Thread: Backsaw saw back mortises: Tight or loose, which is better?

What? Nobody cares? Why am I spending so much time trying to get a close fit if it doesn't matter?

Originally Posted by rob streeper

What? Nobody cares? Why am I spending so much time trying to get a close fit if it doesn't matter?

Nobody knows? Probably to get a valid indication you would need access to quite a large number of saws and be able to try them all to see if there is indeed any variation in their performance. From an aesthetic viewpoint the tight ones look better to me anyway.

Originally Posted by burraboy

Nobody knows? Probably to get a valid indication you would need access to quite a large number of saws and be able to try them all to see if there is indeed any variation in their performance. From an aesthetic viewpoint the tight ones look better to me anyway.

I like the tight slots too. In my experience with the saws pictured I can't really tell any functional difference. The older saws with the sloppier mortises feel pretty much the same, perhaps because they all have plates that are thicker than is usually seen on the modern boutique saws.
In making my own saws however I find that the thin tall plates absolutely require a tight fit of the back to the plate and of the back to the mortise in the handle. If everything is not tight the plates will flex in use causing the kerf to wander unpredictably or to bow causing an outboard drift.
More generally I wonder about the relative importance of aesthetics and function in the boutique tools market. Is appearance more important than function or the converse? Maybe a subject for a poll on the site?

Originally Posted by rob streeper

More generally I wonder about the relative importance of aesthetics and function in the boutique tools market. Is appearance more important than function or the converse? Maybe a subject for a poll on the site?

Can of worms there Rob, I'll excuse myself now.

Originally Posted by burraboy

Can of worms there Rob, I'll excuse myself now.

I appreciate your concerns. I however am generally not squeamish about bringing up such subjects and would genuinely like to hear the views of others on the forum.

Hi Rob. I prefer a slide fit. Yes aesthetics is an important element to consider when building a backsaw. But it should always be considered a secondary priority over performance. Good saw makers through ongoing experience reach a level where they are able to include both these elements within their work.

Stewie;

Originally Posted by planemaker

Hi Rob. I prefer a slide fit. Yes aesthetics is an important element to consider when building a backsaw. But it should always be considered a secondary priority over performance. Good saw makers through ongoing experience reach a level where they are able to include both these elements within their work.

Stewie;

I agree. I've always admired Bridge City Toolworks products for their fine appearance but I'm much more impressed by their functional excellence despite the eye-watering prices they command.
I think a closer fit, unless a loose one is technically justified, is a mark of high craftsmanship. My mortices are particularly difficult to make because the profile of my folded backs is somewhat like a teardrop. Takes lots of test fitting to get it right.

closeup #14.jpg

I think Rob
You have just answered your own question
It's a sign off Quilty.
I personally think a saw with a lose back is only going to get loser over time due to parts moving wobbling a bit.
Tho one with a back stuffed in the mortise is going to put undue stress on the mortise possibly breaking out at a week spot in the grain.
I think a buyer of bespoke saws is going to look for things such as the fit of the back to the mortise.
I know I would.
Partly because I understand from doing it
It is a fine line from in my opinion from looking truly great to looking like it was hacked out with blunt chisel the back to mortise fit that is and Ye I know it's hard thing to do well.
I also think it may be hard to explain to the novice saw buyer the reason for the loose fit of the back even if we add a feasible reason.
It may be seemed to be sloppy workmanship considering the price a handmade saw will sell for compared to the hardware chain stores $9.99 do all saw.

