Every printed part has tiny hills and valleys on its surface. If those bumps are too big, air leaks, gears grind, or paint peels. You need a clear way to measure the bumps so the part works the first time. Many makers compare two common scores and ask the same question: What is the real story behind RZ vs RA Roughness, and which one helps my 3-D prints most?

Success: RZ vs RA Roughness tells two sides of one story. Ra is the average height of all bumps. Rz is the height from the tallest peak to the deepest valley. Knowing both keeps seals tight, wears low, and costs down.

In this guide, you will learn what Ra means, what Rz means, the big RA and Rz difference, and how each number changes across popular printing methods. Simple tips and a clear table will help you pick the right test every time.

What is Ra in Rz vs Ra roughness?

When you’re differentiating among the Rz vs Ra roughness, know that Ra is the most commonly used surface score. Imagine a tiny stylus gliding over a short line on your part. It notes how far each point sits above or below a midline, adds all heights, and divides by how many of points it took. Because it averages, one tall spike does not change the result much. This makes Ra great for spotting general trends in print quality.

Definition And Analogy

Ra is the arithmetic mean of absolute heights from the center line. Picture coins on a table. If you stack them unevenly, measure each stack, add the heights, then divide by the number of stacks, you get the average stack height. That value is like Ra for the part.

Calculation (Simplified)

A profilometer records yiy_iyi​ for each point. Take ∣yi∣|y_i|∣yi​∣ to ignore up or down, add the values, divide by the count nnn. The answer is Ra in microns. Info Box: Many handheld testers show Ra in under ten seconds.

Pros Of Ra

• Easy to test, fast.
• Smooths random dust or single flaws.
• Widely listed in printer data sheets.
• Good for tracking small process changes.
• Works on flat or curved parts.
  

Cons Of Ra

Ra hides single deep pits or sharp peaks that can cut seals. It may say “good” even when one defect could fail in use.

Application In AM

Most FDM brackets and display models are checked by Ra alone. A builder can lower Ra by using thinner layers, smaller nozzles, or warm bed settings that blend lines.

What Is Rz (Maximum Peak-to-Valley Height)?

Rz looks at the same line but picks only the five highest peaks and the five deepest valleys. It averages those two groups and adds them. That value shows the worst-case gap on the surface. RZ surface finish values warn you about leaks, scratches, or weak spots that Ra may miss.

Definition

Rz is sometimes called the “ten-point height.” It is the sum of the average peak height and the average valley depth over the test length.

Calculation (Simplified)

Sort all measured points. Take the top five, find their mean. Take the bottom five, find their mean. Add those two means. Software in the tester handles the math.

Pros Of Rz

• Spot single big flaws fast.
• Great for sealing, wear, and optics.
• Sensitive to process drift that leaves big blobs.
  
• Complements Ra by adding safety.
• Easy to tie to “no leak” rules.

Cons Of Rz

One harmless speck can raise Rz and force rework. Tests take a bit longer than Ra because extreme points must be sorted.

Application In AM

Metal SLM pump housings and air-tight SLA tanks use RZ in surface Roughness checks to stop leaks. When peak height must be under 5 µm, Rz is the go-to control value.

Ra vs Rz: Key Differences Summarized

Here is the first RA and Rz difference: Ra tells the average texture; Rz tells the worst spot. Think of a forest. Ra is the average tree height—good for a quick idea. Rz is the tallest tree minus the deepest hole—critical if you fly low. Because Rz is more sensitive, you may see a big jump with only one nasty spike, while Ra stays almost the same. This is why RZ vs RA Roughness gives a full picture when paired. Use Ra when you need a broad sense of smoothness, use Rz when the biggest bump can cause a leak or scratch. Together, they answer both comfort and safety.

Sensitivity To Outliers

Ra is calm; Rz is sharp-eyed. If one powder bead sticks to a metal print, Ra may barely move, but Rz will shoot up. That jump warns you before the part reaches a seal.

