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.

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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.

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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.

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(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

Chipload = Feedrate / (RPM x Number of Flutes)

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.

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Design Modifications

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Following are the settings I changed :

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

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

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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