5 Things to Know Before Buying bead blasted surface finish specification

From the wide selection of available media to the applied pressure and technique, many variables can influence the appearance of your bead blast finished parts. Suppose you are looking to achieve a particular finish. In that case, it is essential to provide specifications to control some of these variables to guide the intended result. When striving for consistent results across production runs or multiple batches, specifications are critical.

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When ordering bead blasted parts through Xometry's Instant Quoting Engine, if no further instructions are provided, we will defer to using glass bead media and sufficient pressure to remove tool marks and smooth the surface without damaging the part. The sections below go over tips and best practices you can incorporate to achieve greater control of the process and more predictable and consistent bead blast finishes.

Media Specification

The type of media or abrasive used will significantly influence the look and feel of your finish. For instance, fine glass beads will produce a consistent, satin-like finish, whereas aluminum oxide will yield a uniform but duller appearance. It may take some experimentation to determine what media works best for your project, but you will want to include that information in your order or part drawings notes section once you do. In addition to the type of media, be sure to specify its shape if there are multiple options to choose from.

Abrasive Grade / Grit Size

Another variable that plays a role in how your finish comes out is the particle size of your chosen media. Suppliers often refer to the abrasion grade or grit. The grit is very similar to what you would find when shopping for sandpaper at your local hardware store. The lower the grit, the larger and more coarse the particles are. On the other hand, higher grits will be of finer particles. Mesh size is often referred to as well. The easiest way to understand mesh size is if you were to observe a 1"x1" screen made up of equally sized holes. With a mesh size of 20, there would be 20 holes in the screen, and particles smaller than those holes will pass through, while larger ones will be blocked. With a mesh size of 200, there would be 200 holes in the same 1"x1" area, and thus would be much smaller and only allow finer particles to pass through.

Media size is typically broken down into coarse, medium, fine, and very fine grades. Calling out a grade will simplify your notes and prevent being overly specific, which can limit a manufacturer's options.

Avoid Tight Surface Roughness Callouts

By the nature of the blasting process, the part's surface roughness will be directly impacted. It can be challenging for shops to maintain tight surface roughness requirements while also applying a media blasted finish. When these requirements mix, it is often the case that a shop will want to pause the project and get clarification on what the expectations are.

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We recommend limiting surface roughness to no lower than 32 µin Ra when you need a smooth bead blasted part. We do not recommend blasting surfaces that must be lower than 32 µin Ra and instead call out for masking on just those surfaces. Take note that masking requirements can significantly increase labor time and thus cost.

Provide a Boundary Sample

A boundary sample can be invaluable for manufacturers when producing a finish that meets expectations. If your project has strict finishing criteria or detailed notes, a boundary sample can help resolve concerns and provide clarity. Manufacturers can use their knowledge and expertise to produce an output that matches provided examples, reducing the need for detailed notes. If a physical sample is not available, the next best thing would be high-resolution photos taken at various angles. Xometry's engineers and case managers can help you make such arrangements with our manufacturers after placing your order.

Masking Callouts for Critical Features

Lastly, we recommend including masking notes if your part has any critical features or surfaces that should be guarded against abrasion. Examples of these features are sealing surfaces and o-ring grooves. Although most shops already do this per shop practice, we recommend adding masking requirements for threaded features, especially for small or fine pitch threads.

Example drawing notes for bead blasted finishes:

• "Bead blast all surfaces using fine grade glass bead media."
• "Mask indicated surfaces and tapped holes prior to media blasting. Media blast un-masked surfaces with medium grade aluminum oxide."

The figure above clearly shows what is involved in selecting the correct abrasive. Here you can see schematically what a so-called anchor profile is. It ensures that the paint adheres. In the case of the figure above, the primer layer thickness must in any case be greater than the roughness.

This will give you a quick idea of how to determine the desired roughness. Note: these are all rules of thumb. Let us personally advise you and also by your supplier of the paint or coating.

Akzo Nobel shows in its epoxy spec sheet which anchor profile is desired. Click here for an example. It can be seen here that the roughness in this case is 75 microns at a typical (dry) layer thickness of 200 µm.

Contact us to discuss your requirements of glass bead 80 grit abrasive media. Our experienced sales team can help you identify the options that best suit your needs.