Turning issues

[knucklebones]

Hello all.
I am currently running a Samsung cnc lathe, making bushes.
Occasionally I'll get a bushe or 2 with an odd knurling on them, but as of yesterday they are all coming out with it.
The lathe was only serviced and rechecked a couple of months ago.
All the inserts are new.
I've attached a photo.
Could anyone shed some light on this?

[Muzzer]

That's chatter or judder. The usual cure is to increase or decrease the speed and / or feed until it stops. Often people reduce the feed or speed but you may find that increasing them is a actually better solution.

As you are machining brass, you may need to use tools with little or no top rake. In a twist drill this would result in flutes with no twist and on a lathe tool, the top of the cutting edge would be on the same plane as the centre line. Normal angles intended for cutting steel or aluminium would be 5-15 degrees but when these are used on brass, they want to dig in. If you want an exciting time, try drilling brass with a sharp steel cutting drill!

They call it "face rake" here but either way, it should be a small angle or zero.
https://en.wikipedia.org/wiki/Rake_angle#/media/File:Metal_Cut_diag.svg

[JohnHaine]

What grade brass is it? Did you change to new stock recently? Hard brass, cz121, generally machines very well with normal tools but more ductile types can be horrid.

[knucklebones]

Hello,
Thanks for the replies, and sorry for the delay getting back.
It's actually copper.
Same stock as I've run for years.
The tooling is the same also.
Strangely enough, as of today it seems to have stopped happening.
The other issue was the 13mm drill was making a hell of a noise, but that has also stopped.
I've made these parts for Several years with no issue.

[knucklebones]

Hello again.
Problem has returned.
Machine has been stripped down and cleaned, including coolant.
The issue now only seems to be on the bore.