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JOEL Joel's Blog

Sherlock: The Saw Tooth Detective


Closeup of saw teeth
Closeup of saw teeth

A draft blog post about saw sharpening, including a macro shot of saw teeth, inspired me to think about the saw's teeth. The saw in the picture is a mid-late 19th century backsaw by Norris. It's a collectible saw from the period when Norris was a general hardware store company, not the specialty infill plane maker that we remember today. Let's take a look at these teeth. Note: Look here for an explanation on the terms I use and an explanation of saw geometry.

The saw (pictured below) is 7" long with 14 TPI or 15 Pt, which makes it a smaller and a finer tooth backsaw, of the type typically used for dovetailing and small work. Late 18th century saws of this size would have a finer pitch, but by this time period, this saw's filing is pretty typical. As we see in early texts, the term "dovetail saw" was certainly used in tool catalogs, but as we know from "The Joiner and Cabinetmaker a short, fine-tooth saw was used everywhere one needed a short, fine-toothed saw, not just in dovetailing. Most of these short saws would have been found on a building site, not in a cabinet shop.

We can observe the following:

1 - The set: Alternate teeth are bent towards or away from the camera. We would kind of expect this.

2 - The teeth seem thinner at their tips. This is because the teeth were hammer set, not set using a squeeze-type setting tool. My first reaction was that this was the factory setting. But there really isn't any reason to believe that. The saw has a fair amount of use, and was probably used professionally and regularly sharpened professionally. A professional saw sharpener would typically hammer set freehand. Later in the nineteenth century, various hammer setting devices to aid setting were available, but I would actually bet that this saw was hammer set freehand because the hammer marks on each tooth are slightly different. This saw's teeth are bigger than those of a finer pitched dovetail saw from the 18th century, making hammer setting freehand much easier - it's a hand skill that you learn by repetition.

3 - You can see the difference between sharp and dull because the trailing edges of the teeth, which are untouched when sawing, are still crisp from the sharpening file. The leading edges of the teeth, which actually cut the wood, are rounded over from wear.

4 - Almost all the teeth are blunted at the tips, which probably resulted from the saw getting kicked around or misused rather than routine wear. Some of the teeth look less blunted than others, but I think that might be an optical illusion.

5 - There is a tiny bit of fleam, which means that when sharp, the saw would work on both rip and crosscut. I'm guessing that whatever fleam is on the teeth was put there as a habit of saw sharpening, not an explicit plan for a combo filed saw (like our sash saw).

6 - The gullets between the teeth are pretty wide,which suggests a saw file was used for sharpening. Saw files, unlike needle files and engineer's three-square files, have big flat arrises. Arrises are the edges where the sides of the file meet and are sharp on needle and three-square files. By putting flats at the arrises, you end up with a much less expensive file to make. Cost was a critical factor in making the files affordable for the itinerant saw sharpeners, who were paid penurious wages. The tradeoff is easy to see in this picture. There is a big flat in the gully, which means that for a given pitch of the saw, there just isn't that much space for sawdust. A finer saw filed like this will clog and cut slower than a saw sharpened using a file with smaller arrises (and consequently deeper gullets). More importantly, the smallest saw file is a 4" double extra slim which, with its flat arries, limits the pitch of saw on which it can be used. If your saw were 18 or 20 pt, like an eighteen century dovetail saw, you would have very little tooth left because of the wide gullets. This, I believe, is why a 15 pt pitch became the 19th century standard for short saws, even though finer saws leave a finer surface and can cut faster and with less effort if the gullets are deep enough.

7 - The problem with a 15 pt saw is that it will be hard to start on thin stock, like thin material used for drawers, and it would be jumpy in use. The solution is seen in this saw. There is a decent amount of rake on the teeth, which is a typical filing for saws like this. The relaxed rake makes the saw easier to start and keep going on thin stock. But this easing is done at the expense of cutting speed and the force needed to make the saw cut, not glide over the teeth. We here at TFWW prefer a more aggressive rake. Our saws are finer in pitch and we use needle files to sharpen them. Most other modern dovetail saws are filed in the same relaxed manner of this antique. Some people find with our older style of filing a little too aggressive until they get us to it (and the saw wears in a bit).

I'm probably missing other obvious deductions one could make about this saw. Share your observations in the comments.

Norris dovetail saw c. 1880
Norris dovetail saw c. 1880

Join the conversation
06/29/2022 Michael O’Brien
Thanks Joel. I really enjoyed that detailed “saw detective” examination and explanation of what could be learned from examining this Vintage saw’s teeth. I always learn something from your blogs. Well done and most appreciated.
06/29/2022 Tom Heller
Joel: i bought an old and cheap 18 inch panel saw, 10 tip, for $10.00 and decided to experiment based on many filing articles and Youtube videos. I think saw sharpening instructions, from the late 19th century on, are generally overwrought demanding precision in set, rake and fleam angle that is not necessary. I filed all teeth for rip although there's a little fleam from normal hand variation and the fact i'm an amateur saw filer. There's no rake on the first inch and slightly more on the next inch. The rest of the saw has a slightly positive rake. I use this saw for crosscutting and ripping and it seems to work well - tenon shoulders, dovetails, etc. I'd be very interested in what others and yourself think of my thoughts on saw filing - i admit i could be mistaken. Thank you
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