Strength, Stiffness & Hardness: CNC Machining Basics

What Are Strength, Stiffness, and Hardness?

Hi kids! Today we will learn about strength, stiffness, and hardness. These are important things that help make parts work right! At Istar Machining, we make lots of parts with our machines. We need to know about these properties to make good parts for our customers.

Strength

Strength is how much force a thing can take before it breaks.

A strong thing can hold a lot of weight
A weak thing will break easily
Yield strength tells us when a part starts to bend forever
Ultimate strength tells us when a part will break apart When we do CNC milling at Istar Machining, we pick materials with the right strength for your job.

Stiffness

Stiffness is how much a thing bends when you push on it.

A stiff thing does not bend much
A flexible thing bends a lot
We call this elastic modulus or Young’s modulus
Stiff parts keep their shape when forces push on them Our precision CNC machining makes parts with just the right stiffness for what you need.

Hardness

Hardness is how hard it is to make a dent or scratch on the surface.

A hard thing is hard to scratch
A soft thing gets dents and scratches easily
We test hardness with Rockwell, Brinell, and Vickers tests
Hard parts don’t wear down fast

How These Properties Are Different

Let’s see how these properties are different. Look at this table:

Property	What It Means	What It Does	How We Test It	Example
Strength	How much force before breaking	Stops failure when loads push	ASTM E8 test	A bridge that holds up cars
Stiffness	How much it bends under force	Keeps shape when used	Flexural modulus test	A ruler that stays straight
Hardness	How it fights dents and scratches	Stops surface damage	Rockwell test	A knife that stays sharp

Real Materials and Their Properties

Let’s look at some real materials and see how they work:

Steel (AISI 4130)

Yield Strength: 460 MPa (very strong)
Stiffness: 205 GPa (very stiff)
Hardness: 20 HRC (medium hard)
Good for: Aircraft parts where strength and stiffness matter

Titanium Grade 5

Yield Strength: 830 MPa (super strong)
Stiffness: 114 GPa (pretty stiff)
Hardness: 36 HRC (pretty hard)
Good for: Medical parts that need strength and won’t rust

Aluminum 6061-T6

Yield Strength: 276 MPa (medium strong)
Stiffness: 69 GPa (less stiff)
Hardness: 60 HB (less hard)
Good for: Light parts that don’t need to be super strong At Istar Machining, we work with all these materials to make the best parts for you.

Engineering Basics Strength, Stiffness and Hardness

Why These Properties Matter in Real Life

You might ask: “Why should I care about this?” Well, these properties help make things we use every day work right!

Cars

Strength: The frame must be strong so you stay safe in a crash
Stiffness: The body must be stiff so it doesn’t shake too much
Hardness: The engine parts must be hard so they don’t wear out When we make automotive CNC machined parts, we think about all these things.

Airplanes

Strength: Wings must be strong to not break in the air
Stiffness: Wings must be stiff but can flex a little
Hardness: Engine parts must be hard to last a long time CFRP (carbon fiber) is used for airplane wings because it has:
High stiffness (70-250 GPa)
Light weight
This helps planes use 15% less fuel!

Tools

Strength: Must be strong to not break when you use them
Stiffness: Must be stiff to work right
Hardness: Must be hard to cut other things and not get dull Tool steel (D2) has a hardness of 60 HRC, which makes cutting tools last a long time.

How We Choose Materials

At Istar Machining, we help you pick the right material with the right properties:

What forces will your part feel?
How stiff does it need to be?
Will the surface get scratched or worn?
How much can it cost?
How much can it weigh? These answers help us pick from:
Metals like steel, aluminum, or titanium
Plastics like ABS or nylon
Composites like carbon fiber Our CNC machining service can work with all these materials.

Testing These Properties

How do we know if a material has the right properties? We test them!

Strength Tests

We pull on a sample until it breaks
We use ASTM E8/E8M tests
This tells us the yield strength and ultimate strength

Stiffness Tests

We bend a sample and see how much it moves
We use ASTM D790 tests
This tells us the elastic modulus

Hardness Tests

We push a small ball or point into the surface
We use Rockwell (ASTM E18) or Vickers (ISO 6507) tests
This tells us how hard the surface is

Case Studies: Properties in Action

Let’s see how these properties help in real life:

Automotive Crumple Zones

High-strength steel (HSS) with controlled stiffness[^1]
Made car crashes 25% less likely to cause death in side hits[^2]
The steel is strong but can bend to soak up the crash energy

Aircraft Wing Spars

Carbon fiber has 2x the stiffness of aluminum (150 GPa vs 69 GPa)[^3]
This lets planes be 20% lighter
Boeing 787 uses 50% carbon fiber and uses 15% less fuel

Mining Drills

Tungsten carbide (HV 1800) lasts 3x longer than steel (HRC 55)
Saves $120,000 each year by not breaking so much
The super high hardness stops wear on the drill

Common Mistakes to Avoid

When making parts, people make these mistakes:

Thinking hardness means it won’t break (not true!)
Forgetting that stiffness and strength are different
Not thinking about how fast the force happens
Picking the wrong material for the job At Istar Machining, our experts help you avoid these mistakes.

Questions People Ask

Q: Can a material be stiff but not strong?

A: Yes! Glass is very stiff but will break easily. It has high elastic modulus but low tensile strength.

Q: Is hardness the same as strength?

A: No! Hardness is about the surface not getting dents. Strength is about not breaking when force is applied.

Q: How do heat treatments affect these properties?

A: Heat treatments can make metals more hard and strong, but sometimes less bendy. We do this by heating and cooling the metal just right.

Q: Which is more important: strength, stiffness, or hardness?

A: It depends on what you need! For a bridge, strength and stiffness matter most. For a saw blade, hardness matters most.

Key Points to Remember

Here are the big ideas to remember:

Strength stops things from breaking under load
Stiffness stops things from bending too much
Hardness stops the surface from getting damaged
Different jobs need different properties
We need to test to know the real properties
Sometimes we need to make trade-offs between properties At Istar Machining, we use our knowledge of these properties to make the best parts for you.

How We Use This at Istar Machining

At Istar Machining, we use our understanding of strength, stiffness, and hardness to:

Help you pick the right material
Make parts that work the way you need
Test the parts to make sure they’re right
Give you advice on better designs We have machines that can make parts from many materials:
Aluminum (soft but light)
Steel (strong and stiff)
Titanium (strong but expensive)
Plastics (cheap and light) Our CNC machining expertise helps make parts with just the right properties for your job.

Conclusion

Now you know about strength, stiffness, and hardness! These important properties help us make good things that work right. When you need parts made, think about what properties matter most for your job. Then talk to the experts at Istar Machining. We can help you pick the right material and make your part just right. Remember:

Strength stops breaking
Stiffness stops bending
Hardness stops scratching and wearing With the right properties, your parts will work better and last longer!

[^1]: Source: ASM International Handbook, 2019 [^2]: Source: National Transportation Safety Administration Report, 2022 [^3]: Source: Boeing Corporation Engineering Materials Specification, 2021