Can you retension a saw blade?

[Dennis McDonaugh]

I bought a pretty nice Disston nr. 12 panel saw off of e-bay the other day. The blade was dark, but not rusty or pitted and the handle was pretty pristine. However, the blade was wavy and not nearly as rigid as most handsaw blades I've seen. I think it had lost its tension due to misuse. I contacted the seller and he took it back, but I'm wondering if retensioning a blade is something you can attempt at home.

[Bruce Shiverdecker]

Sorry to seem a little dense, but do you mean the "Temper"? The only types of blades that I'm aware of that require "Tensioning" are Bandsaw, copingsaw, Scrollsaw, and others like them, NOT handsaws. Keep in mind, I'm NO expert here.

Bruce

[Tim Leo]

I bought a pretty nice Disston nr. 12 panel saw off of e-bay the other day. The blade was dark, but not rusty or pitted and the handle was pretty pristine. However, the blade was wavy and not nearly as rigid as most handsaw blades I've seen. I think it had lost its tension due to misuse. I contacted the seller and he took it back, but I'm wondering if retensioning a blade is something you can attempt at home.

I picked up a saw the other day on the bay, and when I got it I found the blade to be kinda 'floppy', i.e. it had no stiffness or tension to the blade.

Maybe some of the others with blacksmithing skills can help?

[Dennis McDonaugh]

Sorry to seem a little dense, but do you mean the "Temper"? The only types of blades that I'm aware of that require "Tensioning" are Bandsaw, copingsaw, Scrollsaw, and others like them, NOT handsaws. Keep in mind, I'm NO expert here.

Bruce

Bruce, a handsaw has to be tensioned or it doesn't have enough stiffness to withstand the thrusting of the cutting action. An untensioned handsaw blade will be very floppy as Tim observed below. Originally, the blades were tensioned at the factory by hammering. I have no idea how to go about it though. The saw I was looking at had a very pronounced "S" curve to it and you could move the curve from side to side by simply grasping it in two fingers and tugging on it.

[Dennis McDonaugh]

Here's a section of an article on the Disstonian Institute web page.

"The next step is that of "Smithing." in this the blades are flattened and made perfectly straight, all inequalities being taken out by the skillful hammering of the mechanics.

The blades are next "Ground" to gauge and to a taper so that the back will be thinner than the cutting edge. The back of the hand saw blade is ground to taper from the teeth to the back and from handle to point, the tooth-edge being of even thickness from end to end. A saw not ground to proper taper cannot be ranked as a high-class tool.

After being ground they are returned to the smithers for "Looking-over" and preparing for next operation.

Now follows "Tensioning." In this the blades are hammered so that they shall not be too "fast" or too "loose;'' but shall possess the proper tension, spring or character. If the blade is what is termed "fast" the metal is too long on the edge and needs expanding through the centre, or, if too "loose" the metal must be stretched on the edge. A saw not properly tensioned will run out of its course, in other words it will not cut straight and true. "

[Bruce Shiverdecker]

OK! Thanks for the info. I think????????????? I understand, but have a friend that is a blacksmith. Maybe he can get through the fog between my ears.

Bruce

[Dave Burnard]

I have an old western sawmakers anvil that was used to do that tensioning on both hand saws and circular saw blanks.

A saw can be retensioned, (assuming it's not mangled) but there aren't many people left in the world who can do it. High quality handmade japanese saws are tensioned by hand, lesser saws are just tweaked with hammers that make sharp little pecks in the surface - but that's not really tensioning - more like stress relief or local detensioning.

I'm know some enterprising galoots out there have dabbled with retensioning and/or saw straightening, I remember Pete Taran did and I've read some posts about it from other galoots on forums. On the japanese saw side, Mark Grable and Scott Nehring can also do some retensioning and straightening when they re-sharpen a saw.

It is time consuming though, and in Japan one of the key things that separates a $200 saw from a $500 saw is the amount of time it takes to tension the blade. You can actually see the difference on the surface of the blade. Since time is money, and fewer people can either afford or appreciate the difference, there aren't many makers out there who still hand tension their saws. Yataiki (Miyano Dai Endo) still hand scrapes, hand tensions, and hand sharpens all of his saws. He is the last sawmaker still working this way... :(

[Dennis McDonaugh]

Dave, it looks like something I probably won't want to try on a saw worth anything. In this case, I think I did the right thing by sending it back since the saw is unusable in its present condition.

[Bob Smalser]

Truing and tensioning a circular saw blade is difficult, and doing it by hand was artful. These days it's done by a machine that spins the blade, measures the runout with an integral dial indicator and hits them with a hammer/anvil integral to the machine. You can see the marks when they come back from the sharpeners. A blade even slightly out of true gets worse fast at 3000 ft/min blade speed.

