SPI Surface Finish Standards
The table below describes all twelve SPI surface finishes, including finishing method and typical surface roughness.For injection molds that do not need one of these surface finishes, the SPI standard specifies an as-machined finish with a typical surface roughness of 3.20 RA and visible machining marks.
| SPI Standard | Finish (Grade) | Finish(Type) | Finishing Method | Typical Surface Roughness |
| A-1 | Glossy | Super | Grade #3, 6000-Grit Diamond | 0.012 to 0.025 |
| A-2 | Glossy | High | Grade #6, 3000-Grit Diamond | 0.025 to 0.05 |
| A-3 | Glossy | Normal | Grade #15, 1200-Grit Diamond | 0.05 to 0.10 |
| B-1 | Semi-Glossy | Fine | 600-Grit Paper | 0.05 to 0.10 |
| B-2 | Semi-Glossy | Medium | 400-Grit Paper | 0.10 to 0.15 |
| B-3 | Semi-Glossy | Normal | 320-Grit Paper | 0.28 to 0.32 |
| C-1 | Matte | Fine | 600-Grit Stone | 0.35 to 0.40 |
| C-2 | Matte | Medium | 400-Grit Stone | 0.45 to 0.55 |
| C-3 | Matte | Normal | 320-Grit Stone | 0.63 to 0.70 |
| D-1 | Textured | Satin | Dry Blast Glass Bead #11 | 0.80 to 1.00 |
| D-2 | Textured | Dull | Dry Blast #240 Aluminum Oxide | 1.00 to 2.80 |
| D-3 | Textured | Rough | Dry Blast #24 Aluminum Oxide | 3.20 to 18.0 |
CNC Machining Surface Finishes
Surface finishes not only add functionality to a finished part, they also add beauty. Whether you require a mirror-like gloss finish or a textured finish to improve grip and usability, we have a solution for your project. RCT MFG offers a variety of surface finish options for CNC machining.
1. As Machined– Is a quick affordable choice often used for non show applications such as mounting brackets. However, cutter marks from the CNC Machining process will be present.
2. B3 320 Paper – A light sand paper will remove the cutter marks left over from the cnc machining process, but will leave behind a uniform grit (scratch pattern) from the sand paper.
3. B2 400 Paper – A smoother alternative to 320 Paper
4. MT-11010 Light Bead Blast – This will remove light cutter marks and light sand paper scratches, leaving behind a smooth uniform sheen.
5. MT-11020 Heavy Bead Blast – this will remove heavier cutter marks and scratched left behind by sand paper and will have a light texture appearance.
6. A3 Smooth Finish – This is typically 600 grit paper and canned be polished
7. A2 Optical Smooth Finish – This is similar to A3 but higher grit paper (1500 and up) are used before polishing.
8. Anodizing – Corrosion Prevention and aesthetics.
9. Chrome Plating – Mirror like hard Finish
10. Custom Coatings & Finishes – Available upon request
There are many custom finishes that can be applied to your cnc machined part. Give our Company a try, our experienced sales engineers are ready to quote your project today. Because of our online quote system, your quote will be turned around in 24 hours or less. Check out our CNC Service Offers and get a quote today.
Contact RCT MFG Design Consultants
Explore over 15 surface finish options for your CNC projects. Ready to start your CNC machining project?