Here is some good reading on different types of surfacing machines that are used:
http://www.aa1car.com/library/2004/eb40444.htm
Excerpt from it:
The surfacing needs of today's engines are as complex as the engines themselves. Multi-valve aluminum cylinder heads on cast iron blocks are a tough combination to seal because of the difference in expansion rates between the two metals. If the surface finishes on the head and block are too rough, lateral shearing forces which occur as the engine heats up and cools down can literally scrub a head gasket to death.
To handle such forces, a growing number of late model engines now use multi-layer steel (MLS) head gaskets. These include many Ford, Chrysler, Honda/Acura, Mazda, Mitsubishi, Nissan, Subaru and Toyota engines, plus Jeep 4.7L V8 and Volkswagen 2.0L SOHC engines. MLS gaskets typically have 3 to 5 layers of steel and are extremely tough. But they have little conformability and require a very smooth finish on both the block and cylinder head to seal properly. OEM specifications vary, but surface finish requirements for MLS head gaskets typically range from 15 to 30 microinches Ra or less.
The ability to duplicate OEM surface finishes is a must on these applications, especially if OEM style MLS gaskets are being installed in the engine. Some aftermarket replacement gaskets for these engines have a thicker surface coating that can accommodate a more "traditional" surface finish of up to 60 microinches Ra. But most MLS replacement gaskets require the same surface finish as the original - which means engine builders must have surfacing equipment that can duplicate a finish that meets OEM specifications.
MLS gaskets have also become very popular in racing. Most NASCAR engines today are running MLS gaskets, and the people who are putting these engines together typically want finishes in the low teens or even single digits! That level of smoothness can only be obtained with equipment that has the rigidity, accuracy and control flexibility to produce an ultra-smooth finish.
A number of things are required to achieve a really smooth finish. One is that a surfacer must allow the right combination of cutting speed and feed rate to achieve really low numbers. If the feed rate is too fast or the cutter spins too slowly, the finish may be too rough. Surfacers with multi-bit cutters can handle much faster feed rates than a surfacer with only a single bit cutter - but the tradeoff is higher tooling and maintenance costs, plus more time to set up and adjust multiple cutting bits.
A feed rate of two inches per minute at 1,000 rpm with a two-bladed cutter will typically give a surface finish in the low teens. Increase the feed rate, decrease the rpm or use a single-bladed cutter and the numbers go up. To get the same finish with a single cutter head, the feed rate has to be cut in half to one inch per minute at the same 1,000 rpm cutting speed.
Smooth surface finishes can be achieved with any kind of cutting bit including carbide, CBN or PCD, but carbide obviously wears faster and requires more frequent replacement. The economics of carbide versus PCD or CBN will depend on the volume of work that's being done, the type of heads being resurfaced (cast iron, aluminum or both), the number of bits per cutter and the cost per bit. Many shops have gone away from carbide and are now using CBN and/or PCD. In the past, PCD was recommended for aluminum and CBN for cast iron. But many shops report having success using CBN alone for both metals.
guess the beast will be off the road for longer...
