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VAN NISER

Router Bits for CNC Mills

Guidelines for choosing the correct bit for the job.

By Van Niser

This column has almost exclusively dedicated its content to issues relative to routing machines. The concepts offered covered subjects ranging from cutting tools for hand routing to the most sophisticated CNC router. However, in the marketplace today there is a whole group of companies utilizing CNC mills or machining centers.

Traditionally, these shops utilized endmills, which ran at relatively slow spindle speeds and feed rates compared to CNC routers. Endmills are typically toleranced for these slower speeds and manufactured as a robust cutting tool for heavy loads. However, a limited flute area interferes with the process of clearing stringy chips associated with plastic machining. Endmills are frequently designed with low clearance angles, which can aggravate melting and rewelding problems common in plastic cutting applications. Since the endmill tends to push the chip off the material, multiple passes are usually the norm to achieve a satisfactory finished part. Clearly, these tools are designed for ferrous applications, but endmills have been used on plastic because of availability, cost, and of course, tradition.

CNC mills and machining centers manufactured by companies such as Fadal, Haas, Mazak, Mitsubishi, Cincinnati, Bridgeport, Daewoo, Okuma, Mori Seki and Milltronics, to name a few, have drastically changed over the years with spindle speeds reaching 15,000 RPMs and above accompanied by feed rates in excess of 600 IPM. Since routing speed by definition is around 8,000 RPMs and above, it places these machines in the realm of router type tooling. This exposes the CNC milling operation to a whole new concept in high speed machining, which utilizes tools designed for specific plastic materials. These tools have an open flute area for adequate chip removal capabilities with rake and clearance angles varying by the type of plastic being machined. The problems of melting of chips and multiple passes to achieve maximum finish requirements are eliminated and production time is minimized. (See Figure 1.)

Tool Selection

Figure 1
Figure 2
Figure 3
Figure 4
Selecting the correct tool for the job is relatively simple in the initial stages. Plastic tends to be placed into two general categories: flexible or rigid. The router bits of choice for the flexible materials usually involves the use of single or double edge “O” flute tools in straight or spiral flute configurations. Rigid plastics lean more toward double edge “V” flutes, spiral “O” flutes with hard plastic geometry and two and three flute finishers. All styles of these tools are readily available in solid micro grain carbide.

The key issues after tool selection are chipload and part finishes. The optimum chipload to achieve the best finish seems to be in the range of 0.004 to 0.012. This narrow range provides the best finish through the continuous generation of properly curled or sized chips. Soft plastic chips curl during machining and inadequate chiploads can lead to knife marks. The use of an “O” flute with high rake and low clearance, along with proper chiploads, can eliminate the knife marks by slightly rubbing the part during machining.

In the case of hard plastic, the removal of equally-sized chips of material avoids the cratering effect, which occurs when the process exceeds the shear strength of the material. There does not appear to be any minimum combinations of feeds and speed as long as the proper chipload is maintained. However, the ability of router tooling to run at high feeds and speeds, and the obvious increase in productivity, would encourage the user to maintain the proper chipload at an optimum level. (See Figures 2 and 3.)

The Machining Process

The CNC milling/machining centers have a tremendous advantage in the area of rigidity and accuracy. The mere size and weight of the centers, along with hard clamping devices with mechanical or pneumatic vice-style clamps, inhibits vibration and provides rock-solid part hold-down. These conditions enhance the geometry of the high-speed routing tool and produce better chiploads and finishes in less cycle time.

However, further improvements in the overall machining process can be made by taking a serious look at conventional milling versus climb cutting tools. This would be particularly advantageous in situations involving straight trim passes of scrap. The end result would be a one-pass finish cut as opposed to the rough and finish passes involved in climb cutting. The chiploads provided by the router tools with the correct geometry would also eliminate the need for coolant because the heat would be dissipated by the creation of a larger chip. Once again, productivity and tool life would be increased simultaneously.

The whole area of tooling for the CNC milling/machining center market continues to be a dynamic process. Other products besides high-speed cutters continue to arrive on the scene. One such product is a drill designed expressly for plastic. The plastic machining industry has been at the mercy of inadequately designed drills for years. The jobber drill and similar tools were inappropriate in terms of providing clean holes in plastic material. A new style drill is now available which allows fast plunge speed with reduction of chip wrap in soft plastic and crazing in hard plastic. The 60-degree point and flat face rake reduces the stresses introduced into the hole walls and will provide a clean hole surface without clouding or crazing typical in standard drills. These drills are equally operational in the CNC machining center or the air driven hand drill. (See Figure 4)

CNC milling/machining concerns throughout the country involved in fabricating, forming and molding are busy producing nylon gears, parts for the food service industry, machine components, medical industry parts, electronic mounting points, electronic washing machines, brackets, clips and an infinite variety of other products from plastic. The tooling is readily available and specifically tailored to the vast array of materials being machined in this industry. As an assessment is made into the productivity of the operation and the quality of the finished parts, possibly router tooling with high spindle speed and high feed rate capability should be considered.

Van Niser’s next article will be dealing with frequently asked questions in the routing of plastics. If you would like to contribute a question for this article, please submit the question by e-mail to VanNiser@Onsrud.com or by fax to (847) 362-5028.

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