By Todd Miller, Manager, Rotating Products
If you’ve ever had premature insert failure, poor surface finish or work hardening issues when milling, it’s often the result of not having the correct average chip thickness. By calculating average chip thickness and finding the proper feed per tooth, you can eliminate problems with finish, vibration and deflection as well as achieve peak productivity and tool life.
It has been my experience, however, that people either overlook or misunderstand this basic milling principle. They often don’t realize how chip formation differs between turning and milling. In turning, chip formation remains the same throughout the cut because the feed rate remains constant. Chip formation in milling, on the other hand, varies as the insert moves in and out of the cut.
Chip thickness is the thickness of the non-deformed chip at the right angles of the cutting edge, and it is influenced by the radial engagement, edge preparation of the insert and feed per tooth. Keep in mind, however, that different radial widths of cut and different lead angles require feed rate adjustments to maintain proper chip thickness. For example, when using a 90 degree lead indexable milling cutter, if your radial depth of cut is less than 50 percent of the cutter diameter, you must increase your feed rate to maintain appropriate average chip thickness.
|Chip thickness is the thickness of the non-deformed|
chip at the right angles of the cutting edge.
Furthermore, it’s important to have the thickness of the chip equal or exceed the edge preparation on your insert, and there are charts and formulas you can use to calculate the correct feed rate for maintaining average chip thickness. These charts and formulas are in the back of our milling catalog. However, there are some key things you need to consider before making average chip thickness calculations.
• Know the average chip thickness (Hm) value associated with your insert. We assign designations to our inserts to make determining this value easy. If you have an M15, for example, the “M” stands for medium operations and the “15” indicates the average chip thickness in metric (.15 mm) based on the insert’s edge design. Your average chip thickness value is a key indicator in determining your minimum feed rates.
• Evaluate the radial width of your cut and determine what percentage of radial engagement you must have. The smaller the radial engagement the larger the feeds you will have.
• Know the lead angle of your cutter, which is the angle that is formed between the outer edge of the insert and the center axis of the cutter. A lead angle is typically 90-, 60- or 45-degrees, and the smaller the angle of engagement, the larger the feed.
We ask our customers to do the average chip thickness calculations because we know it will improve their productivity and tool life. If you have any questions about these calculations, please don't hesitate to contact me.
Todd is the manager of rotating products for NAFTA, responsible for solutions and applications involving face, square shoulder and disc milling. Todd and his team of product experts are dedicated to providing a consistent, high-level of support to Seco customers throughout the United States, Canada and Mexico. In his spare time, Todd likes to bowl and cheer on the University of Michigan football team.