Haas Five-Axis CNC Machine - Cutting It Close

As the probe is moved throughout the ports, the Verisurf software displays a 3-D model onscreen, which enables the operator to gauge how much progress is being made. Since the probe will inevitably pass over the same spot multiple times, redundant data points can be filtered out after digitization is complete. A huge perk of digitizing the ports is that dimensions that are difficult to calculate manually, such as the average cross-sectional area, can be precisely measured.

The path in which the CNC tool must travel to create the desired port shape is called the tool path. After creating a 3-D model of the ports and chamber, the projected tool path is put into motion on screen to check for potential hang-ups. The tool path is then run through a post processor to write the actual program, which is called the G-code. Different CNC machines have different axes and ranges of travel, so post processing software is specific to each machine.

The digitizing process is repeated for the combustion chamber. Since the chambers have very complex contours with tight radii, it's often necessary to attach a tiny 3mm probe onto the FARO arm. The digitized chamber data can also be sent to the piston manufacturer to help create a dome that fits perfectly inside the chamber.

With symmetrical heads, such as GM Gen III/IV castings, the prototype ports and combustion chamber are cloned for the remaining three cylinders. This ensures that every single port and chamber in the head is identical, a feat that's nearly impossible when porting by hand. The port on the left is a hand-ported port, while the others have been cloned using Verisurf software based on bore spacing of the block. The software is so precise that it allows modifying a hand-ported surface even further.

In addition to mapping out the ports and chamber, the FARO arm creates virtual planes based on the angle and position of the deck, intake port, exhaust port, and valve cover rail surfaces. The dowel locations and front and rear surfaces of the head are mapped out as well. This enables the FARO arm to orient the location of the ports and chambers in relation to the rest of the head.

The digitized data from the Verisurf software is run through a post processor to create lines of G-code compatible with the Haas CNC machine. That data is then loaded up into the CNC machine's control station via a USB drive, which interprets the code using Mastercam software to determine the tool path. With a five-axis mill, where the A- and B-axes meet is called the point of origin. The final step before machining begins is making sure that the computer program and the CNC machine have the same point of origin.

With race-style raw head castings, the sequence in which various sections of the ports and chambers are cut is critical. Since the raw Victor LS-R castings have no combustion chambers, accessing the short-turn radius through the chamber side of the head is impossible. Consequently, the chamber must be cut first before the short-turn radius can be machined.

The proof is in the flow numbers. The prototype LS-R ports flowed 460 cfm on the intake side and 280 cfm on the exhaust side. The CNC ports that were machined during our visit flowed 460 cfm and 281 cfm, respectively. The R&D work involved to design one set of prototype ports and one chamber can take two weeks. A CNC'd port can be cut in one hour.

With a proven CNC program, machining a head is as easy as bolting it to the fixture and pressing "Start." However, running a new program for the first time requires periodic checks to make sure the cutting speed, spindle rpm, and finish on the port and chamber surfaces are all within established parameters. The order of machining operations can be changed on the fly as well. The maximum amount of material removed in one pass should be limited to 1/4 inch. The step-over distance of the tool path determines the size of the ridges that are usually visible with CNC-ported heads. A 0.080- to 0.100-inch step-over is typically used when roughing in a port, and a 0.020- to 0.030-inch step-over is common on the finishing pass. SAM cut its LS-R heads with an ultra-fine 0.005-inch step-over, which creates a smooth finish almost indistinguishable from a hand-ported head.

The precision of the CNC-machined chambers is equally as impressive. They measured in within one tenth of a cubic centimeter of the prototype chamber.