retrofit on EMCO PC turn 55

What parts do you have? If you still have the original stepper drives, and if they take step/dir signals then it is a pretty easy conversion.
My shopping list would probably be
Intel BOXD525MW $80
8 GB SATA DOM $40
60W PicoPSU $30
Mesa 5i25 $80
Mesa 7i76 $120

Then keep the existing drives and stepper motors.

Alternatives include the Pico PPMC USC. for £250 for the control card. You might also consider the new Intel DN2800MT motherboard which runs direct from a 12V supply (which probably exists in the case). It will work with the PPMC / USC as that uses the parallel port for communication, but as it has PCIe rather then PCI you would need to change the 5i25 to a 6i25 if going the Mesa route.

www.newegg.com/Product/Product.aspx?Item...813121442&Tpk=D525MW
www.mesanet.com/ (Anything IO FPGA cards section)
www.pico-systems.com/univstep.html

If the motors are 5-wire and all the wires are used, then that means that they are being drive by Unipolar drives. This isn't a bad thing, necessarily, as it was clearly adequate when the machine was built. You can run a 5-wire motor with a bipolar supply simply by ignoring the centre tap as described here: www.stepperworld.com/Tutorials/pgBipolarTutorial.htm but I doubt that the increased performance would justify the expense, and might just lead to overheated motors.

chuck1024 wrote:

Also keep in mind the original motors are 5 wire motors and you may have trouble finding drives to run them

As I said, you can run a 5-wire motor on any drive. On a bipolar drive you ignore the centre tap wire and leave it unconnected. On a unipolar drive, it is the 0v return wire.

There are two HAL components for controlling different types of that class of lathe tool holder here:
wiki.linuxcnc.org/cgi-bin/wiki.pl?Contri...oolchanger_component

Are the original motor drives step/dir or do they use an alternative (serial?) interface like the Boxfords originally did?

jrkeat wrote:

So I shouldn't have trouble running the original motors, as long as I don't mind the original slower speed?

Yes. Which sounds like it might be easier if you don't have a way to make adapter plates.

As far as the computer is concerened, what are the constraints? For example, could I use on of my laptops and just buy a pair of USB-> DB25 parallel adapters?.

The main constraint is, no USB. It has too high a latency (and the USB-parallel convertors don't give a proper parallal port, they are only useful for printers as the pins are not (typically) individually addressable

I would look at fitting a mini-ITX board inside the existing case. The Intel BOXD425MW is very cheap and known to work well with LinuxCNC. It has one onboard parallel port. You would need to count pins carefully to see if that is enough.

One possible upgrade would be to add a Mesa 5i25 and the 7i76 board (for more IO, much faster step rates and built-in VFD control.)

If it was my lathe, and because I have some nice little NEMA23 brushless servos, I would be looking at a D525MW, 5i23 and 7i39 combination.

jrkeat wrote:

How would the D525MW work? Chuck says the job requires a DB25 for the turret, and either the Gecko or the 7i39 requires another. So two DB25 parallel ports, which seems like its one more than the D525MW can be set up for.

You may need to add a PCI parallel port card in addition to the onboard one. As I said, you will need to count pins to work out what is required. The Gecko is wired to use a complete parport, but individual drives can be wired individually.
The parport has either 12 outputs and 5 inputs or 4 outputs and 13 inputs.
4 outputs is not enough to drive s axes and the turret, so the question is whether 5 inputs is enough to handle the axis limits and the turret. You need 2 lines for the X limit and Z limit (though it is actually possible to share them, if you really need to). That leaves 3 lines for the turret. I don't know how many it needs.

The 7i39 actually uses a connector on a Mesa PCI card, so is a completely different thing. (It is a brushless motor drive for connection to the Mesa 5i20, 5i22, 5i23 or 7i43 cards)

If you actually meant the 5i25 then the point is that 5i25 connector _looks_ like a DB25, but is actually rather different and configurable. You can connect (for example) a 7i76 with 5 channels of step/dir stepper control, 48 lines of digital IO, and a +/-10V line for the spindle VFD. Then you can connect another 48 IO card to the expansion port on the 7i76, and if you really want to get silly, add another 8 x 48 channel IO cards to the expansion header on the 5i25.
Or the pinout of that DB25 can be identical to a Parport and drive the Gecko G540 drive mentioned earlier, but you still get the option of adding another 8 cards to the expansion header, or a second Gecko G540. It's all down to the magic of firmware.

TBH I don't see much point adding extra parport cards when the $80 5i25 offers so much more in the same footprint. But also bear in mind that many PCI parport cards are dual-headed.

Also I want to make it clear that the 7i76 board does not drive stepper motors directly, it is more of a smart breakout board to which stepper drives such as Gecko / Leadshine / TB6540 / Original Emco drives are connected.

I think you are going to need to spend a minimum of $120 to get the machine working. You can spend a lot more, for an incrementally better result. I have been trying to describe the least-cost method.