Machine Design #06: Aluminum Extrusion Frame vs. Welded Steel Frame for Machine Bases

In FA (factory automation) machine design, the machine base is not just the part that "holds the equipment". It is the foundation that determines the rigidity, stability, accuracy, vibration resistance, and even the maintainability of the whole system.

When you start designing a new machine, a common question is:

Should I use an aluminum extrusion frame for speed, or a welded steel frame to ensure rigidity and accuracy?

An aluminum extrusion frame has the advantages of being fast, clean, light, easy to change, and easy to assemble. A welded steel frame, by contrast, has high rigidity and suits equipment that demands accuracy and a high load capacity, but it comes with higher cost, machining time, and a higher risk when the design changes.

In practice, there are many cases where equipment vibrates, loses accuracy, or is hard to align because the base structure was not chosen appropriately. Some positions that should have used a welded frame ended up with an aluminum frame just because it is "easy to design, easy to buy, easy to assemble". Conversely, some equipment that did not require very high rigidity used a welded frame that was too heavy, needlessly increasing cost and stretching the lead time.

This article gathers the points to weigh when comparing an aluminum extrusion frame and a welded steel frame, from a practical machine-design point of view: static rigidity, dynamic rigidity, vibration, thermal expansion, machining accuracy, cost, fabrication time, and the hybrid-structure option.

1. The basis for evaluating a machine base, and the role of JIS standards

In machine-base design, you should not judge only by feel, such as "it is probably rigid enough" or "it seems to vibrate a bit". For equipment that demands accuracy, you need to turn that feel into quantitative criteria.

For example:

At a load of how many N, what is the allowable deflection in mm or µm?
When a robot or an axis moves fast, what is the allowable vibration?
Do the mounting positions of the linear guide, camera, sensor, or drive axis need a machined reference surface?
After the real load is fitted, does the deformation affect operating accuracy?

In the field of machine tools and precision equipment, standards such as JIS B 6201 are often referenced to evaluate static and dynamic rigidity. Although this standard applies mainly to machine tools, the way of thinking — evaluating by load and deformation — is still very useful for automation machine design.

For vibration, standards related to measuring and evaluating the vibration of the non-rotating parts of machinery are often used as a reference basis too. When there is a requirement from the customer — especially in the automotive, semiconductor, or export-equipment fields — it is very important to explain the choice of structure on a clear engineering basis.

The point to remember is this: the choice between an aluminum frame and a welded frame should not rest on design habit alone, but on the requirements for load, accuracy, vibration, environment, and fabrication time.

2. Young's modulus and material rigidity

When we talk about the rigidity of a machine base, the most basic physical parameter is Young's modulus, or the longitudinal modulus of elasticity.

This parameter expresses how hard it is to deform a material. The higher a material's Young's modulus, the less it deforms under the same applied force.

A typical comparison:

Material	Reference Young's modulus
SS400 steel	about 206 GPa
A6063-T5 alloy	about 69 GPa
SUS304 stainless	about 193 GPa

From this table you can see that steel has a material rigidity about 3 times that of aluminum. That is, with the same shape, the same dimensions, and the same load, aluminum deforms considerably more than steel.

However, the conclusion "aluminum is weak, so it cannot be used" is too simplistic. In structural design, bending rigidity depends not only on the material but on the product:

E × I

Where:

E is the material's Young's modulus.
I is the second moment of area of the cross section.

In other words, even though aluminum has a lower Young's modulus, if you increase the cross-sectional size or use a section with many stiffening ribs, you can still achieve relatively good rigidity.

This is why aluminum extrusion makers usually design a cross section with many internal ribs. The aim is to increase the second moment of area while keeping the advantages of being light and easy to assemble.

When choosing an aluminum extrusion, do not look only at the outer size such as 40×40 or 80×80. You also need to check:

The second moment of area in each direction.
The mass per meter.
The connection method at the corners.
The span between support legs.
The load placed on the frame.
The allowable deflection.

3. Comparing the basic properties of steel, aluminum, and stainless steel

The table below is a reference comparison of the properties commonly used in machine-base design.

