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  1. #1
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    Default Revised version of a trenching saw

    A couple of years ago I made myself a saw for cutting the sides of dado or sliding dovetail trenches. It provoked quite a discussion, and a few very reasonable questions, like "Why would you want to build a saw like this when an ordinary backsaw will do the job?" I have a couple of answers to that, the flippant one was "Because I can", but the serious answers are that a saw like this has a low centre of gravity which makes it easier for me to keep against the guide when sawing. The wooden 'back' to this saw also acts as a depth stop, so the saw cut ends up an even & repeatable depth all along (except for the last 40mm or so in a stopped trench - see later). The amount of blade exposure is easily adjustable by loosening the 3 retaining bolts.

    That saw was a prototype, but I have this habit of not getting around to making the 'production' model for some considerable time (or never, in some cases!). Well, I decided to move quickly this time, so in just a shade over two years (), here is its successor (which could be a record for me): New S_D.jpg

    I made some 'improvements' on this saw. It is a teeny bit longer, and I set the hang angle of the grip a little higher (i.e. closer to the horizontal). If you are wondering why the blade has that funny sticky-out bit at the front, it's because I wanted to see if a bit more crowning on the end would be helpful in clearing sawdust. I wasn't sure it was going to work as I envisaged, so I added the bit extra, reasoning I could cut it off and reshape the end, if it did what I hoped. It doesn't seem to add any value, so it's coming off. Here is the original compared with the new: saws cf.jpg

    The original handle was set at about 25 deg. from vertical, the new one is set closer to 40 deg. I took that angle off a very comfy backsaw I use a lot on a bench hook. As it turns out (just a few trial cuts so far), it's actually a bit high. There is a slight difference in the way I lean to make these trenching cuts, and the handle could benefit from being a trifle lower, somewhere around 35 would be the Goldilocks angle, I think. It's still pretty good, and works fine as-is, I'm just being a persnickerty old goat - I will almost certainly not make another, but if I do.... Just goes to show, I can theorise all I like about how to establish the perfect hang angles (see the long discussion here!), but it still boils down to some trial & error for a specific saw in a specific application.

    And about cutting a stopped trench. I found with my old saw that I could cut all but the last 20-30mm to full depth if I just made a short hole at the 'stop' end: Stopped trench1.jpg

    The bit that the saw couldn't cut because of the slight crowning of the end was easily fixed by cutting down on the sides with a wide chisel, registering it against the sides. However, the extra crowning on the new saw means the cut is a bit shallower still, & leaves a longer length to remove: Stopped trench2.jpg

    So, as it doesn't seem to be of any material help in any other way, it's going to be removed & I will go back to the slight crowning of the original, which did a perfectly good job after all!

    Was it worth the effort? Not really, the old saw works nearly as well, & certainly well enough to do the job. My newer saw is a bit smoother (I'm better at forming teeth & sharpening), a bit more comfortable to hold, but that's it. BTW, the cuts look a bit ragged in the pic, that's because I was using some very soft & crumbly old Hoop scrap, and also because I was being lazy, and only clamped the guide with a single hold-down, so the guide moved a bit on a couple of cuts. I wouldn't do that if I was trenching some expensive cabinet part....

    Cheers
    IW

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  3. #2
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    Hi Ian,

    Great work. Kind of a hybrid of a back saw and a stair saw. How did you tooth these?

    Cheers,
    Rob

  4. #3
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    Hi Rob, thanks. Sorry, forgot to include the vital statistics. It's filed 10tpi, with 10 degrees negative rake, 20 degrees of fleam, and about 15 degrees of slope. I use a home-made saw vise that doesn't lay over, so my slope is judged by eye & limited by the wide jaws of my vise. The tooth size & pattern reflect what it's used for, which is making long, shallow crosscuts, in medium-hard woods - comparable to your Walnut or Cherry, say. I settled on the larger tooth size because the saw has to carry the sawdust for a long stretch on a wide board, & in fact the centre teeth sometimes can't exit the cut & I have to remove the saw & clear them, though not as often as you might expect - depends on the wood.

