Thread: Hammering Hand Saw Blades

Thanks for the post Rob. Did you hammer both sides equally to get rid of the curvature in the 26" blade?

I frequently find that both sides need hammering. My preferred method is to hammer first the convex side. If the plate straightens up I check the droop. If the droop is uniform on both side I quit. Sometimes I find that I need to hammer the convex side until it becomes slightly concave then I flip the blade over and hammer the formerly concave side to bring the plate back straight.

Wow. This is an intersting thread. A lot of work too

Dave TTC
Turning Wood Into Art

Originally Posted by DaveTTC

Wow. This is an intersting thread. A lot of work too

Dave TTC
Turning Wood Into Art

Thanks Dave. This is fairly easy to do though the 0.042" steel is a bit of a work-out. I didn't document all of the blades I did yesterday, only two of the three Harvey Peace type. I also cut blanks for and hammered 6 blades in 0.035" steel. The other blades were not profiled or ground however.

I see there is something for me to learn here

Dave TTC
Turning Wood Into Art

Is the hammering that you show being done to "tension" the blade or to flatten it?

Originally Posted by Isaac S

Is the hammering that you show being done to "tension" the blade or to flatten it?

Hi Isaac,

The hammering is doing both as best I can tell. Flattening can be accomplished by simply hammering out the curve from one side only. As I mentioned above, I often find myself needing to hammer both sides. After doing this I can see changes in the droop of the tip of the blade. In addition the blade feels 'snappier' when flexed. The tensioned blades also exhibit better resilience when flexed to extremity.
In my thread on blade hardening I also measured a stress induced increase in apparent hardness in areas adjacent to the areas that are hammered. This stress hardening is better known today in the guise of shot-peening, sometimes used to work harden the connecting rods of engines for example.

Cheers,
Rob

Assuming for the moment that the hardness is affected by the hammering, what does that translate to in use? I guess I am having a hard time seeing what this does beyond flattening the blade.

Shot or hammer peening, as I understand it, results in a compressed layer of material at the peened surface. This layer of compressed material counteracts applied tensile forces, preventing tensile cracks from forming on the surface. Every reference I have found says that peening is used for stress relief and fatigue resistance, and does a very good job of that.

So far, I understand all of this, at least at a beginner's level. Here's what I've always had a hard time understanding. There are two material properties that are of particular interest to me as a potential handsaw maker: yield strength and Young's modulus (stiffness). Yield strength dictates the degree to which a saw can be bent before being permanently deformed, while Young's modulus tells me how hard it is to bend it that far.

I can sort of see that peening will increase the yield strength through work hardening. My only reservation is that the hammering is done on such a small portion of the blade that its overall effect can't be very large.

Where I have real problem understanding is how hammering increases the stiffness. Young's modulus (for steel) is practically independent of alloy, hardness, and yield strength. I have never seen a claim that peening has any effect (in a quick online search, the one reference I found showed no effect). I'm not sure how "snappiness" could be measured, but my best guess is that it is related to stiffness.

In your pictures measuring the droop, one side measures about 1 3/4", and the other about 4 1/2", although that measurement looks like it may actually be a bit less. That gives a total droop of approximately 6 1/4", or an average of 3 1/8".I'm not sure how careful you were taking those measurements, since they were just for illustrative purposes. But if those are indeed fairly accurate, I would say that you haven't changed the stiffness to any meaningful degree.

To me, a really interesting experiment would be to take a piece of spring steel (hopefully flat, but I'm not sure if that is necessary) and subject it to a three point bending test. Then take that same piece of steel and hammer it just as you have done, then repeat the test to see if stiffness has increased or not. I have no access to the testing equipment needed to do this, but might be able to rig something. I'm thinking of two cylinders at a fixed distance apart that the test strip can rest on, with a dial indicator beneath the strip at the center. The only way I can think of to load the test strip is with fixed weights. If you're up to doing some hammering, I'd be willing to try setting that up. I can supply a few test strips.

http://www.crosscutsawyer.com/viewtopic.php?f=4&t=750

Originally Posted by Isaac S

Assuming for the moment that the hardness is affected by the hammering, what does that translate to in use? I guess I am having a hard time seeing what this does beyond flattening the blade.

