Let Istar help you get started on your project with our experience and know-how!

Upload your design files and production requirements and we will get back to you within 30 minutes!

Rz vs Ra Roughness: What You Need to Know

What is Surface Roughness?

Surface roughness is how bumpy or smooth a part is. When we make things in a shop, we need to check how smooth they are. Two big ways to do this are Ra and Rz. Let’s learn what they mean.

Ra: The Average Roughness

Ra is the arithmetic mean deviation. This is a fancy way to say it’s the average of all the bumps and dips on a surface.

Ra looks at the whole surface and gives you one number. It tells you how rough a part is overall.

Ra is like looking at a beach and saying how bumpy the sand is on average.

Rz: The Maximum Height

Rz is the maximum height roughness. It measures the biggest bumps and deepest valleys on your part.

Rz finds the 5 highest peaks and 5 lowest valleys and figures out their average. This tells you about the extreme spots on your surface.

Rz is like looking at a beach and measuring just the tallest sand castle and the deepest hole.

CNC machining services can help you get the right surface roughness for your parts.

Surface roughness Ra measurement profile visualization

Key Differences in a Simple Table

What It IsRaRz
MeasuresAverage of all bumpsBiggest bumps and dips
Good ForOverall smoothnessFinding bad spots
Number SizeSmaller numbersBigger numbers
Used ForRegular partsParts that must not fail

When to Use Ra

Use Ra when:

  • You need to check overall quality
  • You make lots of the same part
  • You need parts that work well together
  • You make seals or bearings
  • You need to follow ISO 4287 standards

Ra is good for general use. Most people use Ra because it’s easy to measure and understand.

When to Use Rz

Use Rz when:

  • You need to find bad spots
  • You make parts that must not break (like aircraft parts)
  • You want to check for tool problems
  • You need to meet JIS B 0601 rules
  • You make parts where the worst spot matters a lot

Surface grinding services can help you get the right Rz for critical parts.

The Math Behind Ra and Rz

Ra Formula: It adds up all the bumps and dips, takes their absolute value (makes them all positive), and finds the average.

Rz Formula: It finds the 5 highest peaks and 5 lowest valleys, measures the distance between them, and takes the average.

Common Mistakes People Make

  1. Thinking Ra and Rz are the same thing
  2. Using the wrong standard (ISO vs JIS)
  3. Trying to change Ra to Rz with a simple formula
  4. Using old ways to measure (RMS instead of Ra)
  5. Not checking the right length of surface

Ra to Rz: Can You Convert?

Many people try to convert between Ra and Rz. They say things like “Rz is about 7 times Ra.”

Be careful! This is not always true. The ratio changes based on:

  • What you’re making the part from
  • How you make the part
  • What tools you use

It’s better to measure both Ra and Rz when you need to know both.

What Happens When You Get It Wrong

Here are some real problems that happened:

  1. A car company used the wrong Rz measurement. Their piston rings wore out too fast.
  2. An aerospace company mixed up Ra and Rz. Their parts failed tests and cost them lots of money.
  3. A medical device was rejected because they only checked Ra but not Rz.

Medical CNC machining requires very precise surface finishes for safe medical devices.

Standards You Should Know

Different places use different rules:

  • ISO 4287: Used in most places globally
  • JIS B 0601: Used in Japan
  • ASME B46.1: Used in the United States
  • DIN 4768: Used in Germany

These standards don’t all measure Rz the same way! This causes big problems when working with companies in other countries.

Ra vs Rz by Industry

Different jobs need different measures:

IndustryWhat They UseWhy
AerospaceRzParts must not break
MedicalRaSmooth surfaces keep germs away
AutomotiveBothRa for seals, Rz for engine parts
TechRzFor perfect fits in small devices

How to Measure Ra and Rz

You can use these tools:

  • Profilometer: A tool that drags a tiny needle across the surface
  • Optical systems: Uses light to check the surface
  • Reference specimens: Samples you can feel to compare

Surface Texture Parameters Beyond Ra and Rz

Ra and Rz aren’t the only ways to measure surfaces. There are more:

  1. Rq: Root mean square roughness
  2. Rsk: Skewness (if peaks or valleys are more common)
  3. Rku: Kurtosis (how pointy the peaks are)
  4. Rp: Maximum peak height
  5. Rv: Maximum valley depth

These help you know even more about your surface.

CNC machining creating surface texture patterns

Tips for Designers

If you design parts:

  1. Say which standard you’re using (ISO, JIS, etc.)
  2. Use both Ra and Rz for important parts
  3. Know that different making methods give different surface textures
  4. Put surface finish symbols on your drawings

Tips for Quality Control

If you check parts:

  1. Make sure your tools are set up right
  2. Check your tools with test pieces
  3. Measure in many spots, not just one
  4. Know which standard you should use

Real Case Studies

Case 1: Automotive Failure

A company made engine parts using only Ra checks. The parts passed Ra tests but failed in real use. When they checked Rz, they found big peaks that caused wear problems.

Case 2: Aerospace Success

An aerospace company used both Ra and Rz to check turbine blades. This helped them make parts that last much longer than before.

Case 3: Medical Device Problem

A medical company only checked Ra on implants. FDA tests found rough spots (high Rz) that could cause problems in patients.

Questions People Often Ask

Q: Can I use just Ra or just Rz?

A: For most parts, Ra works fine. For critical parts like in planes or medical devices, use both.

Q: Which is better for CNC machining?

A: For most CNC parts machining, Ra is enough. If the part must not fail, check Rz too.

Q: Do I need to know both?

A: Yes, if you work in making parts, you should know both. They tell you different things about your surface.

Q: How smooth is “smooth enough”?

A: It depends on what the part does. A bearing needs Ra around 0.2 µm. A regular part might be fine with Ra of 3.2 µm.

Key Points to Remember

  1. Ra measures average roughness
  2. Rz measures extreme roughness
  3. You often need both for important parts
  4. Different standards measure Rz differently
  5. Don’t try to convert between them using a simple rule
  6. Use a profilometer to measure correctly
  7. Check which standard you should follow

Conclusion

Ra and Rz both tell you important things about your surfaces. Ra gives you the big picture of how smooth a part is. Rz shows you the worst spots that might cause problems.

For most parts, checking Ra is enough. For parts that must not fail, check both Ra and Rz.

Remember: Ra for averages, Rz for extremes – and know what standard you’re using!

The next time you need parts with the right surface finish, make sure to specify both Ra and Rz correctly.


Looking for precision parts with perfect surface finish? Precision CNC machining can deliver exactly what you need.

分享你的喜爱
Cheney
Cheney

A dedicated Senior Application Engineer at Istar Machining
with a strong passion for precision manufacturing. He holds a background in Mechanical Engineering and possesses extensive hands-on CNC experience. At Istar Machining, Cheney focuses on optimizing machining processes and applying innovative techniques to achieve high-quality results.

New Product Brochure

Please enter your email address below and we will send you the latest brochure!