Cheshirechappie
Established Member
We had a long and rather rambling thread on the subject recently, which some may recall with sinking heart - panel-saw-tensioning-t96004.html
After much diligent searching, Corneel discovered a German text, dealing with machine saw blades, that outlined a method to check bandsaw blades for 'tension' put into them by hammer or roller during manufacture or servicing. The blade is held in a gentle curve, and a short straightedge applied across the width. The 'tensioned' areas will be revealed by slight out-of-flatness - very slight humps - of the blade when so curved. When the blade is flat, no such humps are detectable. A hump height in the order of about 0.4mm was mentioned, detected by looking for light under the straightedge.
Corneel tried this on a couple of handsaws, and found some evidence that the same thing happening (go to the end of the thread for photographs).
I tried the same, and - rather indistinctly - also found 'something' on a couple of saws, though no evidence on others.
On another forum, Rob Streeper has documented his work to investigate 'tension' by hardness-mapping old saw blades. He found two lines of extra hardness on some vintage saws, one about an inch above the toothline, and one following the line of the blade back. (Those lines correspond to the 'humps' Corneel observed on his saws, leading to the possible conclusion that they were the same thing.) He subsequently hammered a new saw blade along those regions, and observed a slight change in behaviour of the blade in service - it seemed less 'whippy' at the toe end on the back stroke, and more inclined to return to straight quickly.
Additional hardness (slight work hardening, in effect) would be caused by extra hammering along the regions identified, so things do begin to look promising in the hunt for 'saw tensioning'.
We know also that surface work to sheet materials introduces residual stresses (long and boring explanations in previous thread), and that hammering or rolling have such an effect.
I've been doing a bit of rummaging. One of my old textbooks is "Principles of Industrial Metalworking Processes" by G.W.Rowe. In his chapter on "Metallurgical Factors in Metalworking" he discusses residual stresses.
He notes the following, "Residual sresses arise wherever there has been inhomogeneous plastic deformation, but it is important to recognise that they are elastic stresses due directly to differences in elastic strain and cannot exceed the yield stress of the material." (page 322). In plainer English, if you work a piece of metal, you'll get some, but unless the metal changes it's physical shape, they'll always be limited to the yield stress of the material.
Perhaps more interestingly, he also notes (page 323), "After a long time at room temperature, or a shorter time if heated, the residual strain and hence the residual stress will be relaxed by processes involving the diffusion of crystallographic defects."
In other words - tension might not last in a saw blade. It will gradually relax itself, in a process rather like the seasoning of a casting. I've absolutely no idea how long that might take, but it could well be that vintage saws that were 'hammer tensioned' a century ago have long ago lost whatever tension they had by a slow, gradual, natural relaxation of internal stresses.
After much diligent searching, Corneel discovered a German text, dealing with machine saw blades, that outlined a method to check bandsaw blades for 'tension' put into them by hammer or roller during manufacture or servicing. The blade is held in a gentle curve, and a short straightedge applied across the width. The 'tensioned' areas will be revealed by slight out-of-flatness - very slight humps - of the blade when so curved. When the blade is flat, no such humps are detectable. A hump height in the order of about 0.4mm was mentioned, detected by looking for light under the straightedge.
Corneel tried this on a couple of handsaws, and found some evidence that the same thing happening (go to the end of the thread for photographs).
I tried the same, and - rather indistinctly - also found 'something' on a couple of saws, though no evidence on others.
On another forum, Rob Streeper has documented his work to investigate 'tension' by hardness-mapping old saw blades. He found two lines of extra hardness on some vintage saws, one about an inch above the toothline, and one following the line of the blade back. (Those lines correspond to the 'humps' Corneel observed on his saws, leading to the possible conclusion that they were the same thing.) He subsequently hammered a new saw blade along those regions, and observed a slight change in behaviour of the blade in service - it seemed less 'whippy' at the toe end on the back stroke, and more inclined to return to straight quickly.
Additional hardness (slight work hardening, in effect) would be caused by extra hammering along the regions identified, so things do begin to look promising in the hunt for 'saw tensioning'.
We know also that surface work to sheet materials introduces residual stresses (long and boring explanations in previous thread), and that hammering or rolling have such an effect.
I've been doing a bit of rummaging. One of my old textbooks is "Principles of Industrial Metalworking Processes" by G.W.Rowe. In his chapter on "Metallurgical Factors in Metalworking" he discusses residual stresses.
He notes the following, "Residual sresses arise wherever there has been inhomogeneous plastic deformation, but it is important to recognise that they are elastic stresses due directly to differences in elastic strain and cannot exceed the yield stress of the material." (page 322). In plainer English, if you work a piece of metal, you'll get some, but unless the metal changes it's physical shape, they'll always be limited to the yield stress of the material.
Perhaps more interestingly, he also notes (page 323), "After a long time at room temperature, or a shorter time if heated, the residual strain and hence the residual stress will be relaxed by processes involving the diffusion of crystallographic defects."
In other words - tension might not last in a saw blade. It will gradually relax itself, in a process rather like the seasoning of a casting. I've absolutely no idea how long that might take, but it could well be that vintage saws that were 'hammer tensioned' a century ago have long ago lost whatever tension they had by a slow, gradual, natural relaxation of internal stresses.
