We have wired up our first pass of a stepper motor controlled Z-axis, with a stand-alone stepper motor driver powered by a stand-alone 110V AC to 12V DC power supply unit. All connected to a mechanical assembly that was formerly the X-axis of a Monoprice Mini 3D printer.

First test was to verify electrical functionality. The entire assembly was connected to our Grbl ESP32 controller while sitting on the XY table, and we verified there were no escapes of magic smoke. (Yay!) We then used bCNC on the laptop to command Z-axis movement, and verified the direction was correct. (The distance was not, but we can figure that out later.) At this point we were running short on time available at the shop, so we followed up the electrical test with a quick and dirty mechanical test.

The machine has a vertical aluminum beam where we had taped our sharpie markers earlier, and fastened our servo Z-axis for that series of tests. Now, with a mechanical assembly almost as tall as the beam, we’ll need to decide if we still need that vertical beam for permanent installation.

But for a temporary test while we were short on time, the vertical beam is great for us to zip-tie the mechanical assembly in place. It is crooked (not nearly vertical) but it was enough for us to gain confidence this assembly is actually going to work for us. There are still things to be figured out: a rigid mounting solution that doesn’t involve zip ties, plus Grbl parameters to correct movement distance.

But for today, we’re just happy to run through homing routine on all three axis before proceeding further.

The Monoprice Select Mini 3D Printer is an impressive demonstration of how costs could be wrung out of a basic cartesian 3D printer design. We could debate whether the tradeoffs were worthwhile, but the level of integration resulting in parts count reduction is indisputable. I have extracted the X-axis from a non-functioning Mini printer intending to leverage its highly integrated mechanical assembly. But it turned out the advantageous mechanical integration was balanced by the disadvantages of electronics integration.

The Mini 3D main board is a single monolithic circuit board, with stepper motor driver chips surface mounted directly to the PCB. This meant electronics associated with the X-axis mechanical assembly could not be easily extracted and reused, and we had to use a stand-alone stepper motor driver.

While I was busy routing wires for X and Y axis and cleaning up the tangles, Emily stepped up to handle the task of wiring this Z-axis. In the shop she found a stepper motor driver module gathering dust and got approval to use it in this project. The motor presumably runs best on 12V as that was the power supplied by the Mini’s AC adapter, but there was no existing 12V power rail on our machine to tap into. Our Grbl ESP32 controller ran on USB 5V from the laptop, which was itself running on standard Dell 19V power. The Parker ZETA4 controllers plugged directly to 110V AC and each had its own internal power supply.

So Emily also had to dig up an 110V AC to 12V DC power supply to wire up to the stepper driver. It was also gathering dust and had amperage capacity far higher than what we needed for the motor, but it was easily available so we went with it. Once everything was wired up, it’s time to test what we have.

The major components of the SGVHAK CNC project now all have assigned mounted locations, the Grbl ESP32 controller board being the most recent item. No longer sitting on whatever surface is convenient, a tidier arrangement of components allow a tidier arrangement of wires.

The repurposed aluminum extrusion beams have channels that turn out to be quite convenient for running wire bundles. And as long as we stay away from the cut ends of these beams, there are no sharp corners to raise wire damage concerns. So they were stuffed into a channel, and held in place with zip ties. This may or may not be a good long term idea, we’ll find out as we go.

Under the table, there is a metal cross beam just behind the metal plate where we’ve mounted the Parker ZETA4 motor drivers. This beam is an ideal location for extra length of wire to be coiled up and zip-tied out of the way.

The leaves what we should do about the wire that can’t be conveniently tucked into an extrusion nor tied to the cross beam, and the solution is spiral wrap. The downside of spiral wrap is the time consumed in their installation, a lot of labor just winding them around and around the wire bundle. This spool was available precisely because it was so tedious to install. The owner switched to a different and easier-to-install style of cable sleeves and donated these spiral wraps to the project. It stays with the theme of building this project out of salvaged and repurposed parts, and besides, beggars can’t be choosers!

Once we’ve done our housekeeping and the wires are much neater than before, we can return to feature work.

We now have good homes for the Parker ZETA4 driving our X and Y axis, and the laptop control console has a nice throne of its own. But we can’t start working to tame our nest of wires unless we also find a home for the prototype Grbl ESP32 controller board. Every wire on this machine leads to the controller board, so its location is literally key to the wiring puzzle.

During the test where everything was held down by tape, this controller lived atop the gantry not far from the laptop computer. This seemed to work well and there may be merit to the thought that lines for high speed digital data should be kept as short as possible. To test this plan, some very minimalist brackets were designed and printed for hooking the board on our gantry rail directly behind the laptop computer.

