VMCs Cut Racing Head Porting Time By 80% (November 1999)

VMCs Cut Racing Head
Porting Time By 80%

Because a material variation of just 0.003" will change air/fuel flow and affect horsepower, accuracy and repeatability are critical when machining racing heads.

According to Mike Chapman, one of the foremost experts on racing head design, producing racing heads requires a particular understanding of a difficult process: intake and exhaust ports must be designed for size and shape, and not just for gross airflow at peak lift. A smooth surface without chatter marks also is important so that the air/fuel mixture moves more efficiently to the combustion chamber. His company, Chapman Racing Heads (Woods Cross, UT) makes aluminum racing heads for almost everything that requires maximum horsepower.

“Porting a cylinder head is still one of the most difficult machining processes in the automotive industry,” says Chapman. “We used to port heads by hand, but it was time-consuming and we couldn’t duplicate heads. Fadal Engineering (Chatsworth, CA) was the only manufacturer we talked to at the time that guaranteed they could set us up for CNC (computer numerical control) machining of heads. With CNC, the advantage is that a combination of port and combustion chamber design that works can be duplicated exactly.”

Chapman Racing Heads runs one of the premier racing head facilities in the world. Each department utilizes a “clean room” environment with white epoxy floors and premium lighting; especially in the CNC machining and digitizing department with its three Fadal 4020 VMCs (vertical machining centers) and cutting-edge flow testing rooms. The 4020s are equipped with 10,000 rpm, 15 hp spindles, PC-based 32MP controls, rigid box ways, Fadal’s exclusive Cool Power™, and Renishaw digitizers.

“Valve guides are the heart of the port,” Chapman explains. “A lot of the heads that we get in here are off by as much as 0.008" to 0.012". We build the parts around the valve guide centerline. The Fadal probing cycle is a vital tool in the process of determining centers. The seat area is the most critical area of the valve guide: that’s where the horsepower is. It’s the position relationship between the valve guide, valve seat, port, and combustion chamber that’s critical in delivering maximum horsepower.”

Cars with Chapman heads have won races and points championships in everything from all the major NASCAR series, including Daytona 500 and the Craftsman Truck series, to boat racing, drag racing, and sprint cars; and not just once, but many times. In fact, Chapman racing heads have won more races than most others; and all the major auto manufacturers, including DaimlerChrysler, Ford, and General Motors, have Chapman-designed ports and combustion chambers for their motorsport divisions.

Speed, Temperature Are Critical for Accuracy

“We can machine a whole raw casting, except for the ends, in one setup by utilizing touch-probe and macro routines,” says Chapman. “The cylinder head is machined faster and much more accurately, because accumulated error from multiple setups is eliminated. We’ve gone from 50 hours to port a set of racing heads to nine hours; and that’s hard to beat.”

With the complexity of racing head machining, moving data quickly is extremely important. Controls with slow processing speeds suffer from data starvation, which leads to missed moves and inaccurate toolpaths. The Fadal 32MP control with Pentium® processor and color monitor provides a shop-floor interface with the 4020 that enables using third-party software on the control and facilitates networking. Fadal’s multiprocessor CNC has a speed of 3,000 blocks per second, processing 128 bits of information at a time (the equivalent of four 32-bit processors).

Chapman utilizes a touch probe and macros to identify the datum points on the racing head casting and to set tool offsets, automatically. In addition to identifying a valve guide or port centerline, the probe checks for missing or broken tools. If either of these is detected, a macro puts the VMC into a slide-hold mode and a modem pages the foreman, 24 hours a day.

“There are so many features on the Fadals that most people have no idea what the potential of the machine is,” says Chapman. “For example, the temperature of our machining room is critical. You change the temperature of the fixture 5° and you don’t get the same head. Thermal growth is an issue; if you get ball screw growth like on other machines, it just doesn’t work. If we didn’t have Cool Power, we wouldn’t be able to machine heads as quickly and accurately.”

Cool Power, Fadal’s refrigerated cooling system, circulates refrigerated coolant through the spindle nose, around the spindle cartridge and head stock, and through the center of the gundrilled ball screws. Dow Frost™, a high-performance heat transfer agent, is utilized for its ability to dissipate heat quickly, which helps maintain the temperature of positioning components to ±1.0° of the VMC’s ambient temperature. By monitoring the temperature of the fluid and chilling it as required, the thermally controlled ball screws reduce thermal expansion and ensure positioning accuracy.

Precise, Repeatable Rigid Tapping

“Another thing Fadal VMCs have going for them is smooth axis control,” says Chapman. “When you decelerate into a corner and accelerate out, the corner is accurate and the surface finish is smooth.” The 4020s feature an AC vector drive system that delivers consistent torque in the full speed range, to 10,000 rpm, with closed-loop feedback.

The 15 hp spindles feature a mechanical HI/LO system. The LO range delivers two spindle-motor revolutions for every one revolution of the spindle for high-torque, low-speed cutting operations. The HI range delivers one spindle-motor revolution for every two revolutions of the spindle for high-speed cutting power (such as when cutting aluminum). The mechanical, two-speed gearless system delivers higher performance at a wider range of rpm’s, which results in less noise, heat, vibration, and power loss associated with gear boxes.

“We utilize the full rpm range, from 600 rpm to 10,000 rpm,” says Chapman. “We go in with a ¾" ball endmill and remove a lot of 356 aluminum. We have heads with dime-size holes that end up being big enough to put your fist through, without chatter marks. The Fadals are durable machines; there are faster machines out there, but they won’t take the abuse that the Fadals will. Our Fadals often run 24 hours a day, depending on the part and the backlog. Many times we run the machines unmanned 24 hours a day for weeks with the guys just changing the heads when completed.”

Chapman Racing also uses Fadal’s rigid tapping feature for threading the holes in cylinder heads. Hole/thread sizes range from ¼-20 to 1" diameter pipe threads. Rigid tapping reduces tooling costs by holding the tap in a drill collet, which eliminates the need for spring-loaded tap holders. The Fadal vector drive delivers tapping speeds ranging from 150 rpm to 3,000 rpm, which reduces cycle time. Thread pitch and precise depth control can be maintained within ±0.0005".

With rigid tapping, an encoder is used to synchronize Z-axis motion with the spindle rotation to produce the threads. Rigid tapping delivers higher accuracy and repeatability because side forces (from springs) on the flanks of the threads are eliminated. Pullout or distortion of the first thread, which can happen with spring-loaded tap holders (especially when cutting fine threads or tapping soft materials), is virtually eliminated.

The 4020s feature solid cast iron construction and box way systems for rigidity that delivers 0.0002" accuracy. The box way systems provide a combination of low friction, high stiffness, high load-carrying capacity, and vibration damping characteristics for long life and tighter tolerances. Fadal box ways are hardened and ground, and feature a large surface-contact area that resists side thrust and provides damping, stiffness, and stability. Nonmetallic way lines reduce friction, stick slip, and pickup, while increasing machine life.

“We’ve been exposed to a number of other machines,” says Chapman. “Some are more sophisticated and expensive. We can purchase two Fadals for the price of one of them. Working with the Fadal people, they’ve treated us very well; when we needed another machine, they got it to us very quickly. Fadal factory support is like none other, as well. I’ve got other machines, so I know what factory support is.” Fadal Engineering

- November 1999