Comparison of Aluminum frame and Can making frame in machine stand

Here, we are responsible for the configuration of machine mounts used in FA automatic machines, etc.Aluminum frames vs. can frames."notes.

I am often faced with the choice in the conceptual stage of new equipment: "Should I simply use an aluminum frame for this machine trestle, or should I take the trouble to make it a can structure?

If you choose aluminum frames, you can expect shorter delivery times and lighter weight, but there are concerns about vibration and thermal deformation due to insufficient rigidity.　On the other hand, if a canning frame is selected, reliability is assured, but it carries the risk of increased cost, longer lead time, and difficulty in design modification.

In fact, I have seen many machines at assembly sites and have frequently experienced inaccuracies due to such reasons as "aluminum frames are used for this part even though canning frames are most suitable for this part.　　I think this is why designers prioritize the ease of aluminum frames above all else, but the choice of frame structure is an important point that affects not only the machine, but also the accuracy of the machine in the long run.

This article provides an objective numerical evaluation based on JIS standards, a mechanical basis using Young's modulus and sectional secondary moment, and an in-depth explanation of stress relief annealing and machining realities that are only discussed on the manufacturing floor.

In addition, I also delve deeply into specific design methods for "hybrid structures" that compensate for the shortcomings of both.　Based on my experience in the design field and data from a thorough investigation of the shortcomings, I will note the decision criteria to derive the best option for your project, which I hope will be helpful.

Basics of Machine Trestle Design and JIS Standards

Rigidity evaluation defined by JIS standard

In Japanese industrial machinery design, in order to objectively guarantee the quality of machine mounts, it is essential to evaluate them based on official standards as well as individual empirical rules.

Of particular importance in defining the performance of machine mounts is JIS B 6201 (Machine tools - General rules for operational test methods and rigidity test methods).　　Although this standard is primarily intended for machine tools, it is also referenced as the de facto standard for evaluating static and dynamic stiffness in the design of racks for automatic machines in general, which perform high-precision positioning and machining.

JIS B 6201 provides a method for measuring the amount of displacement when a load is applied, as well as the concept regarding its allowable value.　　By referring to this standard, designers can reduce sensory concerns, such as "it looks like it might shake somehow," to quantitative specifications, such as "displacement must be kept below 0μm for a load of 0N.

For vibration, JIS B 0906 (Mechanical vibration - Measurement and evaluation of mechanical vibration in non-rotating parts) is applied.　　This is consistent with the ISO 10816 series and serves as a standard for measuring vibration velocity (mm/s RMS value), etc. of girders (non-rotating parts) and evaluating the soundness of machines.

By using these standards as the basis for design, it is possible to clearly demonstrate to customers and internal stakeholders the design rationale as to why the frame structure was selected.　　It is not uncommon for rigid designs based on JIS and ISO standards to be included in requirement specifications, especially for equipment intended for overseas markets and for the automotive and semiconductor industries, which require strict quality control.

Young's modulus and sectional secondary moment

When discussing the "strength" of mechanical girders, the most fundamental and dominant physical constants areYoung's modulus (modulus of longitudinal elasticity)　It is.　　Young's modulus is a value that expresses the inherent "resistance to deformation" of a material; the higher this value, the smaller the amount of deformation when the same force is applied.

Comparing steel (e.g. SS400) and aluminum (e.g. A6063), which are the main materials used for machine racks, the Young's modulus of steel is about 206 GPa, while aluminum is about 69 GPa.

This means, as a physical fact, that when a bar of the same shape is pulled or bent with the same force, aluminum is approximately three times more susceptible to deformation than steel.　　However, in actual design, it is a mistake to make the short-cut judgment that aluminum cannot be used because it has only one-third the rigidity of steel.　　This is because the bending rigidity of a structure is determined by the product (EI) of Young's modulus E and the cross-sectional secondary moment I, which is determined by the cross-sectional shape.

Even if aluminum has a low Young's modulus, it is possible to achieve rigidity close to that of steel by increasing the cross-sectional area of the frame or by increasing the cross-sectional secondary moment I by using a different cross-sectional shape.　　Aluminum frame manufacturers are developing cross-sectional shapes with complex internal ribs in order to maximize the cross-sectional secondary moment while maintaining light weight.　　Designers must have the skill to select the appropriate size by checking the cross-sectional secondary moment values listed in the catalog and performing comparative calculations with steel square pipes.