Information Each Value Gives 

Ra describes the fabric feel of the part. Rz describes the tallest feature height. Designers translate those numbers into fit, drag, and wear limits.

When To Use Each 

• Use Ra for demo models, brackets, and non-sealing covers.
• Use Rz for o-rings, bearing races, and piston rods.
• Combine both on critical metal parts in pumps or turbines.

Impact On Additive Manufacturing Processes

Every printing method leaves its own fingerprint on a part’s surface. Laser-cured resin from SLA is already smooth, giving low Ra and low Rz without much effort. FDM stacks thick plastic roads that push Ra higher and send Rz soaring when a blob forms. Powder-bed metals like SLM and SLS sit in the middle; they look fine at first glance, but unmelted beads can spike Rz even when Ra still seems moderate. Knowing these trends helps you choose the right post-step—light sanding for resin, vapor polish for FDM, or shot peen for metal—so roughness numbers fall into the safe zone quickly.

Surface Scores Across AM Methods

Different printers leave different marks. The table shows why RZ vs RA Roughness matters:

SLA: Lower Scores

Thin laser spots cure smooth walls. Ra stays low, so RZ surface finish is easy to tame with one pass of wet sand.

FDM: Layer Lines

FDM lays down roads of plastic. Tall ridges grow, lifting RZ in surface Roughness even when Ra sits mid-range.

SLM/SLS: Metal Powder

Molten pools can leave beads. Peaks push up Rz, showing the need for post-steps like hot isostatic pressing or media blasting.

Post-Processing And How It Changes Ra And Rz

Post-processing is the step where we smooth out the hills and valleys left by the printer. By gently cutting or melting off the tallest peaks and filling the deepest dips, we lower both Ra and Rz. This matters because smoother parts slide more easily, seal tightly, and look better. Different methods work for different materials, but the goal is always the same: shrink the roughness numbers without hurting the shape.

Quick Tip: Test a small coupon first.

Sanding

Use 400-grit, then 800-grit. Ra falls fast. RZ vs RA Roughness gap narrows as peaks level out, making leaks less likely.

Tumbling

Plastic beads roll over the part, smoothing sharp edges. Good for many small prints at once. RA and Rz difference still matters: some valleys remain, so check Rz after.

Polishing

Metal prints shine after electro-polishing. Both Ra and RZ surface finish drop under 2 µm. Warnings Box: Over-polishing can thin walls.

Choosing The Right Parameter For Your AM Part

Write down what the part must do. If it only holds shape, Ra may be enough. If it must seal a pump, add Rz. This simple plan keeps costs low and quality high. Using RZ vs RA Roughness together on the first batch lets you learn which value drives failures. Over time, you can drop the extra test if data shows it adds no safety.

Suggestion Box: Record Ra and Rz on every new material for three builds. The trend chart will guide future jobs.

Case Example: Water Valve 

A printed nylon valve leaked at 2 bar. Ra was okay at 10 µm. Rz was high at 110 µm. After vapor smoothing, Rz fell to 25 µm, and the leak stopped. The RA and rz difference showed the real root cause.

Case Example: Metal Gear 

An SLM gear ran loudly. Ra looked fine. RZ in surface Roughness flagged tall peaks on the tooth tips. Shot peening trimmed peaks; noise dropped. Fact Box: One number saved a costly teardown.

Conclusion: Picking The Right Score

You have seen how RZ vs RA Roughness gives a clear window into your printed surfaces. Ra offers the big picture, while Rz zooms in on the tallest trouble spot. By reading both, you cover comfort and safety without guesswork. Next time you print, let the part’s duty guide determine which value rules your drawing notes. Clear tests make strong parts and happy users.

FAQs
What is RA in simple words?
It is the average height of all tiny bumps.

What is RZ in simple words?
It is the gap from the tallest bump to the deepest dip.

Does post-processing change both?
Yes. Sanding, tumbling, and polishing lowers both scores.

Which is harder to measure?
Rz takes longer because it sorts peaks and valleys.

Do I need fancy tools?
A handheld stylus gauge handles both Ra and Rz.