View Full Version : Conect 121 CNC Lathe Project

29-01-2023, 09:10 PM
Hi All,

I picked up one of these today and plan to get it running for hobby stuff.

I will post some pics during the week but it looks pretty much unused and came with 2 original manuals, bbc computer, control box, monitor, 5 1/4 floppies etc. I don't need to do anything particularly exciting with it and it will probably live at the school I teach at as a training / demo machine only. I do want to be able to cut threads.

Last year I built a cnc plasma using axbbe board and uccnc control software so I have some knowledge although it's fair to say I am good at following instructions rather than fully understanding the electronic principles behind it all.

I want to get this running off of modern software though and ideally (similar to how I run our cnc mill and my plasma) produce code in Fusion 360.

Ideally I also want to be able to use this machine semi-manually, i;e control spindle speed and axis movements from switches.

The spindle motor is a 0.37kw GEC 180v dc motor (I assume original).

My initial thoughts are to use a chinese 400w panel mounted motor controller for speed, and a couple of Nema 23 closed loop steppers for the axes. One thing I am not clear on is whether the spindle needs to be controlled? I appreciate it needs to provide accurate speed via the optical encoder for screwcutting but is that information used to position the axis or also to tweak spindle speed?
I like the axbbe motion controller that I used on the plasma but it is quite expensive - I wanted an ethernet controller on the plasma because of the interference issues from the plasma with usb. I assume this isn't such a problem with a cnc lathe? Can I get away with a cheap chinese usb controller?

Very interested in peoples thoughts on this project. I see a few people on here have got these little machines going so any advice greatly appreciated!


29-01-2023, 11:50 PM
If the original spindle drive still works, then I'd use that.
Failing that, I'd either buy a KBIC board, or a Sprint drive, as they'll give more stable speed control than a cheap chinese controller.
One thing to be aware off, is these types of SCR drives often have non-isolated inputs which sit around half of their supply voltage, so need isolated speed control.

Spindle encoder wise, the original setup on these has a single slotted disc, and a multi slotted disc.
In terms of what you'd need, will depend on what motion controlled you use.

You need to be aware that not all motion controllers support turn/lathes and threading.
I'm not sure what UCNC's current status is regarding lathe functionality, or what they'd require in terms of spindle feedback.

The ballscrew setup on these is very basic, and ultimately rely on the stepper motors for endfloat control.
I personally wouldn't bother with a closed loop setup on one of these, as they won't improve accuracy.
Most accuracy problems I had with mine was due to the noodly Myford ML10 base machine flexing.

30-01-2023, 11:24 AM
The venerable old Mach3 system works well on a lathe, I use it on my converted Super 7. It works for threading using a single slotted index on the spindle. Mach3 just uses this to tell it when to start each threading pass, it doesn't control spindle speed , just assumes there's enough power not to slow down. That works for me, I don't cut very deep threads. You have a 1/2 hp motor, could be fine, just try it and see. I believe Mach4 can use encoder pulses from the spindle for finer control, not sure about that. To reiterate his point about the controller, if you want to control spindle speed from the PC you need to make sure it's isolated. If you're happy with manual control there's not a problem. I have a Denford Novamill of the same vintage, it originally used a Sprint drive but I use a KBIC drive with manual speed control.

As m_c says, there's no point in using closed loop steppers, you might as well stick with the original motors, at least to start with. Upgrade if needed. They will probably work much better with modern microstep drivers.

Regarding the PC interface, I use the CNCDrive UC100 on both my lathe and mill, USB interface to a Win10 mini PC.

30-01-2023, 08:49 PM

How do I go about using the original spindle drive? I believe it was controlled by the computer and not sure what I need to do on the control board to mimic that...31688

30-01-2023, 08:50 PM

30-01-2023, 08:51 PM

30-01-2023, 08:51 PM

30-01-2023, 08:52 PM

30-01-2023, 09:23 PM

30-01-2023, 10:21 PM
That's the early system.

The spindle drive board (in your first picture post), should have everything marked on it.
As it's a NEControl, I'm not sure if a manual will be easily found, but some google searches might turn up something.

