BY MICHAEL KNAUER
Toolmakers and mould makers are among the companies keeping a firm handle on the impact of the coronavirus pandemic. Even so, many companies are asking themselves how they can weather the storm unscathed as much as possible and adapt their capacity quickly once the market improves.
This has placed the spotlight squarely on production methods such as “circle-segment milling,” also known as “Parabolic Performance Cutting (PPC)” or “barrel milling.” PPC tools enable the finishing work for a tool mould to be completed up to nine times faster or, alternatively, the surface quality to be improved up to 80-fold. For example, Koller Formenbau GmbH from Dietfurt in the Altmühl Valley, Germany, has used PPC milling cutters supplied by Hoffmann Group to reduce the finish machining time for geometrically defined surfaces from 100 hours to 15 hours. The PPC method is also ideally suited to finish machining work on 3D-printed parts.
For a Reasonable Investment
The main cutting edge is curved on PPC tools. Compared to a classic ball-nosed slot drill where the effective radius is half the tool diameter, PPC tools have a much larger effective radius – up to 1000 mm – permitting a significantly larger engagement length on the workpiece. However, their more complex geometry means they place higher demands on the CAM software. The software not only needs to offer the “circle-segment milling” strategy, it must also have a tooling database that holds the exact geometries of the PPC tools.
What’s more, since the tools are aligned obliquely to the workpiece, the method can only be employed in conjunction with a 5-axis milling machine. Not all 5 axes need to be in use here. Often, once the tool has been set up, the draft angles can also be finished with 1 to 2 axes clamped. It may also be possible for flat faces and freely accessible surfaces with no interfering contours to be machined on a 3-axis machine.
In the past, there were only a few software programs that offered the “circle-segment milling” functionality. However, that is no longer the case. Koller Formenbau, for instance, already used a 5-axis machine and the Hypermill software, so they only needed the right tool. For a reasonable cost, the company was able to boost productivity by up to 85 percent, thereby increasing its capacity without having to procure a new machine.
Tool diversity increases productivity
PPC is particularly effective when the tool profile fits the workpiece contour precisely. To maximize the productivity gains, different types of main cutting edge are required such as “straight,” “tangential,” “conical,” and “stub conical,” as each of these shapes is specifically designed to machine certain surfaces.
Another factor to consider is the tool’s approach angle to the workpiece. A steeper approach angle, for example, makes it easier to avoid interfering contours. Moreover, some manufacturers like Hoffmann Group also offer precision copy milling cutters with PPC indexable inserts. In certain cases, they may be a viable alternative to solid carbide milling cutters. As a result, when selecting PPC tools it is important to consider whether the supplier is able to offer the corresponding range of products. You also need to factor in all the different materials that need to be machine.
By making a moderate investment in software and tooling, companies can use CAM-controlled circle-segment milling to increase flexibility when scaling up their production capacity and respond to changing market conditions.
|Making production capacity more flexible|
|Application areas||Finishing work on tool moulds
Finishing work on 3D-printed parts
|Production method||Circle-segment milling, Parabolic Performance Cutting (PPC), barrel milling|
|Software||CAM software offering “circle-segment milling” functionality,
database containing precise tool geometries
|Tools||Special tools for circle-segment milling, Parabolic Performance Cutting (PPC), barrel milling|
|Materials||Steel, aluminium, hardened and high-alloy steels|
|Machines||5-axis machine: free-form surfaces, complex surface geometries
3-axis machine: flat faces and freely accessible surfaces with no interfering contours
|Advantages||Up to a 9-fold increase in the stepover or an 80-fold improvement in the surface quality,
productivity boosted by up to 85 percent
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