Hacksaw blade preparation

This describes the basic steps in preparing a used Bahco Sandflex hacksaw blade for use in a knife and is referred to in two write-ups of such knives.

A bi-metal hacksaw blade has the potential to make a thin, scalpel-like blade. The cutting edge is HSS (high-speed steel), giving it great strength and durability, but a complete HSS blade would be costly and brittle. Instead, a strip of HSS only a few millimetres wide is electron-beam-fused to softer, more flexible spring steel. The flexible spring-steel backing supports the thin, brittle HSS strip, greatly reducing the bending stresses that might otherwise cause it to crack and snap.

The blade I used is a Bahco 3906 24 TPI Sandflex, which has a black coating followed by the distinctive orange paint and deep-blue printing. 

 I removed most of the orange with paint stripper but the black coating was tougher and I used wet-and-dry paper for this. One could argue that it would have been better to keep this protective surface but it didn't look that great.

With the majority of the blade being softer (though not soft) steel it could be cut with another hacksaw blade and shaped with a file. The HSS edge was then ground and sharpened with diamond stones. To maximise the HSS remaining behind the edge,  I removed as little material as possible. I used a round file to create a semicircular sharpening choil just before the point at which the blade goes into the handle. This choil ensures that the full extent of the edge can be sharpened.

The bare steel was given a light patination by suspending the degreased blade in a container of a warm salt-and-vinegar solution so that the blade sits in the vapour not the solution. This led to an attractive mottled-grey patina that should give some protection against corrosion.

The wavy line in the tang part of the hacksaw blade is by design; groups of teeth are progressively displaced from one side to the other, producing a gentle wave when viewed along the cutting edge. This helps prevent the blade binding.