Vibration problem

[eski1]

Hi
We are trying to machine a left handed & a right handed thread each thread is 500mm long and the total shaft lenght is 1500mm long, there is a 100mm long section in the middle which is left stock 30mm . The material is EN24T we are using carbide insert tips ,speed has been tried at 500 & 250 rpm ,feed is non adjustable as set by cnc lathe
Pitch is M30 x 2mm , Passes are 15 , Spring passes are 2 , plunge angle is 29.5 degrees. The problem is we are getting alot of vibration and can't get rid of it , net result the thread looks rough
Anyone got any good ideas

Regards
Henry

 

[aamoroso]

It could be a million things;

When was the machine last leveled?
How long is the tool?
How old is the machine?
If the center of the part is left as stock is the length causing a off balance whip?

.....

Fill what's empty. Empty what's full. And scratch where it itches.

 

[Philrock]

That's a long, skinny piece of material. I'm not surprised you're having problems. Have you tried putting a steady rest in the middle?

 

[BillPSU]

The problem is the tool pressures are too great. You will need a follower rest adjusted to the part OD and thread. This is the usual approach to making a thread this long on a shaft.

The other approach is to roll the thread.

 

[eski1]

hi
thanks for the replys
We have now manufactured a new type of support for this job
It is mounted just infront the cutting tip and is basically a large ultrawear bush that is 0.1 clearance on thread od and about 50mm long . The bush completly surrounds the shaft so it hasn't got any chance but to do what its told ( so to speak ) The support for the bush is made from 40mm thick mild steel and the whole thing is bolted to the saddle so runs up and down nicely.
i must admit the whole thing works great and we can easily remove the bush and replace for bigger sizes

Regards
Henry