The Complete Process of Mechanical Design: A Must-Read for Both Beginners and Experts!

General
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  1. Formulating Design Ideas and Refining Overall Strategies

The process of mechanical design is often a process of refining from the whole to the parts. By “whole,” we refer to a general, global perspective. For instance, when taking on a project (designing a machine with a specific function), the first step is to form a general, vague design concept in the mind (at this stage, global factors must be considered). For example, considerations such as the material of the workpieces the customer needs to process (including various physical properties like hardness, strength, yield point, wear resistance, toughness, specific heat, density, etc.), and shape (whether it is a plate, profile, casting, or forging). Additionally, the approximate dimensions of the machine tool to be designed must be considered, including total height, total length, and total width (deciding whether the machine should be shipped as bulk or as a complete unit, based on transportation conditions such as vehicles and roadways).

While considering these fundamental directional elements, it is also essential to consider the initial approach to achieving the machine tool’s functionality. This involves deciding on the process or method for forming the machine, which can be divided into three types based on whether material is removed, added, or left unchanged during the manufacturing process.

Non-material-removal methods include: Casting, forging, extrusion, cold rolling, bending, rolling, coiling, pipe bending, and spinning.

Material-removal methods include: Turning, milling, drilling, planing, grinding, honing, sawing, broaching, punching, shearing, laser cutting, water cutting, flame cutting, plasma cutting, and electrical discharge machining (EDM).

Material-addition methods include: Welding, layered contour processing (3D printing).

From these numerous forming methods, selecting the most appropriate one to establish the basic theoretical framework for the machine tool to be designed is critical.

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  1. Determining the Preliminary Structure of the Machine Tool

Based on the above content, a preliminary layout scheme for the machine tool structure is determined. For example, if the material that the machine designed is to process is a long-profiled material like an I-beam, factors such as convenience and safety for loading and unloading by the customer must be considered during the machining process. Additionally, whether the customer processes one single piece at a time or processes several pieces bundled together must be considered. In this situation, we might lean towards the design approach where the workpiece is fixed (stationary) during machining, while the tool and tool assembly move (similar to a laser cutting machine).

Of course, depending on the actual situation, a moving worktable (or material) design can be adopted (like a gantry milling machine). Once the general design strategy is decided, we must then consider the machine tool’s production efficiency and precision requirements.

Both of these factors must be determined in conjunction with the equipment cost. For example, we need to consider whether to use a multi-station structure, a multi-spindle structure, or whether to speed up certain operations or fast approaches. If the speed is increased, it will raise the power requirements of the drive system. However, higher speeds result in greater inertia in the mechanism, leading to a decrease in positioning accuracy. Using high-power, high-inertia motors directly translates to increased costs. At this point, we also need to balance the overall rigidity of the machine tool, center of gravity, or consider necessary structural resonance frequencies. Additionally, ergonomic requirements must be taken into account—ensuring that positions which require frequent adjustments or operations are designed for ease of use for operators.

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  1. Determining the Structure and Function of Components or Assemblies

Once the forming method and the overall preliminary structure of the machine tool are determined, the next step is to conduct preliminary planning for each part, clarifying the functions of each component and determining which mechanisms will be used to achieve those functions. A critical prerequisite for this planning is to consider the installation and disassembly of the components. For example, for easily damaged parts, consumables, or devices that require frequent calibration, it is essential to consider the convenience of disassembly and maintenance. If replacing a triangular belt requires extensive disassembly akin to “butchering a cow,” then that cannot be considered a good design.

For instance, when we decide to use a common linear feeding mechanism, we must consider the following aspects: clarifying a specific implementation form based on actual efficiency and precision requirements. Common forms include guide rails and lead screws. Whether to use a sliding friction lead screw or a rolling friction lead screw should be determined based on the actual situation. Factors such as transmission efficiency, positioning accuracy, dynamic responsiveness, load conditions, speed characteristics, thread pitch angle, axial load capacity of the screw, lead, and cost must all be comprehensively considered. For intermittent motion mechanisms, before motor control technology was perfected, achieving such mechanisms involved a wide variety of designs (such as slot wheels or non-circular gears).

In my view, the challenge of mechanical design lies in simplifying complex problems. Some designs become overly intricate, featuring mechanisms like cylindrical or disc cam systems, enigmatic spatial mechanisms, endlessly variable four-bar linkages, and gear mechanisms that pose manufacturing challenges.

I do not support designs that intentionally aim to confuse the average user, particularly when simpler, more effective solutions can be achieved through one-dimensional linear motion, two-dimensional planar interpolation, or three-axis linkage. Using servo motors and linear rolling friction-driven guiding devices can perfectly meet functional requirements that necessitate intermittent motion, acceleration, extended travel, and specialized motion curves. Why resort to complex, difficult-to-manufacture, non-standard mechanisms that bewilder maintenance and assembly personnel and often rely on sliding friction? Some might argue that a two-dimensional linear interpolation motion platform overlooks the true essence of mechanical design left by our predecessors.

