Reconstructing a 15th century German frame saw: Back to basics

I recently posted about developing a prototype for a medieval frame saw or bow saw. I was successful in creating a wooden frame that would hold a saw blade, but I was dissatisfied in the excessive bulk and weight of the frame, and when I went back to compare the finished prototype to an early 15th century illustration of a frame saw, I was disappointed to see how different my finished prototype looked from the original. I had been thinking like a modern woodworker, adding bulk to key places for extra strength, generating lots of waste, and willing to make up for all the waste with a more marketable product.

So with due shame, I literally went back to the drawing board and designed a very different saw from a very different way of thinking. I looked very closely at the saw in the picture, studied its curves, studied each detail and considered how each of the saw’s features would have been executed by the craftsman who made it, and how each would have been used by the craftsman who used it. I wanted very much to make an EXACT replica of what I saw in the picture, but there were a few details that I would have to change. I approached those changes very cautiously, and the end result is very pleasing in its appearance, its performance, and its authenticity.

Hausbucher of the Mendelschen Zwolfbruderstiftung (1414), Neurenberg, Germany, folio 21r

Hausbucher of the Mendelschen Zwolfbruderstiftung (1414), Neurenberg, Germany, folio 21r

Since the length of the saw blade was the one thing I was not able to control, that length formed the basis of my other measurements. The 12″ (30.5 cm) blade, adding a little to facilitate access to the pins for easy assembly, meant that the top and bottom ends of the bows should be about 12-1/2″ (31.75 cm) apart. Accounting for the offset of the outward curves, my crossmember would need to be 14″ (35.5 cm) long, plus about 1/2″ to each end for the tenons, so it would need to be cut to 15″ long. I decided to also make my bows 14″ long so I could allow about a 6″ (15.25 cm) space between the blade and the crossmember. I made my best visual approximation of the curves seen in the drawing above, allowing a maximum overall width of 2″ (5 cm) and a length of 14″, and when I cut this out of a manilla folder and repeated the outline on a plank of wood, I found that four of them across an 8″ (20.32 cm) plank left an extra inch along the edge. Assuming that this 1″ remnant could be reused for turning a pair of knobs, this meant there was very little waste wood. I cut these pieces with a scroll saw, just to save some time and frustration, because I have used a coping saw enough to know I could have done it the period way to virtually the same effect, the scroll saw just does a little neater job by maintaining a more uniform vertical angle on the blade and saves me a little time and sweat.

  • CAUTION: White oak is particularly challenging for the tiny teeth on a scroll saw to cut, and your soft, pink fingers are much easier for the scroll saw to cut. Be careful, especially near the end of the cut.
Cutting white oak bows for a medieval bow saw on a scroll saw. These stack together tightly, leaving very little waste.

Cutting white oak bows for a medieval bow saw on a scroll saw. These stack together tightly, leaving very little waste.

Once I had the bows cut out, I cut a plank to 16″ (40.64 cm) long and ripped it into four 1-1/2″ (3.8 cm) widths for the crossmembers of four saws. I took the first one, laid it out with a pair of bows, with the saw blade as an approximate guide, and double checked the length and position of the crossmember.

  • REMEMBER: Measure twice, cut once. If you cut something too long, you can usually trim it, but you’ve wasted valuable wood. If you cut something too short, you have to throw it out and try again, and that may require buying more wood.
Laying out the pieces to determine the precise length of the crossmember.

Laying out the pieces to determine the precise length of the crossmember.

After trimming the crossmember and marking the ends for the tenon joints, I cut away the wood around the tenon shoulders with a coping saw, squared it all up a little with a bastard file, and rounded the tenon with the same bastard file. Then it was time to cut mortises into the inner face of both bows. These should be 1/2″ wide by about 1-1/4″ (3.2 cm) long to provide a snug fit laterally but allow the bows to wobble vertically over the tenon. This prevents the bows from trying to rotate under pressure during use but allows them to adjust as necessary as the string is tightened. The curved shape of the bow, with the crossmember longer than the saw blade and the string, also plays a key role in managing rotational forces and holding the bows in their proper position during use. To cut the mortises in the bows, I placed each bow into a wood vice, marked a 1/2″ x 1-1/4″ rectangle with a pencil, scored it with a 1″ and a 1/2″ chisel, and then used the 1/2″ chisel to cut a groove, alternating strokes to the left and right. I used a mallet to assist in gaining the 1/2″ depth required, and then pushed the chisel by hand for a smoother finishing stroke.

The ends of the crossmember have a tenon, 1/2" wide by about 1-1/4" long, rounded to allow the bows to rock slightly as the string is tightened.

The ends of the crossmember have a tenon, 1/2″ wide by about 1-1/4″ long, rounded to allow the bows to rock slightly as the string is tightened.

