Roubo Joinery Bowsaw Prototype – Plate Anchors
Before I got too deep into making the “bow” of the bowsaw I realized I needed to work out the details of how exactly the saw plate was to be anchored to the bow frame. Given the robustness of the saw plate from Bad Axe and the illustrations and commentary from Roubo I knew this was not a casual thing. The amount of tension required to make the saw plate perform well was considerable given the dimensions of the plate, so the anchors for the plate had to be able to withstand the force requisite for making it function well. And, given the likelihood that any user of such a saw as this might well want to swap out the plate from one utility to another, taking advantage of varying plates that could be available. So, the fitting of the plate to the frame needed to be not only exceedingly stout but also easily reversed or swapped-out.
The clue to the preferred manner of fixing the plate within the bow frame was pretty clearly described by Roubo in his commentary to Plate 12. While the simplest method would be to simply drive a pin through the foot of the bow frame and the end of the plate, there was and is a better way. And he tells us how to do it.
There is still another way to attach the blade in the saw, which is to use stirrups, which are pieces of sheet or flat iron that you fold into the form of a “t”, and that you attach to the two ends [of the blade into the stirrup] with a single nail the same way as above. These are then inserted into grooves in each of the arms, which you take care to fasten tight enough to hold them. This method is very good because the stirrups holding the arms from their back sides make full use of the arms’ strength, without which they might split, and you only use one nail to hold the blade at each end because if there were two it would prevent even blade tension, figures 11 & 12.
I took this information/description and started running with a couple of changes. First, the folding of bar stock into the “T” was not possible with the material I had on hand, although it might be fine with lighter steel flat stock like 1/32″ or 1/16″. I didn’t have that (nor did the local fabrication shop) and I was too impatient to wait for some to be shipped to me. Second, the plates supplied by Bad Axe have two bolt holes, which I believe are necessary to house fittings strong enough to tension the plate. Still, I really liked the concept of a “T” shaped stirrup to affix the plate in the frame.
So instead of folding thin flat stock for this purpose (although I am certainly likely to try it in the future with either thin soft steel flat stock or brass, although I like the solution I came up with for other reasons) I sawed some 1/8″ x 1″ x 1″ angle stock I had on hand. I cut the length such that the body of the plate would be fully housed but the teeth were exposed and unfettered.
Then I cut off most of one side of the angle stock to reduce the arm of the stock from 1″ to 1/2″ so that when two pieces of the cut angle stock were placed together the configuration would be a “T”. I used the finished plate itself to provide the layout holes into the stirrup “T” plates. One half of each pair of the plates was drilled and tapped, the other was drilled and countersunk to fit machine bolts I had in my hardware stash.
Placing the two halves of the “T” stirrup over the pair of bolt holes in the plates, and screwing in the machine bolts, the task of mounting the plate to a stirrup was finished and it was time to move on to the bow frame (this picture is a bit out of sequence and was intended for another purpose but you get the idea).
Hi Don,
I see you choose not to follow the advice:
“and you only use one nail at each end because if you were using two it would prevent even blade tension”.
With a wide blade, with this two pin setup, you might have a blade tension which is not the same on the teeth side and on the back side; with one side slightly buckling as a result. I am curious to know if it happens in practice.
Sylvain
Yes, I chose to go with two pins mostly because the saw plate was pre-drilled with two holes, and I did not want to drill a third hole in the center of them. The stirrups have a bit of play in them due to slightly oversized countersinks, allowing the bolt/pins to seat to the pair of holes just fine and I have not noticed any buckling whatsoever.
Hi Don
Really interesting mounting method.
It is very similar to the one used in my mulesaw (albeit it is considerably larger)
On the blades for that mill, each end has got two hefty pieces of steel riveted to it. These slide into a groove just like on your frame saw.
So I have a lot of faith in that this method will hold up to some heavy use.
Brgds
Jonas