I do understand the basic theory about retro-fitting. It's the details that I am unsure (nervous) about.
To sart with some basic questions:
What is the difference between a DC drive and an AC drive (and motor?) What are the benefits/detractors of each? Can I use my existing stuff (motors, servos, etc.) to keep the cost down? What should I be looking at as far as geckos, break out boards, etc? Obviously, you have Hardinge experience. Would you know exactly what I have on my machine or are there things I need to be looking for (like tags on motors, encoder specifics and such)?
As for the extra room garnered in removing all that GE 1050 controller "stuff", I'm thinking about installing a shop bathroom in the empty enclosure. 
Well I guess the devil is in the detail. But don't worry too much about it.
A lot of Hardinge CNC's used a DC spindle motor and drive. These are quite obvious as they tend to have a tacho fitted to the rear of the motor.
They do work very well, but over the years I have seen a few go bang in a big way! Also with a lot of this stuff they are getting a bit long in the tooth and it's hard to predict how long it's going to last.
Modern Vector AC drives (VFD's, Inverters, VSD's) have come a long long way. The 3Kw Siemens one I fitted to my HXL-S Cost a fair bit £1200 or about $2000 that was for a new Iron Framed motor (4 pole) Drive and braking resistor. The resistor I found I needed later as the basic DC braking in the drive struggled to cope with the high inertia in the spindle drive train (Mainly caused by the air cylinder)
The motor was a drop in replacement after milling the round flange mount to a square one. That figured though as the original motor although DC was also Siemens.
The original Servo's used tacho feedback, and these have caised me some problems in the past on CHNC's with AB Controls. These can be replaced with Encoders with some ingenuity. It may be that you also have encoders already, just depends on how the machine was built originally. Mine had both. tacho's now junked. The CHNC tacho's were gear driven.
For servo drives I'd now recommend CNC Teknix drives from Australia. More expensive than Gecko's but a far more sophisticated product. Support is fanstastic and the Company is a real progressive one. The drive designer came to visit me recently and I have only good things to say about them. They have some other real nice stuff coming out soon too
There is more than likely an encoder on the spindle too, and hopefully soon we will have a way to used them with mach for spindle tracking.
My HXL-s Uses a lot of hall effect switches for limits, homes, and various stuff on the turret. These are 24v devices and you will need to do some level switching to interface with the printer port if thats what you want rto use. Easy though, a cheap opto and 2k2 resiistor works like a charm.
On that subject i personally prefer to have as much of the original 24v levels as possible on the machine signals. As well as differential signals for encoders and step/dir signals if possible. Makes for more reliable operation and costs very little to keep the machines the way they were
built.
The turret will eat up some I/O so a plc may be required. Not anywhere as bad as my VMC's ATC requirements though.
Turret needs 4 outputs for the coils in the valves. Lock, Up & Index, turret stop engage and retract. Turret encoder is 4 ouput but this could be decoded down to 2 inputs with simple logic. It's not BDC output like the CHNC encoder.
There is an additional +z limit for when the tailstock is in use this can be ORed with the stock one.
The flow sensor for the tailstock is there to put the machine in feed hold whilst the quill is moving.
There are various other sensors for low air, lube, spindle lock etc etc..
When mines sorted I'll happily give you all the methods I used!
Wayne....
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