I’ve been bounding around design and work on some different parts of the AUV but had been meaning to get back to working on the bulkhead ring prototypes. I’ve made some tweaks to the design, mostly so that I can mount internal components directly to the inside of the ring, necessitating the creation of some flat spots on the inner surface, where I can drill and tap mounting holes. These should be fairly easy to cut out on the mill when doing the finishing drill steps.
I had some time today to sit down and make some progress on the machining and managed to get most of the lathe work done, minus the external grooves which will seal up against the body tube. For the most part, things went well, although I made a fairly stupid mistake and zeroed off the wrong side of the grooving bit when cutting the face grooves — Luckily, not all the rings will need seals and o-rings, so I can put this one into a “wet” compartment where that mistake won’t matter.
One frustrating battle I did have to fight, however, was with chatter. I had significant chatter on the external turning operation. I think I’ve tracked that down (a little too late) to one of the gibs in the slide, so will need to tweak that before spinning anything more.
One tricky part about the setup is that the part needs to be flipped part way through. To make sure that everything is lined up, I had to place a dial indicator behind the part and very carefully adjust it in the chuck to make sure that I had very little runout. In the end, I got about 2-3 thou peak-to-peak deviation at 2.7″ radius. Not too shabby, and good enough for my purposes.
All in all, however, the g-code works well, and future rings should be much faster/smoother to machine. I ended up settling on a feed speed of 145mm/min and 0.1mm depth of cut with the mill’s motor geared down (~1400rpm with carbide tooling, so a very fast surface speed). I wouldn’t push the machine any harder than this at these large diameters (~5″), especially since the chucking method isn’t super secure, and subject to going out of alignment relatively easily. I suspect I’m feeding too fast with too light of a cut, so on the next ring I’ll experiment with deeper cuts and slower feeds to see if I can improve the chatter/finish/headaches. — Disclaimer: Do not take any of this as good advice. I’m not an expert machinist! This a very non-ideal method for clamping large parts on a small lathe, and can be dangerous if not secured properly.
Next steps will be to finish this part, and work on the next batch. I’ll need about 8 with my current design, so it may take a bit of time to get them all done…