Property	Unit	SS400 steel	A6063-T5 alloy	SUS304 stainless	Point to note
Young's modulus	GPa	206	69	193	Steel about 3× stiffer than aluminum
Density	g/cm³	7.85	2.70	7.93	Aluminum about 1/3 the weight of steel
Thermal expansion	×10⁻⁶/K	11.7	23.4	17.3	Aluminum expands about 2× steel
Thermal conductivity	W/m·K	50–60	200–220	16	Aluminum conducts heat faster

A few important points from this table:

If light weight is the priority, aluminum has a big advantage.
If rigidity and thermal stability are the priority, steel has the edge.
If a corrosive or hygienic environment is the priority, stainless steel can be considered.
If the equipment needs high accuracy in an environment with temperature swings, aluminum must be used very carefully.

4. Static rigidity and deflection calculation

Static rigidity is the ability to resist deformation under a stationary or slowly changing load. In an automated machine, the static load may be:

The weight of a robot.
The weight of a jig.
The weight of a workpiece.
Reaction forces during pressing, assembly, inspection, or machining.
Loads from the drive units mounted on the frame.

If static rigidity is insufficient, the machine base can deflect when the load is placed on it. The consequences are:

A deviation in the table height.
A misaligned linear guide.
A camera losing focus.
A sensor's detection position shifting.
A gripper, robot, or index table no longer at its reference position.
The machine being correct when adjusted without a load, but wrong when the real load is fitted.

With an aluminum extrusion frame, calculating the deflection of the beam is a very important step. For example, for a beam fixed at both ends with a concentrated load at the center, the maximum deflection can be referenced by the formula:

δ = (P × L³) / (192 × E × I)

Where:

δ is the deflection.
P is the load.
L is the distance between the two supports.
E is Young's modulus.
I is the second moment of area.

The notable point is that deflection is proportional to L³. That is, if the span doubles, the deflection can increase about 8 times.

This is a very common mistake when using aluminum extrusion: the frame looks large, but the span is too long, a heavy unit sits in the middle, and it deflects more than expected.

Common countermeasures include:

Increasing the aluminum extrusion size.
Using a high-rigidity profile.
Adding an intermediate support leg to reduce the span.
Reorienting the section to increase the second moment of area in the load-bearing direction.
Using a reinforced base plate.
Switching to a welded steel frame if the rigidity requirement exceeds what aluminum can do.

5. Dynamic rigidity and natural frequency

In today's automated machines, the issue is not only "can it bear the load" but also "does it vibrate when running fast".

When a robot, servo axis, slider, cylinder, or index table runs at high speed, the machine base bears a dynamic load. If the structure has a natural frequency close to the excitation frequency of the motion, resonance can occur.

Natural frequency can be understood simply through the relationship:

fn = (1 / 2π) × √(k / m)

Where:

fn is the natural frequency.
k is the rigidity.
m is the mass.

To raise the natural frequency, you need to increase the rigidity k or reduce the mass m. In real machinery, however, the problem is not as simple as "lighter is always better".

A welded steel frame has high rigidity and large mass, so it is usually more stable when a robot, a fast-moving axis, or a high-acceleration unit is mounted on it. The mass of the steel frame also acts like an "anchor" that helps reduce the effect of vibration in some cases.

An aluminum frame, by contrast, is lighter, but its bolted joints can reduce the overall rigidity. If the design is not good, the natural frequency of the whole frame can be lower than expected, causing vibration when the machine runs — especially during acceleration, deceleration, or a rapid stop.

For equipment that needs high-speed positioning, image inspection, precise measurement, or a robot mounted on the frame, you need to look carefully at dynamic rigidity rather than only checking the static load.

6. The damping characteristics of the structure

When vibration occurs, the ability to make it die out quickly is called the damping characteristic.

An interesting point is that an aluminum extrusion frame sometimes has a certain advantage in structural damping. The reason is not that the aluminum material itself damps better than steel, but that the aluminum structure is assembled with many bolts, brackets, and contact surfaces.

When vibration is transmitted into the frame, very small friction can arise at these contact surfaces. That friction converts part of the vibration energy into heat, helping the vibration die out faster.

