Material Type for CNC Machining

The most easily machined types of metals for CNC machining are soft metals including aluminum and brass.

As the materials get more dense and stronger, they become much more difficult to machine. These materials include steel, stainless steel, titanium, and other exotic alloys. They are much harder to machine and take more working time.

Harder materialsa are less manufacturable through CNC Machining. Most all types of plastic materials are easy to machine, but  the additions of fiberglass or carbon fiber materials can reduce the CNC machining ablility.

Plastics that are particularly soft and more flexible create machinability problems of their own.

Material Form

Machine metals come in all forms, and in the case of aluminum for example, bar stock and plate are 2 of the most common forms in which machined parts are made.

The size and shape of the component to be machined determines which form of material must be used. Engineering drawings often specify which form is to be used.

Bar stock is usually about half the cost of plate, so although the material form isn’t directly related to the geometry of the component, cost can be cut at the design level by specifying the least expensive form of the material for CNC machining.

Tolerances

A large contributing factor to the cost of a machined component is the tolerance in which the parts must be produced.

The tighter the tolerance that is required, the more expensive the component will be to machine. When designing, it is advised to specify the tolerance that will serve the function of the component without increasing the cost.

Tolerances for CNC machining must be specified on each feature. There are creative ways to engineer components with lower tolerances that perform as well as ones with higher tolerances.

Design and Shape

CNC machining is a subtractive process. The time it takes to remove the material is a major factor in determining the machining cost.

The volume, shape, and density of the material to be removed as well as how fast the tools can be fed will determine the CNC machining time.

When using milling cutters, the strength and stiffness of the tool which is determined in part by the length to diameter ratio of the tool will play the largest role in determining that speed.

The shorter the tool is relative to its diameter the faster it can be fed through the material.

A ratio of 3:1 (L:D) or under is optimum. If that ratio cannot be achieved, a solution like this depicted here can be used. For holes, the length to diameter ratio of the tools are less critical, but should still be kept under 10:1.

There are many other types of features which are more or less expensive to machine. Generally chamfers cost less to machine than radii on outer horizontal edges. Undercuts are also more expensive to machine. All the features that require smaller tools are more expensive to machine.

Design for Additive Manufacturing

Additive manufacturing broadens the ability of the designer to optimize the design and save material.

Designs tailored for additive manufacturing are sometimes very different from designs tailored for CNC machining. Forming manufacturing operations ae also different in design.

Due to some size constraints of additive manufacturing machine the related larger designs are split into smaller sections. Self assembly features or fastener locators are inserted.

https://en.wikipedia.org/wiki/Design_for_manufacturability