Hi, Although probably not a Mach related problem this one has me head scratching once again.

I'm hoping one of you kind folks might give me some insight for the cure.


I'm cutting some thickwalled 6082 tube 34mm diameter and 150mm long on my Denford Orac running Mach3.

The tube is located in the chuck on the ID hard against the chuck nose and the other end is supported by the tailstock
with a revolving center and an ally slug fitted into the tube on a 45deg taper.

The turning tool is LH insert tool with insert CCGT 09T304.

When taking a cut from the tailstock end and travelling in Z 100mm the finish of the turned part has undulating banding the
peaks are approxamately 5mm apart. This is most pronounced at the tailstock end and diminishes as the cut gets nearer to the
chuck. If runnning a finger along the part, the finish has a ribbed feel to it.

Things i've tried to correct the problem,

Differing speeds and feeds.
Changing the tool height.
Trying different depths of cut.
Checking flexture of the chucked part.

None of the above made any difference to the depth or pitch of the banding.

So I changed the tool for a TPUN insert and tried again. Same effect even with a 0.2mm depth of cut.

I then had an idea and re-wrote the G-code to take the final pass starting from the chuck end and finishing at the tail stock and
the finish of the part came out perfect.

Any ideas to what the problem might be?

Cheers
John

The turning tool is LH insert tool with insert CCGT 09T304.
When taking a cut from the tailstock end and travelling in Z 100mm ...............
 
 I then had an idea and re-wrote the G-code to take the final pass starting from the chuck end and finishing at the tail stock and
the finish of the part came out perfect.

Hi John,
   At first glance, it appears that you are running the tool backwards. LH should cut fron the chuck towards the tailstock. RH is towards the chuck.

One other point, the slug with a taper along with a live center is little support unless it would be turned to a square shoulder and fitted closley to the tube ID.
What you have could act similar to a ball joint.
(If I am seeing what you are describing)
Russ

how CRUDE !   

You see what I mean though ... right ?

Russ

but then again, not familiar with an Orac. If mounted in the rear pos with spindle CCW it WOULD be LH towards the chuck.
Hope I didn't confuse BOTH of us,
Russ

it sounds like vibration, and a blunt tool.

Hi, Thanks for the quick replys.  I realise I'm running the tool backwards, it's not something I usually do but I was just trying to see if the same problem occured whilst the tool was working in that direction.

I remade a slug with a tight fitting shoulder, made no difference at all. I am also using new inserts, the CCGT are very sharp. I've tried taking light cuts of 0.1mm so not to load the live center end and it makes no difference I still get the banding along the length.

Logically trying to work out the problem, it's worse at the tailstock end so I would expect there's movement there somewhere although I have the live center well tight against the stock.

As i'm using new inserts there may be rubbing going on so I have reset the tool height several times above and below as well as on center.

As I have found that by running the tool backwards I get a very nice finish I am trying to think why, If the problem was with movement of the material I would have expected a poor finish in any direction.

As a picture is worth a thousand whatevers, here's a pic of the tube with the banding.

I'm with you, still suspicious of the tail end.
Is it a good quality center ?
Is there a quill lock on the tailstock ?
Are the head/tail stocks in perfect alignment along their center lines ?

I'm not sure of the quality of the center, it runs smooth and I've used it on other lathes without problems.
There is a quill lock in use.

Is the head/tail stocks in perfect alignment along their center lines?

I checked a while ago and there was 0.015mm in 200mm travel. But that was with a cold rolled stainless roundbar located in the chuck and live center.


It might be somthing to look at closer, I'm also going to try a shallow cut without the tailstock end supported.
and then try to cut some solid bar between centers to see if I get the same effect.

You may want to also try some cutting fluid and  make sure the tool height is correct.
RICH