However, the flip side is this: if the structure has the ability to "micro-slip" to damp vibration, it also means the joint is not perfectly rigid. In other words, better damping does not mean higher rigidity.

A welded steel frame is the opposite. Because the parts are welded into one continuous body, the rigidity can be very high, but without damping measures, vibration can persist longer in some cases.

Measures to increase damping for a steel frame can include:

Inserting a damping material.
Using sand damping inside a hollow frame.
Using anti-vibration feet.
Isolating the vibration source from the main base.
Increasing mass or changing the structure to avoid resonance.

For measuring machines, image-inspection machines, or precision equipment, you need to consider all three factors: rigidity, natural frequency, and damping ability.

7. The effect of thermal expansion

In precision machine design, temperature is a factor that is very easily underestimated.

As shown in the comparison table above, the thermal expansion coefficient of aluminum is about 23.4 × 10⁻⁶/K, while that of steel is about 11.7 × 10⁻⁶/K. That is, under the same temperature change, aluminum can expand nearly twice as much as steel.

For example, for a 2000 mm frame with a temperature change of 10°C:

Steel:    2000 × 11.7 × 10⁻⁶ × 10 = 0.234 mm
Aluminum: 2000 × 23.4 × 10⁻⁶ × 10 = 0.468 mm

A difference of about 0.23 mm may be negligible for a simple conveyor. But for a precision inspection machine, small-part assembly, camera inspection, or a robot placing parts in a tight position, this can be a very large error.

Thermal expansion can:

Shift the sensor position.
Shift the camera focus.
Throw off the robot teaching position.
Introduce extra stress in the linear guide.
Change the clearance or reference position of a jig.

If the equipment is placed in an environment with stable air conditioning, the thermal effect can be reduced. But if the equipment is near a heat source, an entrance, an area with uncontrolled temperature, or a clear hot/cold cycle, be careful about using an aluminum frame for the main structure.

In precision designs, you may need to:

Use a steel frame for the main base.
Design a structure that is "fixed on one side and allows expansion on the other".
Isolate the heat source from the measurement zone.
Use sensor compensation if needed.
Avoid rigidly locking parts with different expansion coefficients in multiple directions.

8. Welded steel frames and stress-relief treatment

When using a welded steel frame, you need to understand that welding is not just "joining steel tubes together".

During welding, the metal is heated locally and then cools. This process creates residual stress inside the structure. Even if the frame looks straight from the outside, unbalanced tensile and compressive forces can still exist inside.

If you take the welded frame straight to milling, drilling, or machining a reference surface without treating the stress, then when material is removed, the stress balance can be broken. The result is a frame that bends, warps, or twists during machining.

More dangerously, part of the stress can be released gradually over time after the machine has been delivered to the customer. Then the machine's accuracy can change after a few months or years.

Therefore, for a welded frame that requires accuracy, you usually need to consider stress relief (SR) treatment, or stress-relief annealing — a heat treatment to remove residual stress.

This process makes the structure more stable, but it brings along:

The cost of heat treatment.
Waiting time for the treatment.
Transport cost to a facility with a suitable furnace.
A cleaning step after heat treatment.
Managing deformation after heating.

In other words, the reliability of a welded frame does not come for free. It needs a correct fabrication process: welding, stress treatment, cleaning, machining, inspection, and coating.

9. The machining accuracy of a welded frame

A very big difference between an aluminum extrusion frame and a welded steel frame is the ability to create an accurate reference surface.

An aluminum extrusion frame is produced by extrusion. The profile itself has tolerances for straightness, twist, and dimension. When assembled into a frame with bolts, the errors of each bar, each bracket, and each mounting surface accumulate.

So by aluminum-extrusion assembly alone, it is very hard to guarantee flatness or parallelism at a high level. You can use shims to align it, but this process depends heavily on skill and on adjustment time.

A welded steel frame is different. After welding and stress treatment, the frame can be put on a large machining machine such as a planer-type milling machine or a 5-face machining machine. The surfaces for mounting the linear guide, motor bracket, robot base, or jig base can be machined into reference surfaces.

