Have you ever thought about why some saw makers add negative rake to the teeth of their rip saws? I have, but when I was drawing a 12 TPI template in Sketchup to re-tooth my Disston No.5 carcass saw, I realized that adding a touch of rake actually increases the volume of space between the teeth.
If you look at a section through a saw file, you’ll see that you have an equilateral triangle (ignoring the rounded corners that define the gullets) and we know that the three angles of a triangle add up to 180 degrees. That means that the angle formed by sides a and c below will always be 60 degrees, irrespective of the rake angle on the front of the tooth. By the way, the white area represents the saw with the teeth pointing up.
You’ll remember from your school days that the area of the triangle abc = the base (a) multiplied by the height (b) divided by 2, so if a = 1 and b = 1.6, the area of the triangle would be 0.8units².
If we add 6 degrees of negative rake to the face of the tooth, you can see that the base of our new triangle (d) increases in length whilst the hypotenuse (f) becomes marginally shorter. If d = 1.2 and e = 1.6, then the area of triangle def is 0.96units². That’s an increase of 0.16units² per tooth. Multiply that by the thickness of the saw plate and you have an increase in volume.
If math isn’t your thing, the following illustration might help you visualize it better. Effectively, by adding 6 degrees of negative rake, you are losing the area shown in green and gaining the area shown in red. In practice, you are increasing the amount of space that the sawdust has to accumulate between the teeth. I say sawdust, but rip teeth actually create tiny shavings as opposed to the much finer dust created by teeth filed for crosscutting. This increase can be important because when the space between each tooth becomes packed with dust, the teeth stop cutting and bottom out. Now you might think that this increase in volume is insignificant, but if you multiply the extra space by the number of teeth on a saw, it soon adds up and could make a difference to the speed of the cut.
Some people find that a rip saw whose teeth have been filed with negative rake is easier to start because the teeth exhibit less of a tendency to grab the wood. Personally, I’ve never found a saw with zero rake difficult to start. If you hold the weight of the saw off the wood so that the teeth just skim the surface until you’ve established a kerf, it really isn’t difficult with a bit of practice. This led me to wonder whether late 19th century and early 20th century saw makers introduced negative rake into their rip tooth geometry to make up for their customers’ inability to saw properly? Could it be that what their customers really needed was not negative rake, but practice at sawing? Is it right for people who are not practiced at sawing to expect to pick up a rip-filed backsaw and get good results first time? At the risk of sounding like I’m hankering after bygone days, maybe it is just that we have come to expect instant gratification from our tools without wanting to expend the necessary time and effort to learn to use them correctly and gain an understanding of what makes them work well. Anyhow, I digress.
Never having used a carcass saw with negative rake, I was interested to find out for myself if adding negative rake was in fact a good thing, so I grabbed that lovely Disston No.5 that I restored in Saw Talk #2. Originally, this saw was filed with 13 teeth per inch (14PPI), but since my Gramercy rip carcass saw is 13TPI, I re-toothed the Disston to 12TPI.
Using a 5” extra slim file, I sharpened it with 6 degrees of rake and 5 degrees of fleam and added about .002” of set either side of the .026” plate.
These little teeth are fiddly to file, but I think I’m starting to get a better feel for filing now. I’m not quick by any means and I still have to remember to breathe while I’m filing, but I think the results are passable. I’m also realizing more and more the importance of getting all of the teeth the same height. Not easy, I can tell you! When I first tested the saw, there were a couple of teeth that were fractionally taller than the rest and the saw just stopped when they hit the wood. Sure I could have forced it to continue, but I wanted it to run smoothly.
I compared the offending teeth against those either side of them and I couldn’t see any difference at all. I tried feeling the difference with my finger, but I couldn’t detect any variation in height. Regardless, I marked the plate just above the teeth with a pencil and put the saw teeth down on a granite plate. The saw rocked very slightly along its length and shining a light behind it enabled me to identify where the fulcrum point was. Lo and behold, it was right where I marked the plate with the pencil. It surprised me how such a small variation can make the difference between smooth and juddery cutting. After I reduced their height, the saw ran more smoothly.
Since it is my intention to use this saw for cutting small tenons, here’s a little video of the saw doing just that. In my view, sawing tenons is more about accuracy than speed. In fact, as you’ll see in the video, I’m holding this saw back more than I’m letting it rip.
After that you’ll see me comparing it to my Gramercy rip saw which is filed 13TPI with zero degrees of rake and zero degrees of fleam. Actually to be honest, I just like sawing thin pieces of softwood with rip saws. It’s the sawing equivalent of planing thin whispy shavings. I should point out that the plate thickness of the Gramercy is .020” compared to the Disston’s .026”, so the Gramercy is removing less wood and should be faster. To make it a fair comparison, I touched up the Gramercy before I used it so that both saws were freshly sharpened. You might like to count the number of strokes it takes me to reach the line with each saw.
I noticed when sawing the tenon cheeks that it could do with a touch more set, so I’ll have to sort that out. At the moment I’d have to say that the jury is still out on whether I think rake is a good idea on a carcass saw intended for ripping, but I’ll live with it for a while and see how I like it. I will probably end up filing the fleam off of it though. Whilst it does enable me to make the odd crosscut with the saw, I don’t really need it to do that since I have other saws that will be filed specifically for crosscutting.
Here’s where this saw’s journey started…
…and here it is now ready for work.
Thanks for your support folks!
-- Andy -- Old Chinese proverb say: "If you think something can't be done, don't interrrupt man who is doing it."