The elusive goal of making money by secondary machining of fiber reinforced plastics (FRP) requires production floor knowledge of cutting principles and proper application of current technology. The most cost effective way to produce a part requires a holistic approach to fabrication. Blending engineering structural requirements, molding capabilities, secondary machining and finishing to produce a least cost part is the goal. The following article will help you understand the roll of the cutting edge in cost reduction and piece part productivity.

Product design, marketing demand, unique physical properties, start up product economies of cost, and the modern economics of down sized production requirements have all contributed to the ever growing use of FRP components. For those who are experienced in the secondary machining requirements of FRP it is a wonderful development. The more parts we can make the greater our job security and the more secure the future of our companies. But, this change in our country's basic manufacturing paradigm does not come for free. The metal cutting mentality of old required very long runs. Fixturing and start up costs were very significant. Extremely cheap parts were made rapidly on automatic equipment. The production skills for this type of metal working industrial manufacturing have been developed over the years until their refinement is a marvel of human ingenuity.

FRP, from the forties, has been plagued with a metal cutting mentality. That is, product designers have tried to use FRP parts as direct replacements for metal parts without regard for physical properties or unique processing benefits and problems that FRP offers. This short coming has been particularly restrictive in the area of secondary machining. Huge amounts of engineering time and creativity have been expended in the search for those parts which do not need secondary machining. No holes, no groves, no edges, no subassemblies has been the design motto.

The problem with this approach is a practical consideration of costs versus benefits. Molding limitations became the frontier of production capability. Huge investments in mold design have reached the limits of production "do ability". Secondary machining has become an issue to be dealt with by top management of successful companies..

Twenty or twenty-five years ago a hole was a hole in FRP and size, location, and ease of manufacture were not considered important. Some how it just got done by manufacturing. No investment in machining science or art was considered necessary. The basic assumption was "Cut it like steel". But then the customer kept complaining about not being able to assemble or having to redrill or rework on the production line.

The best and quickest way to analyze an FRP machining operation is to examine the chips produced in cutting. The ideal chip form for reinforced plastics is a dry, easily moved chip that looks like confectioner's sugar. If the speed of the cutting tool is too high, the heat generated will make the resin sticky, and produce a lumpy chip; if the cutting edge is scraping and not cutting the plastic, the chips will be large and flaky. Either will eventually clog any evacuation system.

Not to many years ago we asked a product engineer what tolerance he wanted on a 3/16" hole. Answer, " Well, a tenth should be close enough." Naturally, we asked about the type of equipment he intended to use etc. We could not believe he was serious about plus or minus that tight a tolerance in FRP. Eventually it dawned on us that he meant plus or minus a tenth of an inch not one ten thousandths of a inch. In other words, "He just needed a hole about here!"

As the customer complaints continued and costs of rework skyrocketed many firms did not survive. We all know of firms with good people and great products who could not deliver a product that was acceptable and they folded. These are needless failures. They hurt all of us and need not have need not have happened.

You can make money with secondary machining. It is not necessary to produce costly engineering marvels that have avoided all secondary machining. Low cost easy to perform secondary machining is available. The technology is readily available to all who approach secondary machining with an open mind.

To understand what secondary machining is we should first visualize the three basic methods of material removal in terms of whittling a wood stick. See figure 2 for basic pictorial concepts. All of the basic material removal techniques can be the technique of choice in certain circumstances. None can be used universally. This is an important point to remember. A given job may require different type of cutting geometry even if only minor changes have been made in the design of the product. Therefore, care needs to exercised from beginning when choosing which technique to use for each job.

The three methods are best described as cutting edge angles which have a negative rake, a neutral rake, or a positive rake.

Negative Rake Angle Tooling:

To visualize the concept pretend that you are cutting a piece of wood with your favorite jack knife held in such a way that the cutting edge to wood angle in the direction of movement is less than 90 degrees. You discover that it takes a great deal of force to remove any wood at all. The pressures you have to exert are significant and there is lots of lost work. Lost work means that extra energy is lost and converted into heat. If you want to remove a very small amount of material with a great deal of control and time is not of the essence then negative rake tools can perform very well in certain limited circumstances. If the piece part is not to flexible and you can support it enough very fine finishes can be obtained with negative rake tooling.

