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Week 7: CNC Machining

Group Assignment

  • Complete your lab’s safety training
  • Test runout, alignment, fixturing, speeds, feeds, materials and toolpaths for your machine
  • Document your work to the group work page and reflect on your individual page what you learned

Lab Safety

For our CNC week we will be using our Lab's Workbee CNC and fortunately it is a fully enclosed CNC machine with sound dampening.

This enclosure makes the machine far safer but there are still some rules about it.
- Always ensure that the enclosure doors are shut when the machine is operational.
- Ensure that the router is turned off when the door is open.
- Safety glasses must be worn when the door is open.
- Keep out of the enclosure when operating the machine with the doors open (e.g. when you are zeroing the head).
- Do not operate the machine if you are alone in the lab.
- Be aware of the fire extinguisher locations.
- Be aware of the emergency stop button on the machine.
- Stop the machine upon hearing any strange or unexpected sounds.
- Ensure that all wood dust is cleaned up - it is a fire hazard.

Machine Set Up and Operation

To test our machine we are going mill a test piece out of our 12 mm plywood. It will have sharp edges, a curved edge, pockets and contours. To start with we made a test 3d model and CAM'd it in Fusion 360 to produce the .nc file we required. We go in depth on our individual pages into the CAMing process.

To get our machine ready we started by loading in our 6 mm flute carbide mill bit. Be careful when handling the milling bit as it is very sharp. Start by unscrewing the chuck off the router. 2 bits will come off - the chuck and the fastener housing. Place the bit in the chuck like so.

Then squarely align and screw the chuck back on with the fastener housing. Once hand tight, fully tighten it with 2 wrenches.

Next we will turn on the CNC controller and the computer attatched to it. We are useing CNCjs for our control software, open it and connect to the serial port connected to the CNC machine.

Now we will place the stock in, aligning it to the 2 front bolts to ensure it is roughly square, and then using the fastening pieces to hold it down. Find a nearby mounting hole in the bed below and use a ratchet to tighten down the piece to the bed.

Next we will zero and home the head on to the stock. This process is really easy on the WorkBee as it has a calibration piece. Place it on the corner of the stock you wish to be the origin and clip the other wire to the bit. Using CNCjs, move the bit to above the centre of the metal block and about 3/4 of an inch above it. Then hit the zeroing function in CNCjs. The bit will lower till it contacts the metal bit and finds the z home, and it will repeat the same process to find the x and y homes. After this the machine is zerod and ready to go.

Now we can import the .nc file we made and hit run in CNCjs, ensuring that the door is closed and the calibration gear is securely fastened away from the milling area.

And after a few minutes our test piece had been cut out.

Machine Characterisation

Runout

Runout is the measure of how far off the centre of rotation the milling bit is - how rotationally out of kilter it is. To test this we milled what should be a 6 mm hole which is the diameter of the bit. And upon measuring we found that the hole was 6.01 mm.

Alignment

Alignment is the measure of how square the cnc axis' are - whether they are orthogonal or not. We cut two 100 mm sides at a 90 degree angle and running it against every square surface we could find, we found no light coming through. We also used pythagorean theorem to calculate that the edge to edge distance these 2 sides make should be 141.42 mm and we measured ours in at 141.41 mm which is pretty darn square.

Joint Tolerances

We printed out a test piece and from previous experience we aimed for a tolerance of + 0.15 mm on top of the thickness of the ply. After milling out slots of that thickness, we found that it provided a nice and tight fit, somewhere between being able to push it in with a mild amount of force and needing a little bit of malletting depending on the inconsistencies in the thickness of the ply.

Speeds and Feeds

To calculate our speeds and feeds we used this handy page which has a great deal of infromation on it. We used the look up tables for 12mm ply wood and specs of our milling bit to get the neccesary data:
- Tool Diameter: 6mm
- Flutes: 2
- Chip load: 0.54 mm
- Surface Speed: 3000 mm/s

Using this we found that we will need a speed of atleast 12500 RPM. We then used the formula for feeds:

  • Feeds = flutes * chip load * RPM

And found that we need a feed rate of 13,586 mm/min.