The only unknown will be what the inhibit (white/black wire) gets switched to. I'd trace that wire, and see if you can work out the circuit it connects to.
As it's connected to Inh (H), I suspect it'll get 5V applied to it to inhibit (disable) the drive, with 0V connected to the speed control L.

The grey and blue wires will go to a circuit that outputs 0-10V (or possibly 0-5V - you can measure the voltage between the L and H terminals, which will give you an idea) to set the spindle speed

Does the original computer power up?
If it does, it would be worth measuring the various voltages, as it'll help eliminate guess work.
It would also let you test the spindle drive, as SCR drives do have a habit of the SCRs failing causing the supply fuses to blow.

It's also worth measuring the voltages on the power supply board (Second photo).
If it's anything like the later board, it'll output ~24V for the stepper drivers, and 5V for the control, as well as providing a pass through for the spindle supply.

Although I'm not sure what the big plastic box does. Is that where the wires from the spindle drive board connect?
If so, it could the isolation circuit.

30-01-2023, 10:45 PM
I plugged the computer in and it immediately started popping and smoking! Suffice to say I think the BBC B is dead. I have a bit of time tomorrow so will power the control box and see what I can find.

I found this on another forum https://www.cnczone.com/forums/general-cnc-machine-related-electronics/178836-cnc.html

Seems to think I can attach a pot to 10, 11 & 12. Not sure what I have to do to the INH - assuming that is inhibit and INH (h) means that it is inhibit when pin 8 has 5v then presumably if it's disconnected then it should run - or maybe I am over simplifying!

31-01-2023, 12:26 AM
You can connect a pot, which if you have one laying around (IIRC they're normally around 100k), I would wire up just to check the drive still works. But as I've already mentioned, be aware those control terminals are probably around half mains voltage.

However I'd aim to keep the spindle computer controlled.
Most breakout boards are capable of 0-10V control, and just need an isolated 12V power source to avoid issues with non-isolated drives. A small DCDC converter is typically used for this, but that power board may already provide an isolated supply.

Inhibits on those boards generally need connected to inhibit. In the case of H, it needs power applied, or in the case of L needs connected to 0V (which may not be 0V in relation to earth)

31-01-2023, 08:48 PM
No luck yet with the motor controller. It has 240v to it, I can measure 12v across pins 12 & 10. I attached a pot to 12 & 10 and 11 and can vary 11 between 0-12v but not seeing any output from the motor controller. Tried it with inh connected and disconnected. The motor turns if I attach a 12v supply directly to it. Is there any more I can try with the original motor controller? There is also a component attached below the motor - not sure hat this is...?


31-01-2023, 10:36 PM
That's a choke. They help smooth/suppress noise from the motor.
It's worth checking it's not failed and gone open circuit (either with a multi-meter, or apply the 12V with the choke in circuit).

Have you checked both fuses on the drive?
It could be the Inhibit needs a certain voltage applied, but without a manual, or being able to work out what the wire gets connected to and fed, it's pretty much guess work.

01-02-2023, 10:36 PM
THanks MC.

How would that choke be wired? Is it in series like a resistor? It's not wired up at the moment.31706

01-02-2023, 11:22 PM
It should be in series with the motor.
The fact it's been disconnected would indicate there's been a problem.

Depending on how much money you're willing to spend, and your modification abilities, if I was to retrofit one of these now, I'd probably fit a 750W servo motor and drive. Certainly more expensive than just a DC drive, but would give more control over the spindle (depending on which controller you fit).

