December 7, 2012

Hobby Computer Numerical Control Milling Machine – control module and important aspect of mechanism design

Filed under: To blow your mind — VIP @ 07:42

Main board:

This is the board that I used to control my machine after I failed to come up with a budget solution to control my machine with a USB signal. I bought it off Ebay, from some undefined Chinese supplier.

An image of my masterpiece. All control module parts are screwed to the main plywood frame while plastic beads were used to raise the boards above the plywood surface. Few components are bolted.

Limit switches are connected to the board

Main interface board: Left connector is to control the mill motor, or in my case a relay that powers the mill motor. Right connector is for the limit switch inputs and other programmable inputs into the board that are sent to the computer through LPT port.
Stepper motor connector for each of three axes.

This connector is for the stepper motor outputs.

Right to left A-, A+, B-, B+. If you reverse the connections, the stepper motor will run in the opposite direction. This can be adjusted in the software.

Various motor regimes.

Switches to change different operation regimes of the stepper motors.
Connectors on the control module

Left to right: AC power connector that is commonly found in computers, ‘my’ DB-25 connector that connects to the electrical components of the CNC mill, a connector that is used for a wired remote control to move the machine components manually, the LPT port that is used to interface the computer.
Control board.

Seen from left to right: software emergency stop button that sends a signal to the computer via LPT port, PWM mill speed control knob that is pressed on the shaft of a variable resistor,

AC power switch for the whole board.

Additional relay to control the mill motor

Relay that toggles that opens and closes the circuit of the DC motor that drives the spindle.
PWM smooth speed adjustment for the mill motor

A smooth PWM DC current control module that is used to change the mill rotation speed. Three wires that are seen on the left lead to the variable resistor that is mounted on the control board.
Power supplies

Two power supplies for outdoor lighting systems: Left power supply outputs 12VDC for the control board, the stepper motors and the warning light, the right power supply outputs 24VDC for the spindle motor.
Power supply breakout

This is how the terminals of the power supply look like.

The ground symbol marks the AC ground, not the DC 0V rail.

Power supply rating

One of the power supplies outputs 24V, 6.5A and another power supply outputs 12V, 13A.
Limit switches

This is how my typical limit switch looks like on the CNC mill. For economic reasons and for the reasons of convenience I used switches that are normally open. Appropriate software adjustments had to be made.
Limit switch override

This is the switch that is Normally Closed. All limit switches in the CNC machine are Normally Open. If I manage to get my machine to move onto a limit switch and cause it to stay closed, causing the software to shut the machine controls off, I can hold this switch and disable the limit switches temporarily to let me move the machine ‘off’ the limit switch so it will open, in order for the proper function of the machine.
DB-25 connector

To quickly connect my CNC mill to the control board, I used the DB-25 LPT connector. I had used it only as a connector so never plug computer equipment into the male or female connector that is used to carry power for the machine operation. This connector is rated for 2A of current for each pin.
Plastic bead standoffs

My traditional way of connecting parallel planes when I build my prototypes. I use a machine screw with standard beads that can be found in craft stores as spacers.
Stepper motor, helical coupling, lead screw

Right to left: stepper motor, helical coupling and a lead screw. The lead screw thread is M6, I believe. Shaft diameter of the motor was 5mm.
Mill motor

Mill motor with a bit chuck and a clamp that holds it against the CNC mill frame.
Mill motor with a coiled power cable

Another view of the mill motor.
Limit switch above the mill motor.

Lead screw and lead nut

This is how the lead nut is attached to the frame components in and relationship to the lead screw.

Notice the white fluoroplast inserts between the moving parts, which are pressed in place by the set screws.


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