Successful chip control
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Chip control is one of the key factors in turning processing, and there are three basic changes in chip breaking:
Self-breaking chips (e.g. gray cast iron)
Impact tool chip breaking
Impact workpiece chip breaking
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self-breaking
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Impact tool chip breaking
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Impact workpiece chip breaking
Factors affecting chip breaking
Insert geometry: more open or tighter chips depending on flute width and micro and macro structural design
Tool nose radius: A small tool nose radius controls chips better than a large tool nose radius
Leading (plunge) angle: Depending on the leading angle, the chips are directed in different directions: towards the shoulder or away from the shoulder
Depth of cut: Depending on the workpiece material, a greater depth of cut will affect chip breaking, resulting in greater cutting forces for chip breaking and chip removal.
Feed: Higher feeds will generally produce stronger chips. May aid in chip breaking and chip control in certain situations
Cutting speed: Changes in cutting speed may affect chip breaking performance
Materials: Short-chip materials (such as cast iron) are generally easier to machine. Chip breaking performance is of greater concern for materials with excellent mechanical strength and resistance to creep (the tendency of a material to move slowly or deform under pressure), such as Inconel
Cutting parameters for turning
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When choosing the correct speed and feed for turning, always consider the machine tool, tool, insert, and material.
Start with low feed rates to ensure insert security and surface quality; then increase feed rates to improve chip breaking
Use a depth of cut greater than the tool nose radius. This minimizes radial deflection of the insert, which is important in internal machining
Setting cutting speed too low will shorten tool life. Be sure to use the recommended cutting speed vc m/min (feet per minute)
Using coolant to improve turned part quality
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When applied correctly, coolant will improve machining security, tool performance and part quality. When using coolant, the following factors should be considered:
Tools with high precision coolant are highly recommended for finishing applications
The coolant pressure required for chip breaking depends on the nozzle diameter (outlet), the material being machined, the depth of cut and the feed
The required coolant flow rate depends on the pressure and the total coolant delivery area of the coolant holes
In semi-finishing and roughing applications, the following coolant is recommended
For finishing operations, it is recommended to use both high-precision upper coolant and lower coolant
Meet a variety of challenges with the correct use of coolant
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Chip Control Issues: Using Overhead Coolant
Dimensional issues: The cause is usually too high a temperature - use both upper and lower coolant and the highest possible coolant pressure
Poor surface quality: If defects are caused by chips, use overhead coolant
Unpredictable tool life in roughing operations: use only lower coolant
Unpredictable tool life in finishing operations: using both upper and lower coolant
Poor chip evacuation in internal turning operations: Use both upper and lower coolants and the highest possible coolant pressure
How to get good surface quality when turning parts
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General rules for surface quality:
Surface quality can often be improved by using higher cutting speeds
Insert geometry (center-mounted, positive and negative rake angles, and positive relief angle) affects surface quality
The choice of blade material has some impact on surface quality
If vibration trends occur, select a smaller tool nose radius
Wiper inserts
Wiper inserts enable turning parts at high feed rates - without losing the ability to produce good surface finishes or chip breaking.
General guideline: double feed rate, same surface quality. At the same feed rate, the surface quality is doubled.
The design of the wiper insert can make the surface processed by the insert smoother when it feeds along the