Ra vs. Rz Rugosité : Quelle est la grande différence pour votre état de surface ?

Many jobs get stopped or go wrong. Often, the trouble was a very small thing on a plan. It was a single note about surface finish. The planners wrote down one kind of roughness, but the people making the part measured a different kind. This little error cost a lot of money.

This article will help you not make that error. We will talk about Ra and Rz. These are two ways to measure the roughness of a surface. You will find out how they are different. You will know which one to use for your parts. I will explain it in a simple way. Reading this will save you time and money. It will make you better at your job.

What is Surface Roughness and Why is it Important?

When you see a piece of metal, it might look very smooth. But if you look at it under a special glass, it looks much different. The surface is not flat. It has very small hills and dips. We call these the peaks and valleys. The pattern of these peaks and valleys is called surface roughness. A surface with big peaks and valleys is a rough surface. A surface with small ones is a smooth surface.

So why is this important? This surface roughness can change many things. It can change the friction when two parts rub together. A rougher surface usually has more friction. The roughness also changes how long a part will work. A rough surface can wear out faster. It can also change how well paint sticks to the surface. This is known as adhesion. For some parts, having the right surface roughness is a very big deal. It helps them work the right way. The manufacturing process is what creates this final surface.

How Do We Find the Measurement for a Surface Finish?

So, how do we know how rough a surface is? We cannot just guess. We must measure it. We use a special tool to measure surface finish. This tool has a very sharp point, like a needle. The point moves along the surface for a certain length. This length is called the sampling length. As the point moves, it goes up and down over the peaks and valleys.

The tool saves this information. This makes a map of the surface. We call this map a roughness profile. It looks like a wavy line. This profile shows us all the small ups and downs on the surface. But a long, wavy line is not a helpful number. So, we use math. We change this roughness profile into one number. This number is the surface roughness measurement. There are many ways to figure out this number. Ra and Rz are two of the most popular ways.

What is Ra Roughness, Exactly?

Let’s talk about Ra. Ra means Roughness Average. It is the most common way to measure surface roughness. Think about it like this. Picture the roughness profile as a line of mountains. Now, picture a straight line going through the middle of the mountains. This is the mean line. Some parts of the surface are above the line (peaks). Some are below the line (valleys).

Ra is the typical distance of every spot on the surface from that middle line. It does not matter if the spot is high or low. We take the total distance and find the average. So, Ra gives you a good general idea of the surface roughness. It is a simple and helpful parameter. Because it is an average, one big peak or one deep valley does not change the Ra number very much. This can be good, but it can also be bad. It tells you the general roughness, but it might hide a large irregularity. The Ra standard is used on many manufacturing plans.

So, What is Rz Roughness?

Now let’s talk about Rz. Rz is not the same. It is not an average of the entire surface. Instead, Rz looks at the biggest and smallest spots on the surface. It wants to know about the highest peaks and the deepest valleys. This is a very different way to measure roughness. It gives you a different type of information about the surface.

There are a few ways to figure out Rz, but a popular one works this way. We look at a few sampling lengths on the surface. In each sampling length, we find the one highest peak and the one lowest valley. We measure the up-and-down distance between them. Then we find the average of these biggest distances. Another popular Rz method finds the average of the five highest peaks and the five deepest valleys over the full measurement length. This tells us the typical top roughness of the surface. Rz is very quick to notice any big scratches or bumps that Ra might not see.

Ra and Rz: What is the Big Difference in How They Measure?

The biggest difference between Ra and Rz is what they measure. Ra measures the average. Rz measures the biggest and smallest heights. Think about two roads. One road is a little bumpy all over. The other road is very smooth, but it has one giant hole. If you measure the Ra of both roads, you might get the same number. The average roughness could be the same.

But if you measure the Rz, the numbers will be very different. The Rz of the bumpy road will be low. The Rz of the road with the hole will be very high. This is because Rz is made to find that hole. It cares about the maximum peak to valley height. Ra smooths it out in the average. This is the most important thing to know. A single measurement of surface roughness can fool you if you use the wrong parameter. The surface profile shows everything, but Ra and Rz give us easy numbers to use.

When is a Good Time to Use Ra for a Metal Surface?

