Continuing Problem With This Lathe.

[alphonso]

http://www.practicalmachinist.com/vb/cnc-machining/chatter-thread-cutting-machine-problem-305924/

Follow up on this thread. I tightened gibs significantly which helped a bunch. Then I changed insert style: ToolFlo FLT-3R-HCB AC22R(a positive rake). All chatter gone. All 18 pieces threaded, about one and a half parts per edge.

Moving on to next operation. Part sticks out of chuck some 45 inches. Now working in area of bed that has no discernible wear. Start to get intermittent Z axis lag errors at variable points along axis. Most consistent error is about .1 into Z feed move. Hhhhhhmmmm, maybe there is a problem with the overtravel cables or stop switches. Switches are a little gummy; a few shots of B-12 remedies that. Still getting error. Pulled cable out of raceway and checked continuity while bending cable from one end to the other. Don't find a problem here. Currently have the overtravel circuits jumped to take the cable out of the mix. Still get error.

Status LEDS on servo drives (Fagor ACS 30) have burned out elements so I can't get a real good reading of the error here. My best guess is either error 5 (motor overload) or error 8(ballast error???). Amp meter at motor wires does not indicate excessive amperage. 7.5 initial amps on a 15 amp motor. Problem doesn't seem to be here.

Just for fun, ran something else with carriage within a foot of the chuck. No error in 20 repetitions of program.

Bring carriage back to the 45-48 inch position. Errors return.

Wonder if the gibs are too tight out here on this end of bed? Kinda thought this might be the problem because I could see very little movement then a sudden jump. Loosened gibs some 4 or 5 turns of screws. Very minute improvement. Did this procedure once while watching the belt between servo and screw. Could see the belt winding up, then relaxing. Tightened the belt. Checked sprockets on motor and ballscrew. They seem to be tight. Stopped the wind up and jump. Checked the thrust bearings and screw pre-load. No backlash.

Checked ballnut to screw play. Not sure what it means, but when prying on carriage in same direction as last move, indicator shows .004 movement. Cannot pry carriage back the other way. Can hear the servo fighting this prying attempt. Same result when doing this in opposite direction. With power off, I can't get any movement of carriage that would indicate ball/screw wear.

This is where things start to get weird. Set backlash parameter to zero. Place indicator on bed to read carriage movement. Move carriage in one direction, zero indicator and move carriage in opposite direction in .01" moves with handwheel. Screen display shows .01" per "click" of handwheel.

Indicator readings:
First move: .001
Second move .003
Third move: .005
Fourth move: .004
Fifth move : .004
Sixth move: .009
Seventh move: .10
Subsequent moves: .01

The sixth and subsequent moves move at rapid to the last thousandth, then creep thru the last thousandth.

I think I have eliminated thrust bearings, ballscrew pre-load/stretch, worn balls/screw, loose belt, loose mounting of any other components in the setup.

This sequence of readings is repeated in either direction of movement.

At this point I can't think of anything else to check other than loosening the gibs some more.

Ideas??

 

[charlie gary]

Loosening the gibs is where I would start. I'm guessing it worked fine until you tightened things up for working in the wear zone, which makes me think loosening them will fix the issue.

 

[alphonso]

Been fucking with this machine for 2 days trying to find the cause of the weird moves. Loosened front gib to the point of it falling out. Made no difference in the moves. Loosened the rear gib in increments until the weird moves stopped. The tightening screw was completely off the machine before the the weird shit stopped. Oddly there is no wear on the rear way, which just a flat way.

Made the notch wider on the rear gib and put it back in. After some 5 hours, I am satisfied with the carriage movement, what a PITA; tighten gib screw a quarter turn, test movement, tighten another quarter turn, test movement, repeat. Back off a little. Tighten again. etc. Then play with backlash. With loose/no gib there is about .004 backlash. It has to be in the balls and/or screw. Most likely the balls.

We'll see what happens tomorrow when I try run a program.

 

[charlie gary]

Where are you putting the indicator when checking for backlash? I wonder if there's anything useful to be learned by putting an indicator on the carriage at a point close to the ball nut and then at a point as far as possible from the ball nut to see if the carriage is racking when it reverses direction.

 

[alphonso]

Where are you putting the indicator when checking for backlash? I wonder if there's anything useful to be learned by putting an indicator on the carriage at a point close to the ball nut and then at a point as far as possible from the ball nut to see if the carriage is racking when it reverses direction.

I actually put 2 indicators on it. One on the tailstock front flat and rear flat ways. Doesn't appear to have any racking. Can't get close to the ball nut - tucked under the carriage. Nut cartridge bolted to housing bolted to carriage.

Run several parts this morning without incident.