Polished Finish Surface: Processes, Types dnd Benefits

Polishing finish

At Aria Manufacturing Limited, we are dedicated to giving you quality CNC machining and sheet metal fabrication technology that’s durable and meets your exact aesthetic needs.

In this article, I will brush up on your understanding of polishing finish to machined metal parts. You will learn what it is, the steps involved, types and benefits.

What Is A Polishing Finish?

Polishing is a process that gradually refines the surface of an object to achieve a smooth, clean surface or flat surfaces finish. It is intended to remove cuts, abrasions and scratches on metal surfaces, giving the surface impeccable shine and appearance.

How does it work?

The CNC machining and grinding process leaves behind scratches, cuts, and rough edges, which can be dangerous and annoying. The essence of polishing machined metals is to remove these difficult and annoying surface defects while giving the final surface a glass-smooth finish and shine.

The Polishing Finish Process Steps

# Step 1: Roughing or Sanding

This step removes scratches and abrasions left behind in cutting and welding metals. It may be done on the entire surface or a component segment.

Some tools used in roughing include a random orbit sander, sandpaper, a burnishing polishing machine, and angle grinders.

The random orbit sander is great for entire surfaces and can handle various metals, including steel and aluminium. The burnishing polishing machine presses through the surface, smoothening any roughness and leaving behind a shiny surface.

Angle grinders are great at smoothening out scratches and nicks along difficult angles. Whether an abrasive tool works best for an entire metal part or a small surface depends on its diameter and the number of grits.

# Step 2: Polishing

polishing wheel

After the surface roughness is dealt with, the next step is to apply the abrasive paste on the surface and have it spread using wool berets and polishing sponges to achieve a more reflective surface finish. You can achieve different grades depending on the hardness of the berets and sponges. There are three different types of polishing finish, and I will discuss them in the sections below.

# Step 3: Buffing or Brightening

Buffing involves using specialized equipment with soft and fine surfaces to enhance the brightness of polished surfaces such as cutlery. An airflow mop may also be applied in the light buffing process to ensure consistency of surface brightness.

# Step 4: Protection

Most metal surfaces get degraded upon exposure to oxygen, water, or high temperatures, causing the surface to rust and lose its shine. This fourth and final step involves applying a protective coat mainly of wax or grease, depending on the component. The ultimate goal of coating this is to lengthen the working life of the component.

Different Types of Polishing Finish and Their Pros and Cons

#1. Mechanical Polishing:

Mechanical Polishing

In mechanical polishing, the notches and abrasions are manually removed from a metal surface using tools like sandpaper, abrasive sticks, felt wheels, polishing sponges and wool berets. High-speed turntables can offer automation to the process and highly precise outcomes. Mechanical polishing usually precedes chemical and electrolytic polishing.

Pros
  • High surface brightness
  • Higher cosmetic appeal
  • Enhanced finish
  • Improved cleaning efficiency
  • Reduced risk of surface contamination
  • Reduced surface area
Cons
  • It isn’t very easy
  • Time-consuming
  • Risk of undesired directional marks
  • Requires a high level of skill
  • The shine may be lost after a period of time
  • Susceptible to rouge deposits and corrosion
  • Complex structural components, as in very soft metals, can’t be treated
  • Physical strength of the surface may be compromised

#2. Chemical Polishing:

Chemical Polishing

Chemical polishing involves dissolving a component into a polishing reagent, washing, and drying appropriately to achieve a smooth surface. It is best applicable when other polishing methods are inappropriate or remove minute deformities from metal surfaces.

Pros
  • Highly effective
  • Several components can be polished at the same time
  • Less investment is required in equipment
  • It can be applied to complex structural parts
  • The chemical polish yields a passivation effect
  • Improves corrosion resistance
Cons
  • The reaction between metal surfaces and the chemicals may release harmful materials
  • It is difficult to adjust polishing chemicals
  • If a chemical medium is not properly constituted, it can lead to uneven lustreu
  • Heating the chemicals to the recommended temperature can be difficult
  • Harmful chemicals can be released into the environment

#3. Electrolytic Polishing (Electropolishing):

Electropolishing

Electropolishing involves the removal of excess material on a metal surface by anodic dissolution, leaving the workpiece smoother and brighter. In anodic dissolution, the cathodes collect compounds released in the reaction process, leaving no deposits on the workpiece.

Pros
  • Removes materials highly susceptible to corrosion, like iron from the surface, thus enhancing resistance to corrosion
  • Provides a brighter appearance than mechanically polished
  • No undesirable grainy polishing lines
  • Smoother surface finish
  • High internal and external colour uniformity
  • No risk of reducing the physical strength of the surface
  • Thorough removal of rouge deposits
  • Cost effective
Cons
  • Requires huge investment in equipment
  • Pre-polishing process is complex

#4. Vapor Polishing

Vapour polishing is a method of removing surface roughness in plastics. A plastic surface is exposed to a highly reactive gas in a controlled environment causing it to melt and fill any voids on the surface.

