Thanks for the additional comments about forming file teeth, Dennis. I still have my Grandfather's hand saw jointer and tooth set. It took a while, but eventually I switched to reaching for a hand saw as a first choice for quick cuts even though I have power tools ready to go.
Just for a quick side comment, years ago I made two back saws just for fun. The readily available material for making the blade is generically sold as 'blue temper steel', which is basically 1095 tempered back to 50ish Rc, right around your spring temper.