As a somewhat ironic joke illustrates: a new recruit, struggling with wilderness survival training and starving after hours of attempting to drill wood to start a fire, eventually gives up and pulls out a lighter and cigarettes. Sitting on a rock to catch his breath while pondering the ancient technique of fire-starting… Of course, some necessary mechanisms cannot entirely conform to modern fast-food design culture. For instance, using eccentric devices to achieve vibration characteristics or employing quick-locking mechanisms on fixtures, or relying on overrunning clutches for reverse failure scenarios, cannot always be simplified.

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4 Part Design
First, consider the production batch of the part. When producing a part in large quantities, it is essential to fully consider the positioning reference and process holes, which might seem like “excessive” factors but are necessary for fixture alignment.

Second, take into account the characteristics of the part’s forming method. For example, in the case of a high-speed rotating disc-shaped part, if it is cast, inevitable defects like porosity or loose structures may arise. Without post-machining dynamic balancing, these defects will lead to excessive centrifugal forces during high-speed rotation, which can indirectly cause bearing overheating, noise, and reduced lifespan. Additionally, when cutting medium-carbon steel or high-carbon steel raw materials via gas cutting to obtain a part blank, it is easy to have the cutting area “accidentally” hardened.

Third, consider the material properties of the part. For instance, when dealing with a thin plate-like part, one may consider using steel plates as material. Of course, the material cutting methods can include laser cutting, water cutting, flame cutting, or even wire cutting or stamping. The best economic approach must be combined with these methods. Whether it’s cutting plates or bars, whether using saw cutting, flame cutting, laser, or water cutting, material utilization (optimization of material yield) should always be a priority.

Fourth, think about the fixture for machining the part. For small-batch or single-piece production, try to avoid situations where special fixtures are required that cannot be processed on general-purpose machine tools.

Fifth, consider the tools used to manufacture the part. When designing a part, it’s important to fully consider the availability of tools on the market that are suitable for the machine tools in use. Avoid custom-made non-standard tools whenever possible. This requires some basic knowledge of national tool standards. For example, if a hole is designed on the part that requires a high degree of cylindrical tolerance and surface finish, the standard process might be drilling or turning the hole, followed by reaming or boring, and then internal grinding. If the diameter of the hole is not close to the standard tool sizes (like series 1 or series 2 tools), it can be very difficult to find suitable drill bits or reamers in the market.

Sixth, consider the machining range of the machine tools. Machine tools have fixed parameters for their processing capabilities. For example, C6132 indicates a horizontal lathe with a maximum turning diameter of 320 mm. M7130*1000 represents a surface grinder with a width of 300 mm and a length of 1000 mm for grinding in a single clamping. Therefore, when designing a part, it is important to consider whether the part can be held or processed based on the machine tool’s parameters.

Seventh, think about the measuring tools. Once the part is manufactured, it must be able to be measured using standard, general-purpose gauges. For example, if designing a conical fit, if you don’t choose a standard Morse taper series or common tapers like 7:24 or 1:50, and instead opt for something like 7:23 or 1:47, it will be difficult to find a corresponding standard taper plug or socket in the market. Of course, if the part is inspected using two-dimensional projection or three-coordinate measuring machines, these limitations would not apply.

Eighth, consider heat treatment requirements. For some newcomers, when dealing with parts that require high wear resistance or good overall mechanical properties, technical requirements like “Material Q235—after quenching, hardness must reach HRC60, with full hardness and uniformity,” or “Material 45# steel—after tempering, it must reach 58 HRC,” often appear on drawings. For such cases, it is recommended to study more about metallurgy and heat treatment knowledge.

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  1. Issues Related to Engineering Drawings

Given the reality of many domestic “shanzhai” (makeshift) factories, the standardization and normalization of design drawings become a somewhat contradictory issue. In a properly organized and scientifically managed mechanical equipment manufacturing plant, the technical department should ideally have dedicated personnel for drafting, drawing review, process planning, tooling and fixture production, as well as specialists such as electrical engineers, hydraulic and pneumatic engineers, or programmers. However, under the influence of the “small-factory mentality,” the staff is often reduced to just one or two people. These individuals may be responsible for all the tasks mentioned above, as well as tasks like preparing equipment manuals, updating product catalogs, and compiling tender documents, among other technical duties.

Here, we will only discuss the practical issues concerning drawings in the context of this multi-functional one or two-person situation (this may not apply to more organized, standardized companies where responsibilities are clearly divided).

First, as a designer, while remaining humble and eager to learn, one must also have independent judgment and opinions. Many people, especially designers, will have encountered the following situation: the boss says one thing, the workshop manager says something else, the production manager has a different opinion, and the customer adds their “reasonable” requirement. If these various opinions and suggestions are not handled well, the drawing file may evolve from “Design Version 1” to “Design Version 11”… If designers do not handle these situations well, it often leads to habitual thinking and dependence. Over time, one may find themselves stuck in a never-ending loop. The boss, workshop manager, and production manager may think of you as someone who is just “pretending to hunt” but only holding a dead rat, while you feel like you’ve lost your design freedom. You are constrained, limited by constant instructions, and trapped in a cycle of endless revisions and late-stage corrections.

