Advanced Saw Filing and Restoration

by Bob Smalser

This article first appeared as a thread on the WoodNet woodworking forum. It was compiled and reproduced here for easier public consumption. All Text and Images are the property of Bob Smalser.


A 6-dollar flea market beater? Yes, but this one is a Disston Acme 120, originally a cabinetmaker's finish saw tapered and hardened to run without set, and one of Disston's finest. So let's see if it can be given another lifetime of use in a slightly different form.

Old saws filed so many times their tips resemble pencil-points usually aren't worth the trouble, as when they get that thin and narrow they are too easily kinked, and this one's no exception. Restoring this in its original 26-inch length isn't a good option for it to survive another generation of use. So I'll shorten it to panel saw length to make it useful again, but that's not as straightforward as it seems if the saw is to please the eye and hand. Panel saws had smaller handles than their full-sized counterparts, and their blades were uniformly contoured to match their smaller proportions; they weren't just stubby versions of full-sized saws.

I don't have a small #120 handle, but I do have an extra Keenkutter panel saw handle and another complete matching saw to use as a pattern. These Keenkutter #88 skewback saws were made by Disston using #16 handles and probably P26 blades from the Harvey Peace factory they bought out, for which Disston offered custom etching in hardware store logos like EC Simmons'. Mr. Simmons knew his saws. These are not only excellent, taper ground saws, their profile pleases my eye. I scribe the new profile onto the #120 blade, and use the bevel gage to duplicate the tip angle. I'll make the #120 a 22-inch saw based on the amount of blade remaining.

Saw steel grinds quickly and relatively cool using a coarse, 8" wheel, with the occasional water dip as the wheel gets close enough to burn what will be the final profile. Here I don't just grind up to the scribed line, I take the line.

I fair the curves by drawfiling using 2d-cut and smoother 1st-cut single-cut files. This is done largely by feel. When I feel a bump I alter the file angle for a more aggressive cut, and finish using my finest single-cut file straight across. The files are chalked and frequently brushed both to keep them from clogging and to prevent stray filings from causing scratches during finishing. I rarely use chalk when jointing and sharpening however, as it often masks what I'm trying to see.

After fairing I ease the sharp edges slightly using the fine file in the drawfile mode.

I fit the new handle and drill the bolt holes using a cobalt bit. The easiest method is to mount the handle and carefully drill through it, indexing each fresh hole with a bolt to insure alignment is maintained. Clamp the saw down during drill extraction to prevent it riding up on the drill.

On the anvil using light and heavy hammers, I remove the saw's tension, hammer out the kink and retension the entire blade as I've detailed in previous articles on permanently removing bends and lumps in saw blades. Saws straightened in a vise, by bending over the knee or even by hammering flat won't remain straight for long without stretching the edge and back to restore the tension put into the blade when it was manufactured.

This blade isn't heavily rusted and isn't pitted, so I forego phosphoric acid to seal the pits and merely clean the steel using 4 grades of Scotchbrite in sequence lubed with mineral spirits.

Sanding is followed by a green rouge buff against the direction of sanding to restore some shine.

Because I've shortened the saw the etch will be off center and unsightly to my eye, so I'll blue the blade to hide it. But this is a good opportunity to demonstrate how to raise an etch like the faint one you see here.

To raise an etch, first avoid sanding it without using a sanding block when you clean the saw. Then degrease after buffing using mineral spirits followed by stronger trichloroethylene, and apply cold gun blue from a sporting goods store to the area of the etch. Follow the instructions on the bottle for curing and oiling.

After the blue cures, simple rubbing with a sanding block and 600-grit wet-or-dry paper lubed with WD-40 or honing oil will reveal any etch remaining. All these bluing solutions will rust if not oiled, some badly, so don't omit it.

I then phosphate blue the entire blade, using the procedures I detail in a previous article on rust proofing tools. This type of blue applies a layer of rust-inhibiting phosphate to the steel, which also slightly fills the scratches and grinding marks present, making the blade a bit slipperier. You can read more about this at Brownells.com.

