Help with a saw that produces curved cuts

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  • Jorge
    14 July 2020 at 1:17 pm #669605

    Hi, hope there is someone there able to help

    I bought a vintage Spear & Jackson brass-backed tenon saw, 12″ 15tpi, the kind of which there are lots on ebay. The saw won’t cut straight, it curves. It is not my sawing technique. After trying many things, mostly fiddling with its teeth setting and sharpening I’ve discovered what the problem is: there is a very slight curve length-wise, on the blade, not the brass back, which is straight. After removing the screws and taking off the blade I see it is straight. It bends only when I tighten the screws back on. To confirm it is this slight blade bend that is causing the problem I’ve tried sawing only with the first front half of the saw (which is straight) and this cuts straight. It is only when I use more or less the full length of the saw that the cut bends.

    So I know what the problem is. Any idea how to fix it?

    Thanks in advance

    • This topic was modified 2 years, 6 months ago by Jorge.
    YrHenSaer
    14 July 2020 at 2:31 pm #669612

    Are you saying that with the normally-straight blade mounted in the handle it develops a bend?

    If so, it suggests that the wood of the handle has distorted. Unusual but not insurmountable.
    The fixes available are:

    1 – Is the saw worth the work? If not, do nothing.
    or
    2 – Toss the handle and make a new one. The advantage is that you can suit the size and shape to your own grip, using all your skill and invention.
    or
    3 – Repair it. If you are happy with the existing handle (except the presumed distortion), mark out the limits of the slot on the handle and take the plate out. Fashion a very thin piece of wood that is wide enough to slip into the slot vacated by the plate. Glue it in place and when it’s dry, round over the end to remove any fragments. With a pin-gauge, run a mark in the exact centre of the handle. Select a saw with a plate that is exactly as thick as the original plate. (Often the original plate works best for this if you tap out the teeth to reduce the set to give zero kerf). Saw to the exact limit marks of the slot and cut a new one in the EXACT centre – don’t go any deeper than the original slot. Run a drill bit that is exactly the size of the holes in the plate, gently through the holes in the handle. The slot needs to be a tight, sliding fit.
    You can now joint, sharpen your plate and add a new set before re-assembling the handle….. don’t forget some thin oil in the slot/plate – prevents corrosion

    Good luck…… let us know how it works out.

    Jorge
    14 July 2020 at 3:16 pm #669618
    YrHenSaer wrote:

    Are you saying that with the normally-straight blade mounted in the handle it develops a bend?

    If so, it suggests that the wood of the handle has distorted. Unusual but not insurmountable.
    The fixes available are:

    1 – Is the saw worth the work? If not, do nothing.
    or
    2 – Toss the handle and make a new one. The advantage is that you can suit the size and shape to your own grip, using all your skill and invention.
    or
    3 – Repair it. If you are happy with the existing handle (except the presumed distortion), mark out the limits of the slot on the handle and take the plate out. Fashion a very thin piece of wood that is wide enough to slip into the slot vacated by the plate. Glue it in place and when it’s dry, round over the end to remove any fragments. With a pin-gauge, run a mark in the exact centre of the handle. Select a saw with a plate that is exactly as thick as the original plate. (Often the original plate works best for this if you tap out the teeth to reduce the set to give zero kerf). Saw to the exact limit marks of the slot and cut a new one in the EXACT centre – don’t go any deeper than the original slot. Run a drill bit that is exactly the size of the holes in the plate, gently through the holes in the handle. The slot needs to be a tight, sliding fit.
    You can now joint, sharpen your plate and add a new set before re-assembling the handle….. don’t forget some thin oil in the slot/plate – prevents corrosion

    Good luck…… let us know how it works out.

    Thanks for replaying, much appreciated.

    The saw is definitely worth saving, and I will possibly make a new handle at some point, but it is not a priority and it wont happen for quite some time. I had thought of something along the lines of 3 as it is in the plate slot that the problem is (slightly misaligned, maybe just one of the sides). The difficulty I see is that whatever the exact fault, it is so slight that I would quite possibly misfire. I was hoping there could be something more trivial I could try….

    Thanks again

    Jorge
    14 July 2020 at 4:18 pm #669627

    In the end the fix was indeed trivial: the exact problem was a VERY slight misalignment of just one of the walls of the mortise for the brass back. It means that it put a bit too much pressure on that side of the plate and this produced the curve. I filed off this mortise wall, just a little bit, with a small saw file. Now the whole plate sits perfectly straight and the saw indeed cuts straight!

    It is amazing to see that such a very small, unnoticeable really, detail can have such an impact on the performance of the tool

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