As a result, a welded frame can achieve much higher geometric accuracy than an ordinary assembled aluminum frame.

A reference comparison:

Method	Reference accuracy	Feature	Suitable application
Welded only, no machining	±0.5 to ±2.0 mm	Affected by thermal deformation	General frames, sub-bases, structures with no accuracy need
Assembled aluminum frame	±0.5 to ±1.0 mm	Extrusion and assembly error accumulate	Covers, safety fences, light lines, test equipment
Welded + machining	±0.01 to ±0.05 mm	Can create an accurate reference surface	Precision stages, robot axes, precision inspection machines

The values above are reference levels only. In real design, you need to base them on the supplier's machining capability, the frame size, the fixturing method, and the machine's tolerance requirements.

10. Cost: aluminum is not always cheap, steel is not always expensive

A common misconception is that "aluminum is expensive, steel is cheap". This is only true if you look purely at material price per kg. In machine fabrication, you need to look at the total cost:

Material.
Cutting, drilling, tapping.
Welding.
Painting.
Heat treatment.
Machining.
Assembly.
Drawing and supplier management.
The risk when the design changes.

For small equipment, test machines, prototype machines, or frames of roughly under 1 m, aluminum extrusion usually has the advantage. The reason is that you only cut and assemble — no welding, no painting, no heat treatment — and it is easy to fix when the design changes.

But for a large frame of several meters, or equipment produced in many identical units, a welded steel frame can be cheaper. If you use aluminum for a large frame, you must choose a very large profile to reach enough rigidity, which sharply raises the material cost. A steel frame, meanwhile, can place material exactly where the force is and use a welding jig to produce many units stably.

So you should not ask "which is cheaper, aluminum or steel?" The better question is:

For this size, this quantity, this accuracy, and this delivery time, which option has the most reasonable total cost?

11. Lead time and the ability to change the design

In terms of deployment speed, an aluminum extrusion frame has a very big advantage.

Aluminum extrusion makers usually keep standard profiles, brackets, nuts, plates, and accessories in stock. If you use catalog items, you can buy fast, assemble fast, and modify fast. In a design phase that is not yet finalized, this is a very big advantage.

An aluminum frame suits:

Test machines.
Prototypes.
Equipment that needs its layout changed many times.
Temporary jigs.
Safety covers.
Sub-frames.
Rush projects that need a base quickly.

A welded steel frame, by contrast, usually needs many steps:

Prepare the material.
Cut the steel tube or plate.
Tack-weld.
Main welding.
Straighten out the deformation.
Stress-relief treatment if needed.
Clean the surface.
Paint.
Machining.
Dimensional inspection.

So the lead time is usually longer, possibly several weeks depending on the size and complexity.

If the project has a clear schedule, requires high accuracy, and has few changes, the welded frame should be planned early. If you wait until the end to decide, the welded frame can become the bottleneck of the whole project.

12. Hybrid design: the right material in the right place

In many real machines, the best option is not "all aluminum" or "all steel", but a hybrid structure.

The basic idea is:

Use steel for the main force path. Use aluminum for auxiliary parts and parts that need flexible adjustment.

For example:

Area	Suitable material	Reason
Robot main base	Welded steel frame	Needs rigidity, mass, and a reference surface
Precision stage base	Welded frame + machining	Needs flatness and parallelism
High-speed axis	Welded frame or steel base	Needs vibration resistance and accuracy
Safety fence	Aluminum extrusion	Light, clean, easy to change
Sensor bracket	Aluminum extrusion	Easy to adjust the position
Cover frame	Aluminum extrusion	Easy to assemble and repair
Cable-duct support	Aluminum extrusion	Does not bear a large load
Operation stand	Aluminum or steel by load	Depends on rigidity and mobility needs

The hybrid approach lets you exploit the advantages of both:

Steel ensures rigidity, accuracy, and stability.
Aluminum ensures assembly speed, expandability, and ease of change.
The total cost can be better optimized than using one material for the whole machine.
The equipment is easier to maintain and improve after it goes into operation.

This is a very practical design direction in today's automated machines.

