I'm thinking of Mini Mill CNC conversion

[rcx132]

I'm thinking of converting my Mini Mill to CNC. The mill and Mini Lathe made all of the parts for my jeep restoration, and I'm now using them for my 4x4 buggy build. These machines really opened up possibilities and are small enough that they fitted in the back of my spare office room. If I knew how useful they would be I would have gone for the bigger versions like the X3 mill.

I've also got a little CNC router which cost £100 and makes plastic parts but won't do metal, so I'm thinking of ditching it and converting the Mini Mill to CNC.

One of the CNC kit suppliers is currently working on a conversion kit that has manual hand wheels. The supplier is a one man band and is very introverted, so comms are difficult, I don't know how exactly the over ride will work, but I think it's worth waiting and seeing. I assume it won't just be double shafted motors with notchy hand wheels.

I've started looking at the stepper motors and what the options are. I can see that they come in unipolar and bipolar options. There's lot of online explanations on what the difference is, but I can't find info on which type should be used on a Mini Mill. Does anyone know ?

I can also see in people's build threads that the electronics are a big hurdle, but I don't get why? My little £100 CNC router has a GRBL black box. The steppers plug into it and the other end plugs into my PC. I upload G code generated by Fusion 360 and that's all there is to it, off the machine goes. Aside from needing additional hardware to handle the bigger stepper motor currents, where exactly is the complexity in the electronics that CNC conversions face ?

Is there anything else I should watch for?

Thanks in advance for your help.

 

[m_c]

It all depends on how much you're willing to spend and what kind of performance you want.

Ideally you'd want to swap the lead screws for ballscrews, to minimise (and potentially eliminate) any backlash.
But you also need to work out how you'd drive the head up and down.

Personally I'd want something better than GRBL on a mill. It's just too limited in terms of changing tools, and other handy features.
I'd also forget the manual use option. With a decent controller, 99% of things you think you'd still want to do manually, you can do just as quick via an MPG/conversational coding/few lines of manual g-code.

I'd suggest doing some searches over on cnczone, as it used to be quite active for benchtop machines - https://www.cnczone.com/forums/benchtop-machines/
If you can find hoss2006's old threads (he used to sell full plans, but I have no idea what his current status is), they had quite a lot of details, and there used to be quite a few other build/conversion threads

 

[Agroshield]

Aside from needing additional hardware to handle the bigger stepper motor currents, where exactly is the complexity that CNC conversions face?

Some slight complexity occurs when the machine you are converting is needed to machine parts for the conversion.

Similarly, if you need to check parts for fit, dismantling and remantling become tedious.

The correct way forward is to buy a second machine.

 

[rcx132]

Some slight complexity occurs when the machine you are converting is needed to machine parts for the conversion.

Similarly, if you need to check parts for fit, dismantling and remantling become tedious.

The correct way forward is to buy a second machine.

I guess this is less of an issue if I buy a kit?

I have been in two minds about whether I should buy the bigger X3 mill and CNC that.

 

[Agroshield]

I guess this is less of an issue if I buy a kit?

My gut feeling is that it will be extremely difficult to buy a kit that bolts on and needs no modification whatsoever to the base machine.

Maybe if it is very well planned, so you know exactly what you have to do, it would lessen the hassle.

With the appropriate jigs and fixtures (e.g. for drilling and tapping holes) and ingenuity, it will surely be possible to do everything with the one machine. Possibly you could do some of the work on the lathe.

The post above was considerably tongue in cheek, a ploy to justify more iron in the garage to the financial controller.

 

[m_c]

I guess this is less of an issue if I buy a kit?

I have been in two minds about whether I should buy the bigger X3 mill and CNC that.

If you can justify an X3 (or one of the similarly sized clones), then I'd strongly suggest that you get one.
As capable as the X2 is, the X3 is a sturdier machine.

I was going to say the extra travel on the X3 is good, but having just checked, it all depends on what spec you buy.

