Curious pattern on lathe cut

Feb 28, 2009 1 Replies
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I was trying out some bits that I had just ground & fooling around with the independent feed on my lathe when I got this strange ripple pattern. The ripple peaks are about .030 apart, but the feed was only .002. And the ripples are rings, not a helix. It's mystery metal.



I'm speculating that there is a rub-then-cut kind of oscillation going on. That is, the .002 feed is not enough to get a bite on the steel, so it rubs a while, without cutting until enough advance occurs then it bites/cuts until the pressure is relieved and goes back to rubbing again. Does that sound right (it's not real clear how that would give a ripple pattern)?



If so, is there a way to avoid it? I thought that I had the bit pretty sharp (it was just a plain RH roughing bit). Could it be lack of rigidity in the lathe?



Thanks, Bob


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B

...

Thanks for all the replies. I've been playing with it some more, based on the replies & have definite improvement: I don't have the ridges, but I can still see a .030 pitch pattern. To eliminate the "mystery metal" variable, I have ordered some 12L14.

To answer some points/suggestions made:

- (Milo) I was using a plain old RH roughing bit, with the cutting edge about 15 deg into the cut. 1/32" tip radius, more or less. 120 FPM

- I was going to try wws's suggestion to set the bit .005 above centerline when I discovered that /somehow/ it was set way low - about .030 below c-l.

- (Carl) I tightened the gibs, although they were pretty tight already

- (Steve) there is plenty of chip clearance, but in examining it I noticed some chip build up right at the cutting tip

- (Jim & Don) the leadscrew pitch is .125. But I also was thinking that the problem could be in the apron. I haven't ever had it apart & this would be a good reason. Also, I am feeding with a DC motor/servo & maybe there's some oscillation in that "stream".

As I said, it's much better now & actually good enough, but I'm still curious about the oscillation. If I ever resolve it, I'll follow up.

Thanks again, Bob

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