Since we have components and wires soldered to the back side of our prototype control board, we need to make sure we keep a healthy distance between those exposed contacts and the very conductive aluminum extrusion beam. M3 threads were tapped in the bracket, and standoffs installed to maintain that distance.

Two identical brackets were printed, with a minor lip to hook over the edge of the extrusion beam. The only thing maintaining the correct distance between these two brackets is the perforated PCB itself. Not the best plan in the world, but this is just for a test to see if all the wires would even reach and function if they do.

If this location doesn’t work out, the next most logical position is on the metal plate below the table, adjacent to the X & Y axis driver modules. We’ll test this location first. And now that a location has been decided, it was time to organize some wires.

Adding functionality to our CNC project is fun, but every once in a while we need to stop and do some housekeeping. Such was the case for a recent work session: we need to start cleaning up our wiring. The big nest of tangles are starting to impede progress, because time we spent sorting through wires is time not doing cool things.

Real industrial CNC machine usually have a big electronics equipment cabinet in the back, made of folded sheet metal. We won’t have that, but we shouldn’t leave the Parker ZETA4 stepper motor drivers sitting loosely on the ground, either. A big metal bracket was available and repurposed as home for the beefy X and Y axis motor drivers, the Parker ZETA4.

First a few holes were drilled in the table to fasten the bracket, then mounting patterns for the ZETA4 were drilled in the bracket and tapped for fastening machine screws. There was one minor oversight: the pattern is technically upside down. The most important part of this vertical orientation is so the driver module heat sinks are oriented properly for convection cooling. Mounted upside-down will reverse the air cooling flow, hopefully the direction matters less than the fact air is flowing.

With the two driver modules fastened to the bracket, we can plan wire paths in some semblance of organization. As a bonus of this mounting, the new locations are closer to the rest of the machine giving us more slack in the wire to reach other components.

Once we added a crude but functioning third axis to our primordial CNC machine, we started testing it by running longer and longer G-code programs. Once we got to a test program that took roughly 25 minutes to complete, we encountered our first system failure: our computer running UGS (Universal G-code Sender) would lose its serial communication connection at unpredictable times.

As we intend to eventually run programs much longer than 25 minutes, this was unacceptable and UGS was removed from contention. We returned to the list of G-code senders in Grbl documentation and chose bCNC as our next candidate.

We are definitely giving up some nice features by moving from UGS to bCNC. We lose the ability to customize the arrangement of our control elements, and we lose the ability to preview our tool path from arbitrary directions. (We’re restricted to a few fixed projections.) But reliability is more important than eye candy, so we’re happy to give them up if it means a system that we can rely upon to run for hours.

To test bCNC, we take advantage of the fact this is an open loop system: the control board will run exactly the same whether it is attached to the salvaged industrial XY stage or not. This allowed me to run long duration tests using just the laptop and the prototype control board, away from the workshop where the machine hardware resides.

To establish our baseline: UGS is brought up to run this 25 minute test program again in the test configuration without hardware. Two attempts both failed with serial connection dropping offline at different times.

Then, using the same desktop test configuration, bCNC is brought up to run the same 25 minute test program. Five attempts ran to successful completion without losing serial connection.

Then we moved on to the challenge level: a G-code program that takes over 7 hours to execute. I would leave the laptop and prototype board running over most of a week. Every time I noticed the program had succeeded, I press “Cycle Start” again. This netted roughly ten attempts, and they were all successful. No serial connections were lost.

It looks like bCNC is the way to go. And with the new candidate software in place, attention returns to machine hardware.

We now have a three-axis machine: aged but still robust industrial motion control linear drives for our X and Y axis, and a new but not at all robust servo holding a Sharpie marker as our Z-axis. It was enough to let us feed three-axis G-code programs and watch them in action.

We started with short programs that would take less than a minute to run, then gradually increased the complexity and duration of jobs. If we wanted to turn this into a CNC mill, it will need to be reliable and dependable for long jobs. We didn’t have enough experience to have a list of potential points of failure, we just ran ever longer jobs and watched to see what goes wrong. We knew it was going to be a process of finding a problem, fixing it, finding the next thing to fail, fix that too, and repeat.

Our first problem: the G-code sender UGS running on the control console laptop. In the middle of running a job of approximately 15 minutes, UGS would drop serial connection. The machine stops moving and UI shows “OFF-LINE”. This is not just annoying, it is plainly unacceptable in a machine that might be expected to run for hours.

