saw sharpening problem

[Jacob]

I've done a few saws over the years, generally OK in a non professional way.
Latest rusty old saw sharpened OK except for 3 or 4 teeth breaking. I assumed this was due to age and pitting etc. Cut nicely though.
Had another go - topping, re cutting, setting, but this time a lot more teeth broke.
The only thing I did differently was to use a different file.
I noticed the gullets aren't so sharp - the corner is rounded making the gullet shallower. Could this be why more teeth broke? Possibly also I should use less set with the Eclipse 77 so the bend is higher and further from the bottom of the tooth?
So which file should I buy for best results (if the not-so-sharp corner is the problem)?
Or is it just too rusty?
It's 10 tpi, very old 14" S&J tenon saw.

 

[Paul Chapman]

rusty old saw

Could be a clue :lol:

Cheers :wink:

Paul

 

[Jacob]

rusty old saw

Could be a clue :lol:

Cheers :wink:

Paul

But wasn't so bad on the first attempt so maybe not the answer? It didn't suddenly get a lot rustier in the intervening three days! It did strike me that the metal was harder than some saws, but it's very subjective.

 

[AndyT]

Afaik a sharply triangular file would make the problem worse, as cracks in the steel are more likely to spread from a sharp corner. Proper saw files make rounded gullets. I've used and liked Bahco saw files, which were the only make on offer in my local tool shop. I believe the conventional advice is to only set the top third of each tooth and don't give as much set as the numbers on the saw set suggest. I lost some teeth on an old saw before I knew those two things.

 

[Jacob]

......I believe the conventional advice is to only set the top third of each tooth and don't give as much set as the numbers on the saw set suggest. ......

Thats probably it then. Less set. It must also be more brittle than my other saws though, as I haven't had this problem before.

 

[Cheshirechappie]

I'm in the process of refurbishing an old panel saw, with the intention of refiling it rip cut. Before doing that, I did a bit of reading around the net, and one or two books, so what follows is 'book knowledge', not experience.

Saw blade steels have changed over the years. Old ones were of 'cast steel', about 1.0% carbon steel. Some more modern ones use 0.9% or even 0.8% carbon steels, which will feel 'softer' to the file. Most modern saws seem to work OK, so the 0.8% is clearly fit for purpose; but that may be one factor in some people liking the older, harder-steeled saws - they probably hold their edge longer. However, with hardness also comes brittleness, so if teeth are inadvertently reset in the opposite direction to their previous set, they won't like it.

It may be an idea to clean up the sawblade near the teeth and see if the rust has gone deep. If there is pitting, it will weaken the metal at a point of maximum stress - were it's bent for setting.

AndyT's point about rounded gullets is also something the literature is quite strong on. Only use a proper sawfile, not an engineer's three-square. You need the rounded gullets that a proper sawfile leaves. I can't see any reason why, if you're recutting teeth, you couldn't do most of the work with a standard three-square, and just finish with the proper sawfile, though.

It might be worth jointing back to take off most of the old teeth. If they've been work-hardened by years of abuse, they will be brittle, and the corrosion won't help. The metal nearer the roots of the existing teeth may not be so badly affected. Most of the old saws I've seen don't have very straight toothlines, either - so straightening things up may get rid of rusty work-hardened old metal, and get you back to better stuff.

 

[Jacob]

Interesting. Thanks Andy and CC. So I'm probably right about the metal being harder but wrong about the file. I'll have another go then.

 

[GazPal]

A dab of oil on the old saw plate can sometimes track and highlight/betray flaws in the same way it does with cracks in plane bodies.

Bahco make decent saw files :wink:

There's a definite chance the teeth were miss-set by a previous owner and your re-set may have simply confirmed this through breakages.