Hi all,
I'm having trouble turning down a shaft. I'm holding it with a 3-jaw scroll chuck and a live center in the tailstock.
The shaft is 1” round 1045 steel, and it protrudes about 14.3” from the 3-jaw, where it's gripped by bored steel soft jaws with .003” copper stock in between to give it some bite. Overall TIR is .002” at the headstock end of the shaft. The task is to simply turn it down to .811”. I have about .7” of stock gripped on that end.
I'm cutting with a 55 degree diamond carbide, using these settings: 400 SFM using CSS – my roughing passes are .020” DoC and .010” crossfeed, with a finish pass at the same DoC but at .005” crossfeed.
So, here's what happened:
I perform the roughing and it goes just fine. Dandy in fact. If it weren't for the fact that it left the shaft too large it would've been almost passable for the customer. This leaves the shaft at .851” D. So, I set up for the final pass. Thanks to the way my Bridgeport Romi handles CSS, I forgot that it had dropped it for a fixed RPM when I switched it to neutral to measure the results of the roughing pass (the OD is a critical dimension). The measurements were good, btw. No taper, no nothing. So for the first part of the final pass, the carbide tears the metal and gives it that nasty finish – but other than that no problem. I reset the finish pass, just to see what the results would be before I started on another piece of stock. I like to verify all operations even if I render something unusable, I screw up less stock that way.
This is when the chatter comes in. Oh my it's a lot of chatter. Awful stuff. So I figure it must be rubbing thanks to the slow feedrate. So I set it up to redo the pass at .010” with an extra .001” offset so I can get a taste of how long smoothing the rest off with emory cloth or sandpaper would take. Chatters even worse. I check the workpiece deflection and it's .001”. I don't know if that's good or bad. Eventually the bar's cut down to 3/4” and I still haven't solved the problem.
I also noticed the live center was backing out/losing contact with the workpiece. Could that be the workpiece working its way backward with the help of chatter? Also, I found that the bar was out of lateral alignment at the end of all the chattering and experiments.
I tried it again, only this time I reduced all excess length (probably something I should have done earlier) and end up with only 12.7" of stickout. I also simply deleted my final pass, replacing it with one at .01 IPR and .009" DoC, hoping to just sand it to smooth it down. It starts chattering at about .840". Light at first, but the final pass chatters like crazy So I solved nothing. None of the other issues happened this time, except I noticed the live center was getting hot. Not incredibly hot (I could still touch it) but hot enough that I didn't want to park my finger on it.
I'm a pretty inexperienced machinist, and this is the first job I'm trying to do professionally on a lathe. What am I doing wrong? Should I be using some kind of steady rest? The material starts out 12.7:1 L to D, and ends up about 15.66:1 L to D, but in the original take it started at 14.3:1 L to D, and my roughing passes seemed okay. I've also considered doing a finish pass at .06" DoC at .005 or .003 IPR, but I don't know if that'll help. I'm burning through stock trying to solve this and it's starting to get expensive.