And there’s another possibly related problem with UGS serial communication: it was not able to connect to Grbl ESP32 if the dev board was connected at boot. We could work around this by unplugging the USB cable to ESP32 and reconnect, but this is rather inconvenient.

We could put up with the startup connection issue if that was the only problem, but randomly halting in the middle of a long job is unacceptable. So UGS is out of contention, and we return to the Grbl G-code sender list to find a replacement.

The next candidate: bCNC.

Soldering up components for a test run at the workshop, I ran out of time to implement something to actually attach to the end of the Z-axis servo. While I was setting up all the components with copious amounts of tape, I asked Emily for help. She has repeatedly demonstrated skill at creatively building a solution in very short time using whatever parts were on hand. The most important episode of this skill saving our bacon was when she fabricated a steering servo bracket for SGVHAK rover.

As I was focused on routing wires and taping them in place, I didn’t watch the magic happen first hand. But I was amazed when Emily walked up less than 15 minutes later with a bracket with parts clearly made of 3D printed plastic. There was not enough time to print such a thing… “How did you do that so fast?”

As it turned out, she drew from a box of rejected & failed 3D prints that was sitting in the back of the shop. The parts were not printed for this project, but their residence in the fail pile made them candidate raw material for today’s quick fabrication. Once Emily installed the Sharpie marker holder to our servo Z-axis, our plotter can lift its pen!

But as we found out very quickly, the lift is not clean. This cheap micro servo has a lot of slop which made it difficult to position it at a height that will make good contact with the paper when down and stay clear of the paper when lifting. Instead of ending a line cleanly, the pen would make a short diagonal mark as it lifted.

No matter, it was enough for us to continue testing this system. We started feeding it increasingly longer programs to see what breaks down first. Our answer: UGS, the G-code sender program running on our control console laptop, was the first to fail.

Completion of soldering circuits for hardware control switches seemed like a good place to take a break and test. It’s fun to keep the momentum going, but it’s also important to stop and verify the work done so far. Off to the shop we go to set things up!

The jury-rigged nature of this prototype test is evident. While I had put a bit of time into building a nice shelf for the computer, none of the other pieces received such attention. Eventually there’ll be an enclosure for the ESP32 running Grbl. A circuit board of the dev module plus all of its associated circuitry, much like the official Grbl ESP32 board. But that board has room for Pololu stepper drivers like those on a 3D printer, and we don’t need that for our Parker motors. That will happen later. For this test, I found a piece of scrap acrylic and taped it to the top extrusion beam to serve as table for the perforated circuit board.

In fact, tape is the dominant element in this test. In addition to the ESP32 platform, tape is held the battery pack for Z-axis servo. Plus all the wires, as it’s not yet clear how we’d want to run wiring for this system. The Parker ZETA4 drivers are still sitting loosely on the bottom shelf of the table, they need to be properly mounted eventually. Once these drivers and the ESP32 are mounted, we will need to consider how to best run wiring to minimize chances of wires getting in the way of our work piece.

In the meantime, everything is held out of the way with tape. Tape tape tape.

Aside from the computer, about the only thing not held by tape was the servo I intended to use to test Z-axis, which received a small 3D printed bracket. However, I ran out of time at home before I could devise something to attach to that servo output shaft.

For that I had to call for help, and Emily answered.

After wiring up for Z-axis servo control, I turned my attention to the remaining empty quadrant of the prototype board.

At the moment I have a small laptop running a G-code sender acting as the CNC control console, from which I can access all functionality of Grbl. The ESP32 port of Grbl also offers a built-in web interface, which is a great choice for compact machines that don’t want the overhead of a full computer. Both of these control options have a communication delay, so some things are better handled by hardware switches for immediate feedback.

By default Grbl ESP32 supports four physical switches, in the order of their pin assignment they are:

1. Feed Hold: On many CNC, this completes execution of the current G-code command but holds off running the next one. Some machines can pause mid-command but this is not guaranteed. Axis motion stops, but spindle does not. This is a good way to have a machine pause, useful for example when the operator wants to visually check to make sure position and upcoming commands look reasonable.
2. Cycle Start: Begins executing the loaded G-code program. Also used to resume execution after Feed Hold.
3. Reset: This commands an immediate stop, of everything. Unlike Feed Hold, it will not wait for the current command to complete. It will also turn off spindle in addition to axis motion. Since it is an immediate cutoff of execution, the controller ends up in an unpredictable state. There’s no resuming after a reset. The machine has to be re-homed and all coordinate offsets recalculated.
4. Door: Designed to be wired up to a door open detection switch so the machine can take actions to ensure operator safety. On some machines this is effectively a Feed Hold, others treat it as an emergency Reset, most machines treat it as somewhere in between. I don’t know yet how Grbl reacts.