Second, designers must know not only what to include in the drawings but also why each element is there. Many novice designers produce “clean” and “neat” drawings that lack roughness markings, shape and position tolerances, technical notes, consistent line thickness, missing or redundant dimensions, closed dimension chains, and unclear processing or measuring references. As you gain more experience, you might start to notice that the tolerances and shape/position requirements on the drawings become incomprehensible or even overwhelming.

For example, when designing a shaft (especially in the case of a bearing arrangement), you must understand that the reference for the grinding process at the end of the shaft is the center hole of both ends, while when using a dial indicator to measure the runout on a segment of the shaft after assembly, the reference is the centerline between the bearing positions A and B. Therefore, do you think it’s appropriate to arbitrarily mark the runout tolerance on any segment of the shaft on the drawing?

Moreover, if two parts designed for a flange connection cannot align as expected, don’t immediately blame the machining or process personnel. Instead, review the drawing you issued. Have you provided a proper assembly stop? Did you miss the “fit” technical requirement? Have you forgotten to include the hole location tolerance?

In such cases, the designer’s role extends beyond simply producing “neat” drawings. You need to ensure all critical design aspects are considered and clearly indicated on the drawings. It’s about understanding how the components are going to be assembled and processed, and how measurements and tolerances are tied into these processes.

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  1. The Soul of Design — Calculations and Verification

Designing a mechanical system involves more than just sketching and creating blueprints. It requires a deep understanding of the underlying calculations and logic that dictate how the system functions and how it can be manufactured efficiently. Here are some key points regarding the calculation and verification process in mechanical design:

(1) Support for PLCs, CNC Systems, and Motion Control Cards

When designing systems such as robotic arms or automated machines, we often need to consider algorithms for motion control. A common example would be solving the algorithm for an N-axis robotic arm that moves its joints in various planes, such as shoulder joints moving in a plane, elbow joints rotating, wrist joints rotating, and finger joints moving. Each of these movements must adhere to specific trajectory equations. These calculations are primarily mathematical and do not involve physical phenomena directly. Understanding how to program these control systems, ensuring they follow accurate motion paths, and integrating them into the larger system are essential parts of the design process.

(2) Calculations Tied to Physical Phenomena

This involves the application of principles from mechanics, such as statics, material mechanics, elasticity, and fluid mechanics.

  • Statics: The study of forces in equilibrium, crucial for determining how loads are distributed across a structure.
  • Material Mechanics: It addresses how materials deform under various loads, including stress, strain, and strain energy. It helps in selecting materials that can withstand specific forces without failure.
  • Elasticity: Deals with the reversible deformation of materials when subject to stress.
  • Fluid Mechanics: Relevant if the system involves hydraulic or pneumatic components.

For any part in the system, you need to first understand the tasks it needs to accomplish and its load-bearing requirements. After determining the part’s function, shape, and movement constraints, you’ll calculate its stress, material properties, rotational speeds, thermal deformations, and lifespan. This way, you can define the dimensions and configurations of each component appropriately.

(3) Calculations for Manufacturing and Assembly

These are practical calculations for optimizing the manufacturing process. For example:

  • Sheet Metal Cutting: You need to perform calculations to optimize the arrangement of parts on a metal sheet, ensuring minimal waste.
  • Machining Parameters: Different types of machining (milling, turning, grinding, etc.) have varying parameters such as cutting speed, feed rate, depth of cut, and tool wear. These need to be carefully calculated to ensure efficiency and part quality.
  • Processing Time: For each manufacturing step, you must calculate how long it will take to produce a part based on the material, tooling, machine, and part complexity.

The analogy can be drawn to martial arts—without mastering the internal skills, no matter how strong or powerful the external techniques, they will never reach the level of mastery. In mechanical design, both theoretical knowledge and practical calculation skills are essential for truly successful designs.

Reverse Engineering for Those Lacking in Logical Thinking

For those who may not have strong logical reasoning skills, reverse engineering can help. Modern software tools and sensor technology allow us to bypass some of the traditional steps of calculation and verification. For instance, if you want to determine the torque output of a shaft at different speeds, you can use a torque meter to directly measure the values at various speeds, bypassing complex theoretical calculations involving motor power factor, friction, and acceleration forces.

While this may seem like a shortcut, it can yield more precise and practical results in certain cases where sensor data can be directly measured. But the key is that you still need to understand basic physical concepts to correctly interpret the results.

CAE Analysis Challenges

I have also experimented with Computer-Aided Engineering (CAE) analysis in the past. However, I found that CAE is highly sensitive to material property data, grid division, and model creation standards. Moreover, the placement of constraints and loads can drastically affect the accuracy of results. Unfortunately, because of uncertainties in verifying the results with both theoretical and practical tests, and due to time constraints and my own understanding of the subject, I have not been able to delve deeply into CAE.

In summary, calculations and verification are crucial for the design process. Whether through direct mathematical algorithms, applying mechanical principles, or utilizing modern sensor technologies, having a solid grasp on these methods ensures that designs are both feasible and efficient in real-world applications.