Can't cure chatter - machining long shaft on lathe

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Thread: Can't cure chatter - machining long shaft on lathe

  1. #1
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    Default Can't cure chatter - machining long shaft on lathe

    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.

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  2. #2
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    Default Re: Can't cure chatter - machining long shaft on lathe

    first use HSS and grind it so you only have about 4* hook and just a really small rad on the tip like .005 max. run about 200 rpm and .004 feed with oil. leave about .015 on the stock for the finish cut. I do some shafts in the cnc about that same stick out and have not found carbide sharp enough to get away with it, that's why I`m saying swap over to hi speed steel and SLOW down. normally to stop chatter you lower the speed and jump the feed. the other trick is to put rubber bands tight on the shaft and then cut throe them, the rubber bands stop the shaft from chattering .



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    Default Re: Can't cure chatter - machining long shaft on lathe

    Thanks a bunch! I'll give it a shot.



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    Default Re: Can't cure chatter - machining long shaft on lathe

    You want as small a noise radios as you can get. I'd you're using inserted tooling a DNGP??1 would work really well. Try slowing down to about 200-250 sfm and feed around .005".

    Make sure your center is tight, that's probably where most of the chatter is coming from.



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    Default Re: Can't cure chatter - machining long shaft on lathe

    I've done work like this and agree w/ viking on the nose radius but would consider finishing with a very positive rake insert with a sharp land and start at the chuck and finish going towards the tail stock, that way it won't push into the chuck and if you have to use a lead hammer resting on the part to reduce vibration until you get the feeds and speeds right. Most of the time I use half the nose radius as my finish doc( unless it's a really small dia.) and rpm is your enemy. High speed works because you can make a sharp edge and slow rpms down. Most folks use carbide because of wear and cycle time. Remember you want to shear the mat'l off, less strees on part and insert.



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    Default Re: Can't cure chatter - machining long shaft on lathe

    Quote Originally Posted by viking73 View Post
    You want as small a noise radios as you can get. I'd you're using inserted tooling a DNGP??1 would work really well. Try slowing down to about 200-250 sfm and feed around .005".

    Make sure your center is tight, that's probably where most of the chatter is coming from.
    I didn't have a DNGP handy but I did have a CNMP, and using the feeds and speeds you suggested I cured the chatter! I also found out that I in fact wasn't at .02" DoC, I was at .01"! So I dug in a bit deeper as well and it seems to have paid off. I turned one of my bad shafts down to .67" (I only need .811") and it still didn't chatter. Thanks viking!

    Quote Originally Posted by tom bryant View Post
    I've done work like this and agree w/ viking on the nose radius but would consider finishing with a very positive rake insert with a sharp land and start at the chuck and finish going towards the tail stock, that way it won't push into the chuck and if you have to use a lead hammer resting on the part to reduce vibration until you get the feeds and speeds right. Most of the time I use half the nose radius as my finish doc( unless it's a really small dia.) and rpm is your enemy. High speed works because you can make a sharp edge and slow rpms down. Most folks use carbide because of wear and cycle time. Remember you want to shear the mat'l off, less strees on part and insert.

    I haven't tried it yet, but I found a lefthand toolholder for the insert above. If I run into more issues I'll try it for sure.



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    Default Re: Can't cure chatter - machining long shaft on lathe

    Glad it worked out for you, keep up the good work!



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Can't cure chatter - machining long shaft on lathe

Can't cure chatter - machining long shaft on lathe