The following table compares the physical properties of typical structural materials.

Reference source: Prepared from JIS Handbook Iron and Steel / Nonferrous Metals (Japanese Standards Association)

Importance and calculation of static stiffness

Static stiffness refers to the resistance to deformation under static or slowly varying loads.　　In automatic machines, static load includes the dead weight of the robot itself, the weight of the workpiece, and reaction force during machining.　　If the static rigidity is insufficient, the frame deflects the moment a heavy workpiece is placed on it, causing the transfer level to shift and resulting in transfer errors, or problems directly linked to the focal distance of the inspection camera being out of focus.

especiallyCalculating beam deflection is an essential process when using aluminum frames　The first two are the following.　　If a concentrated load P is applied to the center of a beam fixed at both ends, the maximum deflection δ is expressed by the following equation

where L is the distance between the fulcrums (span).　　As can be seen from this equation, deflection increases rapidly as the cube of the span L.　　Because aluminum frames have a low Young's modulus E, this effect is more pronounced when used over long spans.　　For example, if the span is doubled, the deflection increases eightfold.

The designer must set the allowable deflection (e.g., 1/1000 of the span) in advance and work backwards to select the required cross-sectional size.　　If the standard aluminum frame is not rigid enough, measures should be taken to shorten the span L by changing to a thicker, more rigid type of frame or by adding a strut in the middle.

Securing static rigidity is a fundamental element for a machine to perform according to specifications, and it is a point that should not be easily compromised for cost reduction reasons.

Dynamic stiffness and natural frequency control

In recent years, automated machines are required to operate at high speed to shorten takt time,More important than static strength is "dynamic sway resistance," or dynamic stiffness.　　The key to managing dynamic stiffness is "natural frequency.　　Every structure has a specific frequency (natural frequency) at which it tends to vibrate, and when external vibrations (motor speed, slider reciprocation, etc.) match this frequency, a violent vibration amplification phenomenon called "resonance" occurs.

The natural frequency fn can be expressed in simplified form using the following relationship with stiffness k and mass m

This equation indicates that a higher stiffness k and lighter mass m will result in a higher natural frequency.　　Setting the natural frequency high keeps the resonance point away from the typical operating frequency band and ensures stable operation.The welded one-piece structure of the can frame has a very high rigidity k and a moderate mass m, which allows for a high degree of freedom in design and easy vibration control.

On the other hand, the aluminum frame is lightweight, so its mass m is small, but the overall rigidity k tends to decrease due to lower rigidity of bolted joints, resulting in a risk of lower natural frequency.　　A low natural frequency can cause resonance during low-speed operation of the equipment, resulting in prolonged shaking (settling time) when the equipment is stopped.　　Therefore,Canning frames are often chosen for the mounts of high-speed, high-precision positioning equipment because they are easy to design with high natural frequencies.

Differences in damping characteristics depending on structure

When vibration occurs, the ability to control the shaking as quickly as possible is called the "damping characteristic.　　Unlike stiffness and natural frequencies,Aluminum frame construction may have a unique advantage with respect to damping characteristics.　　This is not due to the properties of the material itself, but to a mechanism called "structural damping.

Aluminum frame racks are assembled with numerous brackets, bolts, and nuts. As vibration energy is input to the frame, minute sliding friction occurs on these numerous contact surfaces.This frictional action converts and dissipates vibration energy into thermal energy, resulting in a quicker vibration reduction.　　In contrast, canning frames have a perfectly continuous metallic structure due to welding, which means that internal friction is extremely low, and once struck, the vibration tends to remain as long as a bell.

However,This structural damping of the aluminum frame is also the flip side of the fact that "the joints are not completely rigid (micro-movement).　　In other words, static rigidity is sacrificed in exchange for damping performance.　　If high damping performance is desired with a canning frame, additional measures such as "sand damping," in which the inside of the frame is filled with sand or vibration-damping material, or the use of anti-vibration rubber for the installation legs, are effective.　　In selecting a mounting frame for precision measuring instruments and other equipment that do not tolerate vibration, it is necessary to take into account not only the rigidity but also the difference in these damping characteristics.