I secure the saw in the vise and lightly joint it to see what I have. While the joint and teeth look well-maintained to the naked eye, this saw has faults typical of both commercial filing, and filing for sharp points at the expense of the joint. One row of teeth is shorter than the other in several places where one side was filed past the flats on the teeth made by the joint, and the saw was filed straight across by machine using a file larger than optimum for these 11 tpi crosscut teeth. The gullets are perfectly uniform, but are too large, making the teeth too short.

Jointing insures all the teeth perform the same amount of work during cutting and involves more than just regulating tooth height. When the tooth line is wavy as shown above, the saw drags as teeth on the forward slope of a bump bite wood, and those teeth wear more quickly.

This tooth line isn't perfect either close to the tip, but it's more than adequate for efficient sawing. Keep in mind as we go that my goal isn't perfect saws, it's perfect sawing, and there are differences between them.

Jointing is where you add or subtract "breast", the amount of crown in the tooth line. Crown breasted saws are more difficult to joint, but rake sawdust more efficiently and crowns are always most pronounced on saws for green wood with gummy sawdust like the old one and two-man falling and bucking saws once used to harvest trees. Here I show a saw with a light breast on the left, a full breast in the center, and a straight or unbreasted saw on the right.

Before we look at filing, let's look at files. Saw files are tapered triangular files with 60-degree corners, come in several confusing sizes, and always have "1st-cut, single cut" teeth as these cut slowly but produce a finer finish than coarser "2nd-cut" and multi-tooth files. Vintage Saws and file manufacturer websites like Simonds International have helpful guides for what size file is best for each pitch of saw, usually expressed in tpi or teeth per inch. But what if you don't have the recommended file? Can you substitute?

Sure you can substitute. There are two factors guiding recommended file size. The first is you want the narrowest file possible so you can see the saw teeth better, yet you don't want the file to use more than half its width when filing a tooth. That way you have three completely-sharp cutting surfaces on each file to use as each wears out.

The second factor is gullet width. The wider the file, the wider the flat at each corner, the wider the gullet it will produce, and the wider the gullet, the shorter the tooth. Of course there are limits. A knife edge file would minimize the gullet, producing a taller tooth, but the blade would also be prone to cracking at the gullet, and there are a couple hundred years of trial and error behind those file size recommendations that shouldn't be ignored.

But you can certainly substitute files. New files of acceptable quality are expensive, and excellent values can often be had buying boxes of NOS taper files in off sizes and 2nd-cut files. You can use the coarser 2nd-cuts to shape the teeth and finish with your 1st-cut files. 3-inch Regular Taper, 1st-cut files can be currently had for pennies each, and substitute nicely for 3 and 4-dollar 5-inch Slim Tapers and 6-inch Double Slim Taper files. Just be aware that switching file sizes on the same saw can change the shape of the teeth due to even minor differences in gullet width, and the same can be true when switching manufacturers of the same size file.

Saws are filed in the stages detailed in several on-line articles on basic saw filing, like Pete Taran's at the Vintage Saws website:

  1. Jointing to even tooth height and straighten or fair the tooth line or breast.
  2. Shaping the teeth to even the size of the flats made by the joint and set the rake angle.
  3. Filing the "fleam" or cutting edge bevel angle until the flats disappear, creating sharp teeth of uniform height.

The major benefit of sharpening your own saws is you can tune them for how you use them. I usually work outdoors in damp, air dried wood and use hand saws where the mass of the work piece and the required angles and finish often preclude the use of power tools. Often 30 linear feet at a time too, driving study and experimentation in what makes the most efficient saw for my tasks. Accordingly, my saws are generally filed with the full crowned breasts and sloped gullets my boatbuilding family has used for generations.

