Thread: Hammering Hand Saw Blades

For large circular saws tensioning the blade is a common requirement, I worked on sawmills for a bit, ( not on doctoring saw blades however ) they had a guy who would periodically come along and "tension" the blades that needed it. The saw doctor, would hammer the center in a pattern to provide some tension so that when the blade heated up while in a cut the blade would still track rather than become "floppy" in the center and run off track. The saw mill operators described it as a properly tensioned blade would "stand up" through the cut, whereas a blade not tensioned would bend off to one side and not track.

As to how this translates to hand saws, I guess it's the same principle, but more to do with stretching the steel to provide tension along the tooth line.

Full marks to Rob for going to the extra effort.

Originally Posted by Isaac S

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

Isaac,

I think that I've learned that the word 'tensioning' has a different meaning when applied to a backsaw in that the tension is applied to a backsaw blade by the back and in a non-backed blade by the lines of hammering.

Another factor that likely contributes to the snappy feel of a tensioned handsaw blade is that the blade is no longer flat top to bottom. The distortion is not easily or casually visible but if you look at an older saw blade in the right light you'll see what I mean. Analogous to the situation in corrugated tin sheets. Without the corrugations the flat metal sheet is very flexible and it gains a lot of longitudinal stiffness with them.

In my hardness mapping of the older blades that DW sent I was able to measure the pattern of hammer blows on the No.7 plate. The pattern is reflected in my paint strikes made on these plates to guide my hammering.

Cheers,
Rob

Originally Posted by RayG

For large circular saws tensioning the blade is a common requirement, I worked on sawmills for a bit, ( not on doctoring saw blades however ) they had a guy who would periodically come along and "tension" the blades that needed it. The saw doctor, would hammer the center in a pattern to provide some tension so that when the blade heated up while in a cut the blade would still track rather than become "floppy" in the center and run off track. The saw mill operators described it as a properly tensioned blade would "stand up" through the cut, whereas a blade not tensioned would bend off to one side and not track.

And not just heat, but centrifugal forces expands the steel at the rim more than at the center.

Originally Posted by rob streeper

I think that I've learned that the word 'tensioning' has a different meaning when applied to a backsaw in that the tension is applied to a backsaw blade by the back and in a non-backed blade by the lines of hammering.

For all of the words in the English language, it can really be used imprecisely.

Originally Posted by rob streeper

Another factor that likely contributes to the snappy feel of a tensioned handsaw blade is that the blade is no longer flat top to bottom. The distortion is not easily or casually visible but if you look at an older saw blade in the right light you'll see what I mean. Analogous to the situation in corrugated tin sheets. Without the corrugations the flat metal sheet is very flexible and it gains a lot of longitudinal stiffness with them.

I don't think this is a good analogy. The bulk of the stiffness of corrugated sheet metal is due to its cross-sectional shape. The corrugations are orders of magnitude larger than the thickness of the metal sheet. In a saw, any variations are be on the order of 1%, give or take a bit. If there is any appreciable effect on stiffness, is has to come from either internal stresses in the saw plate or changes to the material properties itself.

I managed to get Harvey Peace #1 sharpened and put on the scratch coat today.

Harry Peace no 1 cut test.jpg

Harry Peace no 1 cut test. bJPG.jpg

Some clean-up of the lines, finishing and off to the laser guy.

What a great job...
And thank you for the info on how to tension the blade.
Have an old Disston TH saw, has a slight bend near the tip, same concept should work to straighten it to some degree.

That saw is just beautiful.

I think that this is what happens when a saw plate is hammered and then ground. I've been able to produce this effect repeatedly.hammer tensioning and grinding graphic.jpg

This also provides a degree of understanding about the sequence of steps used by the Disston works described here: http://www.disstonianinstitute.com/factorytour.html referring specifically to the smithing ->grinding -> tensioning (as described above) -> drawing -> blocking -> polishing -> stiffening steps.

The stiffening step, apparently a liquid phase process, is still a mystery. Perhaps a precipitation hardening step?

For all that we know about saws, there is still so much we don't know about how they made saws 100 years ago. We will probably never know.

Some arts truly do get lost, Wootz steel for example (maybe rediscovered?).

Saw making in the 19th century was very much a dark art and most of that knowledge was never written down. Disston began operations before there were standard hardness measuring techniques for example so there must have been a group of experienced workers who knew how hard a saw blade should be and could tell without instruments.
Handsaw manufacture has largely been left to the hobbyists as there's insufficient profit incentive for big industry. Consider that in the years since the effective dissolution of the traditional Disston handsaw production line there have been only two notable technological advances in handsaw manufacture. The first of these is hardpoint saws and the second is slotted backs.

