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Machining Plastics: Optimizing Cutting Tool Productivity

Use of the correct tool design and material can turn production processes into profits.

Editor's note: This is the first in a series of articles on cutting tool optimization. Later articles will address troubleshooting techniques and cutting tool solutions for specific materials.

Productivity -- it's the name of the game. Because of this, routing has become the preferred method of material removal with practically all plastics. But to achieve productivity, two things must occur: The process must be performed quickly and efficiently, and the finish must be acceptable. To achieve quality production, factors such as the type of plastic being cut, cutting tool materials and design, tool wear, fixturing and colleting, need to be considered.

Cutting tool materials
Four groups of cutting tool materials will be considered for this analysis: high-speed steel, carbide, ceramic and diamond.

High-speed steel (HSS) has been a low-cost, yet highly productive tool material for cutting both rigid plastics and flexible plastics. It is particularly well suited for hand-held air or electric routers because of the shank strength and resistance to breakage.

Typically, HSS's tool life is not as long as carbide, particularly in cutting Group R materials. However, there are several factors that can work in HSS's favor if the setup is difficult to fixture and the operating environment is harsh.

It is worth noting that HSS will not perform on fiber-reinforced materials.

Carbide, on the other hand, is a much more dense and harder cutting tool material than HSS and must be used with Group F materials. Carbide-tipped tools -- where carbide is brazed to tool steel at the cutting area -- are also used to cut Group R materials. Solid carbide tools -- where the tool is ground from a single piece of carbide-- can be used to cut all groups effectively.

The downside of carbide is that it is more expensive than HSS; solid carbide bits are roughly three times more expensive than HSS, and carbide-tipped is roughly twice the price. However, the trade-off of when to use HSS or carbide with Group R is one of tool life and feed rates.

When machining Group X, the decision between carbide and HSS also becomes one of price/performance and whether CNC or hand-held machines are used.

Ceramic and diamond tooling is worth consideration when carbide's tool life is not acceptable. This is typically the case with more exotic formulations of Group F. (See figure 3)

Tool design
There is a theory that states, "If a cutting tool is tough enough to cut steel or wood composites, certainly it will cut plastics."

Wrong. This fallacy is perhaps the single biggest reason there are so many routers cutting plastic materials today at less than 50 percent potential productivity. Each plastic category, as well as many specific sub-categories, demand different tool geometry for optimum router performance and productivity.

The design of the flute and the included angle of the cutting edge are what differentiates a plastic router bit from a generic cutting tool. Plastics have a unique characteristic in that chips have the ability to reweld or stick themselves back on to the piece part or cutting tool once cut. Tool geometry must be designed to exit the chips away from the cut, not only to resolve this problem, but to protect the finish.

Multi-flute solid carbide spiral tools are often preferred for machining fiber-reinforced materials, if a CNC router is being used. If hand routers are used, the least-cost alternative is a carbide-tipped tool with plastic geometry.

Rigid plastics are most often cut with straight flute solid carbide or high-speed tools with Z flute plastic geometry. Carbide-tipped tools are also used where cost effective. Tipped tools, however, are limited in their geometry when compared with HSS or solid carbide. Acrylics can be cut very effectively with a new concept in plastic router tooling called three-flute finishing tools.

Flexible plastics are often most productively cut with single-edge O flute tools made from HSS or solid carbide. These flutes may be straight, shear or with a very slow helix.

Tool wear
Heat is the enemy of tool life. Preventing the tool from becoming hot will lead to longer life.

The first step in accomplishing this is to match the proper tool material to the type of plastic to be routed. The second is to select a cutting tool with the proper plastic geometry for the material. It is essential that this tool has a sharp edge to reduce heat. As the tool edge dulls, it will slow down in the cut and the finish will degrade.

The third principle is to take big chips -- get the heat out with the chip. This is accomplished by increasing the feed rate and/or lowering the spindle speed until the finish is no longer acceptable. Then, slow the feed rate and/or lower the spindle rpm until the finish is acceptable again. Do not dwell in the cut. Always feed the tool when it is in contact with the plastic being cut.

Slow moving tools recut the same surface and further cut the chips as they exit. Both generate heat. And each time the tool impacts against the material, it has one less revolution in its life.

Other tool life considerations are the rigidity of the router, the rigidity of the setup and fixturing. Material that moves when it is cut, for whatever reason, will cause premature tool failure. Rubbing will cause both vibration and heat. Stacking thin materials also makes an investment in good fixturing pay off, provided the volume is available.

Many users select tools without regard to the importance of adequately holding them in the collet. Both CNC and air routers need 80 percent of the collet to be filled, preferably with a full shank, if the tool is to perform consistently. Stay off the flute fadeout area of the bit as much as possible.

Collets should be changed every 500 to 700 hours run time, on the average. Over time they can lose elasticity and harden, which in turn leads to runout, and in time, uneven cutting action and poor finish.

Equally as important as changing the collet is cleaning it with a wire brush, each and every time a tool is changed. Chips, chemicals and resins, all made fluid by heat, can migrate up the slits in the collet and deposit themselves wherever they find an opening. If not removed at each tool change, these deposits will continue to build up, resulting in runout and tool failure. The key clue is this: If brown marks on the shank of the tool become visible at the mouth of the collet -- problems are about to begin.

CNC routing has made possible new advances in tool life. Feed rates, multi-axis cutting action, spindle balance, rigidity and horsepower all have been improved. However, the right tool must be in the spindle to achieve design productivity.

Van Niser is director of Plastic Application Engineering at Onsrud Cutter. Based in Libertyville, IL, Onsrud Cutter is a manufacturer of a wide range of cutting tools for the plastics industry. For more information, Niser can be reached at (847) 362-1560.

Plastics Machining & Fabricating
P: (847) 362-1560
F: (847) 362-5028