Building a hacksaw-bladed bow saw from <$10 in materials.

This is a classic design, a simple H-frame held together by some simple mortise and tenon joints and tensioned with some twisted string. This tension is important because these blades are narrow and would likely not stay straight enough to make a clean cut on their own. Other saws stay straight by being relatively thick and heavy (i.e. carpenter's saw), having a stiffener on the back (the back in back saw), or by cutting on the PULL rather than PUSH stroke (Japanese saws, like the one I used to cut the pieces for this project). Frame and bow saws trace their origins back through at least the Middle Ages, used in traditional woodworking even to the modern day.

So, I needed to cut some O1 tool steel (to make a new blade for my jack plane, but that's another Instructable) and my old, tiny hacksaw had had it. Could I buy a brand new hacksaw from the big box store? Absolutely. This is an exercise in making your own tools rather than buying them. I intend to use this as practice in general woodworking and for building a larger bow saw for unplugged woodworking. Everyone ought to have a hobby.

3/15 Edit: I am finally posting pictures of my (much!) larger bow saw, fitted with a somewhat more expensive, 600 mm crosscutting blade (~$20 on Amazon). All the skills you need to build this saw are already in this Instructable, read along to learn! More pictures at the end.

Step 1: Materials and Tools

Materials.

Fir (?) 1"x2"x4'* - I originally bought this from a big box store, choosing the least-warped, fewest-knot, straightest piece I could; the extra length means you can cut around any weird defects in your wood
Note that 1"x2" dimensional lumber like the cheap stick you need for this project is actually about 0.75"x1.5", this is fine.
Hacksaw Blades - This was a two-pack, you can find inexpensive hacksaw blades pretty much everywhere; you can also use a wood-cutting bow saw blade
Machine Screws & Nuts / Screws / Nails / Pins - I happened to have some odd machine screws lying around in my odds and ends; you just need two pieces of hardware that fit through the holes in the ends of your saw blades
String - I used cotton butcher's twine; you will want something that doesn't have a lot of stretch

Tools.

I have given two pictures as examples of the tool you will need. I used the first set, but could certainly have made do with something like the second. There is something beautiful about using hand tools to make more hand tools. If you didn't have a woodworking saw, you can make one!

Measuring & Marking Tools - I like a pencil, eraser, and combination square; the added advantage to using a square is being able to make lines perpendicular to the edge of your wood, very handy for this project
Drill & 1/8" Bit - you need to make two holes, the bit should fit in the holes at the ends of your saw blade
Saw - yes, you need a saw to make this saw; I suppose any method you like for that would work
Chisel & Mallet - we will be making mortises (pockets in wood) here, they could be cut some other way but this is the easiest one I know for making nice, vertical edges on those pockets

Disclaimer.

You are responsible for your safety. Many of these tools have sharp edges and if you are uncomfortable handling any of them, please seek out assistance. They are not hard to learn. Protect your eyes, hands, feet, and brain.

* - You can definitely use different woods for this project, I had this scrap lying around and I was able to shape and work it extremely rapidly given its inherent softness. Hardwoods are definitely more traditional. I intend to use this very same process to make larger and finer saws in the future.

Step 2: Cut the Wood to Length

You will want to make nice, square, cuts. This material in particular is prone to having knots and defects all over. 4" gave me enough material to work around the worst of these. The saw is just four (4) pieces of wood:

(1) Blade Length ~10" - measure between the holes of your saw blade, this is the longest piece you will need for the saw

(2) 8.5" - this length fits my hand well with this particular blade, you will need TWO of these

(1) 4.5" - from whatever you have left, find a piece about 4.5" long and thin it to about 0.5" thick

Step 3: Fitting the Blade

Make a mark at 0.5" and 5" from the ends of both of the 8.5" pieces.
Find the middle of the 0.5" line and make a tick mark.
Place the hacksaw blade hole on top of the 0.5" and mark the top edge of the blade.
Continue the mark around the sides and mark a centerline from narrow edge to narrow edge.
Using a saw, cut down the middle of the centerline until you reach the "top of blade" line.
Drill holes the same size (1/8" here) as the holes in your hacksaw blade on the tick marks from earlier, keep as vertical as possible!
Test fit the blade in the holes, I used the shank of the drill bit and installed the blade using some machine screws I happened to have.

