Understanding Material Properties for Better Machining Outcomes

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    During the CNC machining process, various materials, tools, and geometries interplay to create precision parts and components using sophisticated CAD/CAM software and detailed designs. However, the physical properties of a work material often determine that material’s machinability. So, it’s important to understand material properties for better machining outcomes.

    Different material characteristics, sometimes called “conditions,” can either influence a machining process individually or affect the conditions of other materials around it. Alternatively, certain materials and material combinations can increase machining efficacy or decrease part accuracy. Thus, machinists and designers must know the factors that affect material machinability.

    The 8 factors that affect material machinability

    Determining a material’s practicality for different CNC machining processes isn’t always easy. But these are the 8 most common things that can affect the machinability of a workpiece material:

    #1. Microstructure

    This refers to the grain or crystal structure of a material’s natural, etched, or polished surface as examined through a microscope. Interestingly, a singular workpiece can have multiple microstructures, so a thorough evaluation is critical.

    #2. Chemical Composition

    While a material’s composition isn’t always obvious, especially when working with an alloy, certain generalizations about the chemical make-up can serve as vital details. However, it’s important to note that such generalizations cannot be made about nonferrous alloys.

    #3. Heat Treatment

    Some materials must be put through heat treatments while in a solid-state to create desire properties like increase strength and better surface resistance. Unfortunately, not all workpieces can handle such processed, so their characteristics must be considered.

    #4. Grain Size

    Grain size is often used as a general indicator of a material’s machinability. Tiny, uniform grains are usually easier to work with. However, grain size and material harness must be coordinated for optimal machining results, although both can be altered with heat treatments.

    #5. Hardness

    A material’s tendency to resist deformation is the textbook definition of hardness, and its often measured on a scale. To determine a material’s hardness, machinists must embed indenters into the surface for an accurate reading. Thus, some workpieces cannot withstand this production process.

    #6. Yield Strength

    This is a manufacturing test that measures and compares the conditions of different metal workpieces to determine their strength and resiliency.  It defines the material’s ability to withstand pressure, and it also dictates which CNC machining processes a part can go through.

    NOTE: Yield strength is not the same as fracture strength, cracking, or breaking.

    #7. Tensile Strength

    Tensile strength is the maximum load that a material can carry before deforming, cracking, or breaking. It generally increases alongside yield strength with heat treatment. However, distinct tensile strength levels can exist for each material, so a tensile test is often needed.

    #8. Fabrication

    How a metal, alloy, or other material is fabricated affects its machinability. That’s because it defines the material’s grain size, structure, strength, and hardness. As we’ve learned, each of those factors directly affects a workpiece’s overall machinability.

    By understanding how each factor affects a material’s machinability, part designers, engineers, and CNC machinists can work more efficiently to increase productivity, streamline blueprint creation, and determine more accurate quotes. For more information about a specific material, talk to an expert at Pioneer Cuts.

    Special considerations about material machinability

    Certain physical properties of otherwise workable materials may also affect machining outcomes. Typically, only expert machinists know how to maneuver around these manufacturing limitations. That’s because complex calculations must be done to determine how each factor will play into the finished product. Here’s what’s most commonly considered:

    1. Thermal Conductivity – This is typically described in terms of heat transfer, meaning how quickly a material gets hot or cold under certain conditions. It may ultimately affect a part’s machinability, especially while testing.

    2. Thermal Expansion – This pertains to how likely a material is to increase or decrease in dimensional size when exposed to rising or lowering temperatures. In many cases, it can make or break the machining of a precision part.

    3. Modulus of Elasticity – This is usually determined through a tensile test and can change the way a workpiece interacts with moving parts, force, and pressure. It is not affected by heat treatment, though, because it’s a fixed material property.

    4. Work Hardening – This is the measure of physical hardness after cold work – changing the shape of a workpiece through forming, bending, or rolling. The internal stresses thereof may harden a material, making it more difficult to fabricate workpieces with.

    Because each of these special considerations can work independently or in conjunction with other properties. it’s crucial to consult a professional before conceptualizing your design.

    FUN FACTS: A part’s “machinability” describes how easily it can be machined, and it’s a scale that’s generally compared to 160 Brinell AISI B 1112 free machining low carbon steel. Anything less than 100% is considered significantly more difficult to manufacture than something measuring above 100%.

    How to determine a material’s machinability rating

    A material’s machinability rating is found by simultaneously considering the required cutting speed, tool life, and desired surface finish or geometries of a part. Each factor should be carefully measured and weighed against the final machining outcome for optimal accuracy. That’s why Pioneer Cuts experts are so diligent in their knowledge of various machining material properties and tooling techniques.

    The mechanical and physical characteristics of a material determine how easily any CNC machine shop can fabricate parts from a blueprint. The machinability rating is therefore based on things like grain size, chemical composition, and hardness with some properties being affected by other materials or changes during the testing process. To develop the most accurate and functional components, these factors must not be ignored.

    In summary

    Materials that can’t be machined without wearing down tools or affecting the rest of the design are not considered highly machinable. However, experts can work with a wide variety of different machinability ratings to develop functional, precision parts from unique designs. And while the extra steps may require additional machining costs.  there’s little that a Pioneer Cuts pro can’t do with an open-minded and educated team.

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