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.

[mekanik]

Hi Doddy
Diesel was contemplating building a slant bed lathe some time ago but never got round to it, don't know if he got as far as designing it but might be worth dropping him a pm.
Regards
Mike

[JohnHaine]

I have cnc'd my Super 7 and I'm very glad I did. It's a late PXF version, which has room in the saddle for a 12mm ballscrew, though I had to mount the nut sticking out because there isn't room to mount it conventionally. The 12mm screw has enough rigidity that you don't need a bearing on the back.

For the non-PXF S7 and ML7 the hole in the cross slide is too small for a 12mm screw. If you look at this site:

https://cncyourmyford.com/

...he uses an 8mm ballscrew screw (I think) - these are more expensive and do need another thrust bearing at the back and to keep the screw under some tension. I think the same approach would work for the ML7.

My X ballscrew I think has a 2.5 or 3 mm pitch - whatever it is it's swallowed up in the axis setup in Mach 3. The 8mm screw will have a smaller pitch. I started driving the screw with timing belt and pulleys but went over to direct drive from the stepper when I fitted the ballscrew as I think I was getting cyclic errors from slight pulley eccentricities.

As for the Z screw - so far I haven't found a situation where I can't avoid having to cut in the positive direction, but at some point I would like to convert to a ballscrew. The above site explains how and he also sells some castings to adapt the apron. You need to keep chips off the screw, and he uses a very neat approach using Dyson vac hoses as bellows! I use Tony Jeffree's approach of a stepper on a bracket at the back of the lathe with timing belt drive to a pulley that replaces the Z handwheel. Of course the leadscrew is 1/8" pitch (on my "metric" lathe!) but again Mach 3 takes care of that.

I've done some other mods, mainly removing the flimsy Myford topslide and making a very solid toolpost mounting block for the Dickson toolholder which is dowelled so I can quickly change tools and maintain referencing. I haven't fitted limit switches, I can't see they are much use on the lathe, I do use soft limits in particular to avoid running the crossslide off the ballnut! I've fitted a crosslide reference "switch" that zeros Mach3 at the start of a session - once all my tools are calibrated I can just dial in the tool number and pretty reliably cut to a known diameter.

Happy to answer any further questions if I can help. I love CNC turning!

PS: The way I've done the conversion, I can put the lathe back to its standard state with no issues. In particular I made a new bracket for the cross-slide screw motor mount.

[JohnHaine]

BTW - happy to show you the lathe or provide some photos.

[mekanik]

Photos would be nice

[Doddy]

John,

Good to hear the changes you made - all perfectly sensible. Holes can always be made bigger :) I'd be curious if you have retained a manual operation capability or not, and if you have - do you use it? I look at the manual capability compromising the CNC options.

Would love to see any pics... Cambridge is a bit far to eyeball at this point :)

Mike

[m_c]

I think the big question you need to ask yourself, is what exactly do you intend to make with the lathe?

If you want to knock out multiples of the same part, or items with a lot of profiling, CNC'ing makes sense.
If you only want to make relatively simple parts in single quantities, then CNC can actually take longer.

I've never been a fan of CNC's that still have handles attached. Manual machining with ballscrews is a pain (larger pitch makes accuracy challenging, and they're not selflocking), and CNCing with ACME screws is not ideal.

I still have a manual lathe, and I wouldn't sell it, but then for simple parts I still like being able to take a sketch and just get on with it.
Everything on a manual lathe can be done on a CNC, it's just if I'm going to have to babysit a CNC to ensure tolerances are on spec for a one off part, I'd much rather be spinning the handles than hitting buttons!

Out of curiosity, do you have a DRO on your lathe?
It makes using a lathe far easier.

[Doddy]

Okay, mixed thoughts at the moment. I'd started with the Jeffree conversion for the Z-axis some time ago, and over the last weekend knocked up a more compact X-axis...

Z:

X:


Just for confidence and a break from the mechanics I've slaved up an old controller box (first pic) and an old XP box that had the driver for the ethernet controller for the control box from storage.

It proved the concept, but I'm somewhat puzzled by the noise from the Z stepper...



X is as quiet as you might expect. Z is bloody awful. I've tried fiddling with the stepper driver with micro steps from 1/2/4 and the current from 3.2A through 5A. It just sounds awful. Part of this is transmitted through the bracket onto the lathe bed, but unbolting the motor from the frame its still not great. The motor has been in situ for the last, maybe couple of years, with light manual use of the lathe (so powered traverses using the lathe gearbox) into an unpowered motor, with stripped ends to the motor wiring that may have been shorted from time to time (just rolled up and stowed under the motor bracket). To me, it sounds very mechanical and internal to the motor. I do have a smaller motor available to replace this... in fact I should have an identical one but not seen that for a while. Whilst I tidy up the shed a bit has anyone heard a stepper sound quite so bad - is it likely goosed?

[mekanik]

Sounds like it's the gearbox making the noise, can the leadscrew be disconnected or gears moved to a neutral position, i don't have a gearbox and can just move the quadrant out of engagement.
Mike

[Doddy]

Nah, I've pulled the final gear from the gear chain - it's not that. I've also pulled the belt from the motor and the motor still grinds under power. Removing the motor from the mount and it still feels aggressive under rotation, but it's the coupling of the motor to the mount that generates the awful noise. Perhaps some weird harmonics.

Apologies, btw, for the 80's music in the background. That's my sanity aid :)