Polishing

Why Polish Stainless-Steel Tubing?

Polishing takes away things like welds, scratches, or mill scale that happen during manufacturing. For places where cleanliness is key, a smooth surface stops dirt from sticking, making it easier to clean and ensuring you meet standards like those from the FDA or ASME. In decorative or structural work, polishing gives it a shiny, mirror-like look or a soft, satin finish. You can polish it using different methods, like using sandpaper, belts, or special polishing compounds.

Polishing Stainless Steel Tubing: Decoding Grit and Ra for the Perfect Finish

Stainless steel tubing is a go-to in industries like pharmaceuticals, food processing, automotive, and aerospace. Its corrosion resistance, strength, and hygienic qualities make it perfect for places where cleanliness and durability are super important. But the real magic of stainless-steel tubing often comes down to how smooth its surface is. Polishing not only makes it look great but also makes it easier to clean, less likely for bacteria to stick, and less prone to rust. In this blog, we’ll explore the art of polishing stainless-steel tubing, with a special focus on two important ideas: grit and Ra (Roughness Average). Whether you’re a fabricator, engineer, or someone who likes to do it yourself, understanding these will help you get that perfect finish.

Understanding Grit in Polishing

Grit is all about the size of the abrasive particles in things like sandpaper, belts, or polishing compounds. It’s basically a way to measure how rough something is: smaller grit numbers mean bigger, rougher particles that take off material quickly but leave deeper scratches. Bigger grit numbers mean
finer particles that smooth and polish the surface.

• Coarse Grits (36-80): These are great for getting rid of a lot of material, like smoothing out welds or fixing big mistakes. They create a rough texture but are super important to start with.
• Medium Grits (120-180): These are great for achieving a smooth finish, often giving you a satin or brushed look that works well in many industrial settings.
• Fine Grits (240-600+): If you’re aiming for a high-gloss or mirror-like finish, these grits are perfect. They help reduce scratches and get the surface ready for buffing or other treatments.

When polishing tubing, abrasives are usually applied using belts on rotating machines or handheld tools for smaller tasks. To avoid mixing up scratches, always start with the coarser grits and work your way up to the finer ones.

What is Ra?

Ra, or Roughness Average, is a way to measure how rough a surface is. It’s calculated by looking at how much the peaks and valleys on the surface differ from a straight line, usually in microinches (µin) or micrometers (µm). The lower the Ra, the smoother the surface.

• High Ra (e.g., >50 µin): This means the surface is rough and has a matte finish, often from mill finishes or rough polishing. It’s good for non-critical uses but can collect dirt easily.
• Medium Ra (20-32 µin): This is the standard for sanitary tubing, balancing smoothness with cost.
• Low Ra (<15 µin): This gives you an ultra-smooth surface, often done with electropolishing (EP), which is great for high-purity environments like biotech.

Ra is measured using a profilometer, which helps make sure the quality is consistent. Unlike just looking at the surface, Ra specs give you clear and precise information.

Grit vs. Ra: The Relationship

Grit directly affects Ra—the finer the grit, the smoother the Ra. But it’s not always a simple relationship. Things like how you polish, the pressure you use, and the type of stainless steel you’re working with (like 304 or 316) can also make a difference. Here’s a general idea based on what’s common in the industry (in µin):

For stainless steel tubing, starting with 120 grit might yield an Ra of around 52 µin, but progressing to 180 grit can drop it to 32 µin or less. Electropolishing after mechanical polishing can further reduce Ra by 10-50% without altering dimensions, as it chemically dissolves high points. Remember, coarser grits raise Ra initially but are necessary for leveling; skipping steps can leave underlying scratches, inflating final Ra readings.

Step-by-Step Guide to Polishing Stainless-Steel Tubing

1. Preparation: Clean the tubing with solvent to remove oils or debris. Secure it in a lathe or vise for even access.
2. Initial Grinding (Coarse Grit): Use 60-80 grit belts to remove welds or heavy oxide. Work in one direction to avoid swirl marks.
3. Final Polishing (Fine Grit): Use 180-320 grit for the desired smoothness. For mirror finishes, follow with buffing compounds on cotton wheels.
4. Electropolishing (Optional): Submerge in an acid bath with electric current for ultra-low Ra.
5. Inspection: Measure Ra with a profilometer. Clean and passivate the surface to restore corrosion resistance. For tubing, internal polishing (ID) may require specialized tools like flexible shafts or honing.

Conclusion

Polishing stainless steel tubing is both science and art.

If you specify grit, you’re trusting the process.
If you specify Ra, you’re verifying the outcome.