Digitizer Drives Five-Axis CNC Head Porting To The Performance Redline

Why spend thousands of dollars on a precision machined engine block, crank and pistons, just to bolt on a set of unported cylinder heads with mismatched port volumes and rough wall surfaces? This is where hand porting comes in.

Case Study From: 12/15/1999 Modern Machine Shop

The name of the game in racing and street performance is horsepower, and professional engine builders know that precisely ported heads mean better engine efficiency—not to mention the difference between taking the checkered flag and staring at a competitor's rear bumper for 500 miles. So why spend thousands of dollars on a precision machined engine block, crank and pistons, just to bolt on a set of unported cylinder heads with mismatched port volumes and rough wall surfaces? This is where hand porting comes in.

A set of hand ported heads can look impressive. But from a manufacturing stand point, it is impossible to identically size each port volume, or to accurately replicate an "ideal" port design on another head. It is also time-consuming—inefficient for a high variety, low volume porting business. From a performance stand point, cleaning up ports by hand does assist flow by reducing restrictions, but the head can't deliver its full potential because of mismatched port volumes.

Computer numerical controlled (CNC) machine tools speed throughput in a wide variety of industries. But machining only in three axes leaves tool marks that cross over one another in various directions, requiring hand polishing and blending to smooth surfaces. Not only does this add time to the process, but it also removes additional material, making the port larger than originally designed. And just because a shop has CNC capabilities doesn't mean it can clone a pro engine builder's "perfect" port design.

These were problems that perplexed Bob Hudgins, president of CNC Cylinder Heads (Pinellas Park, Florida). Unfortunately for Mr. Hudgins, he couldn't just open up a machinery catalog and order the exact system he needed—it just didn't exist. So he developed a system to solve his problems. Employing a special combination of high-tech equipment, including vertical CNC machining centers (VMC), custom-built head fixtures with A- and B-axis CNC rotary tables, and specialized CAD/CAM software, he machines aluminum and cast iron heads in five axes simultaneously, producing a smoother finish, without toolmarks, straight off the machine. After years of trial and error, and picking and choosing between vendors, Mr. Hudgins finally perfected his five-axis method.

But what about the duplication dilemma? For that, Mr. Hudgins uses one additional trick piece of equipment—a Renishaw Cyclone digitizer from Renishaw Inc. (Schaumburg, Illinois).

A continuous contact, reverse engineering tool, the Cyclone probes port and combustion chamber wall surfaces, collecting dimensional data to produce a 3D wireframe model of the internal features. A three axis digitizer, the Cyclone is fitted with thin probe styli and a fixture that rotates the head in A (rotation about the X axis), and B (rotation about the Y axis) axes to allow access to hard to reach port areas. From that data, Surfcam CAD/CAM software creates five axis NC code that drives the machine tool through its cutting routines. After machining and cleaning, heads are ready for immediate assembly.

Cyclone duplication needs come to Mr. Hudgins from two fronts. Professional engine builders approach Mr. Hudgins to replicate their new port designs and cut them into numerous other castings. Builders for NASCAR teams may order up to 20 sets at the beginning of each new racing season, but they often come back with new designs throughout the season since their head research and development is ongoing. And when they develop a better design midseason, they need—and get—quick turnaround from Mr. Hudgins.

Mr. Hudgins' in-house engineers are also continuously developing their own port designs on a variety of manufacturers' castings (DART, LTI, and so on) for street performance and racing applications. After a newly designed head passes various flow bench and swirl meter tests, the Cyclone digitizes the finalized design for duplication.

Mr. Hudgins has hundreds of port designs and gigabytes of machining programs stored in a DNC workstation to be downloaded to machine tool at a moment's notice. He sells these heads off the shelf, often packaging them with matching cam, intake and computer chips from other vendors for purpose built racing or street applications that often jump base horsepower by 40 percent.

