Rapid Prototyping and Model Making: Subtractive Manufacturing

Computer Numerical Control (CNC)

A manufacturing process in which the movement of the cutter, is controlled by a NC program.

CNC Machining

A manufacturing process in which pre-programmed computer software dictates the movement of factory tools and machinery.

Parts of CNC Machine

  1. Control Panel
  1. Spindle Unit
  1. Work Table

NC Program

Consists of Alpha-Numeric code, or sometime known as G-code.

How it works

The process is programmable. Tool movement is controlled by NC program.

Remove material from a solid workpiece until the desired shape

Pros of using CNC

Programmable

Can produce a fine surface finish

Cut on hard-metallic material

More accurate

Constraints of using CNC

5-axis Machining Center

Post-processing needed

Work Holding Device

Limitation of cutter size

CAM programming Skill

Trained Machinist

Can only shape the top-half of a design. To machine out a complete 3D shape of the prototype, the workpiece needs to be flipped over manually, and be able to clamp tightly when the second half of the design is being designed.

This is a higher end CNC machine, it is more expensive and more complex to program than a 3-axis machining center.

Sometimes, each half of a 3D part is machined separately, and later glue together to give a complete 3D prototype.

There is a minimum diameter size of a machining cutter. Hence, it is not possible to machine a sharp inner corner. Moreover, the cutter length is shorter when the cutter diameter size is smaller. Hence, programmer need to take note to avoid cutter holder colliding with the part.

To generate the NV program for CNC machining center, we need to use a CAM (Computer Aided Manufacturing) software. Special training in using the CAM software is required.

One must have the knowledge and skill of machining process to use a CNC machine.

Work Flow of CNC: CAD --> CAM --> CNC

How does CAM system works

Toolpath programming

Parameter setting for toolpath (CAM)

Step Over

Feed rate (mm/min), Spindle Speed (rpm)

Step Depth

Depends on the workpiece material

A soft material allows deeper cut, and can remove material faster

Define how deep can the cutter plunge into the workplace

Cutter will need to plunge down more times for hard material, due to smaller depth of cut

Better surface finish, but longer machining time

Roughing and finishing toolpath (CAM)

Finishing: Aim to cut the part to accurate dimension, and good surface finish

Roughing: Aim to remove bulk material at shorter time, ignoring the surface finishing.