If you are considering a bigger mill, I'd suggest looking at Emvio's offering - https://emvioeng.com/shop/machine-t...l-milling-machines/emv-25vbb-milling-machine/
They also offer a CNC conversion kit which is quite expensive, but it does contain pretty high end parts - https://emvioeng.com/shop/emvio-eng...rsions/emv-25vbb-cnc-conversion-kit-hardware/
The listing does contain lots of photos to give you an idea of what would be involved in a conversion.

 

[fizzy]

I bought a DIY converted Sieg X1 clone and it seems pretty good.
Apart from me winding the ballscrew too far and losing my balls Fixed that now though.

 

[rcx132]

I bought a DIY converted Sieg X1 clone and it seems pretty good.
Apart from me winding the ballscrew too far and losing my balls Fixed that now though.

View attachment 379443

Yup, I saw your post, rare find . Those stepper motors look overkill though.

 

[Agroshield]

Those stepper motors look overkill though.

The size is deceptive. Those steppers are quite old, hence why they are so big. Over time, I think the size has reduced for the same power possibly due to changes in the magnet technology (rare earth magnets).

Compare to cordless power tools - the first Ni-Cad versions and a modern Milwaukee.

 

[rcx132]

The size is deceptive. Those steppers are quite old, hence why they are so big. Over time, I think the size has reduced for the same power possibly due to changes in the magnet technology (rare earth magnets).

Compare to cordless power tools - the first Ni-Cad versions and a modern Milwaukee.

Sorry my post got posted before I finished. Oh ok, I didn't know old motors were less powerful.

I've found a post where someone measured the force exerted at the spindle by Nema 23 motors. It immediately reached 130kg where his scale maxed out. He suggested that DIY conversions are using motors that are way more powerful than needed. So I'm trying to figure out what is the minimum motor size I need in order to reduce the notchiness when using the manual hand wheels on the back of the motors (which your conversion doesn't have).

 

[Agroshield]

I do not want to be disagreeable, but I think you are focussing too much on the manual operation bit. Once converted, I do not think you will ever use it. The MPG suggested by m_c above is a much better route to manual control.

If you go for motors that are too small, acceleration and rapids will suffer. Measuring maximum force is not really a good metric. It is the ability to speed up, slow down and change direction (agility) while pushing a metal table around in space that is important.

A way around the notchiness could be to use a timing belt and pulleys between motor and leadscrew, with an easy disconnect mechanism (single pin dog clutch maybe, over-centre belt tensioner perhaps).

With some thought, when you are under stepper control, the handwheel could remain stationary (look at the knee crank handle on a Bridgeport - having that whirling around when you operate the power knee lift would be disconcerting). If not, your handwheels would benefit from being the folding safety type.

 

[fizzy]

The size is deceptive. Those steppers are quite old, hence why they are so big. Over time, I think the size has reduced for the same power possibly due to changes in the magnet technology (rare earth magnets).

Compare to cordless power tools - the first Ni-Cad versions and a modern Milwaukee.

Yes very true - these are Nema 34 motors. Most of the modern ones have about 4x as much torque.
If I can't repair this motor can I just use a generic one? I know a lot of things are standardised with NEMA but what about the wiring and outputs etc?

 

[m_c]

In terms of motor sizing, larger doesn't always mean better. It often means worse.

Nema34 generally have far higher inductance than Nema 23/24.
They may have a far higher stationary torque, but due to the high inductance, it means the back emf (electro motive force - aka voltage) is also far higher.
In terms of electric motors, torque = current. The rated voltage/current of a stepper motor is when it's stationary.
In simple* terms as a motor starts spinning, it generates back emf, which acts against the applied voltage and reduces the current flowing through the motor coils, which in turn reduces the torque. This is why stepper drivers try to work as a constant current source, and require far higher voltages than the stepper motor rating.
At some point, the back emf will increase to the point the stepper driver doesn't have enough voltage to overcome the back emf, which results in current reducing along with available torque.

