Bow saw blades - why not make symmetrical, so they cut straight?

Factual Questions
1

Every year I fight with a bow saw to cut down a Christmas tree. The bow saw wants to cut a circular path with a radius of about three inches. So if it starts horizontally, if the tree is large, the blade curves upward as I try to cut right-to-left, and gets vertical or completely binds before reaching the other side.

You can see the functions of the teeth in the blade. There are teeth that point forward and backward, and have planer-like edges that cut wood from the leading edge of the kerf. There are also triangular teeth that alternately lean right and left, and they are sharpened like a steak knife, with a bevel on one side, so the cutting edge is all the way at the wall of the kerf.

The thing is, the blade doesn’t have symmetry about the plane of the kerf. After the forward and backward planer teeth, it’s always left first, then right, then left, then right. I think the triangular teeth get different access to the wood depending on how far back they are from the planer teeth. If the teeth pointed forward, backward, left, right, left, right, forward, backward, right, left, right, left, and repeated that pattern, then the blade would not have any sidedness and would cut straight.

If I maintain the saw at a constant angle, in a smaller tree trunk it will curve clockwise and go out the far side at an angle. In a larger tree trunk, holding the saw at a constant angle will try to make the cut straight at the surfaces, but in the middle of the trunk the kerf is taking on a saddle shape, a negative curvature, until the flexing of the blade is so severe that I can’t push it any further.

The sidedness in the stroke is because when I push I’m driving the blade further into the wood, whereas when pulling I am taking it out of the work. I try to compensate by trying to force the far end of the blade harder into the wood on the pull stroke, but this is quite difficult - the angle is awkward, and when lying on the ground working under a pine tree it’s hard to use the closer end of the blade.

Why don’t they make symmetric blades?

2

I’m having a tough time following the description of your troubles, (For some reason I just can’t visualize this) but I’ve used bow saws for a ton of work (I was a landscaper for many years) and I’ve never had trouble with them as long as I had room to keep the saw level. If you don’t have that kind of clearance, then use a pruning saw. (Stiffer, curved blade that requires less clearance)

The only times I HAVE had trouble is if the blade is loose for some reason, the saw is dull, a tooth is bent, or the blade is not on perfectly straight. Then, they will curve no matter how much experience you have!

3

I agree. I have a bow saw that I’ve used for many years and it always cuts straight.

4

They start off with symmetrical tooth offsets. What happens is that one side of the blade grazes something impervious, such as a nail or embedded piece of gravel and it deforms (below the hardened point) all the teeth that protrude out to that side. From that moment on, the blade wants to cut an arc.

5

Sounds to me like you’re not keeping the blade and the handle in the same plane, thus twisting the blade and making it wander off.

6

I cut down maybe 30-40 trees a year and never experience what you are describing, and some of the blades I have to use can best be described as UGLY. What size saw are you using? I prefer a small 21" saw if possible as there is much less flex in the blade and a tapered nose helps in tight spaces.

7

Hmmm.

I use 36" Skodko (sp?) bow saws mostly, but have some smaller ones. The smaller ones don’t really test the problem, as they don’t fit around wood that is large enough for the curvature to be a problem.

The problem appears for brand new blades as well as old.

As far as I know the teeth go equally far right and left. I follow the “one side of the blade grazes something impervious”, but I’m having this problem even with blades during their first use.

If I hold the bow in a constant plane, when I start the cut, it cuts in the same plane, but eventually is deviating further and further, which twists the blade. But the bow is trying to twist the blade back into the original direction of the cut, not further from it.

If I can go back and forth on either side of the work - that is, say I’m cutting something that is resting in a sawbuck, and I cut a few strokes from the north side, then walk around and cut a few strokes from the south side, the path will wander gently back and forth, and work out OK, because the angle is never very sharp.

What is especially noteworthy is that if I really work hard at it and make the cut go very far in a big piece, the middle of the cut will curve more than the entrance and exit parts of the cup, so it is creating a surface like part of the bell of a trumpet; that is, it will deviate from a cylindrical surface by flexing the blade first out of the plane and then back in.