Hello Jim, I recently posted this on WoodNet for someone with a bowed hand saw blade:
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If it is a bow/bend without a kink, you can simply bend it gently the opposite direction. Usually you will need to over bend further to the opposite side than it is presently.
I usually grasp the toe and the handle and simply arc the blade to the opposite side. But I as often run the side of the saw blade across one knee, bending the blade, sliding the blade from one end to the other.
On localized bends such as at the toe, you need to be a tad more gentle, but the same principles apply.
Go slow, sight down the blade frequently. To properly sight the blade, have the saw in front of you, teeth away from your body. Hang the saw by placing just a finger or two through the handle. Do not grasp the handle. Look down the tooth line. If you actually hold the handle it is difficult to judge whether you are allowing the saw to hang without the influence of twisting the handle from truly vertical.
Most saws with a bend can be brought back straight in less than a minute or two.
A lot of saws take on a cast such as this from simply being leaned against a wall, handle on the floor. Even at a pretty upright lean, a saw will eventually take on a cast (a bow).
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Now, for what I take as a "hump" along the blade can be simply a more localized bend, in which case the procedure above works fine. If it is due to stretched metal, there is hammering involved. One way to tell the difference between a localized "hump" versus stretched metal is whether this hump goes from the toothline up to the top edge of the hand saw. If it is only at the toothline, then hammering is needed. If it extends up to the top edge, then you can normally bend it back into place.
For the back saw, if it has a kink instead of a bow, the best bet is to simply replace the blade. However, if it isn't kinked metal, one can usually adjust the back (make the back at the toe higher on the saw plate) to straighten out. In these cases, what normally happens is that somehow the toe of the back is forced down onto the plate. Because a back saw blade is held in tension inside the back and at the handle, the back when forced downward forces tension on the toothline and a bow of varying degrees is created.
To drive a back upwards at the toe without removing the handle, I use a piece of wood about 1" square, half again as long as the distance from the toothline to the back. So if there is a usable depth of 3", I use a length of 5" to 6" of 1" x 1" stock. I use a pretty hard wood such as Jatoba, but even Cherry or Oak will work fine. I cut a slot in the middle of the piece of stock a bit longer than the usable depth.
My face vise has a liner against the apron of the bench top to allow this, but if yours does not, simply cut a couple boards to give the needed clearance. The saw is placed in the face vise so as to hold most of the saw plate firmly. The stock of 1" x 1" is slide up onto the saw plate at the toe of the saw so the kerf cut into it straddles the plate. With a good squeeze to hold the stock firmly against the saw plate, use a hammer to tap hard enough to drive the back upwards. Once it moves a little, release the pressure on the vise and retighten. This allows the saw plate to regain some straightness with each adjustment.
Take care, Mike