13. When should you choose an aluminum extrusion frame?

An aluminum extrusion frame suits the following cases:

Short lead time is the priority.
You need to build a frame quickly, within a few days to a week.
The design still changes a lot.
The equipment is at the prototype or test stage.
The load is not too large.
The accuracy requirement is not too strict.
You need a light frame that is easy to move.
You need a clean environment with little paint flaking or dust generation.
You need to easily add sensors, cameras, covers, or brackets later.

However, avoid using an aluminum extrusion for the main structure if:

The span is long.
The load is heavy.
There is a robot or a high-speed axis.
High flatness or parallelism is needed.
The ambient temperature changes a lot.
That position determines the accuracy of the whole machine.

14. When should you choose a welded steel frame?

A welded steel frame suits the following cases:

High rigidity is needed.
An accurately machined reference surface is needed.
There is a heavy robot or a high-speed axis.
Vibration resistance is needed.
The equipment is large.
A large load must be borne.
The equipment is for long-term use and requires high stability.
The design is relatively fixed.
Many identical units are produced.
The operating environment is harsher.

The point to note is that a welded frame must be designed and managed with the correct process. Simply welding steel does not make it automatically accurate. If accuracy is needed, you must account for:

Welding deformation.
Stress-relief treatment.
Machining the reference surface.
Coating.
Dimensional inspection.
The transport and installation method.

15. When should you choose a hybrid structure?

A hybrid structure suits the case where you want to balance performance, cost, and the ability to change.

Consider a hybrid if:

The main base needs to be rigid but the surrounding parts need to be easy to adjust.
The machine has a robot or precision axis, but the sensors and covers change often.
You need to reduce weight in one part while keeping rigidity in the important zone.
You want to shorten assembly time without sacrificing the core accuracy.
You want the machine to be easy to improve after the trial run.

In practice, this is often the most reasonable option for many FA machines: a welded frame for the core, aluminum extrusion for the auxiliary parts.

16. A quick selection checklist

Before deciding between an aluminum frame and a welded frame, check the following questions:

What is the largest load placed on the frame?
What is the longest span of the beam?
What is the allowable deflection?
Is there a robot, a servo axis, or a high-speed mechanism?
Is there a camera, sensor, or measuring mechanism that needs a stable position?
Is a machined reference surface required?
Is the equipment placed in an environment with temperature changes?
Might the machine need its layout changed after installation?
How much lead time is allowed?
Is the build quantity one unit or many?
Is cost more important, or is accuracy more important?
Which part is the main force path?
Which part is only a cover, bracket, or auxiliary structure?

If you cannot answer these questions clearly, the choice of frame material can easily become a decision based on feel.

Conclusion

The aluminum extrusion frame and the welded steel frame each have their own advantages. No option is always right for every piece of equipment.

The aluminum frame is strong in deployment speed, flexibility, light weight, and the ability to change. It suits prototypes, sub-frames, covers, safety fences, brackets, and equipment that does not require very high accuracy.

The welded steel frame is strong in rigidity, load capacity, vibration stability, and the ability to create an accurate reference surface after machining. It suits the main base, robot bases, precision stages, high-speed axes, and equipment that requires long-term stability.

In many modern machine designs, the optimal option is a hybrid: use a welded frame for the part that bears force and determines accuracy, and use aluminum extrusion for the auxiliary parts that need flexibility.

The most important thing is that the designer must be able to explain the reason for the choice:

Why is aluminum used here? Why must a welded frame be used there? How were rigidity, deflection, vibration, temperature, cost, and lead time weighed?

The machine base is not just the supporting frame. It is the foundation that determines the accuracy, stability, and reliability of the whole piece of equipment.

A note from MINATA

MINATA shares these notes as reference material from a practical machine-design point of view. Depending on the requirements for load, accuracy, installation environment, and budget, the aluminum-frame, welded-frame, or hybrid option needs to be considered individually for each machine.

If you would like to discuss the choice of machine-base structure, confirm drawings, compare options, or interpret Japanese–Vietnamese engineering terms during design, feel free to get in touch and we can review it together.

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