Material selection is critical in determining the overall functionality and cost of the part. The designer must define the design’s important material characteristics—hardness, rigidity, chemical resistance, heat treatability, and thermal stability, just to name a few.
The material blank is the size of the material that will be used to create the finished part. For example, if the finished part dimensions are 3.5 in. long × 2 in. wide × 1 in. tall, then the material blank size in its raw form would need to be a minimum of 3.75 in. long × 2.125 in. wide × 1.125 in. tall. Material blank thickness is another area that should be considered during the design process.
A good rule to follow is to account for a blank that is a minimum of 0.125 in. larger than the part size. For example, if the final dimensions are to be 1 in. × 1 in. × 1 in., then the blank for the part would be 1.125 in. × 1.125 in. × 1.125 in. to allow for the variations in the raw material. When designing the part, consider if the form, fit, and function of the part would not be changed if the final part dimensions were 0.875 in. × 0.875 in. × 0.875 in. This way, a standard 1-in. x 1-in. x 1-in. block could be ordered and save some material cost versus a larger starting blank.
As a general rule, softer metals, like aluminum and brass, as well as plastics, machine easily and will take less time to remove material, which in turn reduces time and cost. Harder materials, like stainless steel and carbon steel, must be machined with slower spindle RPMs and machine feed rates, which would increase the cycle times versus the softer materials. As a general rule, aluminum will machine about four times faster than carbon steel, and eight times faster than stainless steel.
The type of material is a critical driver in determining the overall cost of the part. For example, 6061 aluminum bar stock is approximately half the price per pound of aluminum plate, and 7075 aluminum bar stock can be two to three times the cost of 6061 bar stock. Cost for 304 stainless steel is about two to three times that of 6061 aluminum, and about twice as much as 1018 carbon steel.
Depending on the size and geometry of the part, the material cost can assume a significant portion of the overall price of the part. If the design doesn’t warrant the properties of a carbon or stainless steel, consider using 6061 aluminum to minimize the material expense.
Plastic material can be a less expensive alternative to metals if the design doesn’t require the rigidity of metal. Polyethylene is easy to machine, and costs about 1/3 that of 6061 aluminum. In general terms, ABS is about 1½ times the cost of Acetal; nylon and polycarbonate are approximately three times the cost of Acetal. Keep in mind that depending on the geometry, tight tolerances can be harder to hold with plastics, and the parts could warp after machining because of the stress created when material is removed.
Complexity and Limitations
The more complex the part, which means contoured geometry or multiple faces that need to be cut, the more costly it is due to additional setup time and time to cut the part. When a part can be cut in two axes, the setup and machining can be accomplished faster, thus minimizing the cost.
For simple two-axis parts, more material will be removed as the tool moves around the part than with a contoured part. With a more complex part, some areas will need to be cut with X, Y and Z axes moving together.
To create a complex surface with a good surface finish, very small cuts will need to be used. This increases the time and, therefore, price of a part. A general rule to help minimize the cost is to try and design using only two axes cuts, but this isn’t always possible if a certain look or functionality is required. Keeping things consistent, such as internal corner radii and tapped holes, will also help save time and money on parts by reducing the need for tool changes.