THE BASICS - ROUTER BIT SELECTION

How does one determine the best router bit for a specific routing job? The question is best answered if separated into two sub parts, what kind of plastic is being routed and on what type of a router?

The first consideration is what kind of plastic will be machined. The general classification is hard or soft. Most thermosets are hard plastic. (Some fibers reinforced and composite thermosets for aircraft are super hard materials and are not considered in this writing). Thermoplastics can be either soft or hard depending on content and process of formation. Generally speaking acrylics, nylon, phenolics, rigid ABS and PVC as well as solid surface materials are classified as hard plastics. Polycarbonates, polyethylene, polypropylene, PETG, and soft ABS and PVC are classified as soft plastics.

Hard plastics are best cut with solid carbide router tools. They may also be machined with carbide tipped tools, particularly in some hand held router applications when breakage may be a problem due to fixturing. Solid carbide tools, however, because they are ground from a solid piece of carbide on a CNC router, are better suited to machine all plastics. Carbide tip tools are limited to the geometry allowed by brazing carbide onto tool steel. In most cases carbide tip tools are best suited for woodworking and have woodworking geometry. They will work for some hard plastics but at a sacrifice of feed rate or finish on a CNC router. Solid carbide tools can be designed and manufactured for specific plastic materials. Basic tools are available for soft or hard plastics and a good place to start.

Soft plastics are best cut with either high-speed steel or solid carbide tools. Both tools are ground from solid and can be manufactured with plastic geometry. Plastic geometry is different from wood or metal geometry, particularly in soft plastic tools, because of the form and shape of the cutting edge and the flute behind the edge. Plastic geometry allows for faster feed rates and a smoother finish on the work piece when machining with a CNC router. Solid carbide tools are recommended for all CNC routing of soft plastic. High-speed steel or solid carbide tools will work well in hand router situations. Both should be tested to compare price/performance. High-speed tools are less expensive where solid carbide tools will give more tool life. High-speed steel tools are also stronger and may be better in some hand router environments.

The second consideration for tool selection is the router or machining center. There are several types in use, however for this writing CNC machines with spindles turning over 15,000 RPM and hand held air routers will be considered.

Air routers are used in many industries to trim large parts or assemblies. Often fixturing is a problem because of the size or shape of the work part. Other common problems in air router environments are the amount of air available because of the size of the compressor and/or the length and number of air lines to the routers. These factors are important to tool performance, as is the condition of the air router itself. The bearings must be replaced on a routine basis or tool life will suffer. If the air router environment is not controlled effectively (as noted above) high-speed steel or carbide tipped tools are strongly suggested because solid carbide tools will be subject to severe breakage. Conversely, if the air router environment is controlled, fixturing is solid, there is plenty of air and routine maintenance is done to the air routers, then solid carbide tooling is an excellent option.

CNC routers created the market for solid carbide router tooling. Well maintained CNC routers eliminate most environmental inconsistencies that affect productivity. Productivity is not just the production rate. It is that plus the best finish possible on the work part (to eliminate follow up operations) and the ability to repeat such performance each and every time attempted. Of course there are fixturing, hold down, feed rate, spindle speed and most importantly tool selection considerations. These, however, can be addressed and controlled in the CNC environment. The tool selection is a given. Solid carbide tools are the answer and are available in many configurations for both hard and soft plastic materials. Most often O flute tools are recommended. This type of flute was developed to form a larger chip and evacuate it from the cutting action. O flute tools are available in straight or spiral geometry. Spirals are available in upcut (pull chips up) or downcut (direct chips down) geometry. It is recommended that both straight and spiral flutes be tested because results may vary depending on materials. It should be noted that not only do different plastic materials vary when routing, different colors of the same materials may vary. To optimize productivity, find the tool that works best cutting at approximately 18,000 RPM's at 200 inches per minute feed rate. Take that tool and start increasing the feed rate until the finish peaks. At that time, reduce the RPM's and continue to increase the feed rate until the finish peaks again. This can be repeated until the production is optimized for that part and/or material. The proper chip load (feed rate divided by RPM times the number of cutting edges) is critical in plastics. One must stay within a variance of no more than 10% for best finish. Chip loads for plastics will vary from .004 to .020 depending on the plastic material.

The following chart is a recap of the preceding information. These are general guidelines and should be used as a starting point when selecting the proper router bit for a specific job.