To obtain high-quality turned parts, chip control is one of the most important factors to consider. Selecting the correct cutting parameters and applying application techniques can achieve the desired part quality.
Successful Chip Control
Chip control is one of the key factors in turning operations, with three basic types of chip breaking variations:- Self-breaking (e.g., grey cast iron)
- Chip breaking against the tool
- Chip breaking against the workpiece
Factors Influencing Chip Breaking
- Insert geometry: Whether the chip is more open or compact depends on the chip groove width as well as micro and macro structural design
- Nose radius: A small nose radius provides higher chip control than a large nose radius
- Lead angle (cutting edge angle): Depending on the lead angle, the chip is guided in different directions—towards or away from the shoulder
- Depth of cut: Depending on the workpiece material, a larger depth of cut influences chip breaking, creating higher cutting forces to achieve chip breaking and evacuation
- Feed: Higher feed usually generates stronger chips. In some cases, this may help with chip breaking and chip control
- Cutting speed: Changes in cutting speed may influence chip breaking performance
- Material: Short-chipping materials (e.g., cast iron) are generally easy to machine. For materials with excellent mechanical strength and creep resistance (the tendency of a material to slowly deform under stress), such as Inconel, chip breaking performance requires greater attention
Cutting Parameters in Turning
When selecting the correct speed and feed for turning, always consider the machine, tool, insert, and material.- Start with a low feed rate to ensure insert safety and surface quality; then increase feed rate to improve chip breaking performance
- Use a depth of cut larger than the nose radius. This minimizes insert radial deflection, which is especially important in internal turning
- Cutting speed set too low will shorten tool life. Always use the recommended cutting speed vc m/min (ft/min)
Using Coolant to Improve Turned Part Quality
If applied correctly, coolant will improve machining security, tool performance, and part quality. When using coolant, consider the following factors:- Strongly recommended to use precision coolant tools for finishing applications
- The coolant pressure required for chip breaking depends on nozzle diameter (outlet), material being machined, depth of cut, and feed
- Required coolant flow depends on pressure and the total coolant delivery area of coolant holes
- For semi-finishing and roughing applications, use bottom coolant
- For finishing operations, use both precision top coolant and bottom coolant simultaneously
Addressing Various Challenges with Proper Coolant Use
- Chip control problems: Use top coolant
- Dimensional issues: Usually caused by high temperature—use both top and bottom coolant at the highest possible pressure
- Poor surface quality: If defects are caused by chips, use top coolant
- Unpredictable tool life in roughing: Use only bottom coolant
- Unpredictable tool life in finishing: Use both top and bottom coolant
- Poor chip evacuation in internal turning: Use both top and bottom coolant at the highest possible pressure
How to Achieve Good Surface Quality in Turned Parts
General rules for surface quality:- Usually, higher cutting speed improves surface quality
- Insert geometry (neutral, positive rake, negative rake, and clearance angles) affects surface quality
- Insert grade selection has some influence on surface quality
- If there is a tendency for vibration, choose a smaller nose radius
Wiper Inserts
Wiper inserts allow turning at high feed rates—without losing the ability to achieve good surface quality or chip breaking.General rule: Double the feed rate, same surface quality. Same feed rate, double the surface quality.
The design of the Wiper insert makes the surface smoother as the insert feeds along the workpiece. The Wiper effect is mainly designed for straight turning and facing.
External Turning Application Tips
Vibration-prone parts
Single-pass cutting (e.g., tubes)
It is recommended to complete the entire cut in one pass to direct cutting forces axially towards the chuck/spindle.Example:
OD = 25 mm (0.984 in)
ID = 15 mm (0.590 in)
Depth of cut ap = 4.3 mm (0.169 in)
Resulting wall thickness = 0.7 mm (0.028 in)
Use a lead angle close to 90° (approaching 0° entering angle) to direct cutting forces axially, minimizing bending forces on the part.
Two-pass cutting
Simultaneous upper and lower turret machining balances radial cutting forces, avoiding vibration and bending of the part.Slender/thin-walled parts
When turning slender/thin-walled parts, consider the following:- Use a lead angle close to 90° (approaching 0° entering angle). Even small changes (from 91°/-1° to 95°/-5°) affect force direction
- Depth of cut ap should be larger than nose radius RE. A larger depth of cut increases axial force Fz and reduces radial cutting force Fx, minimizing vibration
- Use inserts with sharp cutting edges and small nose radius RE to reduce cutting forces
- Consider cermet or PVD grades for wear resistance and sharp edges—these are preferred in such operations
Shoulder Machining/Shoulder Turning
Follow steps 1–5 to avoid damaging the cutting edge. This method works well with CVD-coated inserts, significantly reducing insert breakage.Steps 1–4:
Each step distance should match the feed to avoid chip jamming.Step 5:
Complete with a vertical cut from OD to ID.If facing a shoulder from ID to OD, chips may wrap around the tool radius. Changing tool path can redirect chips and solve the problem.
Facing
Start with facing (1) and chamfering (2). If possible, and if the part geometry allows, prioritize chamfering (3). Longitudinal turning (4) should be the final step, with smooth entry and exit of the insert.Facing should be the first operation to set a reference point for the next pass.
When the cutting edge exits the workpiece, burrs often form at the end of cut. Exiting on a chamfer or radius (blended radius) minimizes or avoids burrs.
Chamfers allow smoother entry for the cutting edge (whether facing or longitudinal turning).