Thermal properties and manufacturing process of machine mounts

Thermal displacement caused by coefficient of thermal expansion

In precision machine design, expansion and contraction (thermal displacement) of the structure due to temperature changes is a major cause of inaccuracy.　　The magnitude of this thermal displacement is determined byCoefficient of linear expansion (coefficient of thermal expansion)　It is.　　As shown in the aforementioned comparison chart, aluminum has a linear expansion coefficient of about 23.4 × 10^-6 /K, almost twice that of steel, which is about 11.7 × 10^-6 /K.　　This means that under the same temperature change environment, aluminum girders will expand and contract twice as much as steel girders.

As a concrete example, let us calculate the amount of elongation of a 2000 mm-long base frame when there is a 10°C temperature difference between morning and afternoon.

• Steel frame 2000 × 11.7 × 10^-6 × 10 = 0.234 mm
• aluminum frame2000 × 23.4 × 10^-6 × 10 = 0.468 mm

This difference of about 0.23 mm may be tolerable for general conveyor transport, but it is a fatal error in precision inspection machines or in robotic micro-assembly.　　Thermal displacement can cause the straightness of the linear guide to deviate or the detection position of the sensor to shift.

Designers should check the HVAC control conditions of the installation environment, and if high precision is required in an environment with large temperature fluctuations, the use of aluminum frames should be avoided, or a "relief structure (fixed on one side)" should be employed to anticipate thermal deformation.Steel (canning) frames are thermally stable and have long been trusted as the base for precision equipment.

Stress relief annealing essential for can products

Stress Relief Annealing (SR treatment) is an important process that cannot be separated from welding when using can frames, i.e., structures made by welding steel materials. In welding, metals are heated locally to their melting point and joined together, resulting in intense shrinkage forces during cooling and solidification.　　This results in strong tensile stress (residual stress) trapped inside the frame, even though it appears straight.

If the surface is machined without annealing, the internal balance of forces is upset, causing the frame to warp or twist during processing.　　Furthermore, the stresses will be gradually released over several months to years after delivery, which may cause the accuracy of the equipment to deteriorate over time.

This process removes residual stress and stabilizes the microstructure. Annealing, however, is costly in terms of transportation to a partner plant with a large furnace, processing time of several days, and fuel costs.　　In addition, after annealing, the surface is covered with oxide scale, which requires cleaning processes such as shot blasting.Although canning frames are reliable, designers need to understand that there are these unseen costs and labor involved in obtaining them.

High-precision finishing by machining

One of the critical differences between aluminum and can frames is the ability to achieve final geometric tolerances (flatness, squareness, parallelism).　　Since aluminum frames are manufactured by extrusion molding, bending and twisting of a few millimeters per meter are allowed even under JIS standards.Even with a highly rigid aluminum frame, it is extremely difficult to achieve an overall flatness of 0.1 mm or less by bolt assembly alone, and adjustment requires skilled techniques using shim tape and other means.

On the other hand, the true value of canning frames is demonstrated by "machining" (machining) after welding and annealing.　　By machining the mounting surfaces of linear guides and motor brackets using large machine tools such as 5-face milling machines, high-precision reference surfaces with flatness of 0.05 mm/1000 mm and squareness of 0.02 mm can be created.

The following table compares general can manufacturing and machining accuracy guidelines.

Determine the divergence of production costs.

There is a common belief that "aluminum is expensive, but steel is cheap," but this is based only on the unit price of the material (price per kilogram).　　When compared with the total cost (material cost + processing cost + assembly cost + management cost) in actual equipment manufacturing, the superiority of cost advantages is reversed depending on the size and production volume.

Aluminum frames tend to be less expensive for small to medium-sized (up to about 1 m on a side) racks, one-of-a-kind specialized machines, and prototype machines.　　This is because aluminum frames are completed simply by cutting and assembling, requiring fewer man-hours for design and order management, and eliminating fixed costs for intermediate processes such as welding and painting.　　In addition, when changes occur, risk costs can be kept low because only parts need to be replaced.

In contrast, for large girders (several meters in length) and mass production projects where dozens of girders with the same specifications are manufactured, canning frames have an overwhelming advantage.　　Large aluminum frames require a very large cross-sectional area to ensure rigidity, which exponentially increases the cost of expensive aluminum materials.　　With canned products, inexpensive steel materials can be placed where needed and produced efficiently using welding fixtures, which can drastically reduce the unit cost per unit.　　Designers must have a sense of balance to determine the break-even point based on "size" and "quantity.

