Articles

Secondary Machining of FRP is profitable for some

MSDS

CARBIDE WARING

INTRODUCTION:

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.

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.

Twenty or twenty-five years ago a hole was a hole in FRP and size, location, and ease of manufacture were not considered important. Somehow 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 and smaller volume of FRP parts resulted.

From the beginning, machining FRP has been plagued with a metal cutting mentality. 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.

Since the pleasing shapes, unique weight, cost, design flexibility, and ease of fabrication of FRP are now being used with success in thousands of applications, the chemist turned machinist must now develop the skills and experience to utilize modern tooling which has been designed for FRP. Slightly modified cast iron tooling is not acceptable if we want to achieve true potential of FRP. Efficient, high volume, high speed, cost effective drilling of FRP can best be accomplished with tooling engineered to perform productively with long life and to exact tolerances. Modern technology must be used to establish our unquestioned leadership in manufacturing. Imagine, press fitting bearings into FRP material that has only been drilled, it is being done today.

Completely turning our backs on the machining expertise that the metal chip makers have refined and refined and refined into one of the highest art forms and most efficient energy resource skills in the world is not wise. Rather, we as engineering professionals, must test the fundamental assumptions of the metal chip maker to separate the elements into those that are transferable to plastic composite machining and those that are not.

Exotic techniques using lasers and the more familiar piercing and punching have future potential along with high pressure water jets and ultrasonic machining. But, initial costs are great and there are limitations caused by physical application restrictions. Most of these fabrication techniques have not demonstrated an ease of practical and NOW applicability on the current production floors. The tooling we are discussing is AVAILABLE NOW and APPLICABLE with EXISTING WORK FORCE PERSONNEL and is PROFITABLE.

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 hole making with an open mind.

The basic tool for hole making is "The Drill Bit" which has been in use for over five thousand years. In that time the shape has changed little because it is efficient and cost effective. Industrialization has made the art of drilling of a supreme importance because of cost and the need for increased productivity. The major emphasis for improvement of the cutting tool has been on materials, coatings, or different heat treating processes. The cutting tool geometry has been changed or improved little.

The precise nature of hole making is complicated. Tooling used to make holes in FRP products must be designed specifically for FRP not just modified from the older cast iron/metal chip making technology. The tooling must be effective in producing a quality hole and must be cost effective with existing work forces and machine tools.

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.

By definition FRP is a heterogeneous material. This is the most important and basic difference between FRP machining and homogeneous metal chip making. Each element or component of FRP (and even subgroups) must be dealt with individually to get the desired results. The three major FRP elements are, fillers, resins, and fibers.

Most commonly used fillers are chemically inert, abrasive, and insensitive to temperature variations. They tend to be heavy, and once jarred loose from the binding action of the resin system are easily moved out of the cutting zone. The material must be moved out of the hole. The nature of the material makes it unresponsive to cutting tool geometries and accept for its abrasiveness is not a major consideration of tool design. Because of the bulk of the chips developed and the possibility that they could become air borne and therefore harmful to operators, vacuum collection at the source is suggested whenever possible.

All resins are sensitive to heat. They burn, sublimate, or become sticky or tacky when subjected to high frictional heat. Each of these conditions is not desirable and is not necessary with modern tooling. The least heat producing geometry is a high rake shear cut (See Figure 1) with no tool drag. The efficient shearing of a resin is effected by speed (RPM) and feed rate (inches per minute or millimeters per minute) plus tool geometry. Resins do not crumble under cutting pressures. They shear if properly cut. The blends of different types of resins affect the performance of a cutting tool. The addition of 'a little ABS' will achieve different end products and also create different demands on cutting tool performance.

Reinforcing fibers must be separated into two main subgroups, organic and inorganic. The fine edge of cutting tool technology has recently been extended to easily and efficiently machine the aramid fibers made by du PONT. This paper is limited to inorganic fibers made of glass and graphite. As the hybridization of fiber structures become common, each fiber will have to be considered separately.

Fiber orientation has little effect on the performance of a correctly functioning and sharp cutting tool. This is because of the complicated multi-dimensional helical plane path of the cutting edge. The impact between the cutting edge and the fiber is the only condition for which we have developed parameters. The cutting tool does not cut the glass/graphite fibers. The fibers are shattered by impact or crushed by pressure. You do not break a window by leaning on it. You strike it with a hard object. The faster you hit it the more dramatic the fracture. The same is true for modern cutting tools machining FRP.

