The Craftmans Shop > New from Old
The Sequel - Oh Blimey I bought a CNC Lathe (Beaver TC 20)
RussellT:
Hi Andrew
Interesting. I would expect doubling the approach speed to reduce the accuracy by a factor of 4 rather than the 10 I think you found in practice.
I am still wondering why it won't run as slowly and I am curious about the field coil current/voltage. I'm not clear why you can't measure the voltage - if a maximum voltage is specified then surely a method of setting up should be specified too.
Russell
awemawson:
Russell it is feasible to measure the voltage, but it means temporarily removing the cooling fan that I fitted to save the thyristor cooking. I may well add some remote terminals to make the job easier and safer as it's easy to get a nasty shock when probing (ask me how I know!)
On the positive side my Tektronix 2465A arrived this morning to replace my ancient Tektronix 453 that I have been using for decades - it's a very good 'scope but the triggering isn't stable. Probably only a capacitor needs changing, but I wanted a faster one anyway.
Hope to get time over the week end to chase the servo problem, but people keep wanting me to mend things. Ironically I got a desperate email from the bloke I originally got the Traub CNC Lathe from. His Colchester Tornado 300 with Fanuc OT controller has developed a spindle drive fault and can I have a look please !!!! Poor chap has a production back log developing so I'll probably spend some time with him in the next couple of days.
awemawson:
Well I modified the fan mounting plate to allow me to get at the field coil connections while tweaking the field current pot. Taking it all the way up to maximum only got me 165 volts, plate rating is 170. I shudder to think what the current was.
Then I tried the M19 issue and slow running etc. Characteristics WERE slightly different, but the minimum speed for instance was still 9-10 RPM.
Then I issued an M03 S1000 (spindle clockwise 1000 rpm - it started accelerating, the lights dimmed, and gave me a KTK Mentor Spindle Drive fault :bang:
So I wound the pot back to approximately where it had been, crossed my fingers and tried again - phew - back as before.
I do wonder if possibly there is something wrong with the motor field coils, as people in the know commented that they thought that the 23 ohms I'm measuring is rather low.
It is an enormous lump of iron to remove so I hope I don't have to go there ! It looks like this one on eBay but although the power rating is the same, the excitation and running voltages are different
RussellT:
That's interesting, but not especially helpful - except that all information helps. I'm sorry if I diverted you up a blind alley.
I'll be interested to see what you discover with the scope although I'm still thinking that getting the minimum speed down is going to be crucial.
Russell
cnc-it:
I might have missed it in the thread but have you checked the belt tension motor to spindle..presuming it's a toothed belt..might be some back lash there when you change direction or maybe the belt needs replacing if it's been stood a long time?
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