Project Description

The project is based around the commonly available Arduino Uno, which is one of the core boards that Grbl was originally written for. This board can run any version of the Grbl firmware and also has the advantage of having the CNC Shield expansion board, designed to control three stepper motors for the X, Y and Z axes of a CNC machine using Pololu style stepper motor driver boards, giving a choice of stepper motor driver ICs including the A4988 (used here) or the DRV8825.

The CNC Shield board used here (a version 3.00 board as these are the most commonly available) does a good job of meeting the requirements of driving the stepper motors, but has some short-comings when end stops (limit switches) are used due to false triggering. This was seen in practice whilst developing this project, with false triggering causing the Grbl controller to go in to an unintended alarm state, especially when the spindle motor was powered on. This board as designed can only control a spindle motor as a two state (on or off) device and this is far from desirable in a CNC machine as it is very useful to be able to vary the spindle speed for different tooling or materials to be cut. The later versions of the Grbl firmware swap the z-axis limit input on Arduino pin D11 with spindle enable output on pin D12 to allow for a pulse width modulation (PWM) output on pin D11. In practical terms this simply means using the Z+ or Z- end stop connections on the CNC Shield as the spindle enable and the connection labelled SpnEn as the z axis limit input. Some simple wiring modifications can achieve this.

The CNC Shield does not incorporate a spindle motor driver. This has caused some confusion on internet forums but to be clear, the CNC Shield cannot directly drive a DC spindle motor as the spindle enable output is a low current 5V output taken from the ATMEGA328P. This can source a maximum of 40mA - nowhere near enough to power a DC motor. The spindle enable output must therefore be used to control a DC motor driver circuit. A suitable spindle DC motor driver circuit proved surprisingly difficult to source from the usual suppliers. Motor driver circuits are available for hobbyist use, but they all came up short in terms of specification. The circuit must be able to operate from supply voltages up to 36V, handle continuous load currents of up to around 3A and most importantly be able to handle the DC motor's high in-rush currents, which were found to be up to 24A in even a small 775 style spindle motor on a CNC 1610 machine. A few MOSFET driver circuit boards were purchased, but were all found to be unsuitable. They all needed modification in order to be suitable for inductive loads (which generate back EMF) by the addition of a flywheel diode and even then none of the boards was found to be able to handle the peak in-rush currents, despite the published specification claiming they could. For this reason it was decided to design a new PWM Spindle Motor Driver as part of this project. Visit our PWM Spindle DC Motor Driver project page for full details of how to make this circuit board on either stripboard or printed circuit board.

The final part of the project was the power supply requirements. It was decided that the 24V 5A power supply which came with the CNC 1610 machine was to be retained and so a matching DC jack socket was obtained for power input. The Arduino Uno has a maximum power supply voltage of 12V however and so a step down voltage regulator had to be sourced. This requirement was easily met by a commonly available variable voltage regulator based on the LM2596S integrated circuit.

Step 1 - Mounting Plate

Firstly, make an aluminium mounting plate, following the instructions in the article Aluminium Mounting Plate for Arduino / CNC Shield.

The mounting plate shown in this photograph is shorter than the one described in the article as it was used for testing various DC motor driver boards. The design described in the article is the correct size for the PWM Spindle DC Motor Driver built for the final version of this project.

Step 2 - Power In DC Jack Socket and Cables

Attach the DC jack socket to the mounting plate where shown. Then solder on short lengths of power cable to the terminals on the DC jack socket, leaving sufficient length for the cables to reach the voltage regulator board. Ensure that you use heatshrink tubing to cover the soldered terminals. In this project, the centre terminal is positive, but you should check this against the power supply that you intend to use.

Step 3 - Arduino Power DC Jack Plug

The power cables should now be soldered on to the DC jack plug used to power the Arduino Uno. The centre terminal is positive and should be covered with heatshrink tubing once soldered. Heatshrink tubing can also be used to keep the positive and negative cables together throughout their length for neatness. The power cables should be left long enough to reach from the Arduino Uno to the output of the voltage regulator allowing for bends. See later images for guidance.

Step 4 - Solder cables on to the Voltage Regulator board

The power cables from the Arduino Power DC jack plug should now be soldered on to the power out pads on the top side of the voltage regulator board.

Additional power cables should be soldered on to the power in pads on the bottom side of the board. These cables need to be long enough to reach the power in connector on the CNC Shield board (see later images for clarification).

