Application tips for hard part turning
In addition to the general recommendations for turning, there are some key factors to consider for hard part turning (if production process includes own preparation of component before hardening):
- Avoid burrs
- Maintain close dimensional tolerances,
- Chamfer and produce radii in the soft stage
- Do not enter or leave cuts abruptly
- Enter or leave by rolling into or out of the cut
Surface measurementsX-axis: Feature lengthY-axis: Diameter deviation
- Good machine stability, clamping and alignment of workpiece are crucial
- As a guideline, a workpiece length-to-diameter ratio of up to 2:1 is normally acceptable for workpieces that are only supported on one end. If there is additional tailstock support, this ratio can be extended
- Note that a thermally symmetrical headstock and tailstock design will add extra dimensional stability
- Use the Coromant Capto® system
- Minimize all overhangs to maximize system rigidity
- Consider carbide shank boring bars and Silent Tools for internal turning
Insert micro geometry
Two typical edge preparations for CBN inserts are S-type and T-type.
- S-type: Has the best edge line strength. Resistant against micro chipping and ensures consistent surface quality.
- T-type: For best surface finishes in continuous cuts, and minimized burr formation in interrupted cuts. Lower cutting forces.
S-typeChamfer with light honing
T-typeChamfer with no honing
Insert corner geometry
- If conditions are stable, always use wiper geometry for best surface finish.
- Use insert with low entering angle approach when productivity demands are high.
- A normal radius insert should be used only when stability is poor (slender workpiece etc.).
Wet or dry machining
Hard part turning without coolant is the ideal situation, and is entirely feasible. Both CBN and ceramic inserts tolerate high cutting temperatures, which eliminate the costs and difficulties associated with coolants.
Some applications may require coolant, for example, to control the thermal stability of the workpiece. In such cases, ensure a continuous flow of coolant throughout the entire turning operation.
Generally, the heat generated when machining is distributed into the chip (80%), workpiece (10%) and insert (10%). This shows the importance to evacuate the chips from the cutting edge zone.
Cutting data and wear
High heat in the cutting edge zone reduces the cutting forces. Therefore a cutting speed that is too low generates less heat and can cause insert breakage.
Crater wear gradually affects the insert strength, but does not affect the surface finish as much. In contrast, flank wear gradually affects the dimensional tolerance.
Share of tool life determining wear
*) Flank wear **) Crater wear
Insert change criteria
Predetermined surface finish (B) is a frequent and practical insert change criterion. Surface finish is automatically measured in a separate station and a value is given to a specified finish quality.
For an optimized and a more stable process, set a predetermined number of components (A) as an insert change criteria. This value should be 10–20% less than the predetermined surface finish, the exact figure is determined on a case-to-case basis.
A: Predetermined number of componentsB: Predetermined surface finishX-axis: Number of componentY-axis: Surface finishBlue line: Insert wearRed line: Maximum
A one-cut "metal removal" strategy is feasible for both external and internal operations. A stable set-up is important and the tool overhang should not exceed the bar diameter in internal turning (1×D). For good machining, we recommend chamfered, lightly-honed inserts and moderate speed and feed.
- Quickest possible machining time
- One tool position
- Difficulties in meeting stringent dimensional tolerances
- Shorter tool life (than two-cut)
- Tolerance deviations due to relatively rapid wear
A two-cut strategy allows unattended machining of high-quality finished surfaces. We recommend roughing inserts with a 1.2 mm (0.047 inch) radius, and the finishing inserts with a chamfer only. Both inserts should have wiper geometry.
- Tooling optimized for roughing and finishing
- Higher security, closer tolerances and potentially longer runs between tool changes
- Two inserts are needed
- Two tool positions
- One extra tool change