08. Computer controlled machining¶

The assignment this week was to create something “big” using the CNC cutting machine.

What is Milling?¶

Milling is a more specific process that is similar to drilling and cutting. The primary function of a CNC Milling Machine is that your tool will be doing the rotating and moving while your workpiece stays in one spot (generally). These machines can also be either horizontal or vertical, again depending on the tolerance and weight of your workpiece. This process has many axes that allow for a variety of shapes, holes, and slots to be cut into the workpiece at many angles. These axes provide many different maneuvers, either by the spindle or the bed, to cut the part desired to the exact specifications.

In this diagram, we have a CNC Milling Machine with all the aforementioned components as well. The top part is the Spindle, which holds the Tool. This part spins at high RPMs and moves back and forth, left and right, up and down (depending on the number of axes your machine has) to mill away the material.

The workpiece is held in place using clamps or screws. Some CNC Milling Machines have the capability to move the bed in conjunction with the spindle to attain specific shapes at certain degrees. Because of these capabilities, CNC Milling offers cutting solutions for more complex parts, like engine components, custom tooling, intricate mechanisms and enclosures.

Source: Southerfabsales

CNC Router Terminology¶

Lets start with the definitions of some terms we will be using here on:

Chipload: The physical size of the pieces or chips the bit cuts off the material.

Feed Rates: How quickly a machine can laterally move through the material being cut. Typically measured in Inches Per Minute (IPM).

Speed Rates: The speed of the machines spindle. Typically measured in Revolutions Per Minute (RPM)

Flute: The number of cutting edges on a routing bit. Toolpath: A coded route which a machine follows as a guide in order to cut.

Types of Drillbits¶

1. Cutting Endmills : These are the most commonly used drill bit for CNC cutting. This bit features a flat cutting edge and a sharp center point or screw-like threads that can rapidly drill holes. Due to the risk of wandering when drilling thick materials, the flat bit is often coupled with a pumping device that blows away excess sawdust.

2. Ball Nose : These are used for carving large 3d contours

3. V-bit : A 60° or 90° V-bit is great for what’s called V-carving, in which the tip of a V-shaped bit is used to cut into narrow spaces, and the wide bottom is used to cut into larger spaces. V-bits can also create sharp corners that other end mills cannot because of their radiuses.

Types of Cuts¶

Specific bits are designed to work best with specific materials. Once you’ve figured out what materials you will be cutting the next step would be determining your cut direction.

Up cut: Most regularly used type of cut. Sends chips up and out of the way of the cutpath.

Down cut: Used for specific applications such as laminated or thin material. Pushes chips down in spoilboard.

Compression cut: Combines Up and Down cuts. Reduces chipping which leaves a smooth finish on top and bottom of material.

(Note:For most plastic and aluminum applications, a single flute pattern is preferred, while for most wood applications it is ideal to utilize a double flute pattern.)

When it comes to selecting routing bits, remember the thicker the routing bit, typically the deeper the pass. Although, be conscious of the bit diameter in order to not unnecessarily waste material. It is also best practice to match the size of your bit to the size of the material you’re cutting. For example a ¼ inch sheet of plastic would ideally be cut with a ¼ inch bit.

Things to know before using CNC Router¶

• Before you start cutting away at lighting fast speeds, there are a few factors you need to consider.
• First, determine what type of material you will cut, the thickness of it, and the type of application (cutting, engraving, pocketing, etc.) in order to set an outcome.
• Secondly, establish if you want to the toolpath to be a single pass or have multiple passes.
• Lastly, prior to running a cut file, follow the manufactures recommendations for a routing bits recommended chipload. These recommended specs can usually be found on their website, documentation, or manual. The formula provided below will assist you to calculate your chipload.

Chipload = Feedrate / (RPM x Number of Flutes)

• General rule of thumb: If your chips are too large, slow down the feed rate or you risk breaking the bit.

Source : Multicam - CNC Solutions

Steps and processes before CNC¶

For CNC there are multiple steps one needs to follow before actually going to the machining part.

Preparing the design file¶

For normal cutting and creating pockets, CNC takes input from a 2d file (dxf format). Since I wanted to make a chair, I started by working on the side elevation and fixed the angles for the posture I required.

• Next was making the members as per these dimensions.

• Adding pivot points to these members for closing.

• Then I made a seperate elevation for the members at the side to give a slight curve to the seat and backrest for better comfort.

