Straightening Bent Handsaw Blades

[Bob Smalser]

The best Western and Japanese handsaws are quite thin yet in competent hands won’t kink in a cut, and the spring steel they are made from is tempered to allow sharpening with files. They are usually taper ground in two dimensions, so the blade’s cutting edge is thicker than the back, and both the back and the cutting edge taper from the saw’s heel to the saw’s toe. The thinner the cutting edge, the greater the taper, and the higher the polish, the higher the saw’s quality, as taper results in less set required for the teeth, aided by the steel’s polish that inhibits binding in the cut. All these features allow for a narrower kerf requiring less sawing effort.

There are two fundamental choices in manufacturing a thin saw that won’t kink. The least expensive choice is to make the saw stiff by using hard steel and disposable blades, because such saws can’t be economically resharpened. The second choice is to temper the saw so that it can be filed, and to stiffen it by tensioning the blade using hammer and anvil.

When a thin blade is struck on an anvil by a convex-faced round hammer, a dimple is created; often so small it can’t be seen by the eye. Steel from the area around the dimple is pulled inwards toward the point of impact, making the steel in the circular area radiating from the dimple stiffer, or “tensioned” on its surface. Hundreds of such hammer blows applied in certain patterns equally to both sides of a handsaw blade can make it stiffer, can true a warped circular sawblade, or can dish a large bandsaw blade to conform to its wheels while at the same time tensioning the cutting edge. Truing sawblades are not low-order skills, and the major saw factories and filing shacks of logging camps and commercial sawmills was where you found them. Today it’s largely done on computerized machines, except for hand saws. Here you either find an old, retired saw doctor who worked for a big mill, a Japanese saw maker still tensioning by hand, or are on your own because there are few references. I’m not going to make a saw doctor out of you today. But I can get you started with some basics to practice with on old sawblades.



This old saw has a 3/8” kink in the area marked in chalk, and before I do anything else to rehabilitate the saw, I’ll remove this kink and true the cutting edge.



The first step is to remove the handle and bend the blade using your hands in as complete a circle as possible….in both directions. This relieves any recent stress put in the saw, and sometimes makes the existing kink worse or reveals additional problems like bow or twist.



The tools I’ll use to remove the kink are a steel anvil and two hammers, both heavy and light, both with slightly convex faces. I’ll mark the areas to be struck with chalk and using and oily rag, keep all steel surfaces clean and oiled to prevent marking the blade.



Removing a kink or bow requires stretching the steel surface on the concave side of the kink, and compressing the surface on the convex side. Before doing either, the saw’s tension at the cutting edge needs to removed or my attempt may make the kink worse. I accomplish this on the concave side by striking along a line running an inch or slightly less upwards from the tooth gullets. Each “X” represents two light hammer hits. I made identical chalk marks on the opposite side of the saw, but I don’t attempt to strike them yet.



On the identical marks on the convex side of the kink, I’ll accomplish two tasks simultaneously using light hits with the heavy hammer. The heavier hammer strikes will both remove the tension from this side of the saw and straighten out the kink by reversing the conditions that caused it, compressing the near side and stretching the far side of the blade.



I sight down the cutting edge to insure I removed the kink and repeat the previous steps if necessary. Once the kink is removed, I tension the cutting edge using identical light hammer strikes on both sides of the saw in turn, around ¼” to 3/8” above the gullets, insuring I don’t strike the gullets or teeth.



When tensioning is complete, the saw should flex in either direction as I did at the beginning, and return to dead straight.

Continued....

[Bob Smalser]

Only after the blade is straight and tensioned do I continue with rehabilitation. This saw was habitually filed without jointing, and instead of straight or crown-breasted, the cutting edge resembles the hooked nose of the Wicked Witch of the West. Accordingly, I must stamp in new teeth. And guess what? 7 new teeth per inch on a 26” cutting edge require 182 strikes of the stamping dies, and a major retoothing usually bows the blade. So as soon as the blade comes out of the carrier, I again bend it both ways and strike 182 blows on the convex side just above the gullets with the light hammer.

[AlexS]

This should go straight to the 'best of' thread. Thanks for yet another excellent post. I have a saw with a curve in the blade, so I reckon I'll give it a go.

[John Saxton]

Thanks Bob for enlightening us on hand saw blade straightening.I agree with AlexS that this should be mpved to the "best of thread" to give it a place where it won't easily be forgotten.