Pros
  • Can produce a transparent finish
  • The process is short and fast
  • Cost-effective compared to mechanical polishing
Cons
  • Pre-polishing is required
  • Potentially dangerous and should be done by skilled professional
  • Requires a high level of skill
  • If not properly done, the fumes can escape to the atmosphere and contaminate the environment.

What Metals Can Be Polished Finishing?

Stainless steel CNC Machining

Various metals can undergo a polished finishing process to achieve a smooth and shiny surface. Some common metal polishing materials include:

Document
Materials to be polishedPlastic Type of finishing
Steel Roughing, prepolishing, highly polished surface
Stainless steel Roughing, prepolishing, polishing, buffing
Cast iron Roughing, prepolishing, polishing, buffing
Brass Prepolishing, polishing, super finishing
Copper Prepolishing, polishing, super finishing
Aluminum Roughing, prepolishing, polishing, buffing
zinc Roughing, prepolishing, polishing, finishing
Gold Polishing, finishing
Silver Polishing, finishing

Can plastic parts be polished?

plastic injection molding tolerance

Yes, plastic parts can be polished to improve their appearance and smooth out surface imperfections. However, the specific polishing techniques and compounds used for plastic may differ from those used for metals.

The suitability of a plastic part for polishing depends on several factors, including the type of plastic, its hardness, and its resistance to heat. Some common plastics that can be polished include acrylic (PMMA), polycarbonate (PC), polypropylene (PP), and ABS (Acrylonitrile Butadiene Styrene).

Finish Grade Options for Polishing Finishes

The quality of surface smoothness varies with the grit size of the abrasive compounds used in polishing. They range from highly coarse to micron-fine grit with a superb finish. Generally, there are four grit sizes.

Document
Finish Grade Grit size
Coarse 8-24
Medium 30-60
Fine 70-180
Very Fine 220-1200

Based on lustre, finish grade options can be categorized as shown in the table below.

Document
Grade luster Characteristics of luster
Matte A milky sheen absorbing light
Satin Reflects a little of light
Bright Bright reflective sheen
Mirror-bright Highly reflect with mirror transparency

Best Practices for CNC Precision Parts Polishing

When it comes to achieving a polished finish on CNC precision parts, here are some best practices to consider:

# 1: Choose the right polishing method:

There are various polishing methods available, including hand polishing, mechanical polishing, and chemical polishing. Select the method that is most suitable for the material and geometry of the part. Mechanical polishing, such as using polishing wheels, belts, or abrasive pads, is commonly used for CNC precision parts.

# 2: Control the polishing pressure:

Apply consistent and controlled pressure during the polishing process. Too much pressure can lead to excessive material removal or heat generation, while too little pressure may result in an uneven or incomplete finish. Maintain a steady hand or use appropriate polishing equipment to ensure consistent pressure application.

# 3: Follow a systematic approach:

Start with coarser polishing compounds or abrasive pads and gradually progress to finer ones. This progressive approach helps remove scratches or imperfections from the previous stage and achieves a smoother surface. Clean the part thoroughly between each polishing stage to remove any residue.

# 4: Inspect and measure regularly:

Continuously inspect the part during the polishing process to assess the progress and ensure that the desired finish is being achieved. Measure critical dimensions, surface roughness, or other quality parameters to verify conformance to specifications.

# 5: Document and optimize the process:

Keep a record of the polishing parameters, compounds used, and any specific techniques or challenges encountered for each CNC precision part. This documentation can help standardize the process, troubleshoot issues, and optimize the polishing process for future projects.

FAQs

Q: What are polishing compounds?

A: Polishing compounds are substances or mixtures specifically designed to facilitate the polishing process by aiding in the removal of surface imperfections, refining the surface, and enhancing its shine. These compounds are typically abrasive in nature and contain particles or abrasive materials suspended in a carrier medium.

Q: How to choose a felt wheel for polishing?

A: Mechanical polishing leaves behind directional marks. Use electropolishing or chemical polishing to rid your work piece of any directional marks.

Q: What is the difference between buffing and polishing?

A: Buffing aims at giving a surface a smooth, bright and glossy finish while polishing is the general improvement of work piece finishing by use of abrasives.

Q: What is the difference between finishing and polishing?

A: Finishing involves all operations on substrates of any surface condition to improve its texture, brightness, color and corrosion resistance.

Polishing, on the other hand, is a type of finishing operation performed on slightly and finely scratched surfaces with the aim of improving the surface smoothness and brightness.

Q: What is the difference between brushing and polishing?

A: Brushing finish involves the use of a rotating brush or abrasive pad to create a textured or linear pattern, while polishing uses abrasive compounds and tools to achieve a smooth, glossy finish.

Author

Gavin Leo is a technical writer at Aria with 8 years of experience in Engineering, He proficient in machining characteristics and surface finish process of various materials. and participated in the development of more than 100complex injection molding and CNC machining projects. He is passionate about sharing his knowledge and experience.

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