Here are two identical 4 tpi rip saws. Identical except the saw in the foreground clearly has taller teeth, and also a slight back bevel on the rear edge of each tooth. Seen at a different angle they look almost Japanese, and that's probably not a coincidence. It was filed holding the file at a 45-degree angle to the blade, instead of straight across or 90 degrees to the blade like the saw in the background was filed. The geometry of a tri-cornered, 60-degree file is such that the greater the angle it is held at, the taller the tooth it produces. Brent Beach has a website that examines sloped gullets in more detail, including their history and accurate engineering drawings that better demonstrate their geometry.

I find the taller teeth work more efficiently in the wood I use. They bite a bit deeper and rake damp sawdust a bit better, and the back bevel that sloped gullets produce on rip teeth score the wood on the back stroke for easier removal on the front stroke. You'll have to try them and see if there's any benefit with the woods you use. Either way, how to file sloped gullets isn't described anywhere else and they are another tool to place in your kit if you ever feel you need them.

When changing a saw to sloped gullets, I combine the steps of shaping the teeth and filing the fleam angle into one step, and concentrate on "filing to the joint" instead of equalizing the size of the flats made by the joint and counting strokes. Accordingly, I don't use carbon black or other aids to insure I don't skip a tooth, as skipping a tooth doesn't matter when concentrating on the joint. I also tend to file one side less than I would when filing conventional teeth, switching sides more often, and sometimes even rejointing if I make a mistake and file a tooth past its jointing flat. Notice I have my usual fleam angle scribed into the jaws of my saw vise. I prefer a shop-made wooden vise with long jaws because I prefer to file while standing and I can file an entire side without having to move the saw in the vise.

On the first passes I'm hogging off a relatively large amount of steel, and am concentrating more on getting out my gullets, establishing a uniform rake angle and not filing past the jointing flats than I am about the fleam angle. I'll emphasize the fleam later on my final passes when filing is easier, and I may even change the rake angle later after testing the saw on wood.

Keeping your angles constant when changing sides takes practice, and it's also useful to move your work lights so you can clearly see the jointing flats.

Don't continue filing if you can no longer clearly see those jointing flats. Take a break, move your lights or both. Also don't hesitate to rejoint if you make a mistake and file past one. Moreover on saws like this one with short teeth and one entire row of even shorter teeth, you often have to compromise your initial jointing or not leave enough gullet on the short teeth to rest the file in, which is necessary for basic tooth spacing. You can see here that even after a full pass on each side, one row of teeth remains taller than the other, so rather than attempt too many tasks at once with the file, I rejoint and begin again.

I generally set the teeth before my final pass on each side of the saw so any damage caused to the cutting edges by the saw set is removed. Here you can see several small jointing flats remaining on the top of the teeth. When setting, I'm careful to place the plunger of the sawset at the same spot on each tooth, and when possible slightly back from the cutting edge bevel to minimize damage. And yes, although these Acme saws were designed to run without set, if a tad of set makes your saw more efficient on the wood you cut, then you can add a minimal amount without damage by using the lowest setting on your sawset.

Rip saws filed with sloped gullets are set oppositely from crosscut saws. Sloping the gullet puts a small bevel on the back of each tooth, creating a point, and the point must be bent outward to create the desired chisel point of a ripping tooth.

On my final pass to eliminate the remaining vestiges of jointing flats, I do so by leaning the file into the backs of the teeth rather than continue to file rakes and fleams. Uniform rake and fleam angles and their cutting edges are important, the backs of the teeth aren't when your only objective is sharp points on the teeth.

I make some test cuts in relatively hard, air dried Douglas Fir at 12% moisture content to insure the saw cuts smoothly and accurately to a line before stoning off any wire and flash caused by filing. When filing sloped gullets for the first time, you may tend to file in more rake than the blade will take, so be prepared to put the saw back in the vise and make another pass on each side to reduce it.

The finished hybrid panel saw. Cost? Six dollars, one file, and a couple of evening hours.

My short saws don't get used much, but can be handy. They also can be practically free, especially when among the #120's, #12, #16 and #88 shown it's hard to tell which if any began life as a short panel saw.

 


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