Component Procurement vs. Lead Time

In the progress of a project, delivery time (lead time) is as important as quality and cost.　　In this respect, Aluminum Frame has unparalleled speed.　　Major manufacturers (MISUMI, NIC Autotech, SUS, etc.) keep an abundance of standard components in stock, and catalog items can be obtained within a few days from the day after the order is placed.　　With in-house cutting machines and inventory, it is not a dream to assemble a trestle on the same day as the drawings are drawn.

On the other hand, it usually takes about 2 to 4 weeks from the time an order is placed to the time of delivery for canning frames.　　This is due to the need to go through a wide variety of processes, such as "material arrival, cutting, beveling, temporary assembly, main welding, distortion removal, annealing, shot blasting, painting, machining, and inspection.　　In particular, annealing and painting are often outsourced to specialized companies, which adds days to the logistics time.

Therefore,Aluminum frames are ideal for urgent projects with a short development period, or for development projects where specifications have not yet been finalized and design is done while running.　　On the other hand, if the specifications are fixed and the mass production schedule is predetermined, it is standard practice to systematically arrange for highly reliable canning frames.　　Since the difference in lead time affects the timing of the design start itself, it is important to determine the construction method in the early stages of the project.

Hybrid structural design of machine girders

Hybrid structure with the right materials in the right places

In recent years, "hybrid construction," combining aluminum and steel in the right places, has become the mainstream in order to maximize cost performance and performance.　　This is a sophisticated design approach that uses different materials for each functional unit, rather than the dualism of "all aluminum" or "all steel.

The basic philosophy of the hybrid design is that the main force paths (load paths) are made of steel, while accessories and non-structural members are made of aluminum.Specifically, canning frames will be used for "core structures" such as robot feet, precision stage bases, and high-speed transfer axis foundations.　　Here, the weight of the steel acts as an anchor to reduce vibration, and the machined surfaces guarantee geometric tolerances.

On the other hand, aluminum frames will be used for "sub-structures" such as safety fences, sensor brackets, cable duct supports, operation panel stands, and cover frameworks.Since these parts are not subject to large loads and are frequently subject to on-site installation adjustments and future addition or repositioning of cameras, the scalability of aluminum T-slots can be maximized.　　This use of T-slots makes it possible to mutually complement the disadvantages of canned products, which are heavy and cannot be machined, and aluminum, which is less rigid and more expensive.

Let me go back to the beginning,This is a very important concept because if you make a mistake here (try to finish the design easily), you will not be able to guarantee the accuracy that is so important for the machine.

How to choose the right machine frame for your purpose

There is no absolute right answer for the selection of machine mounts.　　There is only the "best compromise" for the constraints of the project (delivery date, cost, accuracy requirements, and environment).　　Summarizing the previous explanations, the following guidelines are provided to help the designer make the final decision.

Your role as a professional mechanical designer is to have a deep understanding of these characteristics and to deliberately choose a balance of risk and benefit that is consistent with the project objectives.　　Aim for a design that you can confidently explain, "We chose this structure for this reason.

summary

• Design of machine girders is based on JIS standards (B 6201, B 0906) to evaluate rigidity by numerical values
• Young's modulus of steel is about 3 times that of aluminum, but the rigidity can be compensated by devising the cross-sectional secondary moment.
• Note that deflection increases in proportion to the cube of the span in the static stiffness calculation.
• Dynamic stiffness is determined by the balance between mass and stiffness, and control of natural frequencies is the key to avoiding resonance.
• Aluminum frames are expected to provide structural damping due to friction at joints
• Thermal expansion coefficient of aluminum is twice that of steel, and thermal displacement can be fatal in precision equipment.
• Canning frames are subjected to stress relief annealing (SR) to prevent distortion and aging after processing
• Machined can products can achieve micron-level geometric tolerances
• Aluminum is more cost-effective for small, single-unit products, while steel is more cost-effective for large, mass-produced products.
• Aluminum has by far the fastest lead time, while canned products require several weeks.
• Hybrid construction is a modern solution that combines the rigidity of steel with the flexibility of aluminum
• Judgment is required to select the appropriate construction method according to the frequency of specification changes and the installation environment.
• Designers should select frames with physical backing, not just cost and delivery date
• Optimal girders design is one of the most important steps in determining overall equipment performance and reliability

That's it.