The least efficient tool geometry for FRP is negative rake and a thick chisel edge, which results in poor chip form and poor chip throwing characteristics. When the chips formed during cutting are either pushed in front of the cutting edge or not pulled out of the immediate cutting edge area, they are forced into a pack that eventually blocks the operation of the tool.

Examples of this cutting geometry are grinding wheels, sanding methods and smooth rotary surfaces. Both silicon and/or diamond particles held by a binder are most often used. The individual particles of grit are cubical in shape and therefore present a negative rake angle to the work piece.

Production rates are usually very slow and unless unusual surface conditions are required it is not usually practical except for very short production runs or one off prototypes. We all have experienced how smooth a finish we can get with this technique. We also have experienced the lost work turning into heat and the FRP resin becoming sticky and gumming up the tool being used. To achieve a specific effect this is sometimes the only way to get the desired results. But it is always slower and therefore more expensive for a given volume of material being removed.

Neutral Rake Angle Tooling:

If you now turn your knife perpendicular to the wood you find that more wood can be removed. The contact pressures are still significant. Lost work, although less, still produces the unwanted heat that causes sticky resin and can only be controlled by slowing down the speed of operation. Never-the-less, material is being removed and progress toward a finished part can be made.

Examples of this type of tooling are tools originally designed for machining cast iron. There are an almost unlimited number of standard tools available from hundreds of sources. The availability and lower cost of these tools can be quite appealing but frequently total costs are not as low as can be achieved.

A great deal of energy is consumed and not quite so smooth a surface finish result is obtained. Most "cast iron" tooling is designed to use this type of geometry. It is a compromise between the other two cutting type as. The reason cast iron tooling uses this technique is because of heat produced lubrication from graphite. The free graphite in cast iron needs about 700 degrees Fahrenheit to become an effective lubricant. This lubrication greatly increases tool life. At 700 degrees Fahrenheit most FRP is oxidizing rapidly (making smoke) . To make this type of geometry work on the FRP production floor the speed of cutting has to be dramatically reduced and frequent tool changes are required to maintain the sharpness of the cutting edge. If these slow operating conditions and frequent tool changes are made then satisfactory results can be obtained, especially for the short runs.

With increasing attention being given to glass and graphite reinforced plastics as quality substitutes for metal parts many people have begun to focus on the various required ancillary production operations required for subsequent assembly. One of the first questions they ask, especially those experienced in metalworking, is " why does it take, so long to drill holes in reinforced plastic parts?" The answer, of course, is that the available tooling is not designed for cutting reinforced plastics. Glass or graphite reinforcement is so abrasive that only tungsten carbide tooling can drill through it for any period of time. And until recently, the only tungsten tooling available has been designed for machining cast iron.

Neutral rake angles tend to push the reinforcing fibers out in front, requiring a great deal of pressure to penetrate the work piece. This pressure causes the fibers to hinge, resulting in furry, undersized holes. The pressure also produces excessive heat, which causes galling and chip clogging in the resin. The release of pressure as the tool bit breaks through the part causes a sudden and momentary increase in feed rate. As the tool plugs through the last few fibers, the cutter shaft, not the cutting edge, removes the remaining material. The result is chipping and cracking.

Simply Stated, the problem is that a properly designed cast iron cutting tip heats the metal to provide the plastic metal flow needed for efficient metal cutting. Reinforced plastic cannot tolerate this heat, so production must be slowed down to keep the heat as low as possible. As shown in Fig. 1, the cutting tip for cast iron has neutral rake angle and a wide chisel point to provide stability. Figure 2 shows the effects of rake angle on cutting action. Note how an edge with neutral rake angled cutting edge scrapes the material and causes it to resist penetration by the cutting edge so that pressure must be exerted by the operator to penetrate the material. This excessive pressure causes the heat buildup.

Another problem presented by tooling designed for cast iron is the length of the drill bit. Most for cast iron are 4, 5, or 6 inches long. Since FRP drilling is usually on parts 1/8 inch or less in thickness and smaller than inch in diameter, longer tools cause vibration and other problems due to their bulk, weight, and lack of straightness. The result is poor hole quality and reduced production.