02-02-2023, 10:17 AM
https://www.ebay.co.uk/itm/550W-Mute-Brushless-SERVO-Motor-for-Industrial-Sewing-Machinery-Energy-Saving/204015227615?_trkparms=amclksrc=ITM&aid=1110006&algo=HOMESPLICE.SIM&ao=1&asc=244770&meid=82d74580bbed463a9bcdc54ed25b8c5c&pid=101195&rk=1&rkt=12&sd=164092634362&itm=204015227615&pmt=1&noa=0&pg=2047675&algv=SimplAMLv11WebTrimmedV3MskuWithRevOptLambda85 KnnRecallV1AndV6ItemNrtInQuery&brand=Unbranded&_trksid=p2047675.c101195.m1851&amdata=cksum:20401522761582d74580bbed463a9bcdc54ed 25b8c5c%7Cenc:AQAHAAABMBTnULwZvkwwvWb54Sp3TESux6kF TXxeByyFeTbBEJTAHELVvJ+npiRCZsaEJO6Sgp1d1wagf52ole bR+jQ/In3hYhYS/5CPdIfcJRHDRpH3FWX7yhPoBx1XQPZ6iqKoBPGzVzWuKBq+/Iy186+RK6TvtTeR9PaoWGRo26O7aiy7a9KJx3HqQCidiul4MWF YbArup/+8Q+T96EpnyLwslh9pCpdbc4BDWiwZVxaQUr6CRgd+2QtPUcMz aAt4iIX9y9R6C+xNgjhn6TtuAFwfYTYmranzB/GmTFk7aDN3pXpQOyFpzC0RoAKCBaMTSeRpA7fgSwkR+smY9dLj vcXYn30OZ45NYeT4tN/mZk7BCzHLSQ7THC0EK5DgIB2e9yxkEdHB2PZGth/C0zC6eBiTDz4=%7Campid:PL_CLK%7Cclp:2047675&pageci=ac910a62-e722-4ca7-aad0-8de89e31ab81&redirect=mobile

02-02-2023, 08:33 PM
That's interesting John. Have you used one?

02-02-2023, 09:00 PM
No but quite a few people over on the Model engineer forum have.

15-02-2023, 09:26 PM
Not had much time but a bit of progress on this.

Replaced the original x axis stepper with a Nema23 which was a direct swap, bolt holes all the same etc.

I ordered a cheap chinese motor controller which arrived today and seems to work well - motor is dead quiet and smooth. It's running off a potentiometer so need to figure out how to get that controlled by software at some point.


I have a fairly simple plan for attaching a Nema 23 to the Z axis so hopefully get that done next week.

16-02-2023, 05:29 PM
That's presumably for the spindle motor? Important to check if the pot is isolated from the mains to determine how to control from Mach.

19-02-2023, 09:33 PM
That's presumably for the spindle motor? Important to check if the pot is isolated from the mains to determine how to control from Mach.

Thanks John, how would I determine that?

19-02-2023, 11:43 PM
Well I would normally say, look at the documentation! But there probably isn't any.

A lot of lower-end motor controllers, especially ones using Triacs, do not have an isolating transformer so that, though the control electronics may be working at "safe" voltages like 12v the actual common side (-ve supply) can be floating at half mains voltage. I have a KBE speed control on my Novamill where this is the case. If you then try to connect the control voltage input and Gnd connection to your properly grounded BoB and motion controller the ground connection puts a dead short across the internal rectifier which blows up - worse it may take the other electronics with it. There are ways to isolate the control electronics using opto couplers so it is operating safely with a proper ground connection. I see that the description includes the cryptic phrase "4, high and low voltage signal is completely isolated, the use of safer" - which is hopeful.

To check, you need an AC voltmeter such as a digital multimeter capable of measuring mains voltage. Have the unit wired up (including to mains earth) and running the motor. Set the meter to its max AC volts range, and using INSULATED probes measure the voltage on each of the 3 terminals connected to the pot - if they are all at or around zero volts it should be OK. And, before doing this test, set the meter to measure ohms and check that the black wire has continuity to mains ground (unit switched off of course).

21-02-2023, 10:59 PM
Hi John,

So I checked all that and all seems t check out ok so pretty confident it's isolated.

I have a attached an image of the terminals on the motor controller.

The board I have is a cheap USB board


I'm not at all sure of how the board will connect to the spindle controller. In the description of the spindle controller it states:

7. the given signal mode (V / mA): the default potentiometer, 0-5V, 0-10V, 4-20mA, PWM, button type


Be very grateful for any ideas!


25-02-2023, 10:30 AM
Sorry for slow reply. This came up in another thread either here or in the Mach support forum and I posted some info gleaned from data sheets & manuals. Some searching should turn it up.