Hand saws, however, are easier, aren't as critical, and there's no reason you can't fix yours at home. If the saw ever kinked during a cut, it probably needs retensioning, and those mysterious small dents in old saws are often from mishits at the sharpening shop.

Striking the blade on an anvil causes a dimple that pulls the steel toward it a tad. The further you get from the dimple, the greater the tension caused, and the closer, the least. Dimples on one side without identical dimples on the other will cause the saw to pull to one side.

http://pic3.picturetrail.com/VOL12/1104763/9131162/128060614.jpg

So the trick is uniformity. Mark both sides of the saw identically near the center of the blade, and alternate sides as you strike. For thin hand saws, a 6oz ball peen hammer is ideal. I used three raps per mark on this #12 blade, and I also rapped between my chalk marks. Some hammer in successive lines all the way from back to edge for max edge tension at the expense of back stiffness, but as I use saws often in awkward positions, I prefer more uniform stiffness with less danger of kinking.

If the edge is badly bent, then hammer a line close to the edge to remove all tension prior to straightening it....then work your way upwards after you hammer the kink out. You'll be much less likely to snap off a tooth that way.

Flex the saw prior to hammering to get a feel for how loose it is...hammer it as I describe...then flex it again and you should feel a difference. Be careful to keep the hammer face flat as you hit and you won't dent the saw.

[Dave Burnard]

Nice Bob, just what the doctor ordered.

Or is it just a little scary... Say if someone next mentions that ancient Inca headache surgery are you already prepared with the do it yourself pictures? ;) "I find a Spoon bit to be the safest because it's so easy to puncture the Dura Mater with the lead screw on a Jennings or Irwin style bit." :eek:

[Dennis McDonaugh]

Thanks for the primer Bob. Am I correct in thinking that a "floppy" blade needs retensioned?

[Bob Smalser]

http://pic3.picturetrail.com/VOL12/1104763/9131162/128125548.jpg

Just so happens I'm writing an article this winter on hand saw selection and restoration.

Yes, hammering down the center equally on both sides will stiffen a "floppy" blade. Picture in your mind how dimpling affects the steel and you'll see it can't do anything else.

[Dave Burnard]

Should be a good article Bob, most stop at electrozapping the rust, so yours will add to our collective wisdom.

The saws that are still hand tensioned in Japan, are of course hammered all over the surface of the saw with zillions of overlapping hits with a polished hammer face on a polished anvil surface. In the end you can't see any individual hammer marks as they all blend together. The net effect is a saw that is significantly stiffer than an untensioned blank of the same material. Too much overall tensioning, or uneven tensioning (too much locally) and the blade will be more susceptible to cracking.

Here's a photo of the tools used in the tensioning process (this was taken at a demonstration here in the US back in 1993)

31055 click and it get's bigger, Whee!

Notice the two anvils, one steel and the other wooden (which has a different effect since the dimple won't be so one sided). You can see the the two main hammers used - with lighter colored handles - the two pictured are for use on larger saws. One has a round face and a cross pein face, the other has two diagonal pien faces. In use the handle is held at 45 degrees to the long axis of the saw (sideways, not tilted up) so the cross pein on the first hammer is at 45 degrees to the length of the saw and can be used to remove twists, The diagonal pein hammer's faces then line up with the length and width of the saw and are use to remove cupping across or along the length.

In this photo, you can also see the tools used to scrape the blade to the proper thickness profile. The scraping table is the inclined bench at the top left, and you can see several of the two handed scrapers, called sen, used to do the final thicknessing on the hardened(!) blade. You can also see several sen with very narrow blades which are used to cut the hollow (ura) in the backs of small chisels and woodworking knives. You don't see that done much anymore - the hollows are nearly always cut with a grinder now unless the smith wants to make a decorative shape.

[Bob Smalser]

Very useful, David...thankyou.

Here in big timber country we still have a few old timers who trued circular saws for the old mills and misery whips for the logging crews, and workers who do similar at the modern Simpson mill at Shelton, doing this for their living. The Sunday breakfast crowd here at the Camp Union Cookhouse includes such a saw doctor now in his '80's, and while he can't do it any more, he can certainly teach. They also still use wooden anvils to abate the hammer marks, as I understand the old handsaw factories like Disston also did in the past. It's been established that the same steel was used in those old saws....I'm certain the only difference between a Disston 120 with a .0033" edge and a Disston 7 with a .0042" edge was the tensioning.

Your craftsmen are making fine saws from scratch, and I wouldn't presume to aspire to such skill involved in tapering and tensioning a blade. But we really don't have to with the amazing number of fine old handsaws still out there in the 30-dollar range...we just have to know how to restore them....and a saw that won't cut true often needs more than its set adjusted.

[Dave Burnard]

We are indeed fortunate that so many of those saws (and other hand tools produced in that era) have survived and that it's not too late to learn from the old timers who used to work on and with them. Keep up the good work.