17. Mechanical design & Machine design

This week I worked on defining my final project idea and started to getting used to the documentation process.

Group project

All of the details are in these two links mechanical design & electronics . The group documentation for electronics has a bit more details than my page.

In the group documentation, we mentioned points related to recognise opportunities for improvements in the design like:

• We would change the thicness of the design to 6mm as this will increase the strength & stabilty of the design.

• We would improve the the press-fit design of some of parts for example, we would create a cover for the pump to make the design looks more industrial

• we would increase the surface area of the structure members.

Research

For this assignment, we discussed & agreed on an idea & disstributed the work between us. My colleage Zahra & me were responsible for the electronic part.

All of the things written in my page are things that I worked on alone (like the part of documentation in my page & flashing grbl & part of the research) or with my colleages (like software & motor testing). We did research about the topic & these are some of the references:

Arduino CNC Shield 1) , Arduino CNC Shield 2 , Arduino CNC Shield 3 , Arduino CNC Shield 4 & Machine Building .

Things we need:

• An Arduino Uno and a PC to upload the firmware.

• Three bipolar stepper driver such as the DRV8825 or the A4988. One for each motor. We got two A4988.

• Three bipolar stepper motor with a dupont connector with 4 pins.

• A power supply for the motors. Normally it will be 12V and at least 5A.

• Arduino CNC Shield (we got Version 3. 0) runs on 12-36V DC.

Flashing GRBL

Grbl is a no-compromise, high performance, low cost alternative to parallel-port-based motion control for CNC milling. This version of Grbl runs on an Arduino with a 328p processor (Uno, Duemilanove, Nano, Micro, etc).

I have downloaded the grbl v1.1 files through their official github link . Then, I have added the (grbl) folder contained within it as a library in the arduino IDE through Sketch > Include Library > Add .ZIP Library

An example called GrblUpload apeared in examples. Then, opened it, compiled and uploaded it.

The settings can be customized to your needs through the config.h. I tried to edit this file by opening the grbl confg.h file (one of the libraries in the arduino grbl code) and comment out any lines regarding Z_homing

#define HOMING_CYCLE_0 (1<<Z_AXIS)to remove the Z-axis homing 

but I got an error, so I returnd it to how it was.

Stepper Motor

A stepper motor is a motor controlled by a series of electromagnetic coils. The center shaft has a series of magnets mounted on it, and the coils surrounding the shaft are alternately given current or not, creating magnetic fields which repulse or attract the magnets on the shaft, causing the motor to rotate.

It converts digital pulses into mechanical shaft rotation. Every revolution of the stepper motor is divided into a discrete number of steps, in many cases 200 steps, and the motor must be sent a separate pulse for each step. The stepper motor can only take one step at a time and each step is the same size. Since each pulse causes the motor to rotate a precise angle, typically 1.8°, the motor’s position can be controlled without any feedback mechanism. As the digital pulses increase in frequency, the step movement changes into continuous rotation, with the speed of rotation directly proportional to the frequency of the pulses. These links, link1 & link2 have some useful information & codes.

We added/changed the motor connectors to increase the length of the wire & to be able to connect it to the shield.

Linear Motion Control

The rotary motion of a stepper motor can be converted to linear motion using a lead screw/worm gear drive system. The lead, or pitch, of the lead screw is the linear distance traveled for one revolution of the screw. If the lead is equal to one inch per revolution, and there are 200 full steps per revolution, then the resolution of the lead screw system is 0.005 inches per step. Even finer resolution is possible by using the step motor/drive system in microstepping mode.

in the group documentation, we mentioned that we found that the steps were not accurate. So, we changed the “V carve sheild” steps by adding jumpers in the right position as shown.

Arduino CNC Shield

Arduino CNC Shield Version 3.0 GRBL 0.9 compatible 4-Axis support (X, Y, Z , A-Can duplicate X,Y,Z or do a full 4th axis with custom firmware using pins D12 and D13) 2 x End stops for each axis (6 in total) Coolant enable Compatible with A4988 or DRV8825 stepper driver Runs on 12-36V DC

It is the PCB that we will need to control the stepper motors. This PCB is perfect to assemble CNC low cost week17 such as a laser cutter, using an Arduino Uno. It is compatible with the GRBL firmware. It has endstops and relays output, and support the speed control by PWM.

Motor Driver

A4988 Stepper Motor Driver Board is a breakout board for Allegro’s A4988 DMOS Micro stepping Driver with Translator and Overcurrent Protection The motor driver takes the pulse signals from the arduino (direction and step pulse) and converts them to motor motion signals outputed to the motor coils to energize them accordingly.

It’s important to pay attention to the orientation of stepper drivers. The potentiometer (pot) on A4988 and DRV8825 are at the opposite side.

Dispenser

We took the dispenser of an old Dettol’s no-touch hand wash system. We connected a DC motor driver directly to the dispenser motor (not to the circuit of the system). We used M3 command to operate the dispenser & M4 to close it.

Duplicate Axis

We tried duplicating X-axis by adding two jumpers to connect the X-axis pins.

Some motor control testing

Software

BCNC

BCNC is a GRBL CNC command sender, autoleveler and g-code editor. It can import/export g-code, dxf and svg files. We had some problems with the connection to bcnc at some times but we were able to connect to it eventually.

Checking motor control using BCNC

Easel

Easel is a web-based software platform that allows you to design and carve from a single, simple program.

Checking motor control using Easel

Below is a list of G-Code commands and what they do. source:

S command: Sets the Spindle speed.(0-1000 with default build)

M3 Command: Sets Spindle Pin to the PWM rate as specified by the S command. (Clockwise) Sets Spindle Direction pin High

M4 Command: Sets Spindle Pin to the PWM rate as specified by the S command.(Counter Clockwise) Sets Spindle Direction pin Low

M5 Command: Sets Spindle pin Low. (PWM 0) Keeps the Direction Pin as is.

In gCode as grbl understands it, there are two commands that are used to turn the spindle on and off .source.

M3S10000: Start the spindle spinning Clockwise (M3) at 10,000RPM (S10000) M5: Stop the spindle

We tried generating g-code for a drawing using Easel & We used M3 command to operate the dispenser & M5 to close it.

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Motor testing

Testing motors with the dispenser

Dispenser control testing by using M3 command to operate the dispenser & M4 to close it.