Positive Rake Angle Tooling:

If you turn your knife so that the angle between the wood and the direction of movement is greater than 90 degrees, say 150 degrees then the cutting conditions change dramatically. First, great chunks of wood can be removed very fast. More power is required for each cut. Fewer passes of the cutting edge are required to remove a given amount of material.

Second, very little work is changed into heat. The exact amount of the material removed with each stroke is hard to control. Only with great concentration can you limit the amount of material being removed and obtain a dimensionally accurate piece part. You soon learn that although you can probably cut it all in one cut it is better to take a series of significant but controllable cuts.

The ability to remove material fast without loss of work (heat) is very appealing to the production minded. Since time is money the faster you can turn out a satisfactory part the better the company bottom line will look. But do you have to spend hundreds of thousands of dollars to achieve these fast controlled cutting results? Generally the answer is no! What is required is a knowledge of the material you are cutting and the way the cutting edge performs in each.

Examples of this type of tooling are usually limited to those proprietary tools which have been designed exclusively for FRP. This kind of tooling usually does not work well, if at all, in composites or materials other than for which it has been designed. The initial cost to purchase tooling made specifically for one type of material is almost always higher.

Positive rake angle tooling requires that fixture designers take feed rates and control of feed rates into careful consideration. Feed rates are not limited to pneumatic to pneumatic over oil, or mechanical machine expectations. Piece part flexibility can dramatically, and often randomly vary individual cutting edge loadings. Over time feed mechanism component wear must also be considered. If your material is being cut at .010 inches per revolution, then a .005 inch flex or wear can cause a 50% increase in tooth loading. Broken, cracked or chipped parts are frequently the result of lack of feed rate control.

COMPOSITES ARE NOT HOMOGENEOUS - Each component should be considered separately.

The very thing that makes FRP so unique and versatile also is its weakness when matching. Since the components are put in the composite for specific physical or chemical reasons then the mechanical engineering necessary to machine these components has to be taken into account as the cutting edge comes in contact with the work piece. This is by far the biggest difficulty for fixture designers. The base pool of machining knowledge in our country has been founded in metal (homogeneous materials) working. The concepts of metal working do not always apply to FRP. And in most cases the metal machine approach is detrimental to successful profitable machining of FRP.

Over Designed Fixtures:

The first problem is a grossly over designed fixture which could withstand the blitzkrieg but does not have adequate chip removal provisions and brutalizes the piece part during loading and clamping. Yes, spindles need to be where you expect them and work holding devises have to hold the piece part but to much pressure crushes the part, and a ten pound piece part does not need a 400,000 pound cast frame to hold it. Light aluminum frames or plastic molding compounds are dimensionally stable enough for all but the most exacting jobs. They are significantly cheaper to construct and maintain.

Location Variations:

The second problem develops during the piece part locating phase. Most FRP is much more variable in thickness than the metal part it is replacing and fixtures designed for metal will not work for FRP. Hole to hole locations, hole to edge locations and similar references are not the same for FRP. In some cases molding variations will cause significant piece part profile variations. All of these need to be taken into account with relatively inexpensive easily maintained secondary machining fixtures and cutting tools.

Each Component Must Be Considered Separately:

The third and probably the least understood problem comes from not designing a cutting edge tool to take into account the unique characteristics of each component of the FRP being used. In many cases the structural engineer used three, four, five different materials in his composite to achieve a specific effect. Each component must be analyzed for its requirements during cutting.

Glass reinforcement is universally used and is now well understood as to its composite contributions. But for cutting tools we need to look at how it is cut. First a slow moving heavy pressure tool (a negative or neutral rake tool) must crush the glass to cause separation. Usually small bits are removed per unit of time and the tool and the glass wear down together. Since glass is a silicon material it is very abrasive and hard on any cutting tool edge.

