ML7 : To convert, or not...

[Doddy]

I'm not sure exactly why I'm posting this, other than perhaps to understand if I'm about to waste a bit of cash.

I have a Myford ML7 lathe. It suits my available workspace and I generally don't hit its limits too often (apart from the spindle bore diameter, I suppose). It's one of the last of the ML7s if the serial number is to be believed and is in "reasonable" condition.

Lathe work feels organic - that is, manual operation kind of suits it, when compared to a mill which in my mind sits firmly in the CNC land.

But, CNC'ing the lathe is attractive from the point of view of avoiding the buggeration with repeated cut/stop/measure and conversion from imperial to metric all the time. Without a powered cross-slide there's a degree of skill and finesse to get a decent surface finish that I rarely can be bothered with.

I'd previously started the "Jeffree" conversion with the saddle available for stepper drive (essentially mounting a stepper on the rear of the bed coupled to the lead-screw), but that does limit the accuracy to the lead-screw and the backlash associated with this and the half-nut. I've got most of the bits to convert the cross slide (just one bit of ali to machine - easy stuff to complete). But that would leave me with, again, the limits of the acme screw/backlash. And the usual faff of the gib strips etc.

I did start to eyeball the cross slide, wondering about replacing the acme thread with a ballscrew, but there's not a lot of space there. I've also eyeballed the main leadscrew with a similar thought to replace the split-nut with a ball-nut and have the Z permanently engaged with the motor (happy to introduce a rotary encoder for hand-operation).

But then I got to thinking. I can get a replacement saddle for fifty quid. I've got plenty of 20mm linear rail and carriages currently collecting dust and spiders in the shed. What's stopping me removing the existing saddle, apron, cross and top slide - and replacing with a new/old saddle with the centre dovetail removed, mount the linear rail onto the original ground surface for the saddle/topslide sliding surface and then mounting onto this a top slide dedicated for a CNC conversion (I have plenty of 1" 6082 plate that should work fine) and fix the usual QCTP. My argument behind this is that it is easier to create the new cross slide from scratch than to retrofit the the existing castings.

The devil on one shoulder is screaming that this would be sacrilege against an old Myford. The devil on the other shoulder doesn't care about that and says that it's an effective solution to CNCing up a lathe that I already have, for very little money, and I get to retain a lot of the old tooling that I have. This little guy is getting more air-time than the first at this time. And typing the above I'm kinda starting to agree with him.

For anyone that has experience with converting a manual lathe to CNC, does this idea of stripping down to the saddle and building up from scratch - using a ballscrew for Z and X axis and a dedicated, simple top slide running on linear rail make sense?, or (and more importantly) are there pitfalls that I'm likely to fall into part way through this conversion? (forefront of my mind is the loss of height with the rail/carriage size).

There's other crazy ideas going through my mind. One is to mount linear rail on the ground surface of the lathe bed and go for a complete custom saddle. That is very much a no-going-back solution, but I do have the rail available to me.I suppose my concern with this is losing more height from the bed to the spindle center. The impractical, silly side of me that believes I've all the time in the world is questioning selling the ML7 and building up a powered spindle/bearing headstock mounted on some ground plate and essentially creating a bespoke CNC lathe from scratch. But it can't be that easy, can it? And that way lies unfinished projects and more upfront costs.

Experiences, concerns, wisdom and outrage all gratefully received.