Thanks to their versatility and agility, CNC machining centers have become a mainstay from job shops to production plants and are even making serious inroads in automotive manufacturing. Many operations traditionally assigned to transfer-type dedicated equipment are now increasingly performed by CNC machining centers--many with high speed spindles. This shift brings into focus the important productivity potential of CNC-dependent combination cutting toolstools that exploit the CNC's capabilities for helical interpolation to perform in a single operation what would otherwise require individual operations.
A good example of such unusual and interesting tools is the Thriller Tool, manufactured by Thriller, Inc. (Dearborn, Michigan), a division of the Turchan Technologies Group. This tool can perform in a single operation what usually calls for a drill, chamfering tool, and tap or thread mill. In so doing, this kind of combination tool reduces the number of tools, toolholders and tool positionings required, and it eliminates tool changes between operations. Properly applied, the result is a significant cost and time savings, together with improved quality in many cases.
High Speed Machining
The Thriller Tool was developed by Turchan president Manuel Turchan primarily as an enabler for complete high speed machining of nonferrous parts. This tool represents a step toward gaining the same kind of benefits achieved in high speed milling in other high speed operations such as drilling and threading of small diameter holes. These operations have been hindered in the past by the limited availability of spindles with sufficient rpm to run them at milling surface speeds, where the full benefits of high speed adiabatic cutting can be realized. Moreover, even the high speed spindles capable of such speeds do not have sufficient torque to tap holes.
The Thriller Tool relieves these problems, because it never has to stop or reverse. Basically, the tool combines a solid carbide thread mill with a step drill that is used to produce the core diameter and the chamfer. In operation, this tool first cuts and chamfers a hole to depth. It then retracts one thread pitch and feeds radially into the wall of the just-drilled hole. From this position, a single orbit of the tool, using helical interpolation, generates the entire thread form. When the threads are finished, the tool is centered in the hole and withdrawn.
The spindle rotates continuously at the same speed through the entire three-step operation, often at its maximum rpm. According to its developers, the complete operation typically takes six seconds or less on a conventional machining center. At very high speedssay 30,000 rpm or morecycle times of less than one second are achievable, developers say.
The high quality of threads thus produced is a key benefit. Because the Thriller Tool has no tapered lead as required with a tap, a full thread form is produced to the last thread to a class 6H standard. In addition, the tool can produce a given number of full threads with less hole depth than is normally required by a tapping tool.
Thriller Tools are used the most extensively by European car builders in threading operations on aluminum and cast-iron engine and transmission parts. Reported benefits include reduced cycle times and improved thread quality compared to conventional tapping operations, as well as tool life exceeding 10,000 holes per tool.
At one major automaker, this combination tool is used on a new high speed machining center recently installed for machining clutch housings. The machine features direct-drive linear motors in the X, Y and Z axes for maximum acceleration and deceleration, and a motorized spindle capable of speeds up to 24,000 rpm. Use of the Thriller Tool minimizes the number of tools necessary to machine the threaded holes in the part, and it eliminates the repeated acceleration and deceleration normally required for tool changing or tapping of a series of holes.
Similar tools are used to produce spark plug holes in cylinder heads for outboard engines. In this application, two toolholders, a tap adapter, a tap and a special form drilling/chamfering tool were replaced. Machining time was reduced by 1 minute, 5 seconds, per head; and two complete tool changes were eliminated. The tool is making possible improved end-use performance. Because the thread's starting point can be controlled, the spark plug electrode can be precisely and repeatably oriented in the head, enhancing combustion in the new generation of direct-injection, low-emission outboards.
The Thriller Tool performs most effectively in materials that produce easily broken chips, such as aluminum and cast iron. However, it has been used successfully on titanium, steel and a number of aerospace materials. The process is especially suited for threading aerospace composites, because it will not delaminate these materials, as conventional taps sometimes tend to do.
Only two limitations govern applications of the Thriller Tool. One is that it requires a three-axis continuous-path contouring capabilitywhich is available with virtually all modern machining centers and flexible manufacturing systems. The other is that blind hole depths are limited to three times the tool diameter in most materials, due to transverse stresses on the tool during the threading operation. Within these parameters, the drill-chamfer-tap combination process has proved to reduce the time and costs of producing both blind and through-threaded holes--often with improved thread quality.
The manufacturer produces these tools from solid micrograin carbide. Standard inch and metric sizes are stocked, and coated tools, as well as through-hole tools, are available.
Pump Component Case History
A British pump manufacturer put to the test the concept that a combination tool like the Thriller Tool could drill/countersink and thread mill a cast-iron pump casing in a single operation. The test part selected was a casing processed on a machining center in the pump company's end suction unit. Several M10 and M12 internal threads are required on this part.
Previously, the threads were produced by first drilling a hole with a conventional tap drill (HSS-TiN-coated), then using a conventional tap to generate the thread form. Use of the new combination tool eliminated one tool and one tool change, and it reduced cycle time per hole from 40 seconds to 12.
With both the tap drill and tap requiring replacement every 1,000 holes (in contrast to 20,000 holes with its solid-carbide replacement), and based on a machine-time cost of $45.00 per hour, the total cost per thread was cut from $0.56 to $0.18.
With these results, using a single tool for drilling was quickly adopted as the permanent process for threading the casing. In two years, this type of cutter averaged 20,000 parts per tool. In contrast to previous costs of $11,240 to drill and tap 20,000 parts, costs are now $3,600.
Drilling And Threading Cross-Holes
Huron, Inc. (Lexington, Michigan), an automotive components supplier to the Big Three, is using the Thriller Tool to drill and thread a cross-hole in 6005-T5 aluminum parts. The combined operation, performed on a Sugino V8 machining center, eliminates a separate tapping tool and one tool change.
Spindle speed throughout the combined operation is 6,000 rpm. Drilling is performed at a feed rate of 60 ipm; threading (by helical interpolation) at 6 ipm. The 3/8-24 thread form produced is machined to a depth of approximately 11/2 times the thread diameter. Total machining cycle time is 6.1 seconds. Tool life is approximately 15,000 parts.
For the product engineers involved, a plus is that it produces a good thread form without the tearing or load-up associated with tapping aluminum. The combination tool saves space for one or more additional tools in the automatic toolchanger. The machining center used for the cross-hole threading operation has only eight tool positions. Saving even one of those positions for another tool can be a big help in changing over to a new job.
Currently, a spotfacing operation also required in the cross-hole processing is performed by a separate tool. Huron is looking at a tool that will combine the drilling, spotfacing and threading cuts in a single operation.