Rough cutting strategies on optimised toolpaths

Toolpath optimisation through the use of CAM systems has been commonplace for quite some time, especially in the die and mould industry sectors. However, only recently have shops begun to pair that capability with relatively new machining strategies and special designed solid rotary cutting tools to optimise rough-machining operations.

These CAM-based rough-machining, or dynamic-milling, strategies are ones that centre on a cutting tool’s arc of contact and its average chip load. By manipulating a tool’s arc of contact via its CAM-generated toolpath, shops can boost roughing speeds, effectively control process temperature, apply higher feeds per tooth, and take increased depths of cut to significantly shorten overall part machining cycle times — all without placing any additional strain on machine tool spindles.

Arc of contact and thermal load in relation to cutting speeds

A cutting tool’s arc of contact is an independent variable that influences thermal load on the tool and is the key to optimised roughing operations.

Maximum arc of contact on any tool is 180 degrees, or basically its diameter. So at a full arc of contact, the radial cutting depth (or cutting width) is the same as the cutter diameter and represented by ae (radial depth of cut) = Dc (cutter diameter).

In manipulating the arc of contact, shops can reduce the amount of heat generated during roughing operations. As the radial depth of cut decreases, so does a cutter’s arc of contact. A smaller amount of contact results in less friction and, therefore, less heat between the tool’s cutting edges and the workpiece it is machining. What occurs is that the tool’s cutting edges gain more time to cool from the time they exit the cut, revolve around, and re-enter the cut. These lower machining temperatures, in turn, allow for increased cutting speeds and shorter cycle times.

Average chip thickness and physical load

A cutting tool’s average chip thickness (hm) is based on physical load and maintained through a combination of feed per tooth and arc of contact adjustments. Because chip thickness constantly changesd uring cutting, the industry uses the term average chip thickness (hm).

A full 180-degree arc of contact will generate the thickest chips at the centre of the cutter’s width. So, a smaller arc of contact – less than 90 degrees (je, engagement angle) – reduces the chip thickness, thus allowing for increased feed per tooth (fz) as compensation.

For example, consider a 10-mm diameter cutter side roughing at 10-mm ae (full arc of contact). At that ae, the cutter is generating its largest average chip thickness/heaviest physical load. Within the first 90 degrees, the cutter is up milling until a maximum chip thickness (fz) is reached, then once into the second 90 degrees, it is down milling where chip thickness decreases again to zero. But if ae drops (ae < Dc) to 1 mm (10 percent), then average chip thickness will become smaller, allowing for faster roughing by applying increased feed per tooth (fz). While the cutter removes less material, it does so at a much faster pace and with less tool and machine spindle strain as compared with taking heavier radial cuts but at slower feedrates. In slot roughing applications, a lower ae also allows for a heavier ap (depth of cut) for even faster material removal.

Cutter designs for optimised roughing

While most cutting tool suppliers offer products for specNew Arrivals