Drag saw question

[Brian Rupnow]

Drag saws were the fore-runners of chainsaws. They are antiques, and look very "mechanical". I have spent the afternoon watching YouTube videos of them running, and there is something I don't understand. Obviously there is a crank arm which moves the mechanism the saw blade is mounted to back and forth on a pair of "guide rods". What I don't understand is why the "guide rods" are running on an eccentric and moving back and forth as well.-Does anybody know why they do?---Brian

 

[Brian Rupnow]

This is another style of dragsaw with a parallelogram type mechanism. On this one you can clearly se the angle of the saw change on the return stroke

 

[Brian Rupnow]

The steam powered ones were generally a belt drive affair. This is kind of a long video, but the last 1/3 shows the drag saw working. It looks like it even has a ratchet advance on the downward movement of the saw---not just gravity. I have two engines which I could consider using---my Kerzel and my Philip Duclos designed Odds and Ends hit and miss. The Kerzel is about 1/2 of the size of the Odds and Ends, so would probably be better from an "overall size" perspective. I really don't want to build anything more until late fall. However that doesn't stop me from doing research. There is a steam show coming up this weekend in Milton, Ontario, and that's only an hours drive from Barrie, so if it don't rain I may go down and see if they have anything interesting there.

 

[Cogsy]

It's been a while since I saw one of these in operation but it looks to me like the pressure is being taken off the blade on the back stroke. In the first video you posted there looks to be a lot more sawdust generated on the forward stroke than the backstroke. That's my best guess anyway.

 

[goldstar31]

In the days of Yore, I recall Ian Bradley- part of 'Duplex' writing about pressure relieving dashpots on mechanical saws in his many articles for Model Engineer. It was all to do with the backstroke.
As I hadn't a saw, I paid little heed to such ethereal concepts.

Maybe someone else can recall more details.

N

 

[TonyM]

It's definitely as Cogsy says. To relieve pressure on the non cutting stroke. Same way as the hydraulic pot of a mechanical hacksaw.
If I remember correctly it lifts, or just relieves the pressure, on the part of the blade nearest the motor on the forward stroke and drops the blade on the return stroke. The cutting stroke being the return stroke hence the term drag saw.

 

[justintime]

Drag saws were the fore-runners of chainsaws. They are antiques, and look very "mechanical". I have spent the afternoon watching YouTube videos of them running, and there is something I don't understand. Obviously there is a crank arm which moves the mechanism the saw blade is mounted to back and forth on a pair of "guide rods". What I don't understand is why the "guide rods" are running on an eccentric and moving back and forth as well.-Does anybody know why they do?---Brian

Hello, My assumption is ( from viewing the video ) , the eccentric action of the guide rod assembly causes slight lift ( higher angle ) at the rear of the blade on the return stroke, easing the "pull" a bit , hence few chips on return stroke and major chips on forward stroke.

 

[TonyM]

If you slow down the speed to that video it is clear that the lift of the eccentric is on the forward stroke and the drop is on the return stroke.

 

[justintime]

Hello, My assumption is ( from viewing the video ) , the eccentric action of the guide rod assembly causes slight lift ( higher angle ) at the rear of the blade on the return stroke, easing the "pull" a bit , hence few chips on return stroke and major chips on forward stroke.

" Brain in reverse similar to saw action", should have said "easing push stroke", in slo-mo, chips pretty equal both ways. dizzying effect regardless.

 

[a41capt]

Being that the drag saw appears to take the place of a single sawyer (rather than a two man saw) it stands to reason that one of the strokes would have a lift in order to save the set of the teeth. I’m including a picture of the set of a two man saw compared to a standard drag saw. As you can see, the two man crosscut saw is designed to cut on the fore as well as the back stroke. Top photo is the drag saw blade and typical of a single crosscut.

John W
Camp Verde, AZ

 

[reubenT]

if you ever used one by hand, the difference is clearly obvious. a little rocking motion saws much faster than strait back and forth. Even a modern chain saw will do a bit better that way, especially as it gets just a wee bit dull. (but not enough to warrent sharpening yet) less distance of wood cut for the teeth to work on lets it dig in better.

 

[a41capt]

When I was a kid, my dad worked as a finish carpenter, and I remember all too well the butt chewing I got when I used one of his VERY expensive and precise backed crosscut saws improperly. He would then proceed to have me hand file and reset the teeth using a fine round file and brass punch and setting guide. Man there were a lot of teeth on those saws!

I figured out pretty quickly how to use a saw correctly after that!!!

John W
Camp Verde, AZ

 

[ShopShoe]

a41capt,

And you better use the file in the right direction, too. And of course, then I made the next dumb kid mistake by laying the plane down on the sole rather than on its side.

And the final test of a saw's set is seeing if the sewing needle slides down...

It was all part of craftsmanship which is almost gone.

--ShopShoe

 

[a41capt]

a41capt,

And you better use the file in the right direction, too. And of course, then I made the next dumb kid mistake by laying the plane down on the sole rather than on its side.

And the final test of a saw's set is seeing if the sewing needle slides down...

It was all part of craftsmanship which is almost gone.

--ShopShoe

HaHa, nice to meet a kindred soul! That upbringing makes one a much better person, doesn’t it? I’ve had a very healthy respect for my tools since then, and one that’s not seen in most folks these days.

My dad tells a story about his apprenticeship to an old German finish carpenter. He was about to use a wood rasp to back off a miter on an outside corner when the craftsman snatched the rasp out of his hand and threw it off into the woods outside the window. “Rasps are for butchers! Block planes are for finishers!”. My dad said he was afraid the old gent was going to beat him he was so angry!

HaHa, must’ve worked, I never saw him with a rasp again!

 

[stuartblack]

This link seems to be playing up