Deep-cavity Milling

Plunge milling can be performed by making repeated axial cuts at the tooling manufacturer’s recommended step-over.

Many problems can arise in the creation of a part with a deep-cavity feature. The side walls can act as barriers, chip removal can prove problematic the deeper into a part the milling cutter proceeds, and toolholders and adapters can even collide with the part.

A few steps can be taken, however, to ensure that these often difficult-to-produce features are milled efficiently.

A smooth engagement with the part is important during the cutting process. Ramping, helical interpolation, and circular interpolation are examples of milling with smooth engagement that will prolong tool life and therefore reduce machining time and the total cost of production. Helical interpolation is best-suited for the creation of wide cavities.

When the corners of the cavity are formed during a side milling operation, it is important to maintain a constant feed rate without exceeding the maximum radial and axial depths of cut. This helps to prolong tool life by reducing the forces being applied to the insert.

If a five-axis machining center is not being used, plunge milling can be performed by making repeated axial cuts at the tooling manufacturer’s recommended step-over. This type of milling, typically used for rough machining, is initiated by drilling a hole for the plunging tool to enter.

As the hole gets deeper, compressed air or high-pressure coolant, preferably directed through the spindle, helps to evacuate chips, especially when milling at the bottom of the component. If chips are not evacuated successfully, double cutting will occur — a major cause of insert edge failure.

“This is very important to help maintain tool life and to flush the chips away so we don’t recut the chips and damage the inserts,” said Tom Hagan, milling product manager for Iscar Canada. “Through the spindle [coolant delivery] helps to directly position the coolant to the insert’s edge, which is the best case for the productivity of the insert.”

When high-pressure coolant is applied, the chip can be easily broken. This becomes even more important in difficult-to-cut materials, such as 300 series stainless steels, which are very gummy.

The material being cut will affect the tool’s ability to evacuate chips. Selecting the appropriate insert grade and geometry is the first step in achieving acceptable tool life and chip evacuation. Typically, short-chipping materials such as cast iron aren’t much of a problem in terms of chip evacuation. Stainless steels and heat-resistant superalloys (HRSA) are more problematic because of the long chips that are created during the cutting process.

Using chip breakers, rake angles, and cutting conditions that are specific to the material being cut also is important, said Hagan.

Vibration caused by a long overhang of the tool reduces tool life and creates poor surface finishes. Dampened tools should be considered.

Eliminate Chatter

Reducing chatter during cavity milling operations will improve productivity because it increases metal removal rates, creates finer surface finishes in fewer steps, and reduces scrap production.

Side benefits to reducing vibration are less wear to the cutting tools and less stress placed on the machining center, especially the spindle. Proper fixturing, workholding, and machine tool maintenance also all work together to reduce vibration.

“Rigidity of the toolholding is very important, so a good spindle and toolholding system are necessary,” explained Kevin Burton, product and application manager - milling products for Sandvik Coromant.

Machine tool and setup rigidity play big roles in any machining process, but become even more so in a long-overhang situation. The more variables there are that add vibration to the milling process, the more difficult it will be to eliminate them.

There is no single cause of vibration, and that means there is no single cure. Vibration creates uneven wear on cutting tools and thereby shortens tool life. If vibration becomes a problem during deep-cavity milling, one or more of the following should be considered:

• Choose milling cutters with a coarse pitch.
• Remove every second insert so that only two inserts are left to cut with. In a worst-case scenario, remove all but a single insert.
• Use positive, light-cutting insert geometries.
• Cut with thin-coated or uncoated inserts.
• Use the smallest possible milling cutter.
• Maintain a constant feed per tooth rate.
• Reduce the radial and axial cutting depths.
• Use a larger adapter to create more stability.
• Use a dampened adapter.

Vibration caused by a long overhang of the tool reduces tool life and creates poor surface finishes, said Burton. In these situations, dampened tools should be considered.