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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 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 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.
What It Is | Ra | Rz |
---|---|---|
Measures | Average of all bumps | Biggest bumps and dips |
Good For | Overall smoothness | Finding bad spots |
Number Size | Smaller numbers | Bigger numbers |
Used For | Regular parts | Parts that must not fail |
Use Ra when:
Ra is good for general use. Most people use Ra because it’s easy to measure and understand.
Use Rz when:
Surface grinding services can help you get the right Rz for critical parts.
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.
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:
It’s better to measure both Ra and Rz when you need to know both.
Here are some real problems that happened:
Medical CNC machining requires very precise surface finishes for safe medical devices.
Different places use different rules:
These standards don’t all measure Rz the same way! This causes big problems when working with companies in other countries.
Different jobs need different measures:
Industry | What They Use | Why |
---|---|---|
Aerospace | Rz | Parts must not break |
Medical | Ra | Smooth surfaces keep germs away |
Automotive | Both | Ra for seals, Rz for engine parts |
Tech | Rz | For perfect fits in small devices |
You can use these tools:
Ra and Rz aren’t the only ways to measure surfaces. There are more:
These help you know even more about your surface.
If you design parts:
If you check parts:
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.
An aerospace company used both Ra and Rz to check turbine blades. This helped them make parts that last much longer than before.
A medical company only checked Ra on implants. FDA tests found rough spots (high Rz) that could cause problems in patients.
A: For most parts, Ra works fine. For critical parts like in planes or medical devices, use both.
A: For most CNC parts machining, Ra is enough. If the part must not fail, check Rz too.
A: Yes, if you work in making parts, you should know both. They tell you different things about your surface.
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.
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.