In milling, ramping has gradually grown more significant. The speed and precise interpolation of modern CNC machines make it possible for a small tool to mill out a much larger hole or pocket in a relatively short time. Ramping is an important element of doing this. Either the tool ramps from one level of passes to the next within the feature, or else it follows a helical path at a continuous angle all the way down to the feature's depth.
Limitations on the ability to ramp generally result from the tool. Many end mills that are able to ramp were not necessarily designed to emphasize this type of cutting. When the tool is designed with ramping in mind, various features change.
Brian Hoefler, milling product manager with Kennametal (Latrobe, Pennsylvania), offers some elaboration on this. His company's "Mill 1" end mill is an example of a tool designed for ramping and other aspects of milling within holes and pockets.
The most fundamental design consideration is greater clearance beneath the insert, he says. The clearance lets the tool ramp in at a steeper angle and reach the bottom of the feature sooner, potentially reducing machining time.
Other considerations relate to quality. As the tool removes material from the hole or pocket, it also leaves behind an inner wall. If the tool can leave this wall surface smooth enough, then an extra boring or finish milling operation might be avoided.
The squareness of the insert helps determine the tool's ability to generate this smooth wall. If the insert has a straight side edge, it has to be set in the tool body so that the edge is precisely square with the work. If not, then this cocked insert will leave a line between passes at successive depths.
On the Mill 1 tool, two features address this problem. One is an insert clamping screw oriented at an angle. The screw pulls the insert into the pocket to lock it in place more precisely.
The other feature is a wiper face on the side of the insert that, instead of being straight, is crowned by a slight arc. With this form, the insert doesn't have to be perfectly square in its seating. The crown leaves small scallops between passes that can't be seen, says Mr. Hoefler, and effectively can't be measured.
The angled screw also offers one additional benefit that gets back to cutting time. This screw lets the tool run faster. Because of the angle, part of the centrifugal force resulting from the tool's rotation is directed not against the screw's shear strength, but instead against its tensile strength. The tensile strength is stronger. As a result, the tool can be spun at a faster rpm safely. Its top speed is 30 percent faster than what the tool would permit if the same screw were oriented in the traditional way, says Mr. Hoefler.