I've learned that the lost techniques can often be re-discovered by applying effort, thought, knowledge and modern tools. Materials science has made enormous advances in the past century and has revolutionized many products in my lifetime. We sawmakers can apply some of these advances to the benefit of our craft.
I think that we can find more ways to improve handsaws, even if it's only really a hobby.

Originally Posted by rob streeper

....The stiffening step, apparently a liquid phase process, is still a mystery. Perhaps a precipitation hardening step?

Rob, doesn't tensioning stiffen a blade? I always thought that was the principal reason for it. This little bit on the page you pointed to intrigues me:

"The blades then undergo the "Polishing" process, then through the important operation of "Stiffening." As to this latter, the different processes and hammering under which the blades have passed, has altered the arrangement of the molecules in the metal and in order to restore the desired qualities and spring they are stiffened in a special bath, which was originated and is known only to Disston."

I guess I'm an old sceptic, but this sounds like a bit of advertising hype to me. Remember the people writing advertising copy are not always technologically-minded, & it's also said that old Henry was at least as skilled a salesman as he was a sawmaker....

Cheers,

Ian,

I think that there are likely red-herrings in every technical description proffered by a company such as Disston that has a stock of trade secrets to protect. I've seen examples of such intentionally misleading information offered in discussions relating to new age saws as well.

I'm struggling to find a way to quantify the differences I can feel in the blades. First, hammered blades feel a bit snappier, despite the fact that they seem to sag under their own weight when held on side about as much as does an un-hammered blade. They also don't oscillate as much as an un-hammerd blade on the return stroke of cutting. Finally they sound different when tapped having a broader sound than does un-hammered steel. The vibrational characteristics are likely due to the introduction of the random array of nodes over the surface of the blade by the hammer blows.

Cheers,
Rob

I agree with the sales hype nonsense. I find it hard to imagine that anything was done to the saw plate after polishing. My understanding is that this is the absolute last step before the handle is attached.

Regards
Paul

Originally Posted by rob streeper

..... hammered blades feel a bit snappier, despite the fact that they seem to sag under their own weight when held on side about as much as does an un-hammered blade. They also don't oscillate as much as an un-hammerd blade on the return stroke of cutting. Finally they sound different when tapped having a broader sound than does un-hammered steel. The vibrational characteristics are likely due to the introduction of the random array of nodes over the surface of the blade by the hammer blows.....

Rob, I would've expected the blades to sag less after the hammering introduced some tension, so there goes my hypothesis. I'm thinking of a growing tree, in which the sapwood is under tension and stiffens the trunk against bending in wind. P'raps you aren't managing quite enough tension yet? Although I've been sporadically working at it, I certainly haven't acquired this dark art myself, so I watch your efforts with great interest. I have managed to get a little tension & changed note into a couple of experimental blades, after a LOT of gentle hammering. I don't have any way of re-grinding, unfortunately, so the last thing I want is to introduce any visible or palpable indentation. P'raps it's not feasible to get a truly usable degree of tension without causing marks, which is why Disston (& you) do the post-hammmering grind? Anyways, you are clearly on the right track, but it may take a while to re-discover dark arts that took a few hundred years to evolve. Fortunately, you have the advantage of being able to make some accurate (& repeatable!) measurements along the way....

Cheers,

The availability of quality 1095 shim stock has simplified backsaw production.
We just need to convince to manufacturers to make some tapered stock for us for hand saws.
Maybe somewhere there is a great pile of discarded tapered shim stock where the rollers slipped ...

I would have expected that the final step of saw making would have been a cleaning process prior to packing and shipping. Did they ship with any protective coating?

I think the snappiness that I'm sensing is what I'm inducing. Remember, after grinding there is less metal to support the blade, and less mass to respond to gravity so the sag test probably isn't the best, or maybe it isn't sensitive enough. The blades do get measurably harder in the areas struck by the hammer and in the adjacent zones of the blade.

As to hammering, I've taken it all of the way and purposely beat the tar out of a blade, the result is amusing. This is a 22" D-8 blade with heavy over-hammering. It adopts a number of interesting equilibrium states.

over hammered d8 5.jpgover hammered d8 4.jpgover hammered d8 3.jpgover hammered d8 2.jpgover hammered d8 1.jpg


Maybe it could be used for cutting corners?