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28th November 2014, 09:23 AM #1
Backsaw saw back mortises: Tight or loose, which is better?
In my collection of saws I have noted a very wide range of closeness of fit of the saw back to the mortise in the saw handle. Thus I have a question: Which is better, loose and sloppy or tight? I can see arguments both ways. Sloppy looks messy but it may have the very useful characteristic of allowing the saw back to move about while the saw is in use without placing stress on the sides of the mortise thus reducing the risk of breaking out the upper cheeks of the handle. Tight may also be of utility, particularly with thinner saw plates, in that it holds the back more rigidly in line with the cut preventing wandering. Most of the makers on the Forum cut tight mortises. What's better and why?
Here are some examples. First a series of Disston backsaws of varying era.
This is an old Disston #4. Nice and tight and the blade slot does not project back into the handle.
Disston #4.jpg
Old miter box saw. Again the back is fitted nicely.
Disston miter #1.jpg
A newer miter saw. Mortise is tight but the blade slot cuts deeply into the face of the handle.
Disston miter #3.jpg
And what looks like the newest Disston, terrible fit of the back in my opinion.
Disston miter #2.jpg
Now for maker #1. Saw 1 perfect fit.
Maker #2 saw 2.jpg
Likewise for saw 2.
Maker #2 saw 1.jpg
And for saw 3.
Maker #2 saw 3.jpg
Maker #2 saw 1 perfect fit.
Maker #1 saw 2.jpg
Saw 2 is also perfect.
Maker #1 saw 3.jpg
Saw 3 however has a slight over-run of the blade slot into the handle, back fit is excellent.
Maker 1 saw 1.jpg
Maker 3, sloppy fit of the back to the mortise and a deep cut into the face of the handle. This is also the saw with the soft blade I discussed in my thread on hardening saw plates (https://www.woodworkforums.com/f278/hardening-sawplates-182165).
Maker #3 saw 1.jpg
So what do you think? Tight or loose? Why?
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28th November 2014 09:23 AM # ADSGoogle Adsense Advertisement
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2nd December 2014, 01:48 AM #2
What? Nobody cares? Why am I spending so much time trying to get a close fit if it doesn't matter?
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2nd December 2014, 03:04 AM #3
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2nd December 2014, 03:33 AM #4
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?
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2nd December 2014, 03:39 AM #5
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2nd December 2014, 05:23 AM #6
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2nd December 2014, 11:04 AM #7Deceased
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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;
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2nd December 2014, 11:20 AM #8
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
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2nd December 2014, 09:03 PM #9
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.
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3rd December 2014, 09:08 AM #10Member
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fit matters
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.
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3rd December 2014, 01:07 PM #11
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
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3rd December 2014, 02:08 PM #12Member
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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.
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3rd December 2014, 10:06 PM #13
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
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17th February 2015, 12:01 AM #14Deceased
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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;
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17th February 2015, 01:22 AM #15
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...
Innovations are those useful things that, by dint of chance, manage to survive the stupidity and destructive tendencies inherent in human nature.
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