Another problem with drilling in FRP is the faster the cutting edge goes the greater the chance of developing heat which will affect the resin adversely. We recommend a speed between 300 and 600 surface feet per minute with a feed rate of 0.010" inches per revolution (ie. 100 to 200 meters per minute and .25mm feed per revolution). These cutting conditions have worked well in all fiber densities from 65% to 15% and resin densities of 75% to 6%. These conditions will produce clean holes at the rate of 60 inches per minute with a inch cutter. (1.5 meters and 12.7mm) (See Figure 2).

The ideal cutting edge material would be as hard as diamond and as tough as High Speed Steel. The recently perfected sub-micron tungsten carbide can economically fit this description. This new material is harder than a regular tungsten carbide and has greatly improved resistance to bending (almost double conventional tungsten carbide). The newly developed grades of sub-micron tungsten carbide are able to survive the bending moments and shocks of drilling without breaking as easily as older conventional grades. As a result of this development we have been able to achieve very strong tools with acute positive rake angles. The positive rake angles are so acute that a normal tungsten carbide would fracture. Because submicron tungsten carbide so much harder the material has a resistance to wear which is increased by a factor of about four.

The three methods are best described as cutting edge angles which have a negative rake, a neutral rake, or a positive rake.(See Figure 1)

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 too 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 a 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 know 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.

The reason you use this technique with cast iron tooling is because of the 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.

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.

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.

A positive cutting edge is the most effective rake angle, the chisel edge at the very center of the cutter is also very important. If it is too large, it will produce heat and require large thrust loads to push the cutting edge into the work piece. The ability of tungsten carbide to resist compression is well known. With these new grades of carbide, we have been able to grind a very thin chisel edge which reduces the required thrust load. This geometry has proven that it will stand up to production requirements and longer wear life.

The production improvements are most dramatically affected by increased feed rates. Feed rate increases have reduced machining time by several fold. Individual machine operation improvements have been reported to have decreased the floor to floor time by 75%. Or said another way, at the end of a shift four time more parts have been completed. The quality of the hole produced has eliminated the costly hand deburring operations which can result from the use of modified cast iron tooling. Conversion to the high performance cutting tool is direct and does not require modification of existing machines accept for increasing the spindle speed. The acute angle on the cutting tool edge allows the tool to penetrate faster and as a result the thrust required to penetrate drops to about 8 pounds or 4 kilograms. Since the thrust pressure is often less than the force necessary to overcome static friction in the drill feed mechanism, care must be exercised to see that hydro-checks or similar devises are properly maintained and the tool is not allowed to jump into the work piece. If this condition exists, the tool may cut the material for a while but the sudden forces on the cutting edge will eventually bend or break something. Let the cutting tool cut. If you do, the quality of the hole will equal that which is routinely achieved in metal working.

The tooling is available now to greatly improve your fiberglass and graphite drilling operations. Many available tools are proprietary and unique, but by no means "special," since they are available "off the shelf." These tools are designed to make maximum use of the physical strengths of submicron tungsten carbide with extreme positive rake angles and minimal chisel edges. This new tooling can at least double production output. This tooling requires a maximum of 15 pounds pressure for penetration, and most applications require only six to 8 pounds thrust. The machines and fixtures used to hold the guide units now can be much lighter weight than those used in metal working. This results in fixtures and machines which are much less costly to build and maintain.

To achieve the promise of FRP, we must use tooling which is designed to be efficient and cost effective. You can achieve metal working tolerances with the proper tooling and cutting conditions.

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 machining. 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 grossly over designed fixtures 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 too 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 Is 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.

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 a positive rake. The reinforcing fibers are pulled into the work piece and sheared or broken between the cutting edge and the uncut material. A positive rake on the cutting edge removes more material per unit of time and per unit of pressure than a 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, but, both production rate and quality are minimal and 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 a 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 with extreme double positive rake angles and a minimal chisel edge. Many models are offered by different vendors.

IN SUMMARY: 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. 2). Since the new tooling requires a maximum pressure of 15 pounds, and most applications are in the range of six to 8 pounds, the machines can be much lighter than those used for metalworking, 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.

Return to Table of Contents

International Carbide Corporation
Last Update April 30, 2002