I'd say that 64mm/min is slow enough. It's similar to this video:
I have not been able to find out what the leadscrew pitch of the SX3L is, might be 2 mm instead of the 4 mm of my SX4. The feed shown in the video is OK for a roughing cut, but for a finishing cut with a flycutter one would definitely go slower. Normally on a hobby mill you don't want to push it; better take it easy than to break the cutter and ruin the part.
At the other end of the scale, is 1000 mm/min a reasonable feed rate? Maybe some mechanical gearing would be appropriate to match the motor speed to the desired range of feed rates?
Max feed while milling is mostly not relevant (we are not doing HSC here, although Stefan suggests a HSC like strategy for manual milling also
)
It's more about moving the table quickly. The SX4 has 450 mm X travel, at the 240 rpm of the powerfeed, this gives 960 mm/min, so it takes roughly half a minute to traverse the table fully. I would not mind it to be faster, but with powerfeeds there is always a chance of crashing into things, milling or not.
You've already got the power feed, your speed controller is simple to finish with this electronic improvement, and unless you want to sell it (and build a new one) there is not much reason to simply bolt it to your machine and start using it.
I would sell it right away. But the SX4 is relatively rare, so the probability of finding someone who pays a decent price (after possibly reading our thread) is near zero. At least it has a halfway decent clutch mechanism for the feed unit, that I can reuse even if I switch to a stepper.
Regarding the improved circuit by Picuino: I have it on the breadboard only. For actually using it, I'd have to do something about the 7 V dropout, make a halfway decent PCB, find a heatsink that fits and put it all together.
It would be definitely an improvement over the current unit, but I don't see a sufficient effort vs. reward.
BTW when I said 16 rpm that is the output after the 1:7.5 gearbox. At the motor this would be 120 rpm, 7% of its rated speed.
Regarding stepper and direct drive: Given the available choices of stepper motors, I don't think it is necessary to add a belt transmission. The DC motor nameplate says 0.186 Nm which is 1.4 Nm after the gearbox. I measured the torque you need to break the axis loose at about 1.2 Nm. Given the stiffness of this (still as far as mills go) rather lightweight milling machine, not much more will be necessary for milling.
NEMA24 closed loop steppers
https://www.omc-stepperonline.com/closed-loop-stepper-motor?mfp=184-frame-size-mm[Nema%2024%20(60%20x%2060)] should do that, although not with a huge margin. I'd start with one of those (fortunately, the x leadscrew axis is 58 mm below the table surface, so there is plenty of room for a NEMA24 motor (30 mm from axis to housing).
It's a shame to give up but I know the feeling!
I'm not giving up. If this would be a paid customer project, the I'd happily invest in more advanced control strategies, count commutation pulses and put all into some STM32 project.
But as all I want now is get the mill going, I try to assess which option gives me the best return for my invested spare time. And as I am already considering putting a stepper motor on the Z axis (it is currently a reversible 20 rpm 60 W geared AC induction motor), which would mean I would already have 90% of the necessary infrastructure (power supply, housing, pulse generator), thinking of a stepper for X seems a more powerful option. It will enable arbitrarily low feeds together with much faster (dangerous!) jog feeds.
But hey,
@Doctorandus_P, as you seem to have abundant free time - if you come up with a design I can order as assembled PCB from JLCPCB (I can help with selecting the components, if you find that boring), I'll order it and see how it works on my machine!