How to choose CNC Router bits

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Introduction


To use a CNC router you need bits. The bit that you choose determines the type of work you can do, whether it is carving, cutting, surfacing, or grooving. It also determines the finish of your design, and how fast you can cut the material. You achieve different results depending on whether you get up-cut, down-cut straight or compression.

You also have to choose between straight cut, bull-nose, v-groove, or specialized shapes, also whether it is made for speed or accuracy, and they come in diameters from a pinpoint to over 100mm for standard CNC routing. This page will try to take you through the different types of cutters so that you can make the correct desisions when purchasing your routers bits.

Tool Materials


Tools in the industrial market place involve high-speed steel, carbide tipped, solid carbide, and PCD diamond. The choice of tooling depends on the material being machined and the equipment it is being machined with. High-speed steel and carbide tipped tools are generaly used for manually fed routers while solid carbide and PCD diamond mostly used for CNC operations.

As hardness of tool material increases the toughness decreases. The harder materials generally function better in the consistent feeding environment, such as CNC machinery.

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Cutting Aluminium


As this is a brief introduction, we will only look at single sheet, vacuum held applications. Soft aluminium requires single flute cutters. Harder aluminium can have better finish with double flute. In all single cut operations conventional cut path is recommended.

If two or more passes are needed because of tool deflection, higher requirement for part finish, or it is faster, then leave at least 0.75mm of material on the part for the finish cut. Use climb cut direction. Spindle speeds should be between 14,000 and 24,000 RPM. Feed speeds up to 7500 mm/minute. Aluminium always needs cooling to reduce clogging. If the bits are clogging up try reducing spindle RPM, Increasing Feed Rate, harder aluminium, and better cooling.


If you are cutting small parts out on a composite wood vacuum table, then solid carbide tools are required due to the abrasiveness of the table. Skip tabs are recommended for smaller parts because they tend to move after several parts are cut which opens up more area for vacuum loss.

63-918 4mm 14000-16000 RPM 1000mm/Min 3mm PER PASS

63-938 6mm 16000 RPM 3000-6000mm/Min 0.6mm PER PASS

63-944 8mm 16000 RPM 3000-6000mm/Min 0.6mm PER PASS

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Cutting Wood


Wood is can be divided into natural wood, both hard and soft, man-made products, or wood by-products. Natural wood may be soft or hard and the tool geometry must be adjusted accordingly. Basically, the rake angle is adjusted to a higher angle of attack in soft than in hard-wood. Wood has a grain structure, which can aggravate tear-out during routing and necessitate using climb to reduce material damage. Wood also contains moisture and natural oils, which effect tool life. Sutable cutters include:

65-018M 5mm

65-023 ¼”


Man-made products include MDF, particleboard, plywood, and laminate stock. Tool life is an issue because of glues and resins used to bind the materials. Laminated products and sometimes plywood can be a problem as in addition to the glue lines they have a top and bottom finish that should not be damaged. For this reason, tooling is critical and the solution may be a compression cutter.

Examples are:

60-152MW: 6mm, 12000-14000 RPM, 7000mm/Min, 3mm PER PASS

60-155MW: 10mm, 10000–14000 RPM, 10000mm/Min, 5mm PER PASS

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Ridged / Hard Plastics


The hard / rigid materials are best routed with double edge “V” flute tools. When this style of tool is utilized with the proper chipload, the cratering effect at the edge finish can be avoided. This cratering phenomenon occurs when the shear strength of the material is exceeded during the routing process.

 

Soft Plastics


It became evident that the router bits of choice for the soft/flexible material usually involved the use of an “O” flute tool. These types of material are extremely prone to leaving knife marks on the edge when adequate chiploads are not maintained. The “O” flute tools with high rake and low clearance, along with proper chiploads, aid in eliminating the knife marks by slightly rubbing the part during the machining process.

 

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Composites


These materials come with some of the most complex cutting requirements. LMT Onsrud makes an extensive line of cutting tools designed for today’s difficult-to-machine, space-age materials. LMT Onsrud manufactures solid carbide and PCD cutting tools designed specifically for machining CFRP (Carbon Fiber Reinforced Plastic), GRP (Glass Reinforced Plastic), Fiberglass, Phenolic, Kevlar®, Carbon Graphite and other composite materials.