Servo Driven 4th Axis for CNC Mill

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awemawson:
Well Pete that's a good question and perhaps one for suggestions from the forum :scratch:

I'll be very pleased to see it under servo control and being able to shift it back and forth using the TNC355 controller - I expect it'll be simple hex or the usual coarse square screw thread, though I was watching some cylindrical engraving on Youtube that set me thinking . . . . need to read up on that :coffee:

awemawson:
A little bit of progress over the past few days:

I've been researching the power side of the servo system. In the Bridgeport Interact where the Bosch Servo driver card and SEM servo motor originate, there is a whacking great 3 kW auto transformer with enough tapping to confuse a Bletchley Park decrypter:

awemawson:
Now I don't like autotransformers at the best of times, as they offer no isolation, and that number of tappings is silly :scratch:

Going through the Bosch manual for the drive, what it needs is two 180 volt windings aranged 180-0-180, and a further 200 volt winding added to one of the 180's to give 380 volts. The main power is taken by the 180 volt windings, and the 380 volts uses a measly 10 VA, and feeds an internal transformer generating 15-0-15 volts DC and also it uses this winding to get the synchronisation of the thyristor drives.

Searching around there were no suitable transformers 'off the shelf' or available second hand, so I'm having one wound. But what power? Well in a Bridgeport Interact it would feed three axis's and potentially a 4th axis rotary drive, and was 3 kW. Pro-rata that means that my 4th axis only needs 750 VA, and I reckon that's probably over stating it as the main axis drives will take more than the 4th axis.

I've simplified the secondaries to just the three needed ones, but have added a 415 v primary for isolation, with a 240 volt tapping so I can drive it off mains on the bench for testing. I upped the 200 volt winding to 50 VA so that they don't use stupidly thin wire.

As a 1 kVA transformer was not much more than a 750 VA that's what I plumped for:

awemawson:
Meanwhile I've got on with the physical mounting of the Bosch servo board and it's twin wound choke. Nothing too complicated, just a bit of a pain as the space is a bit tight. Back panel of the enclosure is 2 mm steel, so everything is mounted using 6 mm hank bushes as 2 mm although it could be tapped and would be ok for first mounting, would strip after a few 'ins and outs'

awemawson:
The X, Y, and Z axis are driven by Indramat AC servos - it would have been nice to use one for the 4th axis, but they fetch telephone number prices even second hand

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