CNC machining sharp internal corners can be challenging due to the inherent limitations of machining processes and the tool geometry. Here are several methods that can be employed to tackle this issue:

1. **Adding Reliefs**: This is a commonly used method where certain shapes are added to the corners to accommodate the roundness of the milling tool. There are various types of reliefs such as:

– A hole centered in each corner: This is the cheapest solution and involves placing a hole at the corner where two internal edges meet

– Half-Moon relief: A slightly more expensive option than the hole, shaped like a half-circle at the corner

– Internal Radii: This is another form that is equivalent in cost to the half-moon and involves creating a rounded internal corner

2. **Avoidance of Sharp Corners**: Where possible, the design can be altered to avoid sharp corners altogether. Instead, radii are included on all internal edges, using an end mill with a radius slightly smaller than the added edge radius

3. **Dog-Bone or T-Bone Fillets**: These are specific types of undercuts that can be used to extend the corner shape beyond the cut area. This allows the milling tool to either complete a full rotation or to back up slightly to create a sharp angle following the tool path

4. **Electrical Discharge Machining (EDM)**: For cases where sharp internal corners are essential and cannot be avoided, electrical discharge machining can be used. This process involves creating sharp corners by eroding the material with electrical discharges. It is noted, however, that EDM tends to be an expensive method

It is generally suggested to design parts in a way that avoids the need for sharp internal corners due to the complexity and additional cost involved in manufacturing them. The best approach often involves collaboration between designers and machinists to find a manufacturable compromise that meets the design requirements while being feasible to machine.

In addition to the thicker sizes available for aluminum 6061 sheets mentioned earlier, thinner sheets are also widely available in Europe. Suppliers such as TW Metals stock aluminum 6061 in various thin thicknesses. The available thicknesses for these sheets include:

– 0.0120 inches (0.305 mm)

– 0.0160 inches (0.406 mm)

– 0.0200 inches (0.508 mm)

– 0.0250 inches (0.635 mm)

– 0.0320 inches (0.813 mm)

– 0.0400 inches (1.016 mm)

– 0.0500 inches (1.27 mm)

– 0.0630 inches (1.6 mm)

– 0.0710 inches (1.803 mm)

– 0.0800 inches (2.032 mm)

– 0.0900 inches (2.286 mm)

– 0.1000 inches (2.54 mm)

– 0.1250 inches (3.175 mm)

– 0.1600 inches (4.064 mm)

– 0.1900 inches (4.826 mm)

– 0.2490 inches (6.324 mm)

These are available in different tempers and are certified to various standards including ASTM B209. This broad range ensures that for most applications, whether it be for aerospace, automotive, or consumer goods, there is a suitable thickness of aluminum 6061 sheet available in the European market. Combining these with the thicker sizes provided by suppliers like Metalex, it’s clear that manufacturers and engineers have a wide spectrum of thicknesses to choose from for their specific requirements.

The surface quality achievable for POM/Delrin when using standard CNC machines can be quite high, depending on the specific machining processes and finishing techniques applied. The standard as-machined surface roughness is typically around 3.2 µm (125 µin), but with additional finishing passes, this roughness can be reduced to 1.6 µm, 0.8 µm, or even 0.4 µm (63, 32, or 16 µin), although this would increase production costs due to extra machining steps and tighter quality control requirements.

For POM/Delrin parts, post-machining operations such as deburring can be employed to remove attached chips, burrs, and tool marks, improving the surface finish. Bead blasting is another option that can provide a uniform finish by blasting the parts with abrasive media like sand or glass beads. Other finishing options include metallic coatings, painting, hot stamping, and laser marking, which can be selected based on the intended use and desired aesthetic of the final product.

These surface finish operations are crucial for achieving the best surface quality, as they can significantly improve the visual appearance and tactile feel of the CNC-machined parts. However, it is essential to note that smoothing and polishing operations can alter the dimensional tolerances of the parts due to the removal of material.