Choosing the Right Material for Your Machined Parts - A ...

Have you had a part fail because the wrong material was used to manufacture your part(s)? It is crucial to your project that you select the right material for the job. There are 4 main factors to consider when selecting your material type:

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• Mechanical Properties
• Physical Properties
• Application Requirements
• Cost

This article aims to simplify the material selection process by reviewing the 4 primary factors to consider when choosing your project material. 

Step 1: Consider the Mechanical Properties

The mechanical properties of the material you select are a good predictor of your part’s performance. 

Example: Medical equipment needs materials resistant to corrosion and not harmful to human tissue (biocompatible). The Automobile industry requires impact-resistant materials, and the marine industry requires corrosion resistance while also providing immense strength and durability.

Step 2: Look at Physical Property Needs

The physical properties of your selected material will help you understand how they will behave during and after the machining process. Some of the top physical properties to pay attention to are:

• Hardness
• Thermal Conductivity
• Ductility
• Elasticity
• Density
• Abrasiveness

Why are these important?

Not only does your selected material need to be suitable for its end use, but you also need to ensure that it can be manufactured safely. Certain materials are incompatible with specific manufacturing methods due to inherent physical limitations. If you need help selecting the appropriate material for your project,  our team at Bayside Machine would be happy to help.

Step 3: What are Your Application Requirements

Not all materials are equal. Hence why materials are abundant with different mechanical and physical properties. For example, you would not choose food-grade stainless steel to machine a bolt group for a submarine. Instead, you may select an alloy steel or titanium. Alloy steel can withstand the immense pressure of the sea and is most often used for the hull. Where titanium allows for deeper diving because of its strength-to-weight ratio and corrosion resistance, it is also more difficult to machine and is more expensive. So in this instance, if deep sea diving is not the end goal, the steel alloy might be the better material selection.

Step 4: Factor in Cost

We’ve mentioned cost for each deciding factor because cost plays a major role in which material you select. While we would all love to have the best material possible for every project, the reality is we don’t always need the most expensive item to achieve the goals of our project. Sometimes the less expensive solution is not only cost-effective but also easier to work with.

The cost of your raw materials isn’t the end cost of the material you select. You need to consider raw material costs, the cost to machine the part, shipping costs, etc.

At Bayside Machine we have a large dedicated staff to ensure your projects meet your needs. Whether it is looking at tensile strength, flexibility, corrosion resistance, or other properties we are here to help guide you through the process.

If you are ready to get started on your project and need guidance or know exactly what you need, we are here to deliver quality parts with exceptional customer service.

Editor's Note: An updated version of this information can be found here.

What exactly is computer-numerical-controlled (CNC) machining? It’s a means to make parts by removing material via high-speed, precision robotic machines that use an array of cutting tools to create the final design. CNC machines commonly used to create the geometric shapes required by customers are vertical milling machines, horizontal milling machines, and lathes.

To successfully make a part on a CNC machine, programs instruct the machine how it should move. The programmed instructions are encoded using computer-aided-manufacturing (CAM) software in conjunction with the computer-aided-design (CAD) model provided by the customer. The CAD model is loaded into the CAM software and tool paths are created based on the required geometry of the manufactured part. After determining the tool paths, the CAM software creates machine code (G-code) that instructs the machine on how fast it should move, how fast to turn the stock and/or tool, and the location to move in a 5-axis coordinate system.

Complex cylindrical shapes can be manufactured more cost-effectively using a CNC lathe versus a 3- or 5-axis CNC milling machine. With a CNC lathe, cutting tools are stationary and the part stock is turning, whereas on a CNC mill, the tool turns and the stock is fixed. To create the geometry, the CNC computer controls the rotational speed of the stock as well as the movement and feed rates of the stationary tools required to manufacture the part. If square features need to be created on a round part, the round geometry is first created on the CNC lathe and then the square features would be made on a CNC mill.

Because the computer controls the machine movement, the X, Y, and Z axes can all move simultaneously to produce a range of features, from simple straight lines to complex geometric shapes. Some limitations do exist in CNC machining, and not all shapes and features can be created even with the advances made in tooling and CNC controls. The limitations will be discussed later.

General Tolerances

If a drawing or specification sheet has not been provided by the customer, a company may provide general specifications to follow to manufacture a model. These specifications may change from one company to another. In addition, some companies do not have default tolerances and will require the customer to provide the specifications.

Listed below are the specifications Xometry follows when a customer has not provided any.

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• Tolerance for all dimensions will be ±.005 in. for all metal parts and ±.010 in. for all plastic parts.
• The finish will be an as milled finish to a maximum 125 microinches RMS.
• If tapped holes are not added on the quote and a provided drawing, they will not be added to the part and will be machined to the diameter specified in the model.
• No surface treatment (bead blast, anodize, powder coat, etc.) will be applied unless specifically requested by the customer.
• For metal parts, walls should be a minimum 0.030 in. (~0.75 mm) thick.
• For plastic parts, walls should be a minimum of 0.060 in. (~1.5 mm) thick.

Material Selection

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.

Metals

(The cost presented will vary depending on vendor.)

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, aluminum bar stock is approximately half the price per pound of aluminum plate, and aluminum bar stock can be two to three times the cost of bar stock. Cost for 304 stainless steel is about two to three times that of aluminum, and about twice as much as 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 aluminum to minimize the material expense.

Plastics

(The cost presented will vary depending on vendor.)

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 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.

Five-Axis Machining

Five-axis machining capabilities allow for more complex parts to be manufactured in the most cost-effective manner. Five-axis machining means that the machine and the part can be moved in up to five ways simultaneously around multiple axes. The coordinated movement allows for very complex parts to be manufactured more efficiently because it minimizes setups, attains faster cutting speeds, generates more efficient tool paths, and achieves better surface finishes.

By using five-axis technology versus conventional three-axis machining, fewer setups are required to create a part with complex geometry. With three-axis milling, contoured parts, or parts with machining on several faces require multiple setups to create the geometry. Oftentimes, with three-axis machining, complex fixtures must be made in order to hold a part in the orientation necessary to create the feature. Five-axis machining eliminates the need, and thus cost, of creating the fixtures, because the part can be held once and rotated to create complex geometries.