The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)

[BillTodd]

I have fixed simo-drives and lathe turrets in the past, but they were relatively simple servo failures (99% down to capacitors - large and small )  and mechanical (operator driving the turret into the chuck at a bazilllion rpm)

From my experience positional errors are down to misreads of the encoder , we (a very experienced colleague and I that used to maintain about a hundred cnc machines here in essex) would mostly just replace stuff but occasionally the older stuff would be irreplaceable so i would dive in to component level, with mixed success.

1)could be mechanical  - loose or broken couplings - fixed an AGIE wirer where the coupling had turned to cheese
2)knackered bearings in the encoder  - very common on spindle encoders
3)Older 'analogue' encoders used on older bridgeports would just start playing-up then  eventually stop - probably weakening photo-diodes or bulb failures : the machines were replaced before I got to the bottom
4) broken cables - mostly X or Y axis  usually due to being clogged with swarf ,chaffed bare and eaten by coolant

[BillTodd]

After chatting to my colleague Chris. He has had a problem where the servo motor ref is set at , or too near to,  the turret reference which can cause the location to jitter by a whole motor turn. 

Heidenhain recommend timing the motor reference 180 degrees from the turret reference before setting the grid (coupling) offsets .

Other problems can be caused by absolute encoder (ones that store their position) getting corrupted by loss of battery power. ( I  remember having to manually 'walk' one by turning on and off the lathe to get the axis awaay from the roll-over / overflow because no seemed to know how to reset the encoders)

[awemawson]

Thanks for that Bill - food for thought.

So yesterday afternoon I had another session and totally mucked things up - must have turned to pot the wrong way or something as the system just locked up. I walked away for a think.

This morning I dug out one of my spare servo cards (6SC6100-0NA11) - extracted it's parameter card (switches and pots) and measured the setting on the offset pot . Nominally 10 K with 15 turns end to end. I measured pot overall as 9.98K and bottom leg as 4.35K and top leg as 5.98

NB I had tried this card  a few weeks ago and knew it basically worked.

Then I pulled the same card out of the machine and firstly measured where I had set it - wildly wrong as bottom leg = 7.61K and top leg as 2K - miles out.

Then I set is as close as I could ending up at 4.36K bottom leg 5.37K top leg. (Interestingly adjusting the pot also altered it's over all resistance)

Re-installing the card and re-initialising the machine it starts up like a champ and indexes remarkably well - don't fiddle any more Andrew!


https://youtube.com/shorts/UyyZrmppEEk

[vtsteam]

Yay! 

[tom osselton]

 

[awemawson]

Thanks for the encouragement chaps but I really do think I'm going to end up in a mental asylum 

This morning I went to the workshop with the good intention of fully loading the tool disk with it's full complement of tools and holders and making sure that they are evenly balanced but first as is good practice repeat the run with the existing  load of six tools representing a 'half load'. NO - running program #13 that quasi-randomly selects tools it was back to significant rotational positioning errors, so much so that the curvic coupling wouldn't come together.

Trying to bottom out what had changed (nothing!) I then had an issue that I couldn't jog the machine at all. A quick look at PLC error messages told me that the automatic lube pump for the main machine ways was in fault. Check it's fuse - return fuse to wrong slot in holder - why has a good fuse got 240 volts across it? - find the error and take fuse from spare location where i'd put it back into the live location - OK pump now has power -but no oil! Oil float sensor on bottom of reservoir - fill up with 1 litre of Magna 68 way lube. Hooray we can now jog!

But where were we - what were we doing - oh yes program #13 Turret exerciser. Back to it !

But it now runs - if anything turret needs driving slightly further clockwise but in each and every attempt the curvic coupling is mating OK.

And before you ask - no the automatic lubrication system doesn't feed the turret !

[modeng200023]

Real progress 

[AdeV]

But where were we - what were we doing - oh yes program #13 Turret exerciser. Back to it !
But it now runs - if anything turret needs driving slightly further clockwise but in each and every attempt the curvic coupling is mating OK.

