The Breasting of Saws
There is a lot going on in the simple design of a hand saw.
David Linnabary in one of the previous comments suggested we discuss breasting on saws.
Breasting, simply put, is a convex toothline. Breasting is used on more than just hand and panel saws, for instance, flooring, whip and pit saws. Here I am only concerned with hand and panel saws.
Breasting ranges from a fairly light to a heavy convexity depending on the saw’s length and PPI. It works in conjunction with the natural oval motion of one’s arm when sawing and has two main effects as concerns the tooth geometry.
Tooth geometry on a straight toothline remains fairly static along the entire length (excepting progressive raked saws). This means the first tooth and the last tooth will have the same rake. While a breasted saw has the same physical static tooth geometry, breasting alters the rake while sawing. Along the toothline, the toe will be a greater rake than at the heel–and saw makers on low PPI saws created a means to deal with this fact by increasing the number of teeth at the toe by usually 2-3 PPI. Why?
Because of the increased effective rake at the toe of a breasted saw, reentry into the cut using a coarsely-toothed hand saw could hasten kinking of the saw. In general, the higher PPI of a hand saw, the less this PPI change. This is because more teeth are engaged in the cut already. Often, saw makers also lessened the amount of breasting the higher the PPI and or the short length of a saw. For instance, on a 4 ppi rip the amount of breasting is more than on an 8 ppi cross cut and there is also a larger increase of the PPI on the former than the latter example.
Effects while sawing with a breasted saw include a more aggressive nature at the toe and a corresponding relaxed rake resulting in a less aggressive cut at the heel. Can you feel these effects? Yes, you can. However, this effect passes by generally unnoticed if one is sawing for all they are worth.
The natural motion while sawing is a slight oval shape. Embrace the oval. It is the most efficient means of sawing quickly with a hand saw. A purely straight motion is not only unobtainable, it is counter-productive to the geometry of breasting. The slight oval motion more or less equalizes the rake through the middle portion of the stroke making for a fairly consistent resistance. The lessened pressure of an oval motion at the beginning and ending of a stroke aids in not bending the toe and changes the effect of the handle hang. Huh?
Usually the longer the saw the lower the handle hang. Handle hang affects where the force of the sawing motion “over centers” during the forward stroke using a western-style saw. The relaxed rake at the heel of the saw and the breasting aid in allowing a full, forceful stroke using the full, or nearly full, length of the saw and lessens the “grab” at the end of the stroke where the power of the stroke is lessening.
Next up for the blog? More on fleam and rake for cross cut and rip filed Western saws…
Take care, Mike
Posted in Sawing, Tool Information
July 15th, 2009 at 6:02 am
Mike,
That explains a hell of a lot. Both my hand saws (not great quality, but they work well) have breasting and I’ve noticed that in the pull stroke when the toe is almost in the wood, not only does the saw sometimes kink (at the beginning of the subsequent forward stroke) but it rattles and vibrates at the toe as it’s pulled back. What can I do about this rattle? It feels like the saw is slapping from side to side at the toe as it’s pulled into the wood.
July 16th, 2009 at 7:48 am
Hi Jim,
Vibration when I am sawing is almost always due to my not withrawing the saw fully in-line with the kerf. Often when I am sawing quickly, my hand either twists slightly or my hand drifts to one side or the other from the kerf. Either of those situations will cause the saw to vibrate.
Another cause can also be too much pressure on the backstroke. If the teeth remain engaged with much pressure against the bottom of the kerf, the washboard effect of the teeth in the cut can also cause the effect. Especially when combined with those things I listed that I often enough do.
As for the bending at the toe and presuming the saw is sharp, if this is a cross cut, is to increase the fleam angle and or rake angle at the toe and “blend” it in to the stronger portion. This will allow the teeth to more readily slice into the fibers. With a rip, the addition of a little fleam at the toe will also allow the same slicing effect.
Take care, Mike
July 16th, 2009 at 12:03 pm
Thanks for that. I’ll try your advice when they next need sharpening.
Jim
July 29th, 2009 at 6:46 pm
Hi Mike,
I have found that the annoying flapping at the toe end on the backstroke can sometimes be caused by poor setting of the teeth. Have you encountered this?
This is my first time reading your blog. This will become a favorite place for people to come for their saw questions to be answered.
Good luck budgeting your time. *grin*
Marv
July 30th, 2009 at 5:50 am
Hi Marv–welcome!
Yes, excessive set also causes the toe to flop around on the backstroke. Especially in less soft woods. In softer woods, the saw can advance so quickly the ridges and loose fiber left behind can lessen the effect.
Time? What’s that?!
Take care, Mike