Once the ends were trimmed and fitted, I shaved the crossmember with a draw knife. I did just a little shaping on the bows but I certainly could have done more without sacrificing too much strength. The main purpose of shaving these pieces is to reduce weight. It also gives the saw a soft touch for easy handling and prevents chipping by softening hard edges.

Shaving the crossmember with a draw knife reduces weight and gives the piece a pleasing shape.

Shaving the crossmember with a draw knife reduces weight and gives the piece a pleasing shape.

At this point, I was ready to drill the knob mounts, turn the knobs, and mount the blade. Here is where I intentionally deviated a little from the saw in the 15th century picture. In order to mount the saw blade, the pins need to be accessible, so I would have to either: a) put the pins through the block as well as the knob, which would not allow the user to turn the knob, b) somehow accommodate the full width of the blade (teeth and all) to pull it through the mounting block in order to access the pins, though cutting a through slit in the bottom of the mounting block would significantly weaken it, or c) make the knob spindles long enough to access the pins with the knobs in place. I chose to resolve this problem with method c and allow the tension of the saw blade to hold the pins in place during use.

The mounts were measured and a small pilot hole started with a hand auger drill to reduce drill walk when drilling with a spoon bit. I decided to use my 9/16″ (14.3 mm) spoon bit to accommodate the width of the blade ends. I turned the knobs on my lathe, starting with 1″ x 1″ x 6-1/2″ long turning blanks cut from the scraps of my white oak stock. I turned the spindles to 9/16″ diameter, to fit the holes through the mounts, by 2-1/4″ (5.7 cm) long to accommodate the width of the mounting blocks and allow easy access to the pins. This left a 4-1/4″ (10.8 cm) long handle, which need only be rounded. I added a slight narrowing near the neck of the knob as an extra feature, to make the knob a little easier to handle. The trickiest part of the whole process was painstakingly fitting the spindle of each knob to the hole in the mounting block. The spindle had to be exactly the right size to fit the hole snugly. Both needed a little coaxing at first, and a little help provided by a fine sanding sponge, but they fit snugly enough to groan when turned. That is exactly how I like them.

  • TIP: Do not force the knobs into the holes. Tapping them in with a mallet can cause the mounting block to split, and then you’re back to the beginning, cutting out a whole new bow. Just be patient and put it back on the lathe if necessary to shave it down just a little more. I think I put mine back on the lathe about five or six times.
The bows must be drilled through to accommodate the knobs which hold the blade. A spoon bit and brace has a tendency to walk into a triangular pattern, especially when starting a new hole, but starting the hole with a smaller auger drill may reduce the walk of the spoon bit.

The bows must be drilled through to accommodate the knobs which hold the blade. A spoon bit and brace has a tendency to walk into a triangular pattern, especially when starting a new hole, but starting the hole with a smaller auger drill may reduce the walk of the spoon bit.

Once the knobs were fitted, I placed them into the wood vice and cut a narrow slit, just the kerf of my Japanese flush cut saw (which is about the same as the thickness of this bow saw blade), down half the length of the spindle. Then I put the knobs on the drill press to drill a small hole perpendicular to the slit. Then I cut a pair of simple pins from an old nail, filed them down smooth on the ends, and placed those into the holes. All I needed now was some string and a wedge to hold it. In the Middle Ages they used “catgut”. It’s actually a sort of rawhide string made from sheep intestines. No cats were harmed in the making of this string. That I know of. In any case, natural gut string is pretty scarce these days. I know it can be purchased at exorbitant prices at musical instrument shops, but faux gut string is quite affordable at my local Tandy Leather Factory store, and I already have a large spool of it on hand. For a wedge to hold the string, I used the largest scrap available, the void cut from the back of the first bow, rounded the ends with a file, narrowed the top half to make a flat spot for adjustment, and smoothed the whole thing with the fine sanding sponge. The saw was finally ready for mounting.

Finished white oak frame saw, finished with linseed oil, strung and ready for use.

Finished white oak frame saw, finished with linseed oil, strung and ready for use.

I laid out all the pieces on the work table: two bows, a crossmember, string, a wedge, two knobs, two metal pins, and a saw blade. First, I pushed the knobs through the mounting blocks, slipped the saw blade ends into the slots and inserted the pins. Then I aligned the tenon joints at the ends of the crossmember and then wrapped the string several times around the bows. The string should be pulled snug (little to no slack) and tied very securely. The final step was to insert the wedge between the strings and turn it until the strings were wound tight.

  • TIP: When the parts are still a little loose and wobbly, one or both of the pins may slip loose. Check them as you tighten the string.

As a final step, I gave the whole thing a light sanding with a fine sanding sponge to give the whole saw a soft touch and a slight time-worn feel, and then I rubbed in a little linseed oil to give it a period finish.

  • CAUTION: Rags soaked with boiled linseed oil may spontaneously combust. I usually put mine into ziplock baggies to prevent them drying out, which also allows me to reuse the rag within a short period of time. Follow manufacturer’s precautions and recommendations for disposal of linseed oil soaked rags.
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