Retrofitting a 3-axis VMC with DC servos - guidance needed

Hi everyone,

I'm currently working on the retrofit of a 3-axis vertical machining center (from 1995) and have decided to use LinuxCNC with Mesa boards. The machine is in excellent mechanical condition, but the original controller (Fagor 8025M) is beginning to fail. I’m still evaluating whether it’s worth repairing, but I’m leaning toward fully modernizing it instead.

Here are the key technical details of the system:

•  **Machine type**: 3-axis vertical milling machine
•  **Original controller**: Fagor 8025M
•  **Motors**: SEM MT30R4-46 brushed DC servos, each with internal tachometers
•  **Drives**: Axor analog drives (Miniclamp 140, Minispeed, Minispeed 140, Minispeed 200) with ±10 V control input
•  **Spindle drive**: Fanuc A06B-6060-H003#H503 (also analog control)
•  **Feedback**:
•  **Main feedback**: KA300 linear scales (Extent 570D), 5 µm resolution, differential outputs (A/B/Z, A-/B-/Z-)
•  **Velocity loop** is handled inside the servo drives via tachometers
•  **Planned control strategy**: Full closed-loop position control via linear scales in LinuxCNC
•  **I/O needs**: At least 18 digital inputs for limits, emergency stop, lubrication pump, coolant, etc.
•  **Desired precision**: Hundredths of a millimeter (0.01 mm) or better
•  **Budget**: About $1500 or less
•  **Planned Mesa boards**: Leaning toward a 5i25 (PCIe) + 7i77 for analog servo control and I/O

The machine will be used for both production and toolmaking, so reliability and precision are key. The current system performs interpolation and achieves 0.01 mm precision, and I’d like to match or improve upon that.

I’d like to ask:

- Has anyone here done a similar retrofit with SEM DC motors and Axor drives?
- Is it reasonable to expect similar or better precision from LinuxCNC using KA300 scales and Mesa hardware?
- Are there any specific concerns when using analog ±10 V control with older servo amplifiers?
- Any advice or experience you could share with full closed-loop systems based on linear feedback?
- I’m also interested in your thoughts on encoder scaling, latency tuning, and best practices for safety inputs and interlocks.

Info:
digital-readout.com/wp-content/uploads/2...stallation-guide.pdf
www.axorindustries.com/wp-content/upload...ce-Manual-MS-Eng.pdf
www.mroelectric.com/static/app/product/p...a06b-6044-manual.pdf
forum.linuxcnc.org/media/kunena/attachme...17/MT30R4Extract.pdf

Thanks in advance for any help or tips you can share! I’ve already started digging into the documentation but would really value input from those who’ve done similar projects.

Best regards,  
Santiago
 

Sounds like it should be a straight forward analog system conversion. What about things like the spindle controls, does it have a tool changer?

Best advice, is first thing e-stop circuits, then get encoder feedback working, then the servo drive enable\disable, then add safeties (limits etc), then confirm directions are correct, finally tune your position loops.

Excellent, it doesn't have a tool changer, which is a plus. I would like to add one eventually if possible, but that’s more of a future upgrade.

Regarding the spindle control, that’s an area I want to dive deeper into, but I'm not sure where to start.
From what I understand, important aspects would include direction of rotation, speed control, and handling alarms or faults. Is there anything else I should be considering?

I retrofitted a Maho and kept it's DC servos...

forum.linuxcnc.org/12-milling/33035-retr...ing-a-1986-maho-400e

Hi, thanks for sharing your Maho retrofit experience, very interesting project.

my machine uses DC servos with ±10 V analog control.
I’m currently deciding which Mesa hardware to choose, but I’ve noticed that most of the combinations I had in mind are out of stock.

One of my doubts is regarding communication:

Some people suggest parallel-port-based cards for heavy, real-time work.

Others mention Ethernet-based cards might be better for future expansion.

My concern is whether Ethernet introduces more delay or has less data transfer capability for this kind of servo control — or if I’m completely wrong in that assumption.

Could you share your opinion or recommendations on:

Which Mesa cards you would choose for ±10 V DC servos in a retrofit like mine.

Whether you think Ethernet is a good choice for long-term expandability.

Any reliable sources or distributors where I might be able to get the boards (especially in Latin America, but I’m open to others).

Thanks in advance for any insight!