So I thought I’d have a go at sharpening the 14 inch Cowell & Chapman backsaw (which is really a W. Tyzack, Sons & Turner). I’m going to file it 10.5 TPI rip with 9 degrees of rake and 5 degrees of fleam. I was going to add 5 degrees of slope as well, but I figure at this point I should just concentrate on filing the fleam correctly without complicating things further. Remember this one?
This saw has an extra-heavy brass back and therefore there is a considerable amount of weight behind the cut. By adding 9 degrees of rake, the saw should be easier to start and result in a smoother sawing action. In other words, adding the rake angle should reduce the tendency for the teeth to ‘grab’ the wood. This is all new to me, so I’m interested to see if this is in fact the case.
Although this saw was originally filed 10.5 TPI, to shape the teeth I decided to joint the existing teeth off completely and use my method of applying a template to the side of the saw with double-sided tape, like I did for the S&J dovetail saw. This works well for filing new teeth.
With the template affixed to the saw, I mounted the saw in the vise and knocked up a jig to give me my 9 degree rake angle. It is easy to keep the top of the jig horizontal and this means that the side of the file cutting the front of each tooth must be angled at 9 degrees. As you can see, I stupidly wrote fleam instead of rake, but at least I knew what it meant even if I didn’t write it properly.
When shaping, the teeth are filed at 90 degrees to the saw plate. For this reason, you can shape all the teeth from one side. Fleam only comes into play once eveny spaced teeth have been established with equal gullet depth and a consistent rake angle.
So using a 6” double extra slim saw file (suitable for 10 – 11 TPI), I moved along the plate taking 5 or 6 strokes on each tooth. This went well and soon I had all the teeth shaped the way I wanted them.
At this point, I decided to ‘set’ the teeth. Some people prefer to do this after sharpening, but in this case I elected to apply the set now so that I could test the saw before filing in the fleam. This would give me a reference point so I could compare the effect of adding 5 degrees of fleam to the teeth.
I wanted to add about .003” of set per side, so I measured the thickness of the saw plate (.025”) and then adjusted my saw set until the added set gave me a measurement of .031”. The numbers on saw sets are just a rough guide and it is always advisable to test the set on the teeth at the heel of the plate (under the handle), since they don’t actually do any cutting. For each saw I sharpen, I’m noting down which file and saw set I used and how the saw set was set. This means that next time I come to sharpen each saw I’ll know what I’m doing.
Before setting the teeth, I dabbed a permanent marker on the tip of every other tooth. This made it easy to apply set to the marked teeth from one side of the plate and the unmarked teeth from the other side of the plate.
Now I could see how the saw cut without any fleam. First I made a cut as if I was sawing off a tenon cheek, then I made a series of vertical cuts to full depth. It took about 33 strokes to reach full depth (just under 4”), although in fairness the teeth are not all perfectly sharp at this point.
I selected Sapele as my test wood, because if a kerf looks good in Sapele, the chances are it will look even better in most other woods. Sapele is not the easiest wood to saw. It has an interlocking grain structure and often, internal stresses cause the wood to close around the saw as you’re sawing, causing the saw to bind. Anyhow, let’s take a look at the back of the cut. As you can see, there is a fair bit of shredding. Although this is easily removed with a swipe of a plane or sandpaper, in theory adding a little bit of fleam should reduce the amount of shredding and I wanted to see if this was true and whether there was any trade off.
Before sharpening, I lightly jointed the teeth (two passes)
The purpose of jointing before you sharpen is primarily to ensure the teeth are of equal height after shaping. However it also serves another purpose if you apply set before you sharpen the teeth. Setting causes the teeth to bend outwards and twist slightly. This means that the tips of the teeth are no longer perpendicular to the side of the saw plate. Instead they form a shallow inverted ‘V’ shape. Jointing after setting ensures the tips are 90 degrees to the side of the plate again. When ripping, you want the bottom of your kerf to be flat, otherwise when sawing a tenon cheek you will always be left with a little bit of ‘fur’ to clean up right in the corner where the cheek meets the shoulder.
The little flats that jointing produces are known as ‘shiners’ and you sharpen the teeth until you just remove each shiner. Then you know the teeth are sharp. A raking light can really help the ‘shiners’ stand out.
So now it was time to sharpen the teeth and I needed to think about getting a consistent fleam angle. Once more I turned to Sketchup and drew a template which was nothing more than a series of parallel 5 degree lines. Notice that the lines lean the other way on the far side of the teeth. Since I am left-handed, the saw handle is on the left. If you are right-handed, the lines on both templates would lean the other way and the handle would be on your right. I covered the paper with Sellotape so the iron filings wouldn’t stick to it and obscure the lines. By sighting down on the file, it is easy to keep it parallel with one of the lines and this ensures that your fleam angle remains consistent.
So I started filing all the odd numbered teeth from one side. Then after reversing my rake angle jig, I filed all the even numbered teeth from the other side. ”This is a doddle”, I thought. ”I don’t know what all the fuss is about.” Then I looked more closely at the teeth I’d filed. (You have my permission to laugh now chaps.) This is what is known as ‘Cows’ and ‘Calves’ or ‘Big teeth’, ‘Little teeth’ and it basically means you’ve messed up. :-(
At this point, I was pretty hacked off. After doing a great job on the shaping, I’d ruined all my hard work. But what had gone wrong? Time for some introspection and to wrestle with the devil inside.
-- Andy -- Old Chinese proverb say: "If you think something can't be done, don't interrupt man who is doing it."