Fox Alien CNC: Adding a VFD for a Spindle

The 400W spindle that comes with the Fox Alien CNC machines is adequate for a beginner, but for more serious work you need an upgrade. I chose to go with a 800W water-cooled VFD spindle. You get multiple benefits:

1. The extra power allows you to cut faster and deeper
2. Improved accuracy and stability reduces the tool chatter
3. You can control the RPMs directly from the g-code
4. The spindle is significantly quieter, especially if you choose a water-cooled one

Here I’ll describe in detail how I achieved this upgrade. My experience is with the Vasto model, however it should work for any other CNC that has a PWM output for a laser module.

There are multiple VFDs and spindles you can buy. I personally decided to go with the Huanyang brand, which had the most comprehensive documentation and my experience with their warranty support was excellent.

The settings and examples here will be for that model. If you use a different brand, likely the specifics will be different and you will have to consult the documentation that comes with your unit.

This tutorial focuses on the VFD part, and not on the actual spindle wiring. For the spindle you will mainly need a good 4-conductor flexible and shielded cable with a H20-4 aviation male connector. Then simply install the cable inside the drag chain. Many tutorials online cover this part in detail.

The main components

VFD and spindle package: https://www.amazon.com/dp/B07BBHJMQB

PWM to DC converter: https://www.amazon.com/dp/B07XZ836QF Note: This particular model can accept 3.3V and 5V input, which can be selected by a jumper. However the label on the PCB is "24V" instead of "3.3V". This is because the same PCB is used for multiple models with different input voltages

Prototype PCB: https://www.amazon.com/dp/B0778G64QZ

GX16-2 male aviation connector: https://www.aliexpress.us/item/3256804459446780.html (or reuse from the existing spindle cable)

GX12-4 aviation connector pair (or any suitable low-voltage 4-pin pair)

2-pin JST-XH connector

48V relay (other voltage can also be used, see below)

Terminal blocks and connectors: https://www.amazon.com/dp/B09WMMZLQY

Wires, fork terminal connectors, a crimping tool

Modding the VFD

Quieter fan replacement: https://www.amazon.com/dp/B07C2S1MV4

Control panel extension cable: https://www.amazon.com/dp/B07P5LS33N

I made a 4-wire cable with a GX16-2 male connector and a 2 pin JST-XH connector on one end, and a GX12-4 male connector on the other. The first two connectors hook to the spindle and laser outputs of the control box. The other end goes to the VFD assembly (covered in the next step).

The 2 wires from the spindle will control the relay and the 2 wires from the laser will control the speed.

Then I needed to ensure that the spindle voltage matches the relay input requirements at all times. If the voltage doesn’t match the relay, bad things will happen. A higher voltage will burn out the relay. A lower voltage will fail to turn on the spindle, causing the CNC to start moving without spinning, eventually crashing the end mill.

The original spindle speed knob is a voltage divider (a potentiometer) that connects to the Vo ADJ input of the power supply.

You have 3 options:

1. Manually adjust the knob to get the right voltage level, and then try not to touch it. This option requires no modifications to the control box
2. Move the potentiometer inside the box to get it out of reach, or even replace it with a small trim pot with a shorter cable as you only need to adjust it once
3. Short-circuit the left two Vo ADJ pins, which will force the output to the max value of 48V – but then you will need specifically a 48V relay. They are less common than 5V, 9V or 12V relays

I chose the last option. I also 3d-printed a cap to fill the hole from the removed speed knob. The cap has two parts that attach with a simple wood screw.

The laser output from the control box provides a pulse-width modulated (PWM) signal. This means the value changes from OFF to ON in quick pulses. The ratio between the OFF and the ON duration (called "duty cycle") determines the overall spindle power. Unfortunately, VFDs cannot read such signal. They need a constant voltage from 0 to 10V to set the spindle speed.

Therefore you will need a device to convert PWM to constant voltage level. Such device needs a power source for its own electronics. Luckily, VFDs usually have the ability to supply power to accessories. If your VFD does not, you will need to include an additional DC power source. Generally the power voltage needs to be higher than the voltage it needs to provide. In my case I power it with 24V from the VFD, and it outputs a 0-10V signal.

I 3d-printed a box to house the PWM converter and the 48V relay. It has a 4-pin aviation connector on the top to connect the cable from step 1. The relay is mounted on a small prototype board, which serves also as a hub for the wiring. Note: Don't forget the mounting nut and washer before you solder the wires to the board.