We all learned as youngsters that we did not want to break glass by leaning on it. We had to hit it. The harder we hit it the more dramatic the break became. Until we learned we could make hole in it with BB gun. In other words, the faster we contact a glass fiber with a hard object the cleaner and more defined the cut. Tungsten carbide of the forties and fifties would not stand up to the abrasiveness of fiber glass or graphite. The development of micrograin tungsten carbide made relatively delicate tough very hard cutting edge geometries possible which could stand up to abrasive reinforcements. These same cutting edges efficiently shear with minimum work loss both resins and fillers.

The most effective geometry for large quantity material removal in glass and graphite reinforcement is an acute positive rake angle tool made with tungsten carbide micrograin cutting edge material. This is true whether it is a router, drill, saw, countersink or similar cutting tool.

Good tool geometry for FRP drilling starts with positive rake. The reinforcing fibers are pulled into the work piece and sheared or broken between the cutting edge and the uncut material. Positive rake on the cutting edge removes more material per unit of time and per unit of pressure than negative rake, but the more positive the rake, the more sensitive and fragile the cutting edge becomes.

A small chisel edge, the second element of good FRP tool geometry, improves the penetration rate, which translates into more pieces per hour. The optimum chisel edge for FRP is as close to a point as possible.

Good geometry also means a cutting-tool shape that facilitates chip handling, so that the chips are produced and then removed immediately above the entrance of the hole. From this point, a properly designed vacuum system can dispose of the chips in conformance with safety and environmental standards.

Solutions:

Until recently, there has been no economical, practical solution for the problem of drilling holes in reinforced parts. Most production people had to choose among the following compromise solutions, all inadequate because of the costs either in dollars, labor, or production time.

Special tooling:

Modification of tooling designed for cast iron can afford some improvement. Both production rate and quality are minimal, however, because it is very difficult to resharpen the tooling.

Custom, designed-for-the-job tooling:

Single-purpose tooling is not only expensive, but it is not readily available because of the long lead time needed to design and make it. For exceptionally long runs, however, it can pay off.

Skilled machinists:

A third alternative is to use standard tooling, adaptable machine tools, and skilled operators. Production rates are low, hole quality, location, and appearance are variable, and overqualified people are misused. Best use is for emergency and short runs only.

Tooling designed for reinforced plastic:

Tooling is now available that will greatly improve secondary machining operations in glass, graphite, kevlar and combination reinforced plastics. Most such tools are proprietary and unique, but by no means "special," since they are available "off the shelf." The tool bits made of submicron tungsten carbide, which has 50 percent greater resistance to rupture and is substantially harder than standard C-2 grade tungsten carbide are available.

But, the real difference lies in the tooling shapes or geometries. They are designed, as shown in Fig. 3, with extreme double positive rake angles and a minimal chisel edge. Many models are offered by different vendors.

The solid shank style, shown on the left, see figure 4, and the twist drill, on the right, are used in automatic drilling equipment. The drill guide system, called Hole-O-Magic, is designed to be used with an air drill motor. Fitted with a socket adapter, it can drive all drill sizes from 0.118 to 1.0 inch. The internal compression control spring regulator withdraws the tool after drilling. Its pressure control compensates for the "breakthrough lurch," or sudden increase in feed rate as the tool bit breaks through the last few fibers. It also eliminates partially drilled holes, because the operator must depress each unit completely each time. He does not feel a change in pressure as the hole nears completion.

With the right tooling you should expect:

Drill penetration rates of 60 inches per minute from 1/8" to 1"

Router feed rates of 60 inches per minute in normal lay ups

Saws at 20-25 feet per minute

With new technology tooling you can at least double production speed, and in many cases, can quadruple it or more. The recommended speed for tungsten carbide micrograin tooling is 300 to 600 surface feet per minute (Fig. 5). Since the new tooling requires a maximum pressure of 15 pounds, and most applications are in the range of 6 to 8 pounds, the machines can be much lighter than those used for metal working, resulting in substantial energy savings.

It is possible to make money with secondary machining with the right tool applied to the right job. If a holistic approach is followed balancing all of the design, production and finishing requirements better products will be made. More of us will find employment and the future of the successful FRP industry will be dependent upon learning the lessons of polymer chemistry and secondary machining.

International Carbide Corporation
Last Update April 30, 2002