Why do files and saws work the way they do?

G

GarageBoy

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May 23, 2003
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What's the physics behind a saw or a file? A knife I can see, slices through, but does a file simply grab material and rip it off? Thanks
 
The tooth of a file or saw blade is just a miniature chisel. It slices a thin curl of material off the surface. The cut is kept shallow by all the teeth in a row. A thin surface slice does not get wedged into the material since the thin surface material has nothing to push it back down as it is sliced. If you had to push deeper into the material the tooth would get wedged.
 
At a deeper level, the abrasive edges of a file--like the teeth of a saw--produce friction. This heat (even with a few strokes) is enough to break apart the molecules of the object being filed or sawed.

This makes them easy to disrupt in larger chunks, per Mr. Clark's explanation above.

Obviously, as you continue to work the file or saw, the heat buildup can be quite noticeable.

Can't imagine what tavern-side conversation produced that wager... :D
 
You are not ripping apart molecules, some of the materials filed don't even have molecules. Essentially on a basic level it is all the same thing, a saw and file and knife all cut in the same way, they just deal with removing the material cut in different ways. Saws are designed to clear the wood that is cut so the teeth can continue to cut, files do similar.

-Cliff
 
Heat has nothing particular to do with the cutting action. Unless you run a power saw you don't generate enough heat sawing wood to char the sawdust created. Likewise you can file steel, aluminum, or magnesium and you won't burn or oxidize the chips. You can file cold metal with a cold file just as easily as warm metal with a warm file. If you preheat the metal with a forge you would see a difference, but that is a whole different ball game. If you use an abrasive cutt-off saw heat does come into play, but not a toothed metal saw or file.
 
PS. The heat that you generate when sawing does not come from friction. This heat is released by breaking apart the metallic bonds holding the metal together. When you break the electrostatic bonds holding the material together there is sort of a snap-back effect like cutting a rubberband. The material is thrashed with vast numbers of displaced ions and electrons randomly vibrating. We call this kind of random vibration, "heat". The heat is related to the work function to remove electrons from the material. So when you are sawing you expend muscle energy to do mechical work to break the metallic bonds which translates the energy into heat.
 
Also the file or saw if usually harder than the material being filed or sawed. Otherwise the reverse process occcurs and the tool becomes dull, but that's another story...
 
Thanks! Why do saw blades have HSS teeth and carbon steel base? Won't the teeth just fracture?
 
They fracture if they hit something hard, like if you hit a nail while ripping wood you can break off the carbide teeth on a skillsaw blade, but they won't break on materials they are meant to cut as they are ground with enough thickness to give them the necessary durability. If you draw the temper on a HSS hacksaw blade it actually gets more prone to damage as it is much weaker.

-Cliff
 
This article discusses in detail how to sharpen handsaws used for cutting wood.

http://www.vintagesaws.com/cgi-bin/frameset.cgi?left=main&right=/library/library.html

Since one must know how a saw cuts in order to sharpen it effectively, it includes what I find to be an excelent illustrated description of how such tools operate.

I suspect that the basic principles apply to files, rasps, and the like, though the combination of cheap-to-manufacture disposable blades and essentially unlimited power (vs hand-operated) makes me suspect that the old hand tools likely are more efficient and higher quality. Chainsaws and some industrial metal-working tools seem to be an exception.

The same is probably true of many hand tools as well. Has anyone had a handsaw sharpened, or know where to get it done?
 
I've had hand saws resharpened and reset here in the UK. My local ironmongers offer the service for just about any hand tool you care to mention from chisels and plane blades to saws and axes. As you need a "saw set" jig and the right tools to do the job properly on a decent saw, I have them do it for me when my grandfathers old saws need doing. My grandfather was a Master Finisher, and made many of his own chisels, but the saws I inherited from him are the best quality available in their day (Probably better quality than I could afford or even perhaps find now).
 
Saws are able to make cuts straight cuts (or grooves) into a material because all saws have a kerf.....This means, the width of the cutting teeth are either equal to or greater than the thickness of the blade, allowing the blade to follow the teeth down through the cut without binding. You can readily see the kerf on the wood saw of your SAK or Leatherman tool. In general, offset teeth which cut with their points, are more efficient for cutting wood and rows of parallel teeth work more efficiently for a metal saw. The blade of a hacksaw has side-to-side ripples down its length to create a kerf wider than the thickness of the blade so as to inhibit binding in the harder metals.

Normally saws are used to cut grooves and files are used on the surface of materials. Both cut with teeth which need to be pushed against the material as well as pulled or pushed. Some work both ways, depending on the set of the teeth.

Most heat produced in sawing or filing (or drilling) represents wasted effort. When sawing hard metals, sharp teeth and the use of a good lubricant speed up sawing, lessen effort and reduce the buildup of heat. When sawing wood or filing, sharp teeth reduce effort and the production of heat.

Knives have cutting teeth too but knives have no kerf. Their shapes come in variations of a wedge. They work by sawing, pushing material aside, or a combination. Push cuts (steady force) and chops (percussion) get the cut started with the thin edge and then wedge the material apart. Push cuts usually work best on thin materials or near the surface as when one sharpens a stick. Draw cuts use the teeth in a sawing action. If one attempted to use a knife to saw a thick, hard material like an oak branch, it would shortly bind in the cut because the intial cut, made by the edge, cannot accommidate the progressively thicker wedge of the blade. This is just as true in the case of a serrated knife blade.

In general, knives work better on softer materials and saws and files on harder materials although their is some overlap. A knife cannot cut a neat groove through a thick piece of wood (Although you could conceivably baton through it or gnaw through it with dint of much hard labor) and a saw or file would be inefficient in whittling a tent stake, compared to a sharp knife. There are endless examples.

BTW, molecules, which are combinations of elemental atoms, held together with electromagnetic force, are so small, that their destruction or formation is in the realm chemical reaction and not tools or cutlery, which work on much larger particles consisting of molecules (e.g. wood) or atoms (e.g. iron).
 
Thanks! I got a nice swedish saw here that some worker borrowed and proceeded on ripping out 3 teeth. The blade is shiny, but the teeth were blue?
 
Forget what these numbknuts have to say about physics and machinery.
It's Hogwarts induced Magic declassified for us Muggles...
 
All those tools , knives, saws, files are simply the simplest tools known to man the WEDGE ! ...Modern woodworkers hand saws are now disposable when the teeth are worn !!.....Wood working saws are made either for cutting across [crosscut] or with [rip] the grain .The teeth are shaped differently.
 
Also note that the saw or file must be made of a harder material than whatever it's cutting, or you'll simply grind the teeth down.
 
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