Backlash on big CNC Craven Lathe.

assface421 · Mar 21, 2018

Hello everyone, it's been a while since posting.

Working on this Craven lathe, I'm the only cnc person here so I am the only one that operates it. They bought it used from Korea about a year ago. Have only done simple jobs here and there, It's just been sitting idle most of the time. I was getting some weird results. When moving from Positive to negative in Z or vice versa, the display on the controller shows .114 movement before I see any movement on the dial indicator I set up. The same thing is happening on the cross slide but only .030. The controller is a GE Fanuc 21I-T. Is there anyway to compensate with a setting in the controller? Any advice?
-Thank you for your time.

Vancbiker · Mar 21, 2018

There is backlash comp available on the control, but I'd advise looking into the mechanical reasons for having so much lost motion and fixing those before setting the comp.

Ox · Mar 21, 2018

Mine has .010" lash and I think that's a lot.

.114" is a whole new level of "you gotta be kidd'n me!"

You need to checking thrust bearings, couplers, loose belts, loose NUT! ... whatever....

I had a taper pin fall out of my Z axis in a coupler box once.
Still tried to work tho...

--------------------

Think Snow Eh!
Ox

Bobw · Mar 21, 2018

Can that amount of slop (and its not just backlash, that is SLOP) even
be compensated out??? I know a Fadal only lets you go up to .010 I think,
and if ANY machine is going to need more backlash comp, its a Fadal.

I agree with the others.. That is far more than a worn screw, that is
"Lost Motion".. Something is really out of whack.

DMF_TomB · Mar 21, 2018

assface421 said:
Hello everyone, it's been a while since posting.

Working on this Craven lathe, I'm the only cnc person here so I am the only one that operates it. They bought it used from Korea about a year ago. Have only done simple jobs here and there, It's just been sitting idle most of the time. I was getting some weird results. When moving from Positive to negative in Z or vice versa, the display on the controller shows .114 movement before I see any movement on the dial indicator I set up. The same thing is happening on the cross slide but only .030. The controller is a GE Fanuc 21I-T. Is there anyway to compensate with a setting in the controller? Any advice?
-Thank you for your time.

.
just cause its cnc dont mean it has ball screws and thrust bearing at end of ball screw is still tight. often old machine needs repairs even ball screw often need replacement. ball screw has ball nut often with ball return tube. could be a problem with it

lowCountryCamo · Mar 21, 2018

Is that .114mm?

sable · Mar 21, 2018

I am guessing that being a Craven ,this lathe will be somewhere between massive and ginormous and won't have always been CNC. Are the screws ballscrews or have they retrofitted this lathe using the ordinary Acme screws? that explain the backlash.

Myrmidon · Mar 21, 2018

If it's .114 in the imperial system, do not run or even attempt to comp out the lash, there is a major mechanical malfunction that needs addressed now.

There's quite a few things it could be, loose nut, thrust bearings, the screw could be mounted incorrectly. Either way something is proper fucked as others have mentioned.

Running it like that could really be dangerous, I've never seen a small Craven, the big ones can be man eaters. Stay safe bud.

alskdjfhg · Mar 21, 2018

sable said:
I am guessing that being a Craven ,this lathe will be somewhere between massive and ginormous and won't have always been CNC. Are the screws ballscrews or have they retrofitted this lathe using the ordinary Acme screws? that explain the backlash.

+1 on that, don't think Craven made anything small.

pressbrake1 · Mar 21, 2018

alskdjfhg said:
+1 on that, don't think Craven made anything small.

They made a baby of 12" centre height by 144" centres, being a craven still in the double figures in tonnage and I mean real 2240lb tons

bikepete · Mar 21, 2018

Some pics of what I assume is the lathe in question are here.

gregormarwick · Mar 21, 2018

bikepete said:
Some pics of what I assume is the lathe in question are here.

I was going to post earlier about the saddle on this lathe possibly being rack driven but I couldn't recall for sure. Now I see those pictures again it's clear that it is rack driven, and not only but also has a bonus right angle geartrain between the motor and the pinion.

Looks seriously cobbled together - kind of surprised that it has only .114" slop...

Ox · Mar 21, 2018

Well, that's not the lathe that I was thinking that he was in ref to.
I thought that this was a 32" x 96" or something like that.
Some-one installed one like that last year....

------------------------

Think Snow Eh!
Ox

Milland · Mar 21, 2018

If that is a face-driven rack setup, it would be worth checking the pinion gear for wear or check if it's been pushed away from the rack face. Is the backlash a constant, or does it vary with position? The rack may be more worn near the spindle, depending on the type of work the lathe did.

How much money does the company want to put into repairing/improving the beast?

RC Mech · Mar 21, 2018

Agree with the above comments. Get down to the pinion and see what kind of wear it has. Hopefully it's a standard metric gear. Check the remainder of the carriage power train as you tear into it. I'm curious to what component the Z encoder is on.

pressbrake1 · Mar 22, 2018

Id want linear rail feedback on a big sloppy thing and not just rely on encoders guessing position. Ive fitted them to high precision cnc machines and twelve plus year old fanuc has had provisions for it. A good mechanical inspection as other said is a must

machtool · Mar 22, 2018

pressbrake1 said:
Id want linear rail feedback on a big sloppy thing and not just rely on encoders guessing position. Ive fitted them to high precision cnc machines and twelve plus year old fanuc has had provisions for it. A good mechanical inspection as other said is a must

It will go nuts trying to find position, if you use a scale / true position feed back, with all that back lash. Oscillation is the word. The servo will just hunt within that nearly 3mm of backlash. 0.114'. Once upon a time 0.125" used to be called close enough to an eighth of an inch. A closed loop system of feed back, will never control it. We're talking carpenters tolerances here.

I've done numerous Butler ElgaMill retrofits with rack feeds, and Fanuc controls. You need a very good integrator. On one occasion we had to fly in a Japanese engineer. There's an option module that will feed until position, then go out of closed loop. Your still at the vagaries of the machine floating around, within the back lash / lost motion.

Best solution was to fit a second servo & pinion. That was treated like a master / slave drive, where the slave drive opposed the main, picking up the back lash / lost motion. Two servo controls on one axis. Shoot it with a laser for pitch comp

Regards Phil.

assface421 · Mar 22, 2018

.114 of an inch.

assface421 · Mar 22, 2018

lowCountryCamo said:
Is that .114mm?

.114 of an inch.

assface421 · Mar 22, 2018

bikepete said:
Some pics of what I assume is the lathe in question are here.

That is the beast.

Backlash on big CNC Craven Lathe.

Aluminum

Diamond

Diamond

Diamond

Diamond

Stainless

Titanium

Cast Iron

Diamond

Stainless

Cast Iron

Diamond

Diamond

Diamond

Stainless

Stainless

Diamond

Aluminum

Aluminum

Aluminum

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