Can I use a mill converted to CNC in manual mode?

Hi, I'm new to CNC. I purchased a mini lathe and have really got on with it and am thinking of supplementing it with a mini milling machine. I'm also thinking about a CNC conversion so that I can make complex cuts without a rotary table and such specialist gear.

Just looking at the CNC conversion plans listed on eBay, they state once converted, if twin shaft stepper motors are used, then some degree of limited manual use will be possible. Any idea why only some manual use would be possible? What's to stop me putting some dials on the other motor shaft and using it manually?

One problem is that even when they are not powered, stepper motors are quite "notchy" to turn which makes steady movement of a handwheel more difficult. It's possible but not quite as good - you need to get hold of a stepper motor and just turn the spindle to see what it's like. One possibility is to arrange some kind of clutch mechanism. Another alternative is to accept that you have to power up the control box, etc, and use an MPG - a box with a knob on, crudely - to control movement. Typically the MPG will let you select the axis you want to move and the step size you want and you then twiddle away to move an axis. Some people go as far as to fit "electronic handwheels" to replace the manual ones.

One other problem is that depending on how you do the conversion, you might lose the handwheel dials so you have no manual position readout. If you want to use the CNC system in purely manual mode, you won't be able to use the electronic position display unless you use something more sophisticated than simple stepper motors - unless you use the MPG approach.

It's also possible that what I am saying is more applicable to larger, heavier, machines and the problem might not be so bad on a mini-mill size machine - it would be good to hear from users of that style of machine for real experience.

Pretty good summary there Neale. Another issue is that with replacing the trapezoidal screws with ball screws - and the associated increase in efficiency, unless you lock each axis (good practise, in any cases on a manual machine) you'll likely find that the axis will move under the cutting force under load.

The cogging effect of a stepper connected shouldn't be underestimated - I finally removed my cross-slide hand wheel on my lathe conversion (to replace with MPGs) as I realised that it did not afford the finesse that you normally associate with a manual control (plus, it was a stomach-ache [glad the stand is as tall as it is] if working close to the machine when it decides to do a rapid movement).

OP: Twin shaft is one way - another is a belt drive onto the screw - offsetting the motor away from the screw and allowing the original hand wheel to be retained.

But... think hard about why you want to CNC... and why you want to retain manual control. I stripped my mill (SX2.7 long-bed) of all manual controls - except the quill feed lever.

I did my Bridgeport conversion so that I could retain manual operation, thinking I was being clever. I retained the large cast iron handwheels on the X and Y, and had a clamp nut on the Z axis so that I could release the ballnut, rather like Elrod's concept.

Much of that was a waste of time. The large handwheels had massive moments of inertia which really buggered up the servo response (dead sluggish, so I removed them) and I've never used it in manual mode anyway, so the complication on the Z axis ballnut yoke was pointless.

You really need a wireless MPG. That way you still have your "manual handwheels" but they are not stuck on the end of the table, you can change the feedrate from the MPG and you can use the MPG where it is most convenient eg with your nose right next to a tool when touching off etc.

Besides, if you are in manual mode, surely you want to use a DRO readout to know where you are. There's not a lot of manual work you can do freehand where a DRO wouldn't be useful.

The torque on a stepper is a lot higher if the wires are shorted out or if the driver is unpowered, which has a similar effect. If you were to disconnect the steppers you would likely find the drag torque was pretty low. On my machine before I fitted ballscrews, I often had the slide locks lightly nipped up, particularly if climb milling. Afterwards, the servos and belt drives provided enough drag not to need any help when unpowered and when using them in manual mode (ie powered up), they hold position very solidly, so little chance of drift! If you simply fitted ballscrews and nothing else, you could nip up the locks as much or as little as you need, so it shouldn't be an issue.

Muzzer is absolutely right - a wireless MPG is a real boon. My CNC router has no manual overrides but I use the MPG all the time for manual positioning - like the man says, you often want to get right up close to the tool for setting up and being able to tweak the MPG controls without even needing to look makes it all so easy. And no trailing cables catching on things and generally getting in the way.

Retaining manual control is so twentieth century...

I was embarking on a thought exercise about this, regarding my CL300 lathe. It would be useful to have something to CNC cut fancy profiles in brass. My first consideration was a hybrid lathe, but you lose the feedback and feel you get from manual turning. My need for a dual control lathe is somewhat abated by the fact that I have a 30" lathe also.

Then even the thought exercise evaporated with the realisation that I have too much else to do first.