Understanding Machinability: Measurements & Key Factors

Let us begin with the primary question to answer: what is machinability? In simple words, machinability is the ease with which a material can be cut (machined) to achieve the desired part quality. Part quality over here refers to characteristics like dimensional accuracy, tolerances, and surface finish.

Materials with high machinability generally take less time and power to machine, cause less tool wear, and have better surface quality. Understandably, from a production perspective, materials with high machinability are always preferable. However, this may not always match the designer’s perspective, who looks for high strength, performance, and thermal stability, which is not always the case with easy-to-machine materials.

This creates an interesting tradeoff between different engineering requirements, which we will talk more about in this article.

Factors to Affect Machinability

Numerous factors come into play when discussing the machinability of materials. These come from the material’s core properties, its post-production treatments, and cutting conditions.

1. Material Properties

The most important set of characteristics affecting machinability are material properties. With each material having a unique set of properties, engineers must understand the impact each property has on machinability to be able to make informed decisions.

1.1 Hardness

Hardness is a key factor in determining the machinability of materials since it defines how hard it is to ‘cut’ the surface. Since the machining tool mostly interacts with the workpiece surface, hardness is an important characteristic of machinability.

Generally, harder materials like Inconel require more power to cut as the tool needs to exert a higher force. Moreover, tools wear more quick when machining hard materials. In short, high hardness means low machinability.

1.2 Toughness

Toughness is another critical parameter in determining machinability. Materials with high toughness like high-carbon steels are good at absorbing cutting forces and resisting deformation, requiring higher cutting forces and more durable tooling.

Additionally, tough materials produce long, stringy chips due to their high ductility. While this is good for maintaining smooth cutting action and effective heat transfer, long chips frequently get entangled with the tool, causing cutting delays and surface wear to the workpiece.

1.3 Thermal Conductivity

Machining processes generate heat due to the shearing of the material. Therefore, thermal management at the cutting interface is very important for effective cutting processes. In terms of heat transfer, this heavily depends on the thermal conductivity of the material.

Hard-to-cut materials generally have low thermal conductivity, which means that the heat energy generated at the cutting interface does not dissipate away quickly. This causes several negative effects like workpiece and tool thermal softening, lower tool life, and dimensional accuracy. A classic example of such a material is Titanium, which has all these issues.

Low thermal conductivity also prevents the usage of high cutting speeds and feeds, as the heat generated does not effectively transfer away.

2. Cutting Conditions

Machinability is all about how a material behaves when cut. Therefore, apart from the material properties, the cutting conditions also affect the machinability of materials.

2.1 Cutting Parameters

The primary cutting parameters in machining are cutting speed, feed, and depth of cut. Optimizing all three of these is beneficial from a production perspective as it results in a higher material removal rate. However, this is not always possible.

Higher cutting speeds generally make materials less machinable due to excessive heat generation and friction causing tool wear. It does, however, improve the surface finish in most cases. At the same time, increasing the cutting feed results in higher chip loads and cutting forces. This can damage the tool and cause excessive vibrations.

The depth of cut is also positively linked to cutting forces, power consumption, and heat generation. These are impacts to the tool and workpiece. Moreover, a higher depth of cut also negatively impacts surface integrity by creating mechanical and thermal stresses.

Additionally, the depth of cut is also a primary contributor to the dynamic stability of cutting processes. Increasing it beyond a certain limit can cause chatter vibrations, which are harmful to tool and machine.

2.2 Cutting Tool

CNC cutters have intricate geometric features that significantly impact machinability. The most evident example is that of the rake angle (angle of the cutting edge). A negative rake angle reduces cutting loads and improves chip formation, which are signs of high machinability. However, it also makes the tool weaker.

Similarly, another factor is the clearance angle, which impacts machinability indicators like tool wear and heat dissipation.

2.3 Cooling and Lubrication

Machinists often apply coolants and lubricants to the tool-workpiece interface to enhance the machinability of materials. These enhance heat removal and frictional properties of the material, leading to smoother cutting action, better surface finish, and a higher tool life.

2.4 Machine Tool Condition

The condition of the CNC machine is another factor that determines machinability. Older machines generally have play in their axis drives and vibrate more under dynamic cutting loads. This makes machining difficult, making the machine incapable of handling hard-to-cut materials.

What is the Machinability Rating?

With a large variety of machinable materials in an engineer’s toolbox, it can be tricky to compare them in terms of machinability. One of the popular methods to gauge the machinability of materials is through their machinability rating.

A standard feature of machinability ratings is to have a reference material for convenient comparison. For example, one of the standard materials is C36000 Brass with a machinability rating of 100%. As materials get harder to cut, their respective rating decreases. For instance, AISI 1018 has a rating of 70%, indicating an average machinability.

Why A Machinability Chart is Important?

Generally, machinability ratings are documented in a machinability chart, found in every CNC machine shop. With an easy-to-navigate machinability chart in hand, it is quick and easy to compare machinability across the entire spectrum of engineering materials.

The main aim of this rating system is to support engineering decisions. For a design engineer, it offers help in understanding any production complications for a particular material they are choosing. This is helpful in practical situations.

