Very basic questions

You can use a parallel port for stepper motors or step/dir controlled servos.

A Bridgeport is getting a bit big for steppers (though it has to be admitted that when Bridgeport made their own CNC mills, they used steppers)

If you don't want to use steppers then the control electronics moves away from the bit-banged parport, either to EPP-controlled cards, or PCI or Ethernet.

I have generally started at the other end, seeing what motors I could find cheap on eBay, then working backwards towards the PC.

I converted a Harrison milling machine to CNC with 750W servo motors and that works nicely. A friend has converted the same machine with steppers, (NEMA 34) and that seems to be working. But the BP is a fair bit bigger.

I actually have the drives and motors from a CNC Harrison M250 lathe that I could do with getting rid of, but to be honest they are huge weak old steppers and large old unreliable stepper drivers, so can't really recommend them.

If you are converting a mill to CNC you probably need to swap the screws to ball-screws. Even if you don't do that immediately, design your conversion around an assumption ta=hat you will.

Lathes can get away with not having ball-screws a bit better, just because of the way lathes work, little lathe work involves a mid0cur direction reversal.

I have done three conversions of my own now, and one for someone else (in one day, at a meetup in Stuttgart). The first was the typical steppers and aluminium plate mounts, the second was a bit more elegant, and the last one cost me more in custom castings than the total cost of the first one.

So we can advise you how to do this, but it might help to know how much you expect to spend. I doubt you can do the Bridgeport for less than £300. But you could spend 10x that if you wanted.

UKenGB wrote:

my aim is to be able to computer design and then machine fork yokes for motorcycles.

My machines often make motorcycle parts, but not yokes as the bike I make parts for was made in 1921 and doesn't have forks, it has a front swingarm....

Another really basic question then, what is the difference between stepper motors and step/dir controlled servos? Can either/both be driven from the built-in parallel port of my PC? [/quote]

Yes, both can be controlled with the parallel port of a PC. I wouldn't, but you can

Actually, you might as well start of with P-port + cheap breakout board, and some steppers. But you might find that (like me) you learn enough from that to know how you ought to have done it in the first place

Many servo drives require analogue +/-10V velocity control. That is something that you can't readily do with a parallel port. But LinuxCNC has a number of compatible interfaces that can do that, if you get a deal on some motors and drives. The drawback is that they cost $200 or more.