Spiral problem after changing inserts in lathe.

[chibbert]

Sorry if I am posting this in the wrong section - there are so many to choose from and I don't find one specific to my machine. We are machining cylindrical steel parts on larger Nakamura Tome lathes. We are having an issue that when we do an insert change, the next xx many parts will have an unacceptable spiral look to them - usually this may only last 5 pieces but sometimes as many as 20. It then appears that the insert dulls enough to where this goes away. I don't really know that the sharpness of the new insert is causing it - just making an assumption. Our parts aren't cheap and we would really like to eliminate this problem. Is this a common thing or something unique to our situation? The steel is not very hard (don't have the actual specifications but I know it's fairly soft) and I knwo that we are using Kyocera carbide inserts.

Any ideas or recomendations would be greatly appreciated.
Thanks in advance for your time

[beege]

Perhaps you're using TNRC and retracting too close to the finished diameter, not turning off or reversing the TNRC? What does your code look like?

[HuFlungDung]

Is this a chatter problem you are describing? What shape insert is it?

[Geof]

It is not unusual for a new tool to be more prone to some form of chatter than one with a bit of use. I think it is a case that the new sharp edge can easily dig in so when a bit of chatter starts it does dig and then spring back and things build up from there. With a slightly worn tool there is a bit of rubbing and back pressure so the tool is stabilised.

One approach I have taken on manual machines is the hone the edge of the tool to take off the really sharp corner. You must not overdo it otherwise the tool life is drastically shortened, but sometimes a little bit of honing can extend the tool life because it prevents micro-chipping of the sharp edge from the chatter.

Another trick is to mount the tool below center, remembering that on a CNC with the tool at the back upside down raising the tool puts it below center from the point of view of the cutting action.