One does not need to visit many automotive manufacturing facilities before encountering a discouraged engineer who is struggling to achieve the lower costs and improved part quality promised with PCD rotary tooling when machining die cast Aluminum parts. Plagued with high tool breakage, out of tolerance parts, and long lead times for replacement tools, the engineer understandably is struggling to justify the continued use of such expensive tooling.

On the other side of the equation is the automotive plant that has successfully implemented the use of PCD tooling, reducing their over all tooling cost and increasing part quality. The contrast can be like night and day. In the troubled facility one can find large bins of scrapped parts and machines off line for lack of tooling. In the other, the production of parts flows effortlessly, production schedulers are being routinely made, and scrap parts are rare.

In the proper application of PCD rotary tools, it is not uncommon to have tools produce ten of thousands of high quality parts with little attention required from the operating personnel. Stories of PCD tools running for months in high volume production of Aluminum parts are not unusual. The difference between success and failure is understanding a few basic machining practices.

PCD is a remarkable cutting tool material. The exceptional hardness can yield tool life increases of 300 times over tungsten carbide in machining die cast Aluminum parts. When applied with a polished surface, the coefficient of friction of PCD is a fraction of other cutting materials. Further, the core diamond particles that make up over 92 volume percent of the PCD cutting edge, do not form bonds with the Aluminum chip resulting in minimum chip welding. Finally, the heat transfer properties of PCD are 3 to 5 times better than tungsten carbide allowing the heat to move out of the cut more efficiently.

On the down side, PCD has only about half of the tensile strength of tungsten carbide making it far easier to chip. Additionally, the exceptional hardness substantially increases the difficulty of manufacturing cutting tools and in many cases seriously limits the tool geometry available to the end user.

The term 'rotary tools', i.e., tools that rotate such as drills, reamers, and end mills is used to exclude single point cutting such as lathe tools or indexible milling cutters. Single point cutting tools have been successfully applied for over twenty years with many well documented applications. The use of rotary PCD tooling has not been as widely successful over that same period.

So what are the secrets of running PCD rotary tooling? One of the most important is to avoid run out at the cutting edges. Tool run out introduces impact loading of the cutting edge causing the PCD edge to chip or flake. A small amount of run out at a moderate cutting speed can be tolerated. At high speeds (350m/min-1000m/min) that same amount of run out causes edges to chip and fail. Run out also degrades part finishes and bell mouths holes. What is too much run out? A few microns is ideal. Anything above 10 microns combined with high cutting speeds reduces tool life.

Many excellent tool holding systems are now on the market that can consistently hold 6 microns or better. Adjustable tool holding systems can produce even lower values. Why then are PCD tools often being put into high quality holders with runout of 50 microns or higher? Tool holders wear out over time or are damaged when tools break. While tool presetting equipment is becoming more common, few plants routinely check for tool runout. This is a must if one is to be successful with PCD tools.

Newer presetting equipment can automatically measure tool runout as well as set the overall length of the tool. Presetting equipment and knowing how to use the equipment is a common factor in those plants that successfully used PCD. Proper training is a must. In one case we encounter a tool setting system that was out of tolerance by 8 microns. A routine check with the gauge supplied with the presetter would have avoided this problem.

Starting with a PCD tool preset to a few microns of runout only solves part of the problem. The critical measurement of runout is with the tool and tool holder matched to the spindle. Any dirt in the spindle bore or on the mating tool holder taper will affect runout. Cleaning both surfaces before installing the tool holder is another key to good PCD tool life.

Realizing the full potential of PCD tooling normally involves using high surface speeds, often in excess of 400 m/min. When tools such as drills and reamers rotate, they generate frictional heating at the contact points between the tool and the hole wall as well as the cutting edges. Higher cutting speeds increase the generation of heat. Although PCD tools generates less frictional heat than carbide tooling, the proper application of coolant is essential to the successful use of PCD tooling.

Heat softens Aluminum chips and makes chip flow difficult and inconsistent resulting in chip packing and lack of control over part finish and size. Application of the coolant near the cutting edge is the ideal solution. In most cases this involves properly designed through the hole coolant. As obvious as it may seem, good coolant filtration is essential to keeping the coolant passage open. In the evaluation of broken or non performing PCD tools, it is not uncommon to find coolant passages packed with fine chips from the cutting process.

Another common source of inconsistent part finish or size is variation in coolant flow or mixture. Changes in the lubricity of the coolant changes the frictional heat generated in the cutting process. Such changes often are responsible for changes in part size and finish. Wide swings in coolant mixture or flow can result in tool breakage due to chip packing. When machining at high speeds with PCD tooling, the quantity and quality of the coolant can not be ignored.

The use of PCD tooling saves money. Lots of money. Not 10 or 20 percent, but often as much as 800 percent or more. In addition, part finishes are improved and part sizes are more consistent This is the promise that PCD rotary tooling holds for those who take the time to create the environment required for high performance PCD tooling.

The above information was taken from the paper:
"Drilling Aluminum Castings with PCD Drills"
by John Bunting, C.E.O., Precorp, Inc.

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