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I chose 425 O.Z. inch stepper motors because I wanted to have some strength to perform tougher tasks. They came grouped as a kit with a power supply and motor drivers.
Below is a visual representation of oz inch. It literally means that many oz at an inch away from center.
How does this help you? It probably wont because if you have ever worked with a router or table saw before you know that not much force is needed to push the cutting bit into the material.
When building a machine you have to realize that milling metals will be your most difficult challenge. You also have to understand that these motors could mill through any metal on earth if you use a low enough feed rate. But this low feed rate will depend greatly on finding a proper spindle. Not only that but you have to consider the ball screw break away which keep the machine parts from deforming quickly.
Just for fun consider this formula I found for calculating the force your ball screw might be able to supply.
T = L*P/5.65
T= torque in oz inch you need to provide
L = inches the lead screw will move forward for each turn
P = axial load in oz. Axial load is the pressure
Some common motor sizes for the lazy.
166 oz inch for small 3D printing or engraving
250 oz inch for milling wood or larger 3d printer
400 oz inch for milling non ferrous metal
These of course vary depending on your thread pitch and the speed you want the machine to move.
Wiring Your System
Now I went with a kit but basically each stepper motor needs a DC power supply and a motor driver that can handle the amperage and voltage.
Here is a great diagram of how they should basically work. Your motors are wired to drivers. Your drivers decide which coil to power based on the breakout boards directions.
I used the 18 volt wiring you can get at the hardware store for air conditioning wiring. The only thing I don’t agree with is the E-stop being integrated into your circuitry. You should have a regular 120v on/off switch that can instantly shut down your motors and spindle without relying on an integrated circuit.
Your motors should come with directions explaining which wires are paired. I had a 4 wire motor with Black/Green and Red/Blue. These get wired into the drivers.
My drivers are set up active low as opposed to active high. Active low means that a DC circuit is being switched by connecting or disconnecting the negative wire. Active high is done by controlling the positive wire. So in my active low setup I ran my positive 5 volt power supply into each of the drivers positive terminals and connected the 5 volt power supply to the breakout board which will control the flow of power to these circuits. In the picture above you can see the wires exiting the drivers – PULSE and – DIRECTION port into my breakout board.
Lastly set your resolution or steps per rotation. This will determine how many pulses your driver will need to make one complete rotation of the motors shaft. I set mine to 1600 which is fairly common.
You can use 28 gauge cat v wiring for your limit switches and 18v for everything else.
A computer power supply is a great way to get 5 and 12 volt power into the system
Never use the relay built into your breakout board. Add another circuit to help isolate it.
Always solder connections after you are sure they work. Wire nuts will fail eventually.