Understanding a CNC Machine and the Processes Used to Create Precision Parts

Nearly every industry requires precision CNC machined parts for the manufacturing of prototype and production assemblies. Engineers spend untold hours designing and developing these parts, expecting that their specifications will result in precise fabrication. With the performance demands placed on them by industries such as medical, aerospace, and the military, these parts must be built at the highest quality levels.

What isn’t always understood, however, is how these parts are fabricated in the first place. Machine shops like Plethora have a very refined workflow that starts with your initial quote request and concludes with the completed high-quality part being sent back to you. In between, CNC milling and turning machines are put through their paces to create the parts you need. Here is an in-depth look at CNC machining to give you a holistic understanding of the processes required to build your precision parts.

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The Development of CNC Machining

Whether it is aerospace, medical, robotics, or almost any other industry, there will always be a need for precision built parts. These parts may support, hold, or move other parts, and they must be fabricated to precise tolerances to accomplish the designated task. Although there are many manufacturing processes available for building these parts, the most reliable method is with CNC machining.

CNC machining cuts away material to form and create the required part in a process known as subtractive manufacturing. With this operation, rotating cutting tools either mill the material or turn the material on a lathe and cut it with a non-rotating tool. The milling and turning cutting paths are programmed with computer numerical code (CNC) instructions, based on a part’s CAD model. This gives CNC machining the ability to create more complex parts with tighter tolerances than conventional machining.

The CNC machining in use today did not spring up overnight. It resulted from the continual development of manufacturing technology over the course of many generations:

The Beginnings of Machining

Evidence has emerged of lathe-turned objects throughout history, with one of the earliest examples including a wooden bowl from the 6th century BC. In the early 1800s, a number of inventors created tools that led to the development of the modern milling machine. One of those was Eli Whitney, better known for the creation of the cotton gin, who conceived his machine to help with the mass production of weapons for the government. These early machines introduced the ability to uniformly mass-produce machined parts, and small incremental enhancements continued over the next 100 years.

The Introduction of Automation in Machining

In the early 1900s, coordinate dimensioning became popularized and set the standard for machining. Around the same time, tool mounting improvements to the head enhanced machining capabilities even further by allowing it to slide and pivot to better access the work. The introduction of servomechanisms in the 1940s yielded the ability to move the tool head and the workpiece, offering some time-saving versatility. Numerical controlled machines started appearing in the 1950s, which, although inaccurate, improved the safety of the machinists who manually moved the cutting tools and workpieces. All of these improvements paved the way for how parts are machined today.

CNC Machining Today

By the 1960s, computers not only created the calculations for the complex tool paths needed for machining precision parts but drove the servomechanisms. It was here that true CNC machining came into its own. By the 1980s, desktop computers drove the CNC mills and lathes at machine shops. The best machine shops now pair high-tech milling and turning equipment with advanced software that allows for extreme precision in the fabrication of high-quality parts. Today, CNC machining is a regular part of the manufacturing process, and we’ll look more into the equipment specifics of CNC mills and lathes next.

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CNC Machines

Today’s modern machine shop has many pieces of CNC machining equipment running continuously to meet the demand for precision parts. These parts are machined out of different metals including aluminum, steel, stainless steel, copper, brass, and bronze, as well as a variety of plastic materials. Although CNC machines can widely vary, there are two basic types: mills and lathes.

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Standard CNC Machines

The standard equipment used for CNC machining is referred to as 3-axis machines. These systems provide movement in the three-linear axes of the cutting tool in relation to the workpiece. This allows the machine to cut while moving to the left or right, back and forth, and up and down.

CNC Milling

A mill holds the part to be formed, known as the “workpiece,” in place while the drills or cutting tools remove the material around the workpiece. These tools spin at a high speed rate and move in the three axes to cut the paths that the CNC G-codes have programmed into the machine.

The 3-axis milling machine is very accurate and can create parts with tight tolerances, but it is restricted to routine part shapes due to its three axes’ limitations. The workpiece can be moved or rotated manually to create more complex shapes, but this adds time and cost to the milling process. The 3-axis mill is one of the more common CNC machines and has a lower operating cost than other more complex machines.

CNC Lathes

A lathe holds the workpiece on a spindle, which spins at a high rate, while a cutting tool moves radially and lengthwise to remove the outer material. A drill also removes material from the center of the workpiece, and together these processes create the desired part. CNC lathes fabricate more parts at a lower cost than CNC mills but are restricted to shaping cylindrical parts.

For more complex parts, the machining on a lathe has to be combined with a separate milling operation to create the required shapes. Fortunately, alternatives exist for complex parts: multi-axis CNC machines.