Shot or hammer peening, as I understand it, results in a compressed layer of material at the peened surface. This layer of compressed material counteracts applied tensile forces, preventing tensile cracks from forming on the surface. Every reference I have found says that peening is used for stress relief and fatigue resistance, and does a very good job of that.

So far, I understand all of this, at least at a beginner's level. Here's what I've always had a hard time understanding. There are two material properties that are of particular interest to me as a potential handsaw maker: yield strength and Young's modulus (stiffness). Yield strength dictates the degree to which a saw can be bent before being permanently deformed, while Young's modulus tells me how hard it is to bend it that far.

I can sort of see that peening will increase the yield strength through work hardening. My only reservation is that the hammering is done on such a small portion of the blade that its overall effect can't be very large.

Where I have real problem understanding is how hammering increases the stiffness. Young's modulus (for steel) is practically independent of alloy, hardness, and yield strength. I have never seen a claim that peening has any effect (in a quick online search, the one reference I found showed no effect). I'm not sure how "snappiness" could be measured, but my best guess is that it is related to stiffness.

In your pictures measuring the droop, one side measures about 1 3/4", and the other about 4 1/2", although that measurement looks like it may actually be a bit less. That gives a total droop of approximately 6 1/4", or an average of 3 1/8".I'm not sure how careful you were taking those measurements, since they were just for illustrative purposes. But if those are indeed fairly accurate, I would say that you haven't changed the stiffness to any meaningful degree.

To me, a really interesting experiment would be to take a piece of spring steel (hopefully flat, but I'm not sure if that is necessary) and subject it to a three point bending test. Then take that same piece of steel and hammer it just as you have done, then repeat the test to see if stiffness has increased or not. I have no access to the testing equipment needed to do this, but might be able to rig something. I'm thinking of two cylinders at a fixed distance apart that the test strip can rest on, with a dial indicator beneath the strip at the center. The only way I can think of to load the test strip is with fixed weights. If you're up to doing some hammering, I'd be willing to try setting that up. I can supply a few test strips.

During my hardening experiments I happened across this reference: https://en.wikipedia.org/wiki/Bauschinger_effect

My understanding is that the irreversible compressive deformation of the steel that is hammered places adjacent regions under stress.

Originally Posted by planemaker

http://www.crosscutsawyer.com/viewtopic.php?f=4&t=750

I've seen that too. The crosscut saw enthusiasts also have discussed the use of English wheels for blade tensioning.

Making the most of long summer days. Above is Mr. Wenzloff's version, below is mine. 5 1/2 ppi, 4^o rake, mahogany handle, 3X large screws, medallion and stainless bushings. Both saws have breasted toothlines, the Wenzloff is filed crosscut and is, if I remember correctly, 6 1/2 ppi.

Harve Peace comparison.jpg

Originally Posted by planemaker

http://www.crosscutsawyer.com/viewtopic.php?f=4&t=750

Thank you very much for that, Stewie. I stumbled upon that forum last year when I was thinking about this very topic, but do not remember finding that thread. That is the first explanation I have seen or heard that makes any sense to me. Specifically, this quote from Diabolo was very enlightening:

Tensioning is expanding the central portion of a blade. Thus the belly of the saw blade is under compression, whereas the toothline and the back are under traction. On a circular saw blade, tensioning compensates the mechanical and thermal dilatation of the toothline under operation. If not, it becomes "too long", and the saw blades is unstable and wobbles in the kerf. On a handsaw, this is for compensating the thermal dilatation of the toothline (my emphasis). It doesn't warm a lot when cutting, but it is enough to make the saw cut less straight because the blade is thin.

I am aware of this phenomenon on backsaws, but never made the leap to this happening in a handsaw.


Nice looking saw, Rob.

Your welcome Isaac.

Stewie;