Step 4: Mark the Mortises

Make a tick mark in the middle of one end of your (~10") long piece of wood.
Line this up with the mark you made on the 8.5" pieces of wood.
Use a round object, ~3" in diameter (like a roll of duct tape!) and trace an arc that is as wide as the end of the wood (see picture 1).
Continue the ends of the arc across the narrow edge of the wood (optionally, copy the arc on the other side of the wood too)
Mark a rectangle, centered on the narrow edge, the width of your chisel (1/4" here)

Step 5: Chop the Mortises

We are making some tiny, 1/4" wide x 1.5" long x 3/4" deep mortises in some very soft wood. It goes quickly. There are many ways to chop a mortise. If you are brand new to this, I recommend reading Mortise & Tenon Index or watching some of Paul Sellers' videos.

Mark your chisel at 3/4" with a small piece of masking tape.
Chop your mortise, starting just away from one end of the rectangle and taking time to stay vertical and within your lines. I like to keep the bevel of the chisel away from me and walk it 1/8" at a time, hammering with my mallet until it meets resistance or I reach my tape mark. Each time you stop you can lever out a small chip!
Turn the piece around and chop some more until the depth is reached all the way across.
With the flat side of the chisel on the end of the rectangle, cut straight down to make a nice, even, and vertical end to your mortise!
Repeat with the other piece!

Step 6: Pare the Curves

With a wider chisel or knife, cut down the walls of the mortise.

Make a saw cut down the center of the arc. This will help keep the chisel from splitting the wood along the grain and going too deep!
Follow the arcs we drew earlier with your chisel. Shallow, stabbing/scooping strokes. Just pushing, no mallet. Don't be greedy, you can always remove more wood.
Repeat for the other mortise.

Step 7: Mark the Tenons 1

Measure the length of exposed blade between the two wooden ends.
Subtract this length from the length of the long piece of wood.
Divide by two.
Mark this distance on both ends of the long piece.
Line up the carved arcs with the marks you just made on the long piece.
Trace the arcs
Repeat on all faces of both ends

Step 8: Mark the Tenons 2

Place the long piece on top of the mortise. Make sure the edges are flush.
Mark the edges of the mortise.
Pivot the long piece and mark the end of the wood.
Draw lines between the marks all the way around the end of the wood.
Repeat for the other end.

Note: I added letters so I could tell the ends apart. When you do the other end of the long piece, flip the piece end-over-end, not left-to-right.

Step 9: Saw/Pare the Tenons (and Curved Shoulders)

Saw just outside of the lines (the first three pictures show the angles I sawed to follow the curve we traced)
Sawvertically around the curve.
Parevertically with the chisel to make the curve smoother.
Repeat for each side of both tenons.
Test the fit, if the tenon is too thick, pare with a chisel until it does.

The purpose of the curve is to allow the tenon to pivot, if you need more wiggle room, make cuts like I have marked in the last picture. You could easily make those cuts in this soft wood with a chisel or knife if you prefer.

Step 10: Assembly and Testing

Fit the tenons in the mortises.
Wrap some string around the ends of the saw. I only used three full wraps and secured the ends with a square knot.
Twist the string using the small stick we prepared earlier.

As you twist, the torsion creates tension between the ends which pulls the blade tight.

Time to saw some metal!

Step 11: Finishing the Tool

The saw is functional, but could use an artistic touch. It is also a little uncomfortable to hold in its raw state.

Disassemble the saw before making any cuts! The various parts are under some stress from the string and could surprise you if you remove material.

I made some simple marks on the wood, about 1/4" in from the edges on the narrow faces and 3/8" in from the edges of the wider faces. Then I cut some chamfers, ending when I met the lines on either side of my chisel. You can also cut these with a knife, files/rasps, or spokeshave!

To use a chisel, make short, paring/scooping motions along the corners of the wood. Pay attention to the grain of the wood, if you are not careful, it could split deeper in to the piece that you intend.

A little sanding (I only used 220 grit on this project) and the saw looks a lot better. Add a protective finish (like linseed oil or shellac) if you like or leave it bare.

Enjoy your new saw!

Note: When you are done using your saw for the day, unwind it a turn or two!

Note 2: I am pretty sure I could build and embellish this saw in <2 hours if I didn't stop to take pictures every 5 minutes.

3/15 Edit: I have added two pictures of my full sized, wood-cutting bow saw. I actually made it from the dark piece of wood you see behind it (reclaimed wood from some shelves I got from a local, defunct electronics business). I wanted a more medieval profile so after building the saw from squared-off boards like the hacksaw (2.5" wide for the ends), I drew my curves and shaped them like I did in Step 6, with a combination of sawing and paring with a wide chisel. I gave the stretcher the lightest of chamfers with a hand plane and rounded the edges of the rest of the saw using paring strokes with a chisel and changing my angles between cuts (instead of keeping just one like you would for a chamfer). I think it turned out quite handsomely and I look forward to letting it age and develop the same dark patina it did (once before) as an old shelf!