by Brian MacNeil
Even simple shoulder milling can become a problem if you don’t follow the rules
The simple task of milling a quality shoulder can be complicated if we don’t follow some simple rules. The objective for any company producing tools is to offer trouble-free, reliable milling performance with great process security.
Consider the following points to get the best start possible.
Quality toolholding provides you with minimal runout and optimal rigidity, critical with any cutting tool. Runout at the cutting edge reduces tool life drastically, has a negative effect on productive cutting data and results in poor surface finish. Heat shrink systems, hydraulic chucks and mechanical milling chucks are the first choice and offer the best rigidity and runout.
Length-to-tool diameter ratio. Different tool body materials offer different rigidity at different lengths. Steel works best at a ratio of 1:3 times diameter, heavy metal at 1:5 times diameter, and carbide 1:6 times diameter. For length to diameter ratios above seven, use tools with dedicated dampening devices built in. Always give yourself the best opportunity for success by using the shortest possible gauge length for the given tool material.
Choosing grades. CVD (chemical vapour deposition) coatings are typically thicker and provide better protection against heat in large arcs of engagement. The thickness of these coatings tends to round off cutting edges and don’t keep the tool as sharp. This can have a negative impact when doing finish cuts with low arcs of engagement, but again, is beneficial if heat is present in the process. If materials have a tendency to be “sticky or gummy” CVD is not your first choice.
When taking finish passes on shoulders or having a low arc of engagement where sharp edges are required to prevent the tool from deflecting or rubbing, PVD coatings should be your first choice. These thin coatings keep the edge prep closer to being as sharp as it was intended to be. This helps to cut the material rather than push it around. These coatings don’t deal well with heat and perform better in situations where coolant needs to be applied. They’re also the best alternative to CVD in “sticky or gummy” materials.
Choosing geometries. Choosing insert geometry suitable for the material you are cutting can also have a big impact on security and productivity and is a complex area. Inserts are sold either direct pressed or ground. Each is suited to work well in many areas.
Loading inserts into the tool. Square shoulders and quality surfaces rely on minimal runout. Having a quality toolholder can’t make up for bad hygiene at the cutting edge. Make sure to clean pockets with air at every index to remove debris that can cause poor seating of the insert. Ensure you load the inserts with matching corners. Most milling insert corners are identified. Matching these corners in the cutting position reduces mismatch from one insert to the next. Use lubricant or anti-seize on all screw threads and faces.
Process security when loading the insert. Use a wrench capable of providing the correct torque to the screw. This gives the best pre-load designed for the screw and prevents over or under tightening. Both can lead to movement of the insert. This will cause poor quality components and potentially broken inserts.
Speed and Feed. Understanding the material composition you are cutting is critical to understanding the speed that you will be able to run. Use the materials’ guide in the back of the catalog and reference the right composition and hardness. You can then reference the grade you chose and determine a starting speed for the tool.
Feed rates based on chip thickness, also known as “hex” for a given geometry, are the best way to calculate table feed. Referenced in the cutting data section of most catalogs are all of the tool concepts and geometries. Each geometry has an optimized chip thickness relative to all materials. Once we know that, we can calculate the feed per tooth (fz) based on lead angle and arc of engagement.
One more consideration is the arc of engagement on the wall. Do you have the correct feed rate? If your tool engagement is below 25 per cent of the diameter of the tool, you need to increase the feed rate to bring back the chip thickness you lose with the reduced angle on the arc of contact. If you don’t, your chip could be too thin. This can cause heat from rubbing, which will leave a poor surface finish and decrease tool life. Using a milling calculator or app is the best way to calculate the required increase in table feed.
The concept of ‘producing more with less’–an increasingly prevalent industry trend–is facilitated further by multi-purpose end milling cutters capable of delivering top level performance in many material areas. Plan upfront and use good machining practices to ensure success. SMT
Brian MacNeil is milling products and application specialist with Sandvik Coromant in Canada.