Step 5 - Attach the Voltage Regulator board to the Mounting Plate

The voltage regulator board should now be bolted to the mounting plate using M3 socket head screws, nylon spacers, M3 washers and M3 nuts.

The DC jack socket cables can now be soldered on to the top side power in pads on the voltage regulator.

Step 6 - Attach the Arduino Uno to the Mounting Plate

The Arduino Uno board should now be bolted to the mounting plate using M3 socket head screws, nylon spacers, M3 washers and M3 nuts.

Step 7 - Attach the PWM Spindle Motor Driver to the Mounting Plate

The PWM spindle motor driver board should now be bolted to the mounting plate using M2 socket head screws, nylon spacers, M2 washers and M2 nuts.

Step 8 - Mount the CNC Shield on the Arduino Uno

The CNC Shield board can now be mounted on the Arduino Uno and the power in cables from the power in pads on the voltage regulator pushed into place in the power in connector of the CNC Shield. Do not tighten the screws on the power in connector just yet.

Note that if you intend to use the End Stop limit switches, then it is highly recommended that you read the Fitting End Stop Limit Switches to a CNC Machine guide and modify your CNC Shield board as described before completing this step.

Step 9 - CNC Shield to PWM Motor Driver Power Cables

Positive and negative power cables should now be cut to length and connected between the power in connector on the CNC Shield (doubling up on the cable from the power in on the voltage regulator) and the power in connector on the PWM Spindle Motor Driver board. The screw terminals on both boards can now be tightened to hold the cables in place.

Step 10 - Set the Voltage Regulator Output Voltage

Ensure that the DC jack plug is NOT connected to the Arduino before carrying out this step!

Connect the power supply to the DC jack socket on your controller assembly in order to power up the voltage regulator board. Attach your multimeter to the output pads of the voltage regulator board using spring-loaded clips if you have them, then adjust the potentiometer on the voltage regulator board until you get 12.00V output.

Disconnect the power supply and multimeter once you have completed this step.

Step 11 - CNC Shield to PWM Motor Driver Power Cables

Positive and negative power cables should now be cut to length and connected between the power in connector on the CNC Shield (doubling up on the cable from the power in on the voltage regulator) and the power in connector on the PWM Spindle Motor Driver board. The screw terminals on both boards can now be tightened to hold the cables in place.

The stepper motor cables should be connected to the CNC Shield board, noting the correct orientation of the connectors and also ensure the correct placement of the X, Y and Z axes. The spindle motor cables can also be connected to the PWM Spindle Motor Driver board, ensuring that the polarity is correct.

Step 12 - CNC Shield to PWM Motor Driver Signal Cable

A 2 way female to female DuPont connector cable should either be purchased or can be easily made from commonly available kits of parts, including 2 wires stripped from a length of flat ribbon cable. A suitable crimp tool will need to be purchased in order to make up the cables yourself and this is well worth the investment as these types of connecting cables are used quite often.

The cable can then be used to connect the CNC Shield board to the PWM / GND connector on the PWM Spindle Motor Driver board. Note that this cable should only be connected to the SpnEn (Spindle Enable) output on the CNC Shield board if Grbl 0.9 or earlier is used and you do not wish to make use of a PWM variable speed spindle. To enable full variable speed spindle control, you will need to connect the cable to the Z- (or Z+) end stop as shown in the photograph. Ensure that in either case you get the orientation of the cable correct.

Step 13 - Connect the End Stop Limit Switches

The end stop limit switches can now be connected to the CNC Shield board, ensuring the correct axis / end is attached to each input.

Step 14 - Connect the Z-Axis End Stop Limit Switch

If you are not using a PWM variable speed spindle, then the Z-axis end stop limit switches can simply be connected to the inputs labelled as such on the CNC Shield board, in the same way as the X-axis and Y-axis ones.

This project does use a PWM variable speed spindle however and so the two Z-axis end stop limit switches have to be connected to the SpnEn (Spindle Enable) header on the CNC Shield board. In order to do this, a splitter cable was made up as shown in the photograph. The splitter cable was assembled using parts from the DuPont Connector Header Kit and the flat ribbon cable. The ends of the cables to be joined were simply soldered together and covered with heatshrink tubing.

The Z-axis end stop limit switches can now be connected to the SpnEn (Spindle Enable) header on the CNC Shield using the splitter cable.