• Next was converting these elevations to solids using loft command

• Select the parts individually and export them as .stl

• Import this .stl file in the slicer. Enter all the setting for the type of joint(here interlocked slices) the sheet thichness and size, distance between two members, etc before generating a composed dxf sheet layout.

• Instead of directly making the final prototype, I decided to first make a scaled model(1:5) using whatever waste material was available. and then making changes in the final thing accordingly.

• The pieces were even marked automatically using numbers and axes.

• Did the assembly and this was the result.(open condition)

• Cloed condition

• The seat member was extruding too much out of the chair.

Design Modifications¶

• As per the observations in scaled model, I made changes in the final CNC version, starting with the elevation made changes keeping the posture same.

• Made the sectional drawings placed them at their respective sectional positions.

• Used the loft command again to convert the sensions into 3d. And exporting these 3d parts as .stl. Also added the pivot holes using subtract command.

• Import these in slicer software, one at a time, enter the settings as shown and you’ll get the dxf file for each individual components of the chair, the seat, the support and the backrest.

Following are the settings I changed :

• Technique : Interlocked slices
• Slice Distribution : 10 in 1st axis and 1 in 2nd(Move teh 2nd axis member using drag and move on the 3d representation)
• Go to export dxf and check the drawings and the naming and if everything is okay, export the file as dxf.

(Note: Use autocad for re-nesting the output since it can be done better manually and material can be saved when machining)

• Once you have the dxf file ready, its time to move to partworks software. This is a licenced software and all the fablabs with a CNC machine should have access to it. So you might need to move to a Lab Computer from this step.

• This is the screen you will be welcomed with in partworks software. You need to setup the sheet dimensions to start with. I used a 12mm PVC sheet of 2400mm*1200mm.

• Go to File>Import and select the dxf file you created earlier.

• Place the file on the sheet in the orientation as you want to cut it.(The top left is the origin here)

(Note: Try to keep seperate layers for outcut, incut, pockets, drillholes, etc)

• Since, I had outcut and quick engrave, I had two layers.

• Next is creating the machining toolpath. On the right side of the screen, click on create profile toolpath for cutting. The screen should display a list of all the parameters you need to set.

• The below are the parameters I’ve used.
• The cut depth I kept at 12.3 for my 12mm material since there might be material bending in some areas in such a large size board.
• Drillbit was 3mm endmill.
  • Pass Depth : 3.0mm
  • Stepover 40%
  • Spindle speed : 12000 rpm
  • Feed rate : 1.0 inches/sec
  • Plunge rate : 0.3
• Select ‘outcut’

• Add tabs to each member to avoid them from flying off during machining. That can be done either automatically using a fixed number on each closed polyline or manusally by clicking on the polylines.

• Check the toolpath and see if everything is okay before going further.

• Create a toolpath for quick engrave. Once done, you should see the list of toolpaths on the right side of the screen. If the toolpaths are okay, click on save toolpath.

• Name the file and click on save toolpaths. It will save an .sbp file which you can open in the shopbot software.

• It’s time to go to the machine. Before starting make sure you have all the safety gear on.

• Mount your material to the sacrificial board on the CNC Machine. Make sure the material is properly mounted with enough screws and these screws should not on the machining toolpath or the drill bit might break and there is a high risk of injury!

• Keep the emergency switch handy at all times when machining is in progress.

• Open shopbot software. Once it is in the move/cut mode, click on the movement button to open the window for moving the bit in the CNC.

• Use the arrow keys on-screen or keyboard to move the bit to the zero position in x,y,z. You can use the fixed option for tiny adjustments. Once set, click on zero axes and select all x,y,z.

• The CNC is ready to start machining. Go to File>Part File load and select the .sbp file we saved earlier.

• Click on enter, and in a few seconds the screen will display a dialogue box to start the spindle and then press enter to start machining process.

• Check if the dimensions are coming out as set or if they’re more or less.

• Once the machining is done, remove the board from the machine and cut through all the tabs manually and gather all the parts to start the assembly

• Start putting all the press-fit parts together.

• Add the pivot rods and the chair is ready for use. This is the open condition.

• And the closed condition!

• The second angle was added for a second posture which can be achieved by sliding the backrest on one side.

• And thats me sitting on the assembled chair!

• This week was really important for me beacuse I was planning to make something big for my final project and hence I would be using CNC machine.

All the files for this week are attached here

Group Work¶

The Group Assignment was to test runout, alignment, speeds, feeds, and toolpaths for your machine. The group page can be found here