There is a 5-wire ribbon cable from the VFD to the box. The colors are:

Red: 24V - to power the converter

Orange: DCM – ground for power and digital signals

Yellow: FOR - turns on the spindle

Green: VI – Voltage input for the RPMs

Blue: ACM – ground for the analog signals

The VFD and the electronics are bolted onto a 14.5x7.5" piece of half inch plywood. The AC wires are connected via terminal blocks for cleaner and safer installation.

The FA and FB terminals of the VFD are connected by an internal relay when the spindle is on. I used that feature to control the water pump for the cooling. Note: The FA, FB and FC terminals were mislabeled in the documentation. In practice FB is the input, FA is the NO output, and FC is the NC output. If in doubt, use a multimeter to check which pair gets connected when the spindle is running.

I added a banana plug on the side that is connected to ground. I use it to ground the vacuum hose for the dust collection. This fixed all the electric shocks I was getting from the CNC. It will also reduce the static noise that can lead to CNC errors.

I added an extra wire to connect the AC ground to the DC ground. This permanently connects the spindle to the CNC ground, which means you don’t need the alligator clip when running a Z probe. This is represented by a dashed line in the schematic.

I 3d-printed various clips to secure all cables to the base.

And finally, I added a thin plywood cover over the AC area for protection and safety.

The settings here are specific to the Huanyang VFD. You will have to adjust them for the brand you have

PD001 = 1 – this enables the FOR pin to work

PD002 = 1 – this enables the external speed control. There is a jumper on the VFD that switches between VR and VI – make sure it is in the VI (left) position

PD014 = 3 – spin up time in seconds. The default is way too pessimistic

PD015 = 5 – spin down time

PD052 = 01 – connect FA and FB when the spindle is running

PD070 = 0 – use 0-10V input signal

PD144 = 300 – multiplier to convert from Hz to RPM (purely for display purposes). Use the >> button on the front of the VFD to switch between different display modes, like Hz or RPM

Additionally, you need to change the max spindle RPM in Grbl

$30 = 24000

This is the value that the controller uses to convert g-code speed to PWM level.

Keep in mind that the PWM signal is not very precise and goes through multiple conversions before becoming a VFD frequency, and those conversions are not very accurate, and also not very linear. You will notice that the actual speed will be different from the g-code commands.

There are few things you can do to improve the accuracy. The PWM module has a trim pot that you can adjust with a tiny screwdriver to set the output voltage. Make sure it is at 10V when the g-code runs the spindle at max speed. You can also set $31 to a value larger than 0 to account for the minimum frequency that the VFD can achieve.

It is best to disconnect the spindle when running these tests. Especially air-cooled spindles don’t like it when they run at low speed as they may overheat. Just use the RPM display.

Alternatively, find a g-code value that produces the speed you want. For example maybe for 12000 actual RPM you need something more like 12670 in your program.

P.S. If you are using a different brand of VFD, feel free to post your recommended settings in the comments. It will help other readers of this article.

I did 2 modifications to the VFD itself.

First, I replaced the fan to reduce the noise as described in this video: https://www.youtube.com/watch?v=Uc7bQ9DBf_g

A more complex version of this mod is to control the fan based on the temperature, though I have not attempted it: https://www.youtube.com/watch?v=x882vvX_L7Y

The second mod was to relocate the VFD control panel to the front of the CNC table using an extension cable. A 3d-printed bracket mounts it at a convenient angle.

My goal was to create the ultimate VFD solution for my needs. Not everyone will need all the features.

Many people will be happy with a simplified approach.

1. Instead of using a relay, you can permanently connect FOR to DCM, which will turn on the spindle as long as VI receives more than 0 volts. This is the common approach you will find on the Internet. In this case you will only need 2 wires from the control box to the VFD to carry the PWM signal. I personally don’t trust that the PWM converter won’t generate a spontaneous spike, which may jolt the spindle while I’m changing the tool and cut off my finger. It is very likely I am being overly cautious.

2. You may prefer to manually control the speed using the knobs on the VFD instead of from g-code. In this case you only need the relay and can omit the PWM cable and converter. This will greatly simplify the implementation.

3. Connecting the AC ground and the DC ground is optional. I don’t see it discussed frequently for this class of machines, probably because it doesn’t work for stock DC spindles and palm routers. It is incredibly useful, however, as it removes one step from the probing sequence. If you use the paper method instead of a probe, then this is something you can safely skip.