Straighten cupped back saw - curved between toothline and back

[Scott Permar]

Hello All,

Has anyone found a process to help straighten a back saw that is straight along the toothline but has a slight bend/curve between the toothline and back? This causes the saw to progressively bind as the depth of kerf increases. I reset the teeth and there's probably now .005 total set. It actually cuts pretty darn straight considering the bend. The top to bottom curve is fairly even from toe to heal and the back is (now) straight. This is a $7 garage sale generic Warranted Superior back saw 12 inches long 4 inches deep with 11 PPI (approx .035 thick). Learning how to make a saw work is part of the goal of buying these but in the end I'd like the saw to be satisfying to use. Most of the information I've found (wkfine,vintage saw, disstonian) discuss smithing to straighten bends along the tooth line. I've had good success following this to remove a moderate kink/bend in a 26 inch rip saw. I've tried that process (probably too much) on another gents saw that's significantly bent from top to bottom without a noticeable improvement. So I tried this a little bit on the WS saw but stopped after it didn't show any progress. The back has been off an I did some work on it and re-installed it. When I bought the saw I didn't notice this top to bottom bend but you do have to look pretty carefully to see it so I may have missed it. Although I could caused it inadvertently. I cleaned the plate and started refinshing the handle so the saw is looking better but I like my tools to work. Has anyone dealt with this issue or am I just the cause of my own problems?

Thanks,
Scott

One observation I made was to make a test cut to full depth and then reinserted the saw in the kerf with the leading edge (parallel to the tooth line but not engaging any teeth) and to see if it still bound and that direction and it was free. Not sure if it's meaning but I tried that to see how tight the kerf was. Putting the saw back in the usual position in the kerf after cutting it still seemed to be tight after a minute or so.

[george wilson]

Clamp the front edge of the BLADE in a vise. Tap against the BACK of the saw to jerk the blade tighter,to straighten it. I got my favorite closed handle Groves saw cheap since it had a big curve(NOT a kink) in the blade. If you have a sharp creased kink,that's another problem entirely,but smooth curves can be jerked straight. Might apply a little Loctite along the joint of blade and back to keep the blade from slipping again.

[lowell holmes]

After George posted this remedy once. I tried it and it worked!

George, your knowledge is remarkable.

[Mike Siemsen]

Scott,
Check the handle. This is one reason handles are Quartersawn so the handle doesn't throw a cup into the blade

[lowell holmes]

That's good information I had never thought about. I need to determine what I used in a handle I made some time back. It was cherry and I think it was quarter, but it might be rift sawn. Thanks for posting.

[Scott Permar]

Excellent advice. I never really thought of a saw back being able to apply tension to the plate. I briefly tried the tapping technique George described above but the back moved enough the plate became proud and the wooden block contacted by plate and likely nullified any tension I may have created. I reinstalled the back it too far rearward to start with so there was almost no room. I was a bit surprised the back did not seem to originally be installed even with the plate, it was pushed further down in the front but the level of the back on the heal end is limited by the handle. So how far is the plate usually inserted into the back ideally? The saw is apart again to re-position the back and check any handle tightening influences. I'll report back again once I have time to work through carefully. Shop time is sporadic, a 2 year old boy has a vast ability to absorb a large quantity of everything.

-Scott

[Mike Siemsen]

You really only need a 1/4 inch of the plate up in the back. Try to match what is at the handle end where the back sets on the handle and can go on no further.

[george wilson]

Sorry,I didn't understand which direction your saw was cupped. If handle is warped and turning the blade into a cylinder,that's new handle time!!:)

Unless you are epoxying the blade into the back,which I NEVER do,I do not think 1/4" is enough. Some slitted back saws are that way. I guess I'm too traditional,but I always annealed the brass,folded it half way,annealed it again,and pressed to shut thinner than the thickness of the blade where the seam was to insert the blade. I ground a little bevel on the top edge of the blade,so it wouldn't cut the grass while driving it in. The back was slightly horse shoe shaped (VERY slightly),most are. If you squeeze it too shut,It'd spring open at the seam. After inserting the blade,I'd squeeze the back fully tight right at the nose,grind and polish the back. Look at the front end of your OLD Disston back saws,and you'll see what I mean by horse shoe shaped.

I have heard of someone selling cracked brass backs. They did not properly anneal the brass if that's the case. For the large Kenyon tenon saws,we used brass 3/16" thick for their massive backs without cracking the brass due to double annealing. The backs were 3/8" thick,finished,as were the originals.

I have been jumped on for suggesting that POSSIBLY the backs were cast in a "V" shape 200 years ago,then closed. Since most brass was cast in the 18th.C.,and since annealing the rolled brass to fold it renders it as cast structurally, I do not think my suggestion was unreasonable,or beyond possibility. Just a guess on my part,but we just don't have complete documentation on every detail of making everything in the 18th.C..

Of course,my method took equipment(though we made it ourselves) to do. Some prefer to slit the backs about 1/4" deep(I think),and glue them in. That takes a milling machine,or at least a drill press(and watch out for slitting your fingers!!!!!:) with the slitting saw. Metal is EASILY warped when getting it hot from slitting or cutting it,so BE CAREFUL if you try slitting your backs. Warping will induce grabbing,ruining the cut,and possible injury.

[Bryan Schwerer]

I have a 14" tenon saw. The blade flexes at the rear end like an oil can(no kink). I put it in a vise and tapped along the spine and didn't see much change. At the back, I was able to tap it down to the handle hole, but it didn't improve the flex and the front moved up. The spine didn't look straight (up and down, not side to side), so I put a straight edge against it and it cups with in between a 1/16" to 1/8" gap in the middle. Any ideas?

[Scott Permar]

Well, I tried a little bit of everything and it's better, but still seems a little strained going through the kerf (even with wax). When the plate was separate I found most of the distortion is in the front few inches. Re-installing the back seemed to make it a little worse and I also eased off on the tightness of the handle screws to reduce that influence. I used the crescent wrench adjustment to twist the back/plate a bit straighter and then added some extra set. After a set few adjustments it cuts a pretty straight kerf to full depth just sawing a test without a line. Still need to do a careful 'cut to the line' test but if that passes I may just use the saw awhile to get a better gauge of actual results and not just feel. The handle is somewhat small between the horns for my hand so this is not likely to become my favorite saw so far. These backsaws seem like they can be fussy to get spot on and after the effort I put into the saw I wonder when I'm better off just buying a new saw. But I like to do things the hard way and really understand how the tool works to the best of my ability.