|Some Theoretical Issues|
|Copyright (c) 2002-11, Brent Beach|
I hope they clarify many of the issues related to sloping gullets and to saw filing in general. Let me know if you have any quesitons about any of the drawings.
My interest in Saw Filing arose first from a desire to use older saws, all of which were dull when I bought them. Since there was no Saw Filing shop in my area it was learn to file or don't use older saws.
Initially I filed using only a file handle and various visual aids. I had a piece of paper with lines at the bevel angle attached to the saw vise. Also, a block of wood on the file tip to maintain the rake angle. The results were poor.
I eventually found a used saw file holding jig that was a lot like the shop made jig I use today. It had a bevel angle setting adjustment that would not stay set. I eventually built my own version of that jig, then a couple of more versions. These pages include the jigs and how to build and use them, along with other material I discovered along the way.
A short History shows some patented saw filing jigs.
This section ends with a short section explaining The Problem -- why saw filing is difficult.
You can right-click on the front view, then click View Image to see an explanation of how a crosscut saw works.
There are two actions: first, sever the fibres, second, push the severed fibres back and forth until they break off as sawdust.Rip This is a standard filing for a rip saw. The rake is 5 degrees, the bevel is 5 degrees. The model is to scale - a 6 ppi (5 tpi) saw that is 0.04 inches thick.
This saw is used for cutting along the wood fibres. Since you are moving along the fibres, the tips of a crosscut have no fibres to sever - they simpl slide back and forth withuout cutting.
There is only one action: these teeth act like chisels, peeling shavings as they move along diagonally through a wood fibre.
A 26" 8ppi saw (ppi = points per inch) has over 180 teeth. Filing involves putting the file in the gullet between two teeth and shaping the front of one tooth and the back of the next tooth in one operation. So, you really file 180 gullets rather than 180 teeth. Usually you file every second gullet from one side of the saw, flip the saw end for end and file the other gullets from the other side.
To file 180 gullets in 7 minutes allows less than 3 seconds per gullet. That kind of speed requires talent and practice. I never come close to that sort of speed - 5 times as long per gullet is a good speed for me. For most people starting out, you should expect to spend at least half an hour on a 26" saw.
While this level of skill and speed is possible, most people would not achieve either unless they file saws for a living. The problem seems to be that the skill is easily lost between saw filing sessions. By the time you are back in the groove, you have already spoiled half the teeth on the saw.
Using this shop-made jig will reduce the number of variables to a manageable set, even after a long interval between uses.
Even with the jig, saw filing takes time. I can hone 10 planes irons in the time it takes to sharpen one saw. It is best to set aside some time, set up your sharpening station in good light, take it easy as you work through the saw. Look here for the complete saw filing process.
Elias Roth patent, 1876
This saw-filing jig was patented in 1876, patent number 173,866, by Elias Roth, of New Oxford, Pennsylvania. The circular part, B, allows you to set the bevel angle, the temporary guide E to set the rake. The jig slides in slots on the sides of the clamp that holds the saw, implying no ability to change the file slope.
While much praised in articles of the time, this jig did not get saw filers beating a path to Mr. Roth's door. There are many patents for variations on this tool, many much more complicated and none which you can build quickly out of wood.
I had seen pictures of similar jigs and finally realized that the arms are curved for a reason - they allow a full use of the file even when using bevel angles closer to 45 degrees. My Version I got pretty cramped in those cases.Disston December 12, 1893 In 1893, Disston added a filing guide to their standard saw vise, the Disston No. 3, and got a patent for it.
In their 1902 Lumberman's catalog, they advertised this as Especially Designed to Assist Those Not Skilled in the Art of Saw Filing to File a Saw Correctly. I wonder about the psychology of that ad. A person buying this first has to admit to themselves they are not skilled in the art of saw filing. They would have to put aside the advice of their pals who said that real men file freehand. I wonder if Disston shipped in plane brown wrapping paper?
The jig had only three settings. Two corresponding to crosscut filing, one to rip filing. The two cross cut settings were actually the same setting, one for filing from each side of the saw.
There was no help for setting the rake angle. You loosen screw in file handle until file gives the shape tooth wanted. Not very clear, but I guess even those not skilled in the art of saw filing could figure out what Disston intended. Perhaps people who bought this guide made up some small wooden blocks to help them set the rake. This jig does not allow for sloping gullets.
The same ad appeared in later editions up to the 191 edition with only minor changes to the copy.Jame Speed patent, 1950
The first saw filing guide I encountered in the wild was based on this patent by James Speed - the Speed Corp patented Saw Filing Guide. Patent number 2,495,991.
The rake angle is controlled by the rotation of the file. You turn the file handle, #17, then tighten the nut, #21, to hold that angle.
The bevel angle is set by rotating the whole assembly including the upper disc #10 about the lower disc #1 sits on the teeth. The arm #13 on the upper disc has a small boss #14 on the underside which catches small holes #15 in the lower disc.
If you exert any force while filing, or if the file catches, the discs rotate pretty freely, giving you an unexpected new bevel angle.
I used this filing guide a few times before deciding I could do better. My jigs arose directly from this guide.
I gave up the idea of having the guide be adjustable for bevel angle. Instead, I made a small number of single bevel angle sliders that handle the small number of angles I actually use.
Although patented in 1950, this jig missed the design advantage (in the Roth jig above) of angling the the arms that hold the file - increasing the usable range of motions for larger bevel angles.
Saw filing requires that you maintain three angles during the filing action. Worse, you make contact with the saw at a single point on the file, and that point moves down the file on each stroke, so the relative pressure required of your hands changes continuously.
While filing a saw we must keep three angles consistent throughout the operation: the rake, the bevel, the slope.
Defining these 3 angle can be confusing. Maintaining them during filing is very difficult. These definitions are based on having the saw held vertically in a saw vise with the teeth facing upward. So, the blade is vertical, the line of the teeth horizontal.
While most people can file keeping the file more or less horizontal, it is much harder to file while keeping the file tilted upward. To simplify non-zero slope angles, most metal saw vices have a capability to tilt the saw blade from the vertical. To get a 20 degree slope, you tilt the vise away from you 20 degrees and work with the file horizontal.
There was a brief time in the latter third of the 1800s in which tilting the saw during filing was a common practice. A saw filed with non-zero slope has been said to have sloping gullets. Filing with non-zero slope is rarely mentioned today. I have taken some illustrations from the Disston Lumberman Handbook of 1917 and included them in a discussion of Sloping Gullets.