Interrupted Cutting
For interrupted cutting operations:- In high-speed interrupted cutting (e.g., hexagonal bars), use PVD grades for edge toughness
- For large parts and heavy interrupted cuts, use tough CVD grades for overall toughness
- Consider strong chipbreaker geometries to maximize edge resistance
- Turning off coolant may help avoid thermal cracks
Machining Relief Grooves on Finished Parts
Use the largest possible nose radius RE for longitudinal turning and facing to ensure:- High-strength cutting edge and higher reliability
- Good surface quality
- Ability to use high feeds
Do not exceed relief groove width; perform as the final deburring operation.
Internal Turning Application Tips
- Select the largest possible boring bar diameter while leaving sufficient space for chip evacuation between bar and hole
- Ensure cutting parameters promote proper chip evacuation and correct chip types
- Keep overhang as short as possible while ensuring boring bar length achieves recommended clamping length (≥3× bar diameter)
- For vibration-sensitive parts, use dampened boring bars
- Use lead angles close to 90° (approaching 0° entering angle) to direct forces along the bar. Lead angle must not be less than 75° (entering angle no greater than 15°)
- Prefer positive basic insert shapes and positive rake geometries to minimize tool deflection
- Choose nose radius smaller than depth of cut
- Too little cutting edge engagement increases vibration caused by friction. Choose cutting edge engagement greater than nose radius for effective cutting action
- Too much engagement (large depth/feed) increases vibration from tool deflection
- Uncoated or thin-coated inserts generate lower cutting forces than thick-coated inserts. This is especially important at high length-to-diameter ratios. Sharp edges minimize vibration and improve hole quality
- For internal turning, open-style chipbreakers are often more effective
- In some cases, tougher grades may be chosen to cope with chip clogging risks or vibration tendencies
- If chip formation needs improvement, consider modifying tool path
Hard Part Turning Application Tips
In addition to general turning recommendations, hard part turning requires some key considerations (such as soft turning preparation before hardening):- Avoid burrs
- Maintain strict dimensional tolerances
- Perform chamfering and radius machining before heat treatment
- Do not use sudden entry/exit cuts
- Use arc-shaped entry/exit cuts
Clamping
- Good machine stability, proper workpiece clamping, and positioning are critical
- As a rule, for workpieces supported at only one end, a length-to-diameter ratio of no more than 2:1 is recommended. With tailstock support, ratio can be increased
- Note that thermally symmetric chuck and tailstock designs further improve dimensional stability
- Use Coromant Capto® system
- Minimize all overhangs to maximize system rigidity
- For internal turning, consider carbide shank boring bars and Silent Tools™
Insert Edge Preparation
Two typical CBN edge-honed inserts are S-type and T-type.- S-type: Best edge strength. Provides resistance to micro-chipping and ensures consistent surface quality.
- T-type: Best surface quality in continuous cuts, minimizes burrs in interrupted cuts, generates lower cutting forces.
Insert Geometry
- For stable conditions, always use Wiper geometry to ensure best surface quality.
- For high productivity, use small lead angle inserts.
- For poor stability (slender parts, etc.), use standard radius inserts.
Wet or Dry Machining
Dry machining is ideal and fully feasible for hard part turning. CBN and ceramic inserts withstand high cutting temperatures, eliminating coolant-related costs and challenges.Some applications may require coolant, e.g., to control part thermal stability. In such cases, ensure continuous coolant flow throughout the turning operation.
Typically, generated heat distribution: 80% to chip, 10% to workpiece, 10% to insert. This highlights the importance of chip evacuation from the cutting zone.
Cutting Parameters and Wear
High heat in the cutting edge zone reduces cutting forces. Too low cutting speed generates less heat and may cause insert breakage.Crater wear gradually weakens insert strength but does not affect surface quality as much. Flank wear, however, gradually impacts dimensional tolerance.
Tool Change Criteria
Predetermined surface quality (B) is a common and practical tool change criterion. Surface quality is automatically measured at a separate station and compared against specified values.For optimized and more stable machining, set a predetermined number of parts (A) as the tool change criterion. This value should be 10–20% less than average part count, depending on conditions.
Single-Pass Strategy
The single-pass “metal removal” strategy is feasible for both external and internal turning. In internal turning, stable clamping is essential, and tool overhang should not exceed boring bar diameter (1×D). For good results, use chamfered, lightly honed inserts with moderate cutting speed and feed.Advantages
- Fastest machining time
- One tool position
Disadvantages
- Difficult to achieve tight tolerances
- Shorter tool life (compared to two-pass strategy)
- Dimensional deviations occur due to faster wear
Two-Pass Strategy
The two-pass strategy can be used in unmanned production for high surface quality. A roughing insert with 1.2 mm (0.047 in) radius and a T-type finishing insert with one chamfer should be used. Both inserts should have Wiper geometry.Advantages
- Tools optimized for roughing and finishing
- Higher safety, tighter tolerances, and potentially longer tool change intervals
Disadvantages
- Requires two inserts
- Two tool positions
- One tool change
In conclusion, achieving high-quality turned parts requires a balance of chip control, optimized cutting parameters, proper coolant application, and stable tooling strategies. By carefully considering insert geometry, machining conditions, and part-specific challenges, manufacturers can improve surface finish, extend tool life, and ensure consistent dimensional accuracy. Ultimately, successful turning operations depend on combining technical precision with practical application techniques to deliver reliable, efficient, and high-performance results.
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