Problem- I.D. Lathe Tolerance Problem

[Billet Sean]

Has anyone had problems holding I.D. tolerance on a 1995 Femco Durga 25-E. The O.D. tooling holds within .0002" all day long, but the I.D. tooling fluctuates +/-.0015". The movement is random. I'll get 2 or 3 parts that run within .0005", then the next part is out -.0015". Then the following part is +.0015. It jumps all around. I've eliminated the tool holders/tooling/inserts from the problem. So I believe it's a mechanicle issue. It's not the X-axis, because the O.D. tooling holds great. This machine has a double turret. The front turret is for the O.D. and the rear is for the I.D. If you put a indicator on the I.D. tools and move the turret by hand, one side will move +/-.001" and the other side will move +/-.01". I've tried everything I can think of and no solutions. We've had a service guy come out 2 times and I've spent hours and hours on the phone with him and the manufacturer with no solutions. I'm about ready to make the machine a boat anchor if I can't get this solved soon If anyone has any ideas or suggestions, please feel free to give me an ear full. Thanks in advance, Sean

[RICHARD ZASTROW]

When the ID turret is indexed, how is it secured? Some have "Curvic Coupling" locking the turret, some have a shot pin locking into a hole. I had a similar situation on a cheap manual engine lathe. The shot pin hole didn't have a hardened bushing and wore into a sloppy condition in a few weeks. Also, after a "crash" the curvic coupling on a CNC lathe turret was loose because a sacrificial pin was sacrified, allowing the turret to move slightly. Because the boring bars were extending further from the turret, the effect was greater. Leverage and all that.

I don't know if that helps any, just thought I'd mention it.

Dick Z

[Billet Sean]

Our machine does have a "Curvic Coupling". It looks to be ok. It also has 2 shot pins inside the casting. One is for the front turret and one is for the rear turret. They both lock into what looks like 2 Geneva gears. The rear turret rides on a bearing race with 108 (1/4") ball bearings. I replaced the old ball bearings (that were just some carbon steel) with stainless balls. One of the old bearings was cracked in half, so after I replaced the bearings it helped out a lot. There is also a large 114 tooth spur gear inside the rear turret that get driven by a smaller 18 tooth spur gear. We replaced both gears due to excessive wear. We were told that if there's slop in the 2 gears that "could" be our problem. But it wasn't. I am concerned with the bearings and race. When I had it all apart, with the bearings and race installed in the rear turret on the shop floor, there was lateral movement in the race. I guess about .03"-.05". The inner race is directly bolted to the casting, so it's not going to move. The outer race is part of the rear turret. Neither race looks wore. There are small pits on them both, but they're very small. There is a access hole to the bearings on the outside of the rear turret. There's a double set screw that keeps the balls from coming out. I did notice a direct relationship between how tight the set screw is and how much movement there is when I move the rear turret by hand. But with the "Curvic Coupling", when the front turret clamps down, it should lock the rear turret in place. But it doesn't. I'm stumped!

[Billet Sean]

Ok, here's what I've found. If I move my Boring bar 2 stations away, I can hold within .001". Doesn't make sense to me...

[RICHARD ZASTROW]

Are the ID tool sockets worn? A bellmouthed socket might not be secure enough, especially with a heavy cut or long extension. One of a gazillion possibilities.

Dick Z

[Billet Sean]

I've checked the holder, it's within a couple of tenths of perfectly round. It's a darn good fit for my boring bar also. My boring bar is 3/4" dia., using CCMT inserts and it's only sticking out about 1.9". I'm taking .05" per pass and leaving .005" for a finish cut in 6061 aluminum. I've tried leaving anywhere from .015" to .001" for the finish pass using the same tool to rough and finish. We've held .0002" on this job before. We've ran over 10,000 of these parts over the past 6 years.

[RICHARD ZASTROW]

That would seem to eliminate the socket/ID tool theory. Does the problem occur on the same turret position every time it happens? Also, does the problem occur every time that turret position is used? I'm trying to see if this is one of those infamous "intermittent" problems.

Dick Z

[Billet Sean]

Originally Posted by RICHARD ZASTROW Does the problem occur on the same turret position every time it happens? Also, does the problem occur every time that turret position is used? Dick Z

So far, when the boring bar is in pocket 27 I can't hold any tolerance outside +/-.01". I moved the boring bar and tool post holder to pocket 25 and now I can hold +/-.001". As far as every time we us this pocket (27), that's where we've always had the boring bar. We never changed the position. Never had a reason to until January of this year when we started having problems. And since then, we've only ran 2 jobs that required boring. Neither time has it held very good.

I have pictures of every component to the turret from when I had it apart if anyone's interested.