What this means in use, is Nema34 torque drops of far faster.
You can mitigate it to some extent by using high (aka mains) voltage drivers, but at that price point, you can get a set of cheap servos which will out perform steppers.
Compare that to some low inductance Nema23/24 steppers. The stationary torque might be lower, but the torque drop-off is far less, which allows them to maintain useable torque to higher speeds.
Then there is also the benefit that the smaller motors have a far lower inertia, you can often get better acceleration.

Off course on a small mill, speed isn't generally that critical, as long as you can get reasonable acceleration.
Speed is more of a concern on larger machines and routers, where you don't really want rapids that take 30+ seconds to move the length of the bed.


As for modern motors.
Yes, modern square motors have much better performance mostly due to stronger magnets.



*I say simple, as the exact science the inductance creates a delayed response, along with the inefficient design of steppers, result in a non-linear torque curve, unlike something like a standard induction motor where things are far more linear.

 

[fizzy]

In terms of motor sizing, larger doesn't always mean better. It often means worse.

Nema34 generally have far higher inductance than Nema 23/24.
They may have a far higher stationary torque, but due to the high inductance, it means the back emf (electro motive force - aka voltage) is also far higher.
In terms of electric motors, torque = current. The rated voltage/current of a stepper motor is when it's stationary.
In simple* terms as a motor starts spinning, it generates back emf, which acts against the applied voltage and reduces the current flowing through the motor coils, which in turn reduces the torque. This is why stepper drivers try to work as a constant current source, and require far higher voltages than the stepper motor rating.
At some point, the back emf will increase to the point the stepper driver doesn't have enough voltage to overcome the back emf, which results in current reducing along with available torque.

What this means in use, is Nema34 torque drops of far faster.
You can mitigate it to some extent by using high (aka mains) voltage drivers, but at that price point, you can get a set of cheap servos which will out perform steppers.
Compare that to some low inductance Nema23/24 steppers. The stationary torque might be lower, but the torque drop-off is far less, which allows them to maintain useable torque to higher speeds.
Then there is also the benefit that the smaller motors have a far lower inertia, you can often get better acceleration.

Off course on a small mill, speed isn't generally that critical, as long as you can get reasonable acceleration.
Speed is more of a concern on larger machines and routers, where you don't really want rapids that take 30+ seconds to move the length of the bed.


As for modern motors.
Yes, modern square motors have much better performance mostly due to stronger magnets.



*I say simple, as the exact science the inductance creates a delayed response, along with the inefficient design of steppers, result in a non-linear torque curve, unlike something like a standard induction motor where things are far more linear.

Mine are slo syn motors - they are a bit different from the usual I think?

 

[rcx132]

I do not want to be disagreeable, but I think you are focussing too much on the manual operation bit. Once converted, I do not think you will ever use it. The MPG suggested by m_c above is a much better route to manual control.

If you go for motors that are too small, acceleration and rapids will suffer. Measuring maximum force is not really a good metric. It is the ability to speed up, slow down and change direction (agility) while pushing a metal table around in space that is important.

A way around the notchiness could be to use a timing belt and pulleys between motor and leadscrew, with an easy disconnect mechanism (single pin dog clutch maybe, over-centre belt tensioner perhaps).

With some thought, when you are under stepper control, the handwheel could remain stationary (look at the knee crank handle on a Bridgeport - having that whirling around when you operate the power knee lift would be disconcerting). If not, your handwheels would benefit from being the folding safety type.

Sorry sometimes I need to hear things several times before they sink in. Yup I've taken on board that requiring manual control shouldn't be a deal breaker, but I do still feel that for the sake of selecting the right non overpowered motors, having double shafted motors with hand wheels will be nice.

My current thinking is to upgrade the mill first, I think these used mills keep their value and mine has had a very nice DRO set fitted and gas strut, so it should sell easily allowing me to upgrade to the SX3 mill, so that I'm converting something I won't grow out of.

