Thursday, February 27, 2014

Project CNC - more on build materials

A brief look ahead (what my build looks like right now):
Starting calibration

After adding X-axis brace
Stepper Drivers
Initially I thought I'd be able to use L293 chips (dual H-Bridge drivers), since I already have plenty of them. But...I ended up finding out that I didn't understand what “Stepper Drivers” were and how CNC G-Code worked (how information is sent out).

Basically, programs interpreting G-Code send out pulses to control 1.) direction and 2.)distance (in this case - “steps”). Dedicated stepper motor drivers are designed to interpret these 2 pulsed signal to the motors for each axis. With the type mill I'm trying to make that means 3 axis with 2 signal lines to the motor controller for each axis.

Dual H-Bridge drivers are fine for powering and running stepper motors but they do not have the additional circuits to interpret and translate digital signals into power to the stepper motor coils. So, either I would need to program controllers to interpret the signals or better yet - find chips that are already designed to do this.

Surprise, surprise – once you really know what you are looking for/need it becomes much easier to find it.  Long story short – there are a number of different chips and breakout boards that are made specifically to control stepper motors for robotics and computer assisted machining (just what I needed). Some boards have multiple drivers (up to 5 for true 3D milling), different voltage and amperage levels (depending on the size and type of stepper motors you are using).

The most popular seem to be the Easy Driver Board by Sparkfun and the Pololu Stepper Driver boards (I later found out that these boards are used in RepRap 3D Printers and the ShapeOko CNC machines).  These boards control one stepper and each board takes two inputs (direction and distance - in steps) and outputs the signals to bipolar stepper motors on 4 wires. I eventually settled on the Pololu boards, since I was able to find package deals on 3 boards (just the number I would need). These drivers can handle up to 35V and up to 2amps for the motor power supply. These boards work great for the NEMA 17 stepper motors that I have. The design of these boards is open source so you could make them yourself – if you are comfortable with surface mount chips (not me...yet).
3 Pololu Stepper Boards with added Capacitors for spike protection

Received my longer 8mm rods, additional linear slide bearings (originally only had 10 and found that I really needed 12 – 4 for each axis) and a replacement for burnt out stepper driver board (I ordered 2 just in case I burned out another one). Also, I ordered a 24 volt 5 amp power supply (about $16) which also arrived, so I don't need to continue to rely on batteries for the stepper power supply.

Finished my new mount for rotary tool, complete with linear slide bearings.

Finished Mount
I've seen a number of similar configurations but the one I adapted this design from came from searching through the SketchUp 3D Warehouse. Details on the original design can be found at 

Stages of making my mount:
MDF board for mount - routed out areas for HDPE sections
Mounting board lined up to bearings and taped in place

Temp nails hammered in board to mark spots and ensure glide with no binding

Picture after nails removed

Having to deal with setbacks is a consistent part of life and building/making is far from an exception. Just the other day I thought I was so close to being done when...Oops! I tried to move the frame onto its side, with everything attached, while only holding one section of frame – End Result – the screws joining the 2 sides ripped though the MDF on both ends. End result – I had to cut a new section out of the MDF, sand it down, check dimensions, drill new guide holes to attach piece, drill new holes to attach x-axis support (small mistake = bigger repair). When I was running a cut test I attempted to stabilize the x-axis I ended up breaking the same side again - this time I did not have the extra MDF to remake the section so I had to glue it with epoxy.
Glued repair

I'm still trying to catch up to where I'm actually at now in this build.  Currently my machine is functional but there have been a number of issues that have slowed me down, due to having to solve them.

 I am working to catch up on posting this build process - it is difficult to work to solve developing issues while also writing them up.  I know that my posts are delayed but I am endeavoring to write up my entire journey - especially including the obstacles I encounter and the solutions that I am able to eventually find to them.  

Building and making is fraught with challenges, what seems simple and straight forward at first frequently turns out to be more difficult.  But, that is no reason to give up.  While we all want a successful end result I personally feel it is the journey that is most important (it's searching for the prize that is the adventure - not the prize itself).