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CUTTING TOOLS
Advanced roughing strategies – Faster through optimised toolpaths Toolpath optimisation, through the use of CAM systems, has been commonplace for decades. In the last 10 years, shops have begun to pair that capability with relatively new machining strategies and specially designed milling tools to optimise rough-machining operations. By Rob Mulders. Dynamic milling or advanced roughing are typical descriptions of this time- and cost-saving strategy. These CAM-based strategies are ones that centre on a cutting tool’s arc of contact and its average chip load. By adapting the tool’s arc of contact via its CAMgenerated toolpath, roughing speeds are increased, effectively controlling process temperature, applying higher feeds per tooth, and increasing 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. The maximum arc of contact on any tool is 180 degrees (its diameter). 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 adapting the arc of contact, shops can reduce the heat generated during roughing. As radial depth of cut decreases, so does a cutter’s arc of contact. A smaller amount of contact results in less time in cut and, therefore, less heat between the tool’s cutting edges and the workpiece. What occurs at the same time is the tool’s cutting edges have more time to cool from the time they exit the cut, revolve and re-enter the cut. These lower machining temperatures, in turn, allow for increased cutting speeds and shorter cycle times.
based rough-machining strategies, it is this tooling that addresses the key issue of chip control along with necessary flute and length requirements. •
Classic roughing. Jabro-HPM (high-performance machining) cutters are specifically designed to rough machine at their full arc of contact and take heavy depths of cut for extremely high-volume material removal applications. These cutters feature special geometries for high performance in specific materials.
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All-round machining. To cover a wider range of workpiece materials, Seco has developed the geometries of its JabroSolid² 500 series of cutters and the JSE560 series has been developed specifically for optimising rough-machining strategies while respecting the all-round material focus. In the JS560 series, features have been added to provide extra stability and reduce tool deflection, while at the same time secure material removal is guaranteed.
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Dynamic milling / advanced roughing specific geometries. For deep pocket and 3D shape advanced roughing/dynamic milling, tool lengths are typically between three and four times diameter. Since the demand is increasingly present, especially in more challenging materials such as stainless steel and titanium alloys, Seco has also developed the advanced roughing, multi-flute series, JS720. This tool is an excellent choice to utilise the full CAM and machine potential while at the same time guaranteeing a secured advanced roughing process.
Average chip thickness and physical load A cutting tool’s average chip thickness is based on physical load and maintained through a combination of feed per tooth and arc of contact adjustments. Because chip thickness constantly changes during 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 (ⱷe, engagement angle) – reduces the chip thickness and allows for increased feed per tooth (fz) as a compensation. For example, consider a 10mm diameter cutter slotting (full arc of contact), at 50% of its full arc of contact (5mm), the cutter is generating its largest average chip thickness/heaviest physical load. In the first 90 degrees the cutter is up-milling until a maximum chip thickness (fz) is reached and continues with the second 90 degrees in down-milling where the chip thickness decreases again to 0. If, for example, the ae drops (ae < 0.5xDc) to 1mm (10%), average chip thickness will become smaller, allowing faster roughing by applying increased fz. While the cutter removes less material, it does so much faster. Also, there is less tool and machine spindle strain generated when compared to taking heavier radial cuts at slower feed rates. In dynamic milling roughing applications, a lower AEMX (radial depth of cut) also allows for an increased APMX (axial depth of cut) for even faster material removal.
Cutter designs for optimised roughing While most cutting tool suppliers offer products designed for specific materials, others such as Seco Tools also develop tool geometries for advanced machining methods. In the case of CAM-
AMT FEB 2022
When a consistent arc of contact is maintained, end mills typically develop a more evenly distributed wear on their flutes. This results in much more predictable tool life. However, long cutters produce equally long chips that can be difficult to evacuate from cutting zones and from the machine tool. To create chips that are smaller and more manageable, both Seco’s all-round advanced roughing geometry JS564 and specific ISO M&S advanced roughing geometry JS720 cutter designs feature chip splitters. These are tiny grooves on the tool’s cutting edges and reliefs. The grooves spaced apart at a distance equal to 1 X DC (cutting diameter). So a 40mm-long, 10mm-diameter cutter would produce chips no longer than 10mm that are quickly evacuated from the cut zone and eliminate the risk of jamming machine tool chip conveyor belt.
