adding counter-weights onto that extension?

Indeed this will make the overall length of the saw longer.

Chris S.
09-01-2011, 10:38 AM
I emailed this guy to ask how long it took to cut that big steel block. He replied "About an hour". That particular day I watched a sh*t load of youtube power hacksaw vids. This one caught my I because most of the vids showed designs that clanked, clacked, clicked and clopped. This one ran smoothly though. Maybe the ones that made a racket lifted on the back stroke, but don't don't know for sure.

http://www.youtube.com/watch?feature=player_detailpage&v=P5Tu_zoO828

ViKARLL
09-02-2011, 02:13 AM
How about an adaptation of "the hinge trick" that is used as a tool lifter when cutting "T' slots on a Shaper?

Sorry, I am not too familiar with Shapers but in them isn't the reciprocating arm travel virtually on the same horizontal plane ?

This is my worry with the power hacksaw.

Suppose the max cutting capacity is 4" X 4".
At the beginning of the cut, the frame will be a few inches above the crank wheel but as the cut progresses, the frame will drop lower and lower (finally, parallel to the horizontal plane). But our long cam, though eccentric, will be bolted in a fixed position that will not “follow” the saw frame up and down.
Meaning, at the highest position (of the frame) if the gap in between the cam surface and the hinge edge is ‘X++++’, at midway it will be ‘X++’. At the lowest position it will be ‘X’ (in fact, ‘X’ = zero)

Now, do I see a real problem here or have I not understood you and am ‘holding the wrong side of the stick’?