    The handle wood is "She-oak" (Allocasaurina torulosa), no relation whatever to northern hemisphere oaks, but named for the prominent medullary rays (as are several other species in Australia, unrelated to the she-oaks or Quercus spp.) This stuff is very dense, and has much smaller pores than a typical Quercus, so takes a very fine, highly tactile finish - one of my favourite handle woods.

    Yes, I took my inspiration from the stair saw, I suppose. You could also think of it as a backsaw, but with an adjustable wooden back instead of a fixed metal one, and the back serves as a depth stop.

    Cheers,
    IW

  5. #4
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    I like the first one. I think I would be prone to adding more rake and fleam since it is predominately a cross cut saw. This would help keep the edges clean for me. Don't feel bad about the time interval. I have some still waiting for the first one to be made. 2 years later. Thanks for posting.

  6. #5
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    Nice looking saws, I like the little bead on the back of the spine.
    How did you cut the "breasted" section of plate?

  7. #6
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    Quote Originally Posted by hiroller View Post
    ....How did you cut the "breasted" section of plate?
    I just filed away at it with a 10" file 'til it looked about where I wanted it - it's a judgement thing, not precision science.

    The tricky part is laying out the tooth spacings on a curved edge. I have made a set of templates out of scrap for the common tpi's I use (old bandsaw blades are handy for this). These make laying out straight sections a breeze, but setting out the curved section is more of a challenge. There are a few ways you might do it, but the simplest for me is to use a paper template. Stick it on perpendicular to the straight section, then trim it to the curve. That means the spacings will alter slightly as you follow the tangent of the curve, but the difference is too slight to notice, on a shallow curve, and easily compensated for when filing the teeth to depth.

    Quote Originally Posted by hiroller View Post
    .... I like the little bead on the back of the spine...
    A job handled admirably by one of the simplest home-made tools there is: Bead centered S_S.jpg


    Cheers,
    IW

  8. #7
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    Quote Originally Posted by Ron Bontz View Post
    I like the first one. I think I would be prone to adding more rake and fleam since it is predominately a cross cut saw. This would help keep the edges clean for me. Don't feel bad about the time interval. I have some still waiting for the first one to be made. 2 years later. Thanks for posting.
    Hi Ron - yes, in some ways I agree, it's less fussy & looks more like a hard-working tool. But I don't like the top horn, which is rather thin & flat. That saw was made from the very last bit of suitable wood I had at the time, which was a bit too narrow, so I had to modify the top horn a bit to squeeze it out of the board. I try to put comfort & practicality foremost on working tools, but, well, I can't help adding a bit of ornament, & it's a fine judgement as to how much is enough..

    I'm still grappling with rake & fleam angles on smaller crosscutting saws & trying to arrive at the best compromises. The rake angle I used on this saw probably seems a bit aggressive on a crosscut, but the combination I used seems to work well on our hardwoods, or at least the ones I use most often. I theorize (& may be totally wrong!) that it has to do with the high silica content of so many of our woods, which dulls edges quickly. As with a plane blade, you can compensate a bit by raising the sharpening angles to make edges 'tougher', which equates to keeping fleam angles a bit steeper. The higher rake keeps the saw biting a bit better as the tips of the teeth wear. It seems to work for me, but possibly not for the reasons I think....

    I tried sharpening with slope early in my saw-sharpening days, but soon gave it up because I was having enough trouble getting rake & fleam consistent, without adding another complication to the mix. But I have been slowly coming to grips with it over the last year or two. Have to say, I don't notice any real difference in performance, on small saws, but one thing it does is make the teeth look more even when viewed from the side. The bevels on non-sloped gullets can give the illusion that these teeth are smaller than the ones leaning towards your eye.

    I think I could spend two lifetimes making saws, and still be learning on the job!

    Cheers,
    IW

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