 

[m_c]

Mine are slo syn motors - they are a bit different from the usual I think?

Hard to say without seeing a datasheet for them.

Stepper motors have largely been made to NEMA sizing for a long time. There are some odd balls around, but they are rare.
A 'standard' stepper motor is 2 phase and has 200 steps per revolution. These might come in 4, 6, or 8 wire versions. The extra wires allow different combinations of the internal coils. 4 wire are the simplest. 8 wire allow parallel or serial wiring of coils. 6 wire will typically be wired internally as a serial setup, but could be parallel.

You can get 3 phase stepper motors, which are better performing. Just like induction motors, they're smoother due to 3 coils transitioning to push/pull the rotor around, so they eliminate a lot of the resonance problems with stepper motors.
However they're less common and more expensive.

 

[m_c]

Sorry sometimes I need to hear things several times before they sink in. Yup I've taken on board that requiring manual control shouldn't be a deal breaker, but I do still feel that for the sake of selecting the right non overpowered motors, having double shafted motors with hand wheels will be nice.

My current thinking is to upgrade the mill first, I think these used mills keep their value and mine has had a very nice DRO set fitted and gas strut, so it should sell easily allowing me to upgrade to the SX3 mill, so that I'm converting something I won't grow out of.

The big issues with manually controlling a CNC mill, is if you convert it to ball screws, the screw pitch increases, and the screw efficiency greatly improves. IIRC lead screw typical efficiency is around 30-40%, whereas ballscrew efficiency is 95%+
This results in handwheels being less accurate, and screws not being self locking (if you let go of the handwheel, cutting forces will easily push the bed and spin the screw), so you need to continually prevent the screw from turning (either by holding it, or putting a lock on).

 

[fizzy]

Sorry sometimes I need to hear things several times before they sink in. Yup I've taken on board that requiring manual control shouldn't be a deal breaker, but I do still feel that for the sake of selecting the right non overpowered motors, having double shafted motors with hand wheels will be nice.

My current thinking is to upgrade the mill first, I think these used mills keep their value and mine has had a very nice DRO set fitted and gas strut, so it should sell easily allowing me to upgrade to the SX3 mill, so that I'm converting something I won't grow out of.

Use a nice jog shuttle pad and you won't miss the manual control so much.
Once I get mine up and running properly I will post some videos so you can see what a small machine is capable of. Mind you I may be a little biased - I have a Bridgeport and an Elliott Sturdimill which weighs around 3000kg

 

[minimutly]

Hard to say without seeing a datasheet for them.

Stepper motors have largely been made to NEMA sizing for a long time. There are some odd balls around, but they are rare.
A 'standard' stepper motor is 2 phase and has 200 steps per revolution. These might come in 4, 6, or 8 wire versions. The extra wires allow different combinations of the internal coils. 4 wire are the simplest. 8 wire allow parallel or serial wiring of coils. 6 wire will typically be wired internally as a serial setup, but could be parallel.

You can get 3 phase stepper motors, which are better performing. Just like induction motors, they're smoother due to 3 coils transitioning to push/pull the rotor around, so they eliminate a lot of the resonance problems with stepper motors.
However they're less common and more expensive.

My one and only connection with stepper motors was when we (as a sub group doing a Beng. Electronics course) were given a task to make a line following "buggy". Ours got christened the "millenium buggy" so that should tell you when this was.
Anyway, I made the buggy chassis one sunday in the workshop, stuck some radio controlled car wheels to the output shafts of a pair of tiny perastaltic pump steppers and a brass round head bolt front and back to keep it upright. To power these we needed a 12V dc supply, which meant a six AA cell holder each, plus another supply for the optics and control electronics.
What did we all learn? That stepper motors eat current at standstill, but they like whizzing along. The six pack battery holders (nicads in those days) actually started melting at the spring ends.
We got it working, followed the white line "circuit" and beat the others, so obviously we were experts...