Slotted Vs. Folded Backs

[Jelly]

Following on from Stewies Thread...

I'll be honest, my assumption is that slotted backs are popular with small makers and people making their own saws because it's hard to acquire, set-up and even store the kind of equipment needed to accurately bend a 6" to 18" long by 2" wide strip of 1/8" to 3/16" thick brass in two controllably round a saw plate.

This is a pretty important point all by itself.

Isn't a folded brass back supposed to be better, allow for retensioning should it become necessary at some point?

Ever seen a slotted and glued saw, that was in the need of "retensioning"?
I don't and if, it would be easy. take it all apart, clean the spine and glue the blade back.

I think the counterpoint is that the approach of using a slotted back and structural adhesives is still quite new, and the saws which are out there were either made for personal use or were quite costly purchases by experienced users so the saws themselves haven't yet seen the kind of hammer that folded brass and steel backed saws from days gone by have to cause defects requiring retensioning.

I'd also suggest that my experience with these kinds of adhesives is that the kind of damage which would deform the spine in particular would cause the adhesives to experience brittle fractures and fail.

I also suspect that as long as they're well cared for, there is likely to be no noticeable difference in use between slotted and folded backed saws... I'm interested to hear other opinions on this however.

 

[pedder]

I think the counterpoint is that the approach of using a slotted back and structural adhesives is still quite new.

Independent Saws started to make saws in 1996. So these kind of saw making is at least 19 years old.

http://www.popularwoodworking.com/woodw ... ory-lesson

I'd also suggest that my experience with these kinds of adhesives is that the kind of damage which would deform the spine in particular would cause the adhesives to experience brittle fractures and fail.

Again, I yet have to see a damaged saw of that making.

Cheers
Pedder

 

[custard]

Pedder, I've just been admiring your gent's saws. The entire thing is wonderful but the curve at the back of the saw plate is especially delicious!

=D>

 

[Cheshirechappie]

The spine of a backsaw is there to add stiffness and weight. Both folded and slotted spines will do that, so there's no real functional difference, I reckon.

The sort of damage that kinks a saw-spine will probably kink the blade, too. Bending either a folded or slotted spine with a kink straight again would be extremely difficult, so with either construction it's probably going to write the saw off.

 

[Corneel]

For me it was pretty simple, I wanted to copy vintage designs, so I also wanted folded spines.

I think it is easier in a simple back yard shop to make a folded spine. No need for fancy milling equipement. Just a big hammer and something sturdy acting as an anvil. Files to remove the dents and lots of sandpaper. I also used one of these handheld bandsanders, upside down in the vise.

About the possible repairs, a folded back is easier to correct, but it might be more often needed because the blade slips easier inside the spine. A kinked back isn't so difficult to straighten again, but it might need some refiling and sanding again. (but a kinked back is very rare again).

 

[swagman]

There are two types of people: those that talk the talk and those that walk the walk. People who walk the walk sometimes talk the talk but most times they don't talk at all, 'cause they walkin'.

 

[Jelly]

Wisdom begins in wondering

• - Socrates

 

[Trafalgar]

I acquired an older S&J tenon saw some time ago that was advertised as having a 'very straight toothline.' It didn't. I tapped the spine a little here and a little there (all of twenty seconds work) and the plate became straight and has stayed that way. I assume this would not have been possible with a milled spine, but wouldn't have my feelings hurt at all if this is a misunderstanding on my part. Is it?

 

[Tony Zaffuto]

I acquired an older S&J tenon saw some time ago that was advertised as having a 'very straight toothline.' It didn't. I tapped the spine a little here and a little there (all of twenty seconds work) and the plate became straight and has stayed that way. I assume this would not have been possible with a milled spine, but wouldn't have my feelings hurt at all if this is a misunderstanding on my part. Is it?

I have a liking for folded backs also, for similar reasons to Charlie's. BUT...I have a saw, with a folded back, that has been relegated to a wall hanger, with me unable to straighten the plate, as the folded back is more of a folded "sway-back"! I've done everything with the saw, even as far as completely removing & re-inserting the saw plate. Can't remember the name as this moment, but it is of English vintage, with a lovely handle (plus it has been about a half dozen years since I've putzed around with it). It has also been sent to a US saw smith, who also was unable to cure its ills.

Also, from the many saws I have (vintage and current make), I have a liking more for the look of a folded back, and they also seem subjectively a bit heavier towards the toe, which helps my ham-fisted sawing technique!

 

[MIGNAL]

Many years ago I made a very wide kerf saw, it was for a specific task. I laminated two blades together with epoxy. This was then epoxied into a folded brass back. The brass back was effectively a slot because I had deliberately opened it up. When the blades became dull I used to heat the brass back with a propane torch and the blades would pull out easily, blades separated ready for sharpening. I can't think of any problems that it would cause that could not be remedied.

 

[Corneel]

Tony, Blacknurn tools has written on the subject of straightening backsaws:
http://www.blackburntools.com/blog/saw-build-along-assembly-part-ii/

Usually with a combination of bending and twsisting I get a back and the plate straight. Sometimes the handle has induced a small bow at the rear end of the saw under the handle. That is almost impossible to get rid of, but luckilly is of no consequence for the fundtion of the saw. It is more of a cosmetic defect.

 

[pedder]

I acquired an older S&J tenon saw some time ago that was advertised as having a 'very straight toothline.' It didn't. I tapped the spine a little here and a little there (all of twenty seconds work) and the plate became straight and has stayed that way. I assume this would not have been possible with a milled spine, but wouldn't have my feelings hurt at all if this is a misunderstanding on my part. Is it?

