cnc lathe taper problem

[trh400]

The machine is a Mori Seki TL40-Y (4 axis) lathe. Today we had a 2.625 dia. x 8.438" long journal that was .0004 big at the chuck. No big deal, we've been programming taper out of shafts for 20 years. However, after the cut we found the journal was straight for about a inch then tapered undersize by .002. I can't make the machine cut a taper without this step. I called the service dept. at our Mori distributer. They don't have a clue. It seems to me that the x and z axis ae not in "time" with each other. Has anyone here experienced such a problem? or does anyone have any ideas?
Thanks,
Tim

[HuFlungDung]

I'm not sure, but I think that extremely shallow tapers turned on a cnc are still 'digital' in that the X axis moves a bit, pauses while so and so much Z travel occurs, then another X pulse. There is simply no interpolated motion possible when the command is less than a tenth, or whatever your machine resolution is.

Apart from that, have you ruled out relevelling the machine to get a straight cut? .0004 in 8 inches is straight, AFAIC , but a little tweak on the levelling jack on one side of the tailstock end should dial it in straight.

I hope you were turning with coolant all over the part and the toolholder. Only .0002" is the real error!

[trh400]

Thanks for the ideas. The machine distributor sent me to Fanuc. They've asked me to check the backlash on the "X" axis. I can't do that today, the machine is running. Yeah, .0004 in 8.438 is not too bad, but there is a bearing journal +/- .00021 closer to the chuck. It is much easier for us to hold the tolerances if we program-out the taper to make the part cut straight.
We do short runs on this machine, so the tailstock is moved back and forth a lot. It is not practical to check and adjust the taper each time. Programming the taper out usally works just fine.
When I figure it out I'll post the result, incase someone else ever has a similar problem. Both the Mori and Fanuc tech guys were pretty weirded-out over this problem.
Tim

[Chuck Pressure]

Check your x axis way guards, we had a mori tl-1 and had the same problem, one of the telescopic guards got jammed causing a huge x axis taper!

[Chuck Pressure]

Oops! sorry, it was the z axis way cover causing the problem! this was an early 80's vintage machine, 4 axis......the x axis didn't have any covers....

[ctate2000]

Box Way construction? Older machine? You may have a slight dip worn in the ways which can cause the turret to dive into or away from centerline. Chasing tenths can be hard. They are fast and hard to catch.

[mhtom]

Tapper problems can drive you nuts when it's 10th's. I have a old Turnex lathe which is mounted on a 6" thick grante slab. I had the same problem you have and for the hell of it one day I reprogamed the machine to cut from the spindle out and WOW problem solved. We coulded explain it but the tapper was gone. I aint going to say it will solve your's but give it a try.