Turning, milling and drilling techniques are used by manufacturers to machine features on workpieces. However, these processes can also produce burrs and sharp edges at the feature borders, potentially causing material breakage when the part is in use, or structurally weakening it to pose a danger for those handling it. End users reject parts from suppliers based on these burrs or sharp edges.
Traditionally burrs and sharp edges are removed using hand grinders and other manual processes. Apart from being slow, these methods also require that the part be taken out of the machine tool and refixured for the deburring or chamfering operations. Even when performed by skilled craftsmen, these manual processes lack the necessary process consistency from part to part.
Mechanised Edge Profiling (MEP) is a productive alternative to manual deburring processes as it eliminates unacceptable edge conditions by applying an engineered tool and the same equipment that machined the part features. Key benefits of the MEP process include enabling the final edge condition to be exactly defined and programmed through the machine’s CAM system, resulting in maximum repeatability; reducing overall part production time because the part does not have to be removed from the machine and refixtured; and eliminating tolerance stacking and other inconsistencies occurring from setup to setup. In response to this trend, modern cutting tool makers continue to develop new and productive tools that enhance the benefits of the MEP process.
Jet aircraft components are prime candidates for the application of MEP given the aerospace industry’s increasingly rigorous demands for parts accuracy and consistency. Aircraft turbine engine components, for example, are generally categorised as non-rotating and rotating. For MEP of non-rotating engine parts such as drums and casings, edge profiling usually consists of standard chamfer and break edge tooling, applied on the equipment that machined the part.
For critical rotating parts such as fan and compressor discs, end users have higher standards and demand complete elimination of surface imperfections. Edge conditions typically must undergo lab approval and certification. To deburr these parts, toolmakers have developed high-accuracy, fully repeatable, custom MEP tooling.
MEP tool development
Standard deburring and profiling tools, such as those applied on non-rotating components include coated solid-carbide chamfering endmills with 45° and 60° cutting edges, as well as tools that use indexable inserts to produce 45° and 60° chamfers.
For the most critical applications, toolmakers provide tools custom-engineered to profile edges and remove burrs specifically at the entry or exit of a hole. Some tools combine those capabilities and can remove both entry and exit side burrs. These custom tools often feature complex cutting geometries with the most sophisticated ones having edge designs that produce a chamfer with a radiused edge, which is preceded by lead-in and lead-out angles engineered to prevent formation of secondary burrs.
Various application analyses have indicated that MEP tools engineered for removing burrs at the top or entry of a hole provide longer tool life than tools intended for removing burrs at the bottom or exit end of a through-hole. That is because a deburring tool designed to reach through a part to access the hole exit will be longer and smaller in diameter than one intended to do its work from just one side of the hole. A longer and smaller-diameter tool is more prone to instability and vibration, both of which can chip or break a carbide tool. As a result, most shops opt to use separate tools to deburr the entry and exit edges of a hole rather than a single tool that can do both.
Longer, smaller-diameter tools also require more care with regard to choosing cutting parameters. A short, sturdy tool can run faster without vibration or other problems. Part geometry and features make a difference as well. When cutting conditions are stable and cuts are smooth and uninterrupted, more aggressive cutting parameters can be applied. On the other hand, part features such as access holes that interrupt MEP cutting paths force the use of more conservative parameters so as to minimise tool wear and prevent premature failure.
MEP’s ongoing development also includes tools that combine machining of a feature with deburring. For instance, the MEP cutting edge would be located at the top of the endmill so it could simultaneously machine the diameter of the hole and deburr the entry edges.
Many aerospace materials present additional challenges for machining when removing burrs and chamfering sharp edges. Nickel-base alloys used in engine components, for example, are tough and poor conductors of heat, leading to the cutting tool absorbing the heat generated in the cutting process, which accelerates tool wear.
When determining the metallurgy and geometry of a tool, toolmakers have to therefore, strike a balance between edge sharpness and edge strength. A hard carbide substrate material may resist thermal and abrasive wear very well, but it will lack the impact resistance of a substrate that features additions of cobalt or other alloying material to increase its toughness. Likewise, a dead-sharp cutting edge may be more prone to breakage as compared to one that has a hone or other edge-rounding preparation. Toolmakers also fine-tune rake and helix angles as well as tool coatings to achieve the best results with specific workpiece materials.
For processing large holes and edges, toolmakers can design tools of any size using large blanks. There are limits for smaller holes. Currently the smallest radius that can be ground is about 0.2mm, with proportionately smaller lead-in and lead-out angles.
Custom MEP tools have specific radii, chamfers, angles and combinations of these features. The tools commonly have square cutting edges. However, ballnose and lollipop-style tools are also available to profile features of a component whose contours restrict access of a square-edged MEP tool. Applied on a five-axis machine, these tools can scan the line of a complex part profile and create a radius on long contoured edges.
MEP in operation
To maximise accuracy and consistency and save the time spent moving a part from machine to machine, manufacturers usually perform MEP as a portion of the actual part feature machining operation.
Typically, deburring occurs after all machining operations are completed. The CAM program directs the MEP tools to deburr all the holes and break sharp edges in sequence. Some MEP tools can be used to deburr a variety of holes, and some profiling tools can be applied on three or four different locations or features, such as the bottom of a hole as well as the bottom of a scallop contour.
To ensure that the edge profiling takes place in the correct location and to the proper extent, the hole or feature involved must be defined or measured before the MEP operation begins. When part tolerances are very tight, the location of the part surface is well defined and in-process measurement may be unnecessary. However, when tolerances are generous, measurement is necessary after initial machining to determine the location of the edge or feature to be profiled.
The tool must also be measured and located to ensure that it will profile the part correctly. Since the tool radii are so small and unmeasurable, the tool length is specified in the CAM program. The operator can confirm the tool length away from the machine with a presetter or on the machine via a laser or touch probe. Feed rates are calculated relative to the measured dimensions of the part features and the tool. The most sophisticated custom deburring tools are 100 percent measured by their manufacturer to a tolerance of 40 microns on the tool profile, including runout.
The deburring or chamfering operation should be considered as a finishing pass, with the primary focus on quality. Productivity is always important; however pushing the tool to maximise output, especially with expensive aerospace components, can have negative repercussions. It is important to ensure consistency, reliability and elimination of scrap parts.
Parts with out-of-specification sharp edges and burrs are more and more frequently considered to be expensive scrap. This is strongly evident in the aerospace industry, but is a growing trend in some critical applications within the medical, energy and other industries. Manufacturers need a method to deburr components and profile part edges that is consistent, documentable and cost efficient. Mechanised Edge Profiling (MEP) fulfils that need because it replaces manual operations, eliminating inconsistencies, manual labour expense, setup and part handling expense. Some end users have already banned manual deburring because it cannot be documented and certified.
The most efficient and cost-effective MEP represents a combination of engineering development and application expertise. Toolmakers that offer such a total solution will help streamline the aerospace manufacturing process as well as similar processes in other critical industries, producing new levels of quality and productivity in parts manufacturing.
Written by Teun van Asten, Engineer Marketing Services and Jan Willem van Iperen, Application Engineer Solid Milling at Seco Tools.
Seco Tools is a leading global provider of metal cutting solutions for milling, turning, holemaking and toolholding applications.