Simple turning question from "another forum"

[HMF]

No one answered this question adequately in my opinion, so I am asking it here:

"I chucked up a piece of steel about 7/16" diameter. I took about 0.060 cut and it machined down. When I got to the shoulder (and stopped) it continued to cut deeper into the work.

My theory is this:
While turning, the bit is "pushing" the work inwards. And when the power feed stops, it continues to machining DOWN to the "real" depth.
Is this correct? Is my tool to dull? Maybe the work was chuck too far out?
The cut doesn't seem to cutting (into) the shoulder, but in the direction the crossfeed moves.
Is the metal being pushed, bent while turning, and is my tool bit sort of skating across the surface, only to continue cutting until it reaches it's correct depth?
Where would the flex be coming from? It's a Heavy 10 with a new cross feed screw. Gibs are snug and smooth. Phase ll AXA quick change post, 3/8" tool bit.
I make a cut, stop at the shoulder, it continues to cut (into) the diameter and then stabilizes into a sort of "groove". Then if I move the carriage back, it drags across the work,, scratching/scribing. The I can use the power feed and take another cut - without moving the cross feed. "

What do you guys think?

Thanks,


Nelson

 

[u.p. north]

Sounds like your work is flexing. How long is the piece you are turning? Also are you turning between centers? Make sure your cutting tool is set to the center of your work. To me it sounds like your tool is to high. Make a facing cut on a solid piece and make sure you get no nipple on the center of your work.
Thanks,
Tim

 

[rleete]

Agreed. Flexing, compounded by way too deep a cut on thin material.

 

[HMF]

Where would the flex be coming from? It's a Heavy 10 with a new cross feed screw. Gibs are snug and smooth. Phase ll AXA quick change post, 3/8" tool bit.


Thanks,


Nelson

 

[randyjaco]

I am with Dave. That is a pretty deep cut for a 7/16" rod. If it is longer that a few inches, even turning between centers, the work piece will flex. If it is just chucked in, it will flex a lot. Back off a bit and do a .02-.025 cut and them finish with some much lighter passes. I'll bet you are also running carbide. Slow down and take your time, get it right first, them try speeding up.

Randy

 

[Tony Wells]

Yep, pushoff works both ways. If the machine isn't rigid enough, and the work is limber enough, that's what you get. A small nose radius will alleviate some of this effect, but not all of it. A second pass, a "spring pass" will sometimes suffice to minimize pushoff. Outguessing it (or out thinking it) is all part of machining.

Others have noted several causes, and they are right. Lighter passed are typically used when cutting to finished size.

 

[pjf134]

With machine off put a dial gauge on the back side on round bar on center then push the cutter in 0.060 and see reading on dial, it would be a nice test to see how much it deflects in a certain distance between supports. This would give you a good rule of how much to cut in without deflection. It would be nice to take deep cuts without deflection. One pass threading would be nice, no need for a thread dial. Waiting for results from someone with time to kill.
Paul