All four are designed for normally-open switches that activate upon closing the circuit to ground. For reasons I don’t understand, these switches don’t use the ESP32 internal pull-ups and require external pull-up resistors. Normally-open switches don’t necessarily make sense for a situation. For example, reset is usually wired up with a series of normally closed switches and, if any of them should open, the machine halts. But this is easy to address in the physical circuit.

Another concern for these switches is that they tend to have long runs of wire. (Such as  the aforementioned reset switch.) Long wires have a nasty tendency to act as an antenna picking up stray signals. So a real board would require opto-isolation to avoid triggering from line noise. But again, this is a prototype board missing the full set of protective features.

After I wired up jumpers for playing with these four switches, I made a small piece of label to remind me which pin is which. Then it’s time to bring the results of this prototype soldering session to the shop and run a few tests.

I plan to take advantage of a feature in Grbl ESP32, where a RC hobby servo can take the place of a stepper motor for any supported axis. It won’t be nearly as precise, or have the same range of motion, but it’s a good option for specific situations including tests.

The first part of this task was, obviously, to find it in the code repository and know how to compile with this option active. The second is to wire up my ESP32 dev board to emit the 1-2 ms RC control signal at its 3.3V output and convert that to a 5V signal courtesy of the USB port. Then it’s time to connect a servo.

But not directly, because USB doesn’t supply very much current and would not be a good source for servo motor power. Tasks with high stress or moving quickly will draw more than what USB can supply and cause voltage level to sag. Possibly causing the ESP32 to brown out and restart which would ruin everything.

What I needed for a servo test was a separate power supply. I started digging through my collection of salvaged AC to DC power transformer bricks looking for one in an appropriate voltage range before I decided I was overthinking things. This isn’t going to be the long term solution, it’s just for a short test. I don’t need a long term power source.

RC hobby servos are originally designed to run off 4.8-6V of a four-cell pack of batteries, tracing its lineage back to the age of NiCad radio receiver packs. This is roughly the same voltage as a commodity AA battery cell, so I pulled from my stock of 4xAA battery holders (*) for this test. They even have an integrated on/off switch for convenience.

The ground wire was connected to the servo ground wire as well as ESP32 ground. Servo power wire is supplied by this 4xAA tray, and servo control signal from the 3-pin servo header of the prototype controller board. And finally, the servo gets a 3D printed bracket to clip on to the gantry structure’s 40mm extrusion beams.

The Z-axis test arrangement is now complete. But before I wrap up this session of perf board soldering, I wired up provisions for hardware control switches.

(*) Disclosure: As an Amazon Associate I earn from qualifying purchases.

Strictly speaking, it was not critical to add a shelf for the little laptop used to control everything, but it is very nice to resolve the constant irritation of pausing what I wanted to do just to find a place to set it down. It should speed up future tinkiering of this machine, and the next step in that process is to wire up Z-axis capability.

In hindsight I should have packed the X (lower left) and Y axis (lower right) components a little closer together, but learning these lessons this is why we build prototypes. There isn’t quite enough room to duplicate the same pin layout for Z in the upper right, but then again, I don’t really need to. I’m not interfacing with an existing Parker Motion Control product, the Z-axis will be different.

As a result, the JST-XH connectors have been simplified from a 4+3 arrangement to a single 6-pin arrangement. 4 of the pins are the same: a pair of open-drain signals for step and direction. The remaining two pins are for a normally-open switch to home the Z axis stepper control.

But there’s another twist to the Z-axis: a feature of Grbl ESP32 is the ability to use a RC hobby servo for any axis. It requires changing a few #define in the source code and recompiling, something I could do on the little laptop who now has a shelf. I thought a servo-actuated Z-axis would work well as a first test of system functionality, before I go through the effort of building a more functional stepper-controlled Z axis.

But I couldn’t use the same pin for both, because an open-drain circuit is the wrong way to send a control signal to a servo. Instead, a separate transistor was rigged up so the ESP32 control signal (3.3V) can emit a 5V servo signal. The signal pin and ground are exposed via a separate 3-pin header.

The 5V source for the control signal came from the USB port connected to the little laptop. Ideally there’ll be some opto-isolation for the control signal, but this will suffice for a prototype. However, it will not be enough to power the servo motor, which will require its own power source.