Finishing

The final stage of all our projects involves preparing the work for a final finish that will protect the material and show off the quality of our workmanship! Attention to detail at this stage will mean you have a great looking project you can be proud of. You will also need to know about the different types of finish and how to use them for your exam:

  • Preparation of surfaces

  • Planishing, polishing, bluing

  • Machine finishing (ground, milled)

  • Knowledge (but not use) of preparation and application of painting and blue-dip coating


Surface preparation

Before any final finishes are applied to a metalwork project the surface must be properly prepared.

  1. all tool marks and blemishes should be removed so that the final item looks as good as possible;

  2. the surface must be clean and all grease removed to make sure the final finish adheres to the surface properly and doesn't peel off.

In order to remove tool marks, the surface will be polished to an appropriate degree. If a mirror finish is required then polishing will need to follow the steps outlined below. If paint is to be applied, then the surface doesn't need to be quite as smooth.

Planishing

This surface finish gives the metal a texture as it is formed, the surface is usually left bare or might be covered with a clear layer of lacquer to prevent discolouration but still show off the metal underneath.

Polishing

Polishing will take a rough metal and smooth the surface potentially until it is mirror smooth. This process uses abrasive paper or wheels to remove scratches. Initially a rough paper or wheel will be used to take out the large scratches before a finer grade is used to remove smaller scratches. To get a mirror finish, this process will have to be repeated with finer and finer paper before polishing compound and a cloth wheel are used to finish the polishing off.

Bluing

Bluing is a process which gives a small amount of rust protection but also an attractive finish to steel. It is commonly used on tools in the workshop where although they will be used indoors, the surface still needs to be protected from moisture in the atmosphere. During bluing, a chemical reaction occurs which changes the properties of the surface of the metal, as such it is more suitable than painting because it doesn't add any thickness to the part; this is important for precision tools and products.

Hot Bluing

In industry, bluing is often achieved by immersing steel parts in a solution of potassium nitrate, sodium hydroxide, and water which is heated to between 135 °C and 154 °C) depending on the recipe. This is most commonly used for coating firearms.

Oil Bluing

In school, we sometimes use hot oil bluing on our projects. This is a similar process but simply involves heating the part until the desired colour, and then quenching in old engine oil. This also has the effect of adding some hardening to the item.

Oil Bluing like we do in school!

Painting

Many different paints are available for coating metal, these can be applied with a brush or sprayed onto the surface.

Preparation:

  • Surfaces must be degreased and clear of old paint and any processing marks

  • If necessary a primer must be applied to ensure that the top coat sticks to the surface properly. Aluminium is especially difficult to paint and special primers must be used that eat into the surface and create a good base for the top coat

  • Often a clear coat is applied at the end. This can give a really shiny or metallic finish..

Dip Coating

You should be aware of the processes of dip coating in the workshop.

Machine finishing

Sometimes an item is left in the condition that it came off the machine. In this case the final cuts must be very accurate and even so that the product looks ok.

Milled finish

An example can be seen in the picture on the right. If you look closely you can see the marks left by the end mill used to cut the shapes from the solid block. The item will be smooth to touch but the eye can still see the marks. The video below shows how a CNC milling machine can produce a much smoother finish.

Ground finish

Where extra accuracy is needed on a surface, it will be 'ground'. This is often the case in very accurate shafts which need to be very smooth so that they don't wear out too fast. Once the shape has been machined on a mill or lath the Surface grinder finishes the process. These machines use a grinding tool instead of a cutting tool to remove very small amounts of material and ensure absolutely flat sufaces. A surface table will be finished accurately in this way.

This part has a Milled finish, look closely to see the tool marks