Denford Senior 3C Z quill servo?

[Dave MartinParticipant @davemartin29320]

Denford Senior 3C Mill (s/n 3C24037B, manufactured April 1987) – appreciate if anyone can provide info on the Z-drive quill servo motor.

The quill has started “stuttering” and on inspection this is at a regular interval which corresponds to one bout of stuttering per rotation of the Z servo motor. Now traced to what I believe is a shorted commutator or failed winding on the Z servo motor (with the motor just on a non-CNC low-voltage DC supply it will turn through, say, 330 degrees and then jump the remaining 30 degrees). Am urgently trying to find a replacement or to get it rewound.

Unfortunately there is no label or markings on the motor at all.  Denford have searched but it just predates the start of their archives; have also drawn a blank via the Denford forum. 

Appreciate if anyone could advise – anyone have a parts list? or just the motor specs? or even better, a user manual? or even just recognise the maker? no label on the motor at all, castellated top may be distinctive? appears tacho is an integral fitting. Base appears to be NEMA-34 footprint. Current motor (2 brushes) delivers app 30 RPM / volt (minimum start voltage is just under 3v); free-running/no-load current (i.e. motor off machine) is app. 0.2A. Tacho (4 brushes) gives out app 6.5v / 1000 RPM.  Half-inch shaft protuding 1.5″.

 

thanks / Dave

 

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[Dave MartinParticipant @davemartin29320]

[David Clark 13Participant @davidclark13]

Hi There

One for you Mr Stevenson I hope.

 

If you can’t help perhaps you can put Dave in touch with the Denford user group on Yahoo. The address is on my old computer and I can’t access it without a lot of work.

 

regards David

 

[mark vegaParticipant @markvega33700]

You might be better off getting electrical engineer to check it out, or a company that specializes in servos: see local yellow pages or do web search, as the spec/components might not be specific to Denford. Anyway, you might be able to get someone to make up spec to confirm with faulty part if it needs more than a re-wind.

[John Stevenson 1Participant @johnstevenson1]

Sorry Dave, can’t help on the motor as to be honest I have never had any personal experience of a Denford on servos.

I have done about 10 now and these have all been on steppers,

Also seems a funny spec at 240 volts and 10 amp, out of interest where is the encoder ?

 

Only think I can help with is that I do a lot of work for three different motor rewinders and could get a price if needed, may not be cheap but it would be realistic and I can trust them.

 

John S.

[RedpiperbobParticipant @redpiperbob16487]

Hi dave have you tried here

http://www.denfordata.com/bb/viewforum.php?f=4&start=100

 

 

Or here

 

http://www.mycncuk.com/forums/showthread.php?t=822 

 

hope this helps

Bob

 

[Dave MartinParticipant @davemartin29320]

Thanks everybody so far

 

Mark – thanks for the advice but am certain we’ve got a faulty motor. Have disconnected all of the CNC side and just run it off a lab DC supply and it still jumps (it runs OK for most of the revolution, stops momentarily, and then jumps forward). When driven via the built-in ASR Servotron DC servo amplifier/drive, but with +/- control voltage supplied from a manual pot, the jump is faster as the drive, via the tachogenerator, recognises the motor has ‘stalled’ and briefly increases the motor power to try to un-stall it and as a result it ‘leaps’ over the dead spot. With the servo amp driven as intended off the Heidenhain TNC, the situation is far worse as the encoder is then in the loop and the TNC drives it even harder and we end up with an horrendous oscillation as it transits the ‘bad spot’.

Unfortunately the other axis motors are much bigger, and there’s no chance of swapping-in a borrowed unit – we don’t even have an ‘ordinary’ motor rewinder here – its one of the very rare downsides of living on a rock in the middle of the Irish Sea !  I’m sure its not going to be a Denford or Senior specific servomotor, but it makes it a lot easier to replace or get it rewound properly if I could find the manufacturer’s details.

 

Bob – thanks for the links.  Have already aired the subject on the Denford bulletin board without success; and thanks for the link to MYCNCUK – and whilst that thread was about the Starmill on steppers, will also ask there

 

John (and David) thanks for the feedback. Firstly, apologies if the machine’s data plate looked like it was for the Z servo! – the offending servo is completely devoid of any labels at all. Have attached some more general photos below of the machine and the head. Ballscrew is static and the ballnut is driven 2:1 from the motor. Because of the proximity of the ballscrew to the head casting, and I guess desire to limit the overall height, the encoder is also driven off another timing belt.

 

Whilst the XY travels are great, the quill only has limited travel, so if the knee had been built with a ballscrew, I would have jumped now to use the knee as the Z – but as it only has an acme screw, I’m wary of wearing that out, and will persevere with the quill for now.

 

In some ways, I think the quill Z might be more of a candidate for a stepper, rather than a servo (lower speeds, more holding than moving).  I do have a spare G203V from my small Mach3-driven machine – but in this case I really want to keep the TNC control – so I would need to drive the Gecko off a +/- 10v rather than step/dir ….

 

If I can’t source a new motor, or change to stepper, then I’ll be very glad of your help with a rewinder John – its so much nicer to entrust to soemone with a known track.

 

cheers / Dave

 

[John Stevenson 1Participant @johnstevenson1]

Dave, problems I can see is that fitting a stepper as you say, would require a step and direction to analogue card but then the TNC would require information about steps per inch etc.

Not sure how easy this would be to program in.

 

The dead spot could be windings or even a comm to earth problem.

 

Try doing a continuity check on each segment and earth.

Then pick one segment and do a check to the nearly opposite one , say RH side and note the reading.

Move round clockwise and chart the results.

Then do the same, same start point  but the segment to the left and chart the results.

 

That will give you more ammunition.

 

John S.

[David ArmstrongParticipant @davidarmstrong63156]

Hi Dave ,

Well definatly sounds like a canidate for a rewind ! , i hazzard a guess that the motor is a Baldor , but as i know from experience ! , the usual fault with these is exactly what you suspected a winding fault probably shorted due to over heating , but i’d suspect more to do with age than anything else . you may pick one up off Ebay or go for a rewind if you’d still like to keep it running on servo’s . 

 

you should be able to find a suitable motor as a replacement resonabely easy

by the size i’d recon it’s about 6 – 9 Nm 

 changing to steppers is feasable , but more work involved as you say changing the analog driver board , you can still get analog input driver boards to stepper but few and far between and probably over budget !

 

if it were mine i’d go with getting the motor serviced , ok expensive but probably never needed to be done again in your lifetime ! , or pick up a comparable motor on ebay the last ones i got cost me £65

 

Dave 

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