Well!
I was just about to post on needle files when I thought I'd have a quick look in 'ere first!
I have some really cheap ones from either Maplins or Toolstation - it's so long ago I can't remember which. You can also get them on eBay. They're quite invaluable.
Today, the reason for the posting, I 'as bin mainly rescuing worn out blades from my Bosch GOP multi-tool. It turns out they're blued steel, but NOT hardened (as far as I can tell). A triangular diamond needle file recut teeth in a completely worn out 'saw plate' pretty quickly. I should have done before and after pics really.
I started with lumps on one end of the blade (used to be teeth), and just a worn smooth edge on the other end - teeth entirely worn away (note to self - don't use that type of blade for cutting through lath + plaster!).
The result isn't brilliant, but it is functional, and for the 109-year-old in situ skirting board buchery I intend, will be fine.
I also had a go at one of the semicircular saw ones, not so badly worn, with really excellent results. It feels to the fingers to be sharper than it was new.
The issue with my diamond files is that, although they cut fast and last well, they're not very sharp-cornered, so the pitch of teeth I can do is very limited. I was going to ask if someone had sourced better quality ones, but it seems Andy T has done so. I'll look for the Trend ones.
They really are wonderfully useful little things.
I have to say though Marcros, the last time I made anything scratchstock-ish from hacksaw blades (actually a specialist contact extractor tool for the wiring loom connectors on our Seat people carrier), I ground the profile using the corner of my dry high-speed grinding wheel. A bit naughty but it worked a treat, especially as two prongs had to be ground as nearly identical as possible - could just clamp them and grind them as one piece.
E. (wandering off topic as usual)