For example, if they are choosing a hard-to-cut material, they may indicate it in the engineering drawing via a special note or specifically include a surface finish requirement to ensure the machinist fully understands the design intent. For a machinist, it helps with selecting tools, cutting parameters, and lubrication/cooling conditions.

Different Methods to Improve Machinability

Machine shops adopt several strategies to make materials more machinable. This carries several benefits like more productive machining, lower costs, and overall higher quality of products.

Heat Treatment

The dependence of material properties on machinability was covered in detail in the previous sections. Therefore, when we are talking about improving machinability, altering material properties is the foremost idea in the list of actions.

Heat treatment is an effective method to enhance the machinability of materials. For example, common CNC materials like steel and aluminum are oftentimes annealed to reduce their hardness, refine grain structure, and relieve internal stresses.

Material Additives

The use of material additives is another method for improving machinability. The core theme is to incorporate material additives in the lattice structure of the base material to make its mechanical properties machining-friendly.

For instance, the addition of zinc to form copper alloys like brass drastically enhances the machinability of pure copper, allowing for lower forces, friction, and better chip formation. In fact, many machinability rating standards use the zinc-carrying copper alloy C36000 as a reference material due to its high machinability.

Coolant/Lubricant

Optimizing cutting conditions, particularly the application of coolants/lubricants, can remarkably improve machinability. The usage of such agents makes the tribological properties at the tool-workpiece interface, making the workpiece material easier to cut.

Lubricants decrease friction and the consequent heat generation, decreasing factors like tool wear and heat-induced stresses. Additionally, it also allows machinists to use more aggressive cutting parameters, leading to a higher material removal rate.

Coolants enhance the heat dissipation properties at the cutting interface. With more heat being efficiently transferred away from the cutting zone, there are lower thermal stresses, dimensional inaccuracies, and tool breakages.

Cutting Parameter Optimization

Finally, an informed selection of cutting parameters can also positively influence the machinability of materials. Most of the time, the equation is simple. Higher speeds, feeds, and depth of cuts decrease machinability, and vice versa.

There are, however, some counterintuitive cases as well, which require manufacturers to have a strong understanding of the basic principles of metal cutting. The case of built-up edge, for instance. If machinists observe a high rate of material adhesion on their tool inserts, which is bad for tool life, increasing the cutting speed or feed a little bit can be beneficial in terms of less build-up edge and tool wear.

How is Machinability Measured?

While there is no standard way of calculating a material’s machinability, there are a few generally accepted systems. Most of these rely on two major components: having a set of criteria to estimate the machinability of materials and a reference material to rank other materials against for convenience in comparison.

Cutting Tool Life

Cutting tool life is one of the most practical measures of machinability as it has a direct impact on productivity, quality, and finances. The principle is to rate the machinability of materials in terms of how long a cutting tool is usable on a material before it needs replacing or resharpening. This is, of course, considering all other factors like tool geometry constant.

Understandably, materials with high machinability do not cause high tool wear and thermal damage, so the tool life is long. On the other hand, hard-to-cut materials like steel quickly wear down the tool.

One of the methods to mathematically measure this is by using the Taylor’s tool life equation:

Here, Vc and T correspond to cutting speed and tool life, respectively. The other parameters relate to cutting conditions and tool material, which remain fixed for machinability analyses. Materials that allow higher cutting speeds while maintaining a similar tool life as reference material are considered more machinable.

Surface Finish

Surface finish is another common parameter to measure machinability. It is a viable parameter as any change in machinability is most of the time directly reflected in a change in the surface quality. For example, hard materials have low machinability and have a rough surface finish due to chipping and friction.

Moreover, measuring the surface finish itself is also quite convenient. Mostly, it is visibly observable to machinists. Additionally, engineers can also utilize easy-to-use surface testers to quickly map the surface finish of a machined surface.

Power Consumption

Machining consumes power due to cutting forces. Hard-to-cut materials require more force to cut. Therefore, they consume more power. For easily cut materials, it is the opposite case.

Due to this very straightforward relationship between machinability and power consumption, it is a popular measure of the machinability of materials.

Machinability Rating

The machinability rating is another way of measuring the machinability of materials. While not as scientific as the other methods, it is a very practical method that is widely utilized in machine shop environments.

Common CNC Materials and Their Machinability

The entire pool of CNC machining materials is too big to enlist. Therefore, in this section, we are sharing a representative sample of CNC materials and their relative machinability ratings to give a general overview of the machinability of materials.

Material CategoryMaterialMachinability (%)MetalsFree-Cutting Brass (C36000)100Aluminum (6061-T6)90-95Austenitic Stainless Steel (AISI 304)30-40Titanium (Grade 5, Ti-6Al-4V)20-25PlasticsPolyethylene (HDPE)90Polycarbonate80Polyvinyl Chloride (PVC)70CompositesCarbon Fiber Reinforced Polymer40-50Glass Fiber Reinforced Polymer30-40CeramicsAlumina (Aluminum Oxide)30Zirconia (Zirconium Dioxide)15Organic MaterialsSoftwood (e.g., Pine)90Hardwood (e.g., Oak)70