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Saw Talk #14: Disston No.5 - Sharpened and tested

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Blog entry by Brit posted 767 days ago 5711 reads 3 times favorited 26 comments Add to Favorites Watch
« Part 13: Intrepid Sawster Triumphs over Adversity Part 14 of Saw Talk series Part 15: W. Tyzack, Sons & Turner No.120 - Sharpened and Tested »

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 interrupt man who is doing it.



26 comments so far

View ShaneA's profile

ShaneA

5257 posts in 1202 days


#1 posted 767 days ago

Saw is a beauty Andy. Thanks for sharing, very informative.

View llwynog's profile

llwynog

282 posts in 1182 days


#2 posted 767 days ago

Hi Andy,
This saw is now simply gorgeous ! You did a terrific job in restoring it.

Your comment about the rake angle affecting the area between teeth (and thus its sawdust holding capacity) is one I never considered but find highly enlightening. It is an interesting thing to think about when sawing through wide planks that may tend to clog the spaces between teeth.

Your blog along with Mafe’s is one I always look forward to reading when I receive a new post notification.

Cheers,

-- Fabrice - "On est bien bête mais on sent bien quand on se fait mal" - my grandfather

View Don W's profile

Don W

14662 posts in 1171 days


#3 posted 767 days ago

Information overload. I’m going to have to read this one again.

-- Master hand plane hoarder. - http://timetestedtools.com

View Brit's profile

Brit

5107 posts in 1446 days


#4 posted 767 days ago

Thanks guys. Just a snapshot of the kind of stuff I think about. Sad isn’t it?

Don – You’ve got plenty of time, I’m off to Germany in the morning. Here we go again… another project close to home… NOT.

-- Andy -- Old Chinese proverb say: If you think something can't be done, don't interrupt man who is doing it.

View Don W's profile

Don W

14662 posts in 1171 days


#5 posted 766 days ago

Good luck in Germany. Hopefully you’ll have some time to look for some tools over there.

-- Master hand plane hoarder. - http://timetestedtools.com

View Dave's profile

Dave

11142 posts in 1443 days


#6 posted 766 days ago

Andy most informative. The tenon 101 was great. To see the saw go through its rigors making a joint helps me a lot. I am a visual person and learn better that way. You sir are well on you way to saw excellence.
Nice!

-- Superdav "No matter where you go - there you are." http://chiselandforge.com

View steliart's profile

steliart

1807 posts in 1291 days


#7 posted 766 days ago

very nice saw, thanks 4 the video

-- Stelios L.A. Stavrinides: - I am not so rich to buy cheap tools, but... necessity is the mother of inventions - http://www.steliart.com --

View mafe's profile

mafe

9456 posts in 1692 days


#8 posted 766 days ago

Really interesting Andy, thank you for the light.
I am getting more and more happy for not having had the time and now the workshop to start my sawsharpening tour, since you provide so much help that I can hopefully learn before starting my self.
Best of my thoughts,
Mads

-- Mad F, the fanatical rhykenologist and vintage architect. Democraticwoodworking.

View Brit's profile

Brit

5107 posts in 1446 days


#9 posted 766 days ago

Don – Thanks. Just arrived at the hotel. I had to fly via Zurich and my flight from Heathrow to Zurich was delayed. When I landed in Zurich, I had to sprint from one end of the airport to the other and they arranged my own bus to take me out to the plane from Zurich to Stuttgart. I was the last one to board and everyone looked at me as if it was my fault. Talk about cutting it fine. A minute later and they would have unloaded my suitcase.

Dave – It is a lot easier to saw when you have a workbench that doesn’t wobble though. :-) I’ll have to see if I can fix mine as both of the vise screw no longer work. If not, I’ll either make a portable bench or buy another Workmate.

Steliart – You’re welcome.

Mads – Once you develop a feel for filing, it isn’t that difficult. It would also be easier in a workshop with a raking light. Working out in the garden as I’ve been doing, the sun either illuminates my ‘shiners’ or not depending on where the clouds are. LOL.

-- Andy -- Old Chinese proverb say: If you think something can't be done, don't interrupt man who is doing it.

View Dave's profile

Dave

11142 posts in 1443 days


#10 posted 766 days ago

Andy simple construction grade will make a nice starter bench. And have a bit of weight. I am still using the one I built. It will become my secondary when I take the leap to the Roubo.

-- Superdav "No matter where you go - there you are." http://chiselandforge.com

View nobuckle's profile

nobuckle

1120 posts in 1364 days


#11 posted 765 days ago

Your posts on the this topic are very timely for me. I am embarcing on my first attempt to restore some old hand saws that were given to me. I need to learn as much as I can before I tackle sharpening the teeth. Your posts will be an invaluable resource. Thanks.

-- Doug - Make an effort to live by the slogan "We try harder"

View Brit's profile

Brit

5107 posts in 1446 days


#12 posted 765 days ago

Thanks Doug – Please remember though that I’m only one step ahead of you. It is all new to me as well, so you have to take what I say with a pinch of salt. :-) At the very least, I hope this blog causes people to consider some of the things that I’ve mentioned, even if they reach different conclusions.

-- Andy -- Old Chinese proverb say: If you think something can't be done, don't interrupt man who is doing it.

View terryR's profile

terryR

2978 posts in 912 days


#13 posted 765 days ago

ANOTHER excellent post…Andy, there’s NO WAY you could write a flop…you are too good at explaining everything!

...will be thinking of triangles all day now as I continue pull barbed wire…hmmmmmmm…

Thanks for the video on tenon cutting! you certainly make it look easy! Darn it…I wish you’d quit cutting up all that sapele…you must have enough to build yourself a decent bench from it. :-)

-- tr ...see one, do one, teach one...

View ShopTinker's profile

ShopTinker

872 posts in 1372 days


#14 posted 765 days ago

You’re suggesting we need to learn how to use our tools? That made me smile. We are generally in to big a hurry to learn to do that. I remember as a boy, before I was allowed to use power tools, learning how to saw so that the saw didn’t catch. It sure did make sawing less of a chore. It was many years after that before I learned to saw straight and follow my mark.

-- Dan - Valparaiso, Indiana, "A smart man changes his mind, a fool never does."

View Mauricio's profile

Mauricio

6749 posts in 1755 days


#15 posted 765 days ago

Andy, your a wealth of knowledge, the visuals are great. I loved the videos, nice to watch. Sawing with a sharp saw is such a pleasure. Your making me want to buy some old backsaws…

So what is the moral of the story? No rake cuts more aggressively but fills up faster, 6 degree cuts less aggressively but removes more material….

The Gramercy saw got through the pine in 5-6 strokes. I guess its hard to tell if it was due to the 6 degree rake or the saw plate being 30% thinner.

The Diston took 1 or two more strokes. from 6 to 8 strokes is a 25% difference, so how much of that difference is due to the saw plate and how much is due the rake?

My guess is that the rake is just personal preference since the they both cut just as fast?

-- Mauricio - Woodstock, GA - "Confusion is the Womb of Learning, with utter conviction being it's Tomb" Prof. T.O. Nitsch

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