Curious pattern on lathe cut

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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

...

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