Why was it not straight?
I know 3 different cases:

1. Most often, that is because the blade wasn't held in the spine. It slips. And when it slips backwards to the handel, bows in the blade apear. They disapear when you tape the blade back where it belongs. A short cut is to tape the toe of the spine down. That often straights the blade and lead to a canted blade, wich many favour.

A good example how dangerous this trick is, can be find in my workshop. The spine was driven to deep and opened because it touched the blade.
This trick can't be done with a slotted and glued spine. But again: I've never saw such a bow in a glued spine.
(By the way, did they do this carthago thing?)


2. If the blade is bowed because the spine is straight and he bow is in the blade, I can't do more than bow the blade. Often that does not help. One can try to dissasamble the blade from the spine and straighten it, but that is not easy on both methods folded or slotted.

3. If there is a bow in the spine but not the blade, the spine can be straightened. No difference here between slotted and folded spines.

Cheers
Pedder

 

[Trafalgar]

I really didn't spend a lot of time assessing why it had a slow curve in it. Sometimes a blade can get stuck in a cut and pulled slightly down from the spine I suppose. Tapping it in back deeper then fixes it. I'm just happy it's fixed.

 

[Klaus Kretschmar]

Folded backs work. They proved it for 200+ years. Milled backs work as well - proved not that long although. If a glue joint will get loose somewhere down the road, it won't be a big deal to reglue it. Folded backs need relatively stiff blades to work well. Most of the vintage back saws have blades with 0.5 mm or thicker. If you want to go with a thin bladed saw, a folded back probably won't work reliably at least when it's used the normal way by just clamping the blade. A milled back works fine with thin blades, the glue joint will compensate the lack of stiffness the thin blade has. Currently it seems to be a trend to fine bladed dovetail saws that I can understand easily. They cut easier and smoother, just more pleasurable. So my conclusion is that old fashioned thick bladed saws can be made with both of the backs without quality loss. For thin blades a folded back isn't an option.

Pedder and I have another reason to go only with milled backs. It's a consequence of our saw design. An ovally shaped and flush to the top mounted spine you won't get with a folded one, it needs to be a milled one. But that's only a design thing of course.

Klaus

 

[Jelly]

So my conclusion is that old fashioned thick bladed saws can be made with both of the backs without quality loss. For thin blades a folded back isn't an option.

Thanks Klaus!
That's useful information and explains my anecdotal experience with thin-plated Dovetail Saws/Gent's saws.

My Gent's saw is of folded construction with a very thin plate, it keeps coming loose which is a major PITA as getting it secure requires me to soften the brass then re-work it with a big hammer whilst ensuring everything stays straight (the back is growing at a rate of about 1mm every 3 years, so eventually i'll end up thinning it too much with the re-working.) not a 2 minute job. Next time I may just use adhesive, it's a specific loctite compound that's used by yourself and pedder is it not?

I also sharpened an even thinner plated S&J Dovetail saw recently for a friend, in spite of the plate being firmly held by the back, it seemed prone to flex, as if the thin strip where it was held by the back was a fulcrum point.

 

[Klaus Kretschmar]

Next time I may just use adhesive, it's a specific loctite compound that's used by yourself and pedder is it not?

We're using an industrial glue of another brand but Loctite will work fine as well. It's important to choose the right type. First and foremost it must be a very thin fluid glue that is running itself into the slot.

Klaus

 

[Isaac S]

Here's part of my take on the subject. As you read this, keep in mind that I have chosen to use slotted backs in making my backsaws. I don't use them because they are easier or cheaper to make. I use them because I firmly believe they hold several advantages over folded spines.

One of these advantages is the ease of fitting the blade into the spine for test fitting everything together. This may not show up in the use of the saw, but when you are making one offs, this ease of dryfitting everything together is a real luxury.

My method of assembly does not use adhesives, but relies on friction to hold everything together. Just as with a folded back, the slot is pinched together to capture the blade. Unlike a folded back, the clamping force of a slotted spine can be easily manipulated. Want an easy fit that goes together with a few light taps? Just squeeze the slot a little bit. Want a fit that takes real force (along with some courage and luck) to put together, but results in a near permanent assembly? Sqeeze that slot even more to get a bond that no folded back can equal. There are a few pictures and a more lengthy description of this process here and here.

I use an approach that falls between these two extremes. By squeezing the slot a few thousandths of an inch, I get a fit that assembles without too much effort, but with significant holding power. This holding power gives what I consider the greatest advantage of the slotted spine - the ability to truly tension the blade.

Unlike a folded spine, where "tensioning" the blade amounts to little more than straightening it, a slotted spine can be used to impart significant tension to the toothline of a blade. By bending the spine before assembly (this step was not shown in the blog entries above), significant tension can be applied at the toothline when the spine and blade are assembled. This tensioned toothline has at least two benefits: first, the blade is naturally straightened, and second, the tension allows the blade to expand slightly without warping when the blade heats up, as it often does when used in deeper cuts with little set. This keeps the blade tracking straighter.

The drawing below may make what I've said above a bit clearer.

 

[Corneel]

That's very interesting Isaac. But why would that not be possible with a folded spine?

BTW, I like your method of using a friction fit in slotted spines. Modern glues are nice, but they don't have the proven track record of the friction fit folded ones yet.

I don't think the old timers chose for folded spines based on engineering specs. They just made them with the technology they had available.

 

[Isaac S]

That's very interesting Isaac. But why would that not be possible with a folded spine?

Folded spines, in my experience, don't have enough gripping or clamping force to generate the friction needed to pull and keep the spine straight. If you try to install a folded spine that is curved like the one shown above, one end just slides back up as you force the other down.

 

[Corneel]

Allright, thanks for the explanation.