From my perspective, the saw back has a 3d function. Please forgive my lack of eloquence in my attempt to explain this.
1) The primary purpose/ importance of any saw back is to support the saw plate horizontally, ( x axis ), keeping it straight under compression from the toe end. The thinner the plate the more important. As the cut proceeds forward the forces shift from compression to tension along the plate. ie: if you are half way through the stroke, the front half is in tension and the heel half compression. Hence Japanese saws do not need backs always being in tension. This is also why the toe end of the spine should be as tight if not more so than the rest of the spine. To prevent distortion/ wrinkling of the plate. ( A down side to folded backs not made with enough spring force on the plate. Or slotted backs being too loose in the slot. ) I have considered placing a steel pin through the toe of the spine just to see how much difference it would make. But then there would be the problem of removing the pin for what ever reason.
2) The saw back folded or slotted, does very little in the way of vertical support of the plate.( z axis ) The plate thickness and tote slot are responsible for much of that. However, the back adds to that rigidity binding the plate in the center of the tote/ plate slot. ( if that makes any sense ) Hence if the spine is not centered on the plate/ tote mortise you get that nasty bow in your plate. Some may compensate for this misalignment by having the spine fit loosely ( float ). This may also help compensate for a plate slot not being parallel with the tote sides. But then the plate will not be parallel with the tote, vertically or horizontally, when all is said and done. Straight plate but at an angle to the handle.
3) A parallel spine will assist in maintaining a plate parallel with the tote as mentioned but with a loose fit the plate/ spine will move from side to side along the y axis during use. Just as the plate moves from side to side if you only put the spine on the plate up to the cheek and stop. Sooner or later you will kink the plate right at that spot or damage the tote.
Once the tote is tightened down onto the plate, there should be the same amount of pressure on both components. All components should be parallel with the tote vertically and horizontally as well as center alignment. Plate and spine. To loose and the plate may move along the y axis as well as the z axis during the stroke and your cut may not be so easy to keep straight.
Also one should note, a slotted spine will often make contact with the plate 1/4" to 3/8" depending on how deep the slot is cut. So if the slot is not parallel with the sides of the spine, the plate will be skewed, adding to the problem of alignment between the plate slot, mortise and spine, vertically. Laying it on an inspection plate, or other nice flat surface, and pressing down on the spine will show this clearly with the plate being higher on one side than the other.
On the other hand a folded back may not make as much contact with the plate, perhaps 1/8" or less, depending on the shape of the folded back. This allows for some slop to some extent with the contact point being able to rotate slightly, one way or the other. But now you have the problem of getting that tote mortise fitting properly. If you are using a mill/ CNC machine you can angle the cutter to match the folded back. This is assuming every folded back is exactly the same shape/ size. Some of the vintage makers seem to have just preferred to just mill the top of the slot to fit snug, leaving a gap along the bottom. ie: a vertical mortise only. This would allow for a certain amount of misalignment of the plate/ spine mortise. That would also allow for different plate thicknesses. ( Bonus )
Lastly, by having the plate slot cut deeply ( beyond the spine ) it adds some flexibility to the tote itself. Same principle as the folded back but at an adjacent angle. Clothes pin effect, for lack of a better description. So I hope this rambling, at least, makes some sense. So now I have done gone and muddied the waters. AGAIN.

Ron,

Agree with your comments. I've found that the thin plates need a very tight back, especially when taper ground. So tight in fact that the first couple times I fit some up I was afraid that the plate might tear or wrinkle. Never happened so I guess the grip isn't that high. Another observation I've made is that it is good to leave a slight gap, no more than a millimeter or so between the bottom of the back and the mortise. This gap allows for post assembly tensioning of the plate without the risk of splitting or compressing the fibers of the tote.

So that's four votes for nice-and-tight and zero for big-and-sloppy. Anybody else?

Cheers,
Rob

I think I like the tern "slide fit" or snug better. Lie Nielsen is a good example of an excellent fit. Oh, I could only wish I had all his toys.

I always try for a nice neat fit, no gaps.. sometimes I manage to fluke it and get one that fits perfectly.. Never loose..

Ray

As a follow up from Ron's excellent comments, with slotted backs, my preference is to use loc-tite to fully adhere the hardback in position. With the backsaw fully assembled, apply a thin bead of loc-tite to both sides of the slot, and wipe away the excess with a rag before its hardens. Then remove the handle so that the remaining area of the slot can be treated with loc-tite. Wipe away the excess; job done.

Stewie;

Originally Posted by planemaker

As a follow up from Ron's excellent comments, with slotted backs, my preference is to use loc-tite to fully adhere the hardback in position. With the backsaw fully assembled, apply a thin bead of loc-tite to both sides of the slot, and wipe away the excess with a rag before its hardens. Then remove the handle so that the remaining area of the slot can be treated with loc-tite. Wipe away the excess; job done.

Stewie;

Sounds like your saws are permanently assembled. That would open some options on saw bolts/screws because they become essentially non-functional in the sense that there's no further need to remove them once the Loctite hardens. For instance, you could use tapered pins secured with Loctite or compression rivets as used in knife handles. Your design creates a lot of options...