Digitizing deep inside a port—in effect, an angled tube—is more difficult than digitizing the perimeter of an object. Mr. Hudgins designed a special Cyclone fixture that can be rotated in A and B axes manually or by way of servomotors. This rotation, combined with Mr. Hudgins' special thin styli, allows the Cyclone to probe all port nooks and crannies. Rotary encoders register the angular position of the fixture, and a digital display shows position to a thousandth of a degree.

Once all port areas that can be accessed from the home position have been digitized, the fixture is rotated to allow access to other areas of the head, and the new angular position is noted. Separate probing data files, called patches, are stored for each different angular position.

When all areas of the intake port that can be reached from the intake side of the head are probed, the head is rotated to finish probing the intake port through its valve opening, to begin probing the exhaust port through its valve opening, and completely probe the combustion chamber. It is then further rotated to finish probing the exhaust port from the exhaust side of the head. The Cyclone uses Tracecut software to establish the datum, or home position, and to collect the probe data.

The patches are downloaded into Surfcam, which has a provision for inputting patch angular position. Having various mirroring and copying features, Surfcam is used to manipulate and translate the data into five axis NC code for all ports and combustion chambers.

Not every port is digitized. Mr. Hudgins typically digitizes one combustion chamber, intake and exhaust port, and sometimes an additional exhaust port depending on head configuration. Accurate to within 0.002 inch (50 microns), the continuous contact Cyclone scans at a rate of 140 points per second at a maximum scanning speed of 118 ipm (3 m/min.). A small block Chevy head can be scanned on the Cyclone in a couple of hours.

The NC machining code is loaded into a DNC workstation, which stores the program and downloads it to the machine control. Not all machine controls have sufficient processing speed or memory to handle the large five axis part programs. Slow program processing may cause axis lag errors, leading to inaccurate ports.

Heads can be quickly set up on another Hudgins designed fixture, which holds and positions the head on the machine tool. An Aaxis Nikken CNC rotary table is mounted on a Baxis rotary table to supply the fourth and fifth axes of movement. The five axis NC code simultaneously moves the machine tool in X, Y and Z axes and the rotary tables in A and B axes. Ports can be machined in one setup, versus up to five setups for three axis machining. Not only does this provide quicker throughput, but it also eliminates stackup of fixturing errors.

Mr. Hudgins uses standard ball nose end mills and special backcutting tools for hard to reach port areas. As with digitizing, the tools cannot reach all port areas, so the head must be rotated for access to combustion chambers and ports.

It typically takes four hours and two tool passes to machine a NASCAR head, due to the nature of its casting. Those heads are cast with additional material in port areas so that any port design will fit into the head without leaving walls between ports too thin. Street heads, cast with less material, need only one tool pass.

High compression NASCAR engines like a smoother wall finish compared to lower compression street engines. However, Mr. Hudgins doesn't shoot for a specific surface finish spec because it is difficult to determine a very accurate reading on cast aluminum due to material porosity. Rather, he defines surface finish by tool incremental stepover (distance the cutting tool increments into the port after each pass) or cusp measurement (height from bottom to top of cutting groove). For a NASCAR head, tool stepover may be 0.0300.035 inch (0.760.89 mm), where on a street head it would be double that, another reason street heads can be machined quicker. Cusp height, dictated by the tooling chosen, ranges from 0.0002 inch (0.005 mm) on a NASCAR head to 0.005 inch (0.127 mm) on a basic street head.

The Cyclone's work envelope measures 23 inches by 19 inches by 15 inches (600 mm by 500 mm by 400 mm) and it can hold a model weighing up to 500 lbs (225 kg), more than sufficient to support an aluminum or cast iron cylinder head with fixture.

Since the learning curve for this technique is nearly vertical, Mr. Hudgins offers his system to anyone wishing to duplicate his shop's capabilities. He can package everything a job shop needs for five axis cylinder head machining, except the machine tool itself. The system covers the complete spectrum of customer needs, be they single cylinder engines for racing lawn mowers or multivalve Mercedes V12 powerplants.

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