Was the machine switched on for most/all of the time between your first unsuccessful run, and the successful run after mucking about with the oil pump?

If yes: Could be something thermal - maybe there's an iffy PCB trace near a component that gets warm/hot - when cold it's not conducting reliably, once warmed up the expansion is enough to get it working?

Literally clutching at (copper) straws here...

[vtsteam]

Andrew, have you seen anything that kinda looked like this?
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[awemawson]

It?s probably full of them Steve. At the moment I?ve walked away from it to preserve my sanity.
The turret is in an odd jammed state, not being able to move the tool disk forward or backwards. Something is mechanically stuck, but also my oil central heating boiler in   the workshop has sprung a leak requiring total replacement. I?ve had the new one delivered (?3049 !) but not yet installed. I am required to employ a registered installer so that?ll be a few more ?100?s

[tom osselton]

Remember this?

[tom osselton]

Gremlins from the Kremlin 1944

[vtsteam]

It?s probably full of them Steve. At the moment I?ve walked away from it to preserve my sanity.
The turret is in an odd jammed state, not being able to move the tool disk forward or backwards. Something is mechanically stuck, but also my oil central heating boiler in   the workshop has sprung a leak requiring total replacement. I?ve had the new one delivered (?3049 !) but not yet installed. I am required to employ a registered installer so that?ll be a few more ?100?s

Man if it's not one thing, it's another....

[vtsteam]

Wow, Tom, no! Never saw that one. Usually it was just Bugs vs the Gremlin.

Why did they name a car that anyway? 

[awemawson]

OK I've been away from this for a couple of weeks to let things soak in in my brain. Sumarising, the tool turret seems mechanically stuck in that the hydraulic cylinders cannot move the central shaft forwards or backwards. Now I call these hydraulic cylinders but actually they are machined cavities in the casting with the central shaft acting as the piston / rod. There is a powerful spring pushing the shaft rearwards aided by one half of the hydraulics with the other side of the double acting 'cylinder' pushing it forwards when a tool change is called for.

So is it a stuck valve maintaining locking pressure, or something internally physically miss-placed?

As an aid to seeing what is going on I dug out my 'spare' turret and rigged it so that I could try moving the shaft with a crowbar against that spring. The whole thing is plonked on the tool disk face with the central shaft pointing upwards and it's too darn heavy for me to shift it so in fact in this experiment the tool disk and central shaft are staying still and I'm raising the rest of the turret upwards by leverage - this direction corresponding to 'forwards' were it mounted in a lathe.

Rear Hirth coupling disengaged (this is the one that applies rotation to the tool disk) - ignore all the detritus on the coupling, this has been sitting open in the workshop for months:

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Rear Hirth coupling engaged by crowbar:

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So it is possible to mechanically move the shaft on the 'spare' turret with the hydraulics disconnected - I could hear the gurgling of the residual oil as I did it !

This gives me confidence to perform the same experiment on the actual turret in the lathe which is the next step I think.

While I was inside the 'spare' turret I think I've found out why the chap couldn't get good tool alignment with it - there's a massive crack in the casting !

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[vtsteam]

Glad to see you back on this project, Andrew.  From the looks of the discoloration around the bottom of that crack, I'm wondering if the turret was used in that condition for some time.

Andrew, are you considering brazing/repairing that casting?

[awemawson]

No Steve I've no intentions of using that turret - I only got it with a load of spares I bought from a chap in Ireland. It has proved useful as a diagnostic aid though.

When I packed in a couple of weeks ago, the turret disk was stuck mid motion - now it has relaxed into the rearmost (locked) position (presumeable under pressure from that spring), and hydraulically I cannot drive it forward into the fully unlocked position (I'm referring to the front curvic / hirth coupling which is not the one in the pictures above) so I am more convinced that I have a valve issue. First approach is to uncouple the appropriate hydraulic hose but to say that they are hard to get at is an understatement. Somewhere I have a hydraulic piping schematic - hopefully that will let me relax pressure further down the line but it's a bit of a jungle of pipes at the manifold where the valve are.