You're using forged mainsprings (I do like that...), but are using cast frizzen springs and plan to use milled ones later. Is it because there is less stress in the frizzen spring? (Besides the ease of manufacture for such an ornate part).
From what I understand, in a cast part there is no grain flow to speak of, in a milled part you have to take into account the direction of the grain at critical stress points (ask anyone who bends sheet metal with a tight radius), and in a forged part the grain flows with the shape of the forging, which is why forging is preferred for high stress parts - and why forging generally allows for slimmer and trimmer springs.
Yes, that is correct. I want to use a forged mainspring as they are the heart of the "action/lock." A well made forged spring is a thing to behold. Just play with some of the 2-3 hundred year old well made locks and you'll get goosebumps. At least I know I do!
Also yes, for the frizzen spring I am going to use cast springs for now and probably switch over to milled springs at some point. They don't flex near the distances that the mainspring does, but we'll see 2-3 hundred years from now how they do!
Also, Also yes, you are correct about grain structure. When I first started building locks years ago I poo-pooed the cast springs with their (gasp!) homogenous grain structure. I thought they couldn't be any good. It didn't take long to learn that a properly treated cast spring can be a very good spring indeed. We also have 60 plus years of data in the form of 10's of thousands of cast flintlock mainsprings out there. Do they ever fail? Sometimes, but their track record is nothing short of amazing. Milled mainsprings on the other hand we don't have much data on. Sure you can build up the weak areas on a milled mainspring to keep it from catastrophic, or other types, of failure, but that has effects on the rest of the spring and it's action as well.
There is a mountain of information out there on all of this, and 10X more opinions. So in short, I intend to use a forged mainspring on the Ditchburn lock. If I cannot make that work (economically) I'll go with a well made, properly treated cast spring. And I will never say never on milled mainsprings, but options 1 and 2 come first.
Incidentally I am still working on the design of my forging dies for the mainspring. I made a several prototype springs and am working on a few changes to both the spring and the dies right now. I'd hoped to show up at Kempton with a few, but that may be a bit ambitious! I've mentioned before how much my 40 hour (more like 60! most weeks) job cuts into my lock building time. Who knows, maybe one day I can divert the entire shop toward flintlock work. I'd like that!
Best,
Chris E.
