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

Maybe brakes? Do the motors have brakes on them?
The brakes on motors work as a clutch, meaning a friction plate pressed by a metal plate with a spring when braked and an electromagnet to pull the metal part away from the friction plate when released.
Electromagnets are usually shown the same in schematics as relays, they are after all the same wounded wire.

Yes, I think you’re right. I haven’t been able to check the machine again yet since it’s not mine – I’m just trying to help out a friend – but it probably does have a brake, considering the previous controller was only 2.5 axes.
Now, what I’d like to ask for some advice on: if I want to keep the option of adding more expansions in the future and also a control pendant, would the 7i74 be a good choice? I see that it’s mentioned as a supported configuration in the 6i25 manual, so it seems like it could give me more I/O and flexibility.

The 7i74 is a good start, bit doesn't do much by itself.
The 7i73 is a handy little board for control panels and pendants.

You could also look at the Mesa 50-pin cards. I am a fan of the 6i24
They use different daughter boards to the 25-pin cards (eg 7i44 v 7i74 for smart-serial) but they use the same smart-serial accessory boards.

Just be aware that there are two separate daughter-board ecosystems.

An ethernet-based FPGA card might make for easier PC swaps in the future. My mill recently got a 6i24 side-grade when I couldn't find a PC with a full-size PCI slot to suit the existing 5i23. (they still exist, but I had other constraints like 12V power and M2 slot)

Yes, 7i74 will do perfectly for 6i25 and add 8 more SSerial ports.
And yes, i have used many 7i74 (i have one on my Mazak lathe) as they make for cleaner installs as wiring to 7i70 and/or 7i71 is just a simple network cable.
@Andy,
see if you can source HP Z400 or Z420 or Z600 workstations, all have PCI slots, 420 has 1, 600 has 3 i think, and all have plenty of PCI-E. They are usually very cheap, but struck gold yesterday with 5 of Z600 :
forum.linuxcnc.org/18-computer/57073-hp-...or-use-with-linuxcnc

Excellent, thanks again for the help. Now, looking at what @Andy mentioned, what would be the advantage of the 6i24-16 (I understand this is the one being referred to) over the 6i25? I see that it has more expansion possibilities for the future, but in terms of processing capability and other aspects, is there any difference?

From what I’ve checked, I think that with the 6i25, 7i77 and 7i74 I already have a very large working possibility. Also, I couldn’t find a way to use a 4th axis in the future with the 50-pin options, since they come with control for 4 analog servos, and I understand that one of them would have to be used for the spindle. That means I’d have to use one of the 6i24 configurations that allows 8 servos (which in my case seems unnecessary). So I’d like to know what would be the advantages of using a configuration with the 6i24.

@Tommy, thank you very much as well for your reply to Andy. I was checking which workstation models I could use with these configurations, and I noticed that the ones you mentioned are actually available here in my country (Argentina, which has a lot of issues with getting certain hardware). So thanks to both of you, I’ll keep researching the advantages and options.

Hello again,

While checking availability, I noticed that in terms of cost and stock it might be more convenient to go with the 6i24, since it is currently available directly from Mesa USA. Otherwise, I would need to buy everything from Eurosurplus, which ends up being more expensive overall.

Could someone please confirm the main differences between the 6i25 and the 6i24? From what I see, the 6i24 has no limitations on its connectors, so in the future I could also leave an extra port available to add another servo control board if needed.

Thanks a lot for your help!