An insight such as this to a craft of which most of us know little about and in the hands of a good saw doctor a saw usually came back well treated but with little thought of the application applied to redeem it.

AGAIN thanks Bob ...a greenie for a wonderful post.

Cheers

[JDarvall]

This should go straight to the 'best of' thread. Thanks for yet another excellent post. I have a saw with a curve in the blade, so I reckon I'll give it a go.

agree entirely. Haven't seen that described anywhere else. Thanks for going to that effort.

[Hardenfast]

Excellent post, Bob. Even those of us who thought we knew a little about hand saws have learnt something here.

By the way, I'd love to own an original old blacksmith's anvil, but have you seen how much these things sell for?

[Bob Smalser]

Excellent post, Bob. Even those of us who thought we knew a little about hand saws have learnt something here.

By the way, I'd love to own an original old blacksmith's anvil, but have you seen how much these things sell for?

This just a small trial balloon to see if I can satisfactorily explain something significantly more complicated than tuning a block plane. This will eventually be a major magazine article on advanced saw filing to go a step beyond all the good basic filing primers out there.

A good filer can crown or change the crown of the cutting edge, make the teeth taller and the gullets deeper to clear damp sawdust better, and even change the fleam or cutting edge bevel at the heel to make the saw start easier. These are features than can only be done by hand.

Filing with a 60-degree triangular file used at a 22 to 45-degree slope from vertical instead of straight across like the filing machines do produces taller teeth with longer cutting edges along with deeper gullets to better clear sawdust.

A crown-breasted cutting edge:



12-pt crosscut sloped gullets with no set, for hardwoods:



8-pt crosscut sloped gullets with light set, for softwoods:



5-pt rip sloped gullets with full set complete with raindrop, which is why I often phosphate blue the blades:



My focus is on boatbuilders who have no choice about hand saws because of odd angles and 500lb workpieces.

[Groggy]

These threads are great, thanks for the effort Bob!

[Hardenfast]

This just a small trial balloon to see if I can satisfactorily explain something significantly more complicated than tuning a block plane. This will eventually be a major magazine article on advanced saw filing to go a step beyond all the good basic filing primers out there.

A good filer can crown or change the crown of the cutting edge, make the teeth taller and the gullets deeper to clear damp sawdust better, and even change the fleam or cutting edge bevel at the heel to make the saw start easier. These are features than can only be done by hand.

I'd love to have a look at the article when it's released, Bob. Saw sharpening & tuning by hand is fast becoming a forgotten art, as is proper function and use of hand planes, all of which is unfortunate. Hopefully you'll be able to let us know when the article is released?

Wayne

[Bob Smalser]

Part II
Lump and Cup Removal





On saws that have been kinked repeatedly or severely, it’s common to achieve a straight cutting edge by hammering and tensioning as I described in the previous installment, but still not have a saw that will cut smoothly. This Disston #16’s cutting edge was hammered straight, but you can see a small bend remaining in the saw’s back and what appears to be a cup or lump remaining that will bind in the kerf during sawing and cause the saw to wander.



A short straightedge run down the blade allows chalking the lump’s outline on the concave side. Turn the saw over and transfer the marks to the convex side of the lump.



Any tension in the concavity is removed using a small, convex-faced hammer, hammering on the marks from the outside of the circle inwards. Each mark represents two light hammer strikes.



Flipping the blade to the convex side, the lump is hammered out using the large hammer in exactly the same sequence and locations as in the previous step.



The blade is checked again using the straightedge and while there are still lumps and hollows, they are smaller. Rub out the old chalk marks and use the straightedge to make new ones, then repeat the same marking and hammering sequence of using the light hammer on the concave side and the heavy hammer on the convex side. Stop when the straightedge makes full contact with the blade.



Then tension the saw as I did in the first installment using the small hammer. Strikes near the edges provide tension to the edges, strikes near the center flatten the saw. If strikes are not made equally in force, location and number on both sides of the saw, the saw will bow or cup, and this is the basic principle used to tension or stiffen the blade and hammer out defects.



Cupped toes can be difficult and often not worth the effort, so I’ll shorten the saw instead.



Sawblade steel grinds to the line easily on the coarse wheel with an occasional water dip to keep it cool.

Continued….

[Bob Smalser]

And the grinding marks easily cleaned up by drawfiling.