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Multi-Axis CNC Machines

As evidenced, 3-axis CNC milling machines and lathes create commonly shaped parts quickly and efficiently. When it comes to more complex shapes, either the part has to go through multiple processes on different machines, or a multi-axis CNC machine can serve as the alternative. Multi-axis CNC machines are standard mills and lathes enhanced with additional movement in different axes and other tools. While these advancements allow for creating a more complex shaped part, they also come with a higher price tag in both hardware and operator proficiency.

Indexed 5-Axis CNC Milling

While milling, this machine essentially operates the same way as a standard 3-axis mill in that it only moves in the same three axes. However, between milling operations, the tool-head and the bed containing the workpiece can rotate, allowing for different angles of access for the cutting tools. Due to the addition of these two other axes of movement, this mill is often referred to as a 3+2 CNC milling machine. It can machine parts with more complex shapes without manually adjusting the part between milling operations, making it faster than the standard 3-axis mill.

Continuous 5-Axis CNC Milling

This machine is the next step up from the indexed 5-axis machine and can move on all five axes during the milling operation. This allows for the fabrication of parts with complex shapes that can’t be produced accurately with other CNC machining methods. Due to the equipment’s complexity and the need for highly-skilled machinists, parts produced on these machines are usually the most expensive.

CNC Lathes with Live Tooling

This machine is a combination of a CNC lathe with CNC milling tools, allowing both operations to be performed on a part in one continual process. It functions as a 3-axis CNC lathe, but the spindle can also stop spinning and position the part at an angle for the live tooling mills to cut a path. This system is also referred to as “mill-turning,” and it can create complex cylindrical parts more efficiently than other CNC machining processes can.

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Cutting Tools

Both CNC mills and lathes use various cutting tools to remove material during the formation of a part. While the same tools can be used for both milling and turning, each process can warrant its own tools. These cutting tools often resemble a common drill bit, but many other shapes will be used depending on the required cutting path. Due to the generally cylindrical shape of a rotating tool, the internal contours of the cut path always have a radius, which restricts the kinds of shapes available for part manufacturing.

To summarize, standard CNC machining is done with 3-axis milling machines and lathes. These systems produce high-quality parts, but their three axes restrict the complexity of shapes produced. The 5-axis milling machines allow more tool access and movement for fabricating complex-shaped parts but at a greater cost. CNC lathes with live tooling give machine shops the ability to combine milling and turning operations in one continual process.

All of these machines come in different sizes and formats depending on the part that needs manufacturing. The key is to assign each job to the best-suited machine, where the management team can maximize the shop’s overall workflow efficiency. Next, we will look at the process used for running these CNC machines to produce precision parts.

The CNC Machining Process

The manufacturing of precision parts by CNC machine shops like Plethora is a very efficient process that can ship completed parts back to the customer in as little as three days. To accomplish this requires the latest software tools, advanced CNC machining equipment, and human expertise. It also requires a carefully managed workflow, which generally follows these steps:

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1. Quote Request and Part Evaluation

The traditional quote request system in a machine shop has gone through many changes over the years. What used to involve phone calls, in-person meetings, and a paper trail now happens mostly through the use of online request tools and CAD files. This system allows the customer to transfer their CAD data through an online portal to the machine shop, where advanced engineering analysis software immediately evaluates it. Upon passing this initial inspection, the online tools generate the quote for the customer and route the CAD data into the machine shop’s procurement and fabrication systems for part manufacturing.

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2. Material Preparation and Tool Selection

With the customer’s CAD data pulled into the shop’s internal systems, the preparation work for manufacturing can begin. Any specialized materials and tools not currently in stock are ordered. Meanwhile, the shop’s engineering software determines the size of the material and workholding requirements to build the part. Programmers select the necessary tools based on the results of the automated analysis. Once all of the materials, tools, and drawings are ready, they are put together in a “kit,” and delivered to the assigned mill or lathe along with the accompanying machinist.

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3. Part Manufacturing

A modern CNC machine shop like Plethora has multiple mills and lathes in continual operation with skilled machinists. These operators use advanced software tools in the shop, which enables them to double their efficiency over a standard machine shop. The machinists configure the mill or lathe with the materials and tools provided in the kit and then manufacture the customer’s part.

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4. Quality Control

The quality control team works in tandem with the machinists of the machine shop. These specialists use advanced measuring and detection equipment to ensure that the built parts follow their specified dimensions and tolerances. The quality control team also establishes the first article inspection (FAI) of a new production run of parts to set the standard against which all subsequent builds are measured. Once the completed part has passed its quality control checks, the shop ships it back to the customer.

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5. Maintenance

While not part of the actual production process, the CNC cutting tools’ maintenance is essential for ensuring the shop’s continual high manufacturing efficiency standards and quality. In addition to the skills of their technicians, machine shops like Plethora use advanced applications to monitor the life cycle of CNC machining tools. These tools are rated for a specific lifespan depending on the materials they use. The monitoring software alerts the shop about serving or replacing the tools before they wear out of tolerance or outright break.