Design Practice · Material Selection
One of the first things that trips up new machine designers is material selection. When filling out a parts list, questions like “Should this base plate be SS400?” or “Does this shaft need S45C with heat treatment?” can take up a surprising amount of mental energy.
“Using the toughest material available is not always the right answer.”
A designer’s job is to select the optimal material for each part — balancing cost, machinability, and function. In this post, we’ll compare the three most common materials in machine design: SS400, S45C, and AL6061, and walk through how to calculate part weight using material density.
1. SS400 — “The Cheapest and Most Versatile Structural Steel”
SS400 is the most basic structural steel, commonly referred to on the shop floor as “plain steel plate.” The 400 in its name guarantees a minimum tensile strength of 400 MPa.
- Density: 7.85 / Tensile Strength: 400–510 MPa
- Characteristics: Lowest material cost, easy to source, and excellent weldability. However, its low carbon content (approx. 0.15–0.2%) means hardness cannot be improved through heat treatment.
- Typical Applications: Equipment frames, base plates, brackets, covers. If it’s a welded structural component, it’s almost certainly SS400.
2. S45C — “The Go-To for Strength, Wear Resistance, and Heat Treatment”
When a part needs to handle heavy loads or friction that SS400 can’t manage, S45C is the answer. The “45” indicates a carbon content of approximately 0.45% — more than double that of SS400 — which is what makes heat treatment possible.
- Density: 7.85 / Tensile Strength: 570–690 MPa (before heat treatment)
- Characteristics: Can undergo induction hardening or quenching & tempering (Q&T) to significantly increase surface hardness. Note: welding S45C can cause cracking — avoid using it for welded assemblies.
- Typical Applications: Drive shafts, gears, locating pins, guide rails — anywhere rotation or friction is involved.
3. AL6061-T6 — “Lightweight, Machinable, and Great for Surface Finishing”
In modern automation equipment where weight matters, aluminum is indispensable. T6 denotes a heat-treated condition (solution heat treatment + artificial aging) that maximizes the alloy’s strength. Most AL6061 stock available on the market is already T6.
- Density: 2.7 (approximately 1/3 the weight of steel) / Tensile Strength: approx. 310 MPa
- Characteristics: Raw material cost is higher than steel, but it machines quickly and easily — so overall machining costs are often lower. Does not rust, and can be anodized for increased surface hardness and a range of color finishes.
- Typical Applications: Moving plates on drive assemblies, robot arms, cleanroom equipment, external panels.
| Material | Density | Tensile Strength (MPa) | Heat Treatment | Weldability | Cost |
|---|---|---|---|---|---|
| SS400 | 7.85 | 400–510 | Not possible ✕ | Excellent ○ | Very low |
| S45C | 7.85 | 570–690 | Possible ○ | Poor ✕ | Low |
| AL6061-T6 | 2.7 | approx. 310 | Anodizing | Difficult △ | High |
| SUS304 | 7.93 | approx. 520 | Limited | Good ○ | Very high |
* SUS304 (stainless steel) is used when corrosion resistance is the top priority, but it is notoriously difficult to machine and comes with high processing costs.
4. How to Calculate Part Weight Using Density
Knowing a part’s weight is essential for motor sizing and cost estimation. When you only have a drawing, the formula below lets you calculate it directly.
※ Since engineering drawings use millimeters, convert all dimensions to centimeters before calculating to get the result directly in grams.
Example 1: Rectangular plate — 100mm × 200mm × 10mm thick
- Volume: 10cm × 20cm × 1cm = 200 cm³
- SS400: 200 × 7.85 = 1,570g (approx. 1.57 kg)
AL6061: 200 × 2.7 = 540g (approx. 0.54 kg)
Example 2: Round shaft — Ø30mm × 200mm long
- Volume: π × 1.5² × 20 = 141.3 cm³
- S45C: 141.3 × 7.85 = 1,109g (approx. 1.11 kg)
AL6061: 141.3 × 2.7 = 382g (approx. 0.38 kg)
As you can see, simply switching from steel to aluminum cuts the part weight to roughly one-third — without changing anything else. This is why switching to aluminum is always the first option to consider when you need to reduce motor load.
Wrapping Up
Good design isn’t about always using the strongest or most expensive material. It’s about placing the right material in the right place — with a clear reason based on the part’s role and load conditions. That judgment is what separates a competent designer from a great one.
In the next post, we’ll take a deeper look at types of heat treatment — induction hardening, carburizing, nitriding, and more — and when to use each one.