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]

I don't have a DRO - and that's half the problem.

With the stuff I have in the shed it would cost me as much to add DROs as to CNC the lathe. It's one of those binary decisions that I need to make.

I don't intend to make many multiples of the same part, no. But, part of the task to CNC it is for the hell of it. One of the appeal is for more complex shapes - tapers (yes, can be done but...) balls (I inherited a ball-turning device with the lathe, but...). Another possibly mis-guided view is that by removing the top slide I can increase the rigidity of the lathe. Also, those silly little hand wheels are not the most ergonomic with my sausage fingers. Then factor in the imperial nature (not a huge issue, but I'm clinging onto this) and a Mach conversion holds some appeal. I'm currently juggling gib-screw forces with the tightness of the part-worn cross slide acme screw to minimise backlash and rigidity.

I also wonder if throwing a micro controller in there could help produce a stand-alone capable machine with fly-by-wire controls and an easy macro option for simple shapes. I don't want to necessarily be tied into F360 to turn down a radius.

It would be my only lathe, I can't afford more space to have a CNC and manual separate option. I have been keeping an eye out for a cheap (hah!) CNC lathe to replace the Myford, and for that keeping it as-is is appealing.

EDIT:

John, can I ask what you've done with the spindle motor?

[JohnHaine]

No manual operation capability though I would like to add some handwheel encoders to give a sort of manual control, but frankly I don't really miss manual. I do have a, XBox controller for jogging which is useful for quick positioning, but not really for cutting. Even if the drivers are not powered up trying to manually turn steppers is not easy and having to disconnect them is a pain.

There is this persistent myth that CNC is only for repetitive work. Once you have calibrated tools and the ability to home the X axis, it's probably quicker to chuck a bit of bar, find its end (I have a "find bar end" macro with a tool touch sensor to do this), select the wizard to use, and let it rip, than do it manually. Mach 3 wizards let you do turning to a diameter, radiusing a shoulder, tapers, balls (internal and external), and threads. Very occasionally there's something where you need to write G code - for example doing odd profiles on gear cutters. There isn't unfortunately a Mach 3 parting wizard but it's very easy to program parting in the MDI. (One day I'll write a macro for it perhaps.) Mach 3 will thread but only syncs on 1 pulse per rev so you have to take light cuts - but so what, the wizard does it all for you anyway.

I haven't controlled the spindle yet - I keep meaning to fit a 3 phase motor and VFD but too many other things to do. So I just start and stop the lathe manually.

Photos will take a while as I'm off on hols and other things for a couple of weeks.

[Doddy]

I'm a little confused now with what I actually have. I fear a Frankenmyford (remember, it was one of the last ML7s off the production line, according to the serial numbers on lathes.co.uk)



The link to the conversion site that John provided at the head of this thread (bottom of https://cncyourmyford.com/modifications-needed/), suggests the extra bolt-hole on the left of the saddle is that of a Super-7 saddle, not the ML7. And the bore for the cross feed screw is 1/2"