I get this question a lot. When should I write down Ra on my plan? It is a good question. Ra is a great pick for many normal parts. I suggest using Ra when you care about the general, overall finish of a metal surface.

Here are some good times to use Ra:

• For surfaces that are not critical: If the surface is just for looks, or a simple part, Ra is usually okay.
• For processes that are the same every time: If you use a manufacturing process like grinding that makes a very even surface, Ra gives a good idea of the finish.
• When you need to manage friction: For a surface that slides, a steady Ra can help make sure the friction can be guessed.
• As a general check on the process: Many shops use Ra to check that their machine tools are working right and making a steady surface finish.

Ra is like a quick health check. It tells you if the surface is generally okay. For many metal parts, this is all the information you need.

When is Rz a Better Choice to Measure Surface Roughness?

So, when is Ra not enough? You should use Rz when one single surface irregularity can make the part break. A single deep scratch can be a big problem in some cases. The Ra number might look fine, but that one scratch can break everything. Rz is the tool that helps you find that scratch.

Here are some times when you should pick Rz:

• For surfaces that seal: Think of a cylinder in a motor. If there is a scratch on the surface, oil can get out past the seal. Rz helps make sure there are no deep scratches.
• For parts with high pressure: A deep scratch on a tank with high pressure can make a weak spot.
• For surfaces that will be coated: A good surface is needed for paint adhesion. A deep valley can cause issues with the protective layer.
• For bearings: Any single high spot on a bearing surface can make it wear out very fast. Rz helps control the biggest irregularity that is okay.

If the worst spot on the surface is what you care about, then you need to measure Rz.

Is it Possible to Change Ra to Rz?

This is a very common question. People want an easy rule to convert Ra to Rz. I wish it was that simple. The fact is, there is no perfect way to change one to the other. You can find some charts online that give a guess. They might say that Rz is about 4 to 7 times bigger than Ra. But this is just a very rough idea.

The real ratio between Ra and Rz is based on the shape of the surface. And the shape of the surface is based on the manufacturing process. A surface made by a milling machine has a different shape than a surface made by grinding or polishing. They can have the same Ra, but their Rz numbers can be very different. My advice is simple: do not trust a conversion. If you must know the Rz of a surface, then you must measure the Rz. Trying to guess the rz to ra ratio can cause big problems.

How Does the Way a Part is Made Change the Surface Finish?

The way you machine a part makes a big difference to its surface finish. Every machine leaves its own special mark on the part. A part straight from a mold will have a very rough surface. A part cut on a lathe will have a pattern of small lines. A part that is ground and then gets polishing will be very smooth.

When we pick a manufacturing process, we are also picking the kind of surface roughness we will get. For example, polishing is a process we use to get a very low Ra and Rz. If you need a very smooth surface with a low roughness, you must pick a manufacturing process that can make it. You cannot get a shiny finish with a big, rough milling machine. Knowing how the machine changes the surface helps you design better parts and make better manufacturing plans.

Which Rule Should I Use for My Steel Parts?

Let’s put it all together. You are designing a steel part. Which roughness should you write on the plan? Ra or Rz? The answer is based on what that steel part does. First, check your company’s engineering standard. Many companies have rules for this. They may follow an ISO standard.

If there is no rule, think about how the part could break. Ask yourself this question: Is the problem likely to be the average roughness, or a single scratch?

• If the part just needs a good, general finish, and a single scratch will not cause harm, then specify Ra. It is easy, and everyone knows what it is.
• If the part is for a seal, a bearing, or a high-stress job, you should specify Rz. This will make the machine shop look carefully for those random flaws that could cause a break.

Thinking about the job of the surface will always give you the right answer. Do not just copy a finish from an old plan. Think about what you truly need to control on the surface.

Ce qu'il faut retenir

• Ra is the average roughness. It gives you a good, general idea of the surface finish. It is good for general checks.
• Rz measures the biggest differences. It looks at the typical distance between the highest peaks and deepest valleys. It is good for finding single, important flaws.
• A surface can have a good Ra but a bad Rz. A single deep scratch can be hidden in an average measurement.
• You cannot change Ra to Rz in a way you can trust. The connection between them is based on the manufacturing process. It is always better to measure the parameter you need.
• Pick the right measurement for the job. Use Ra for general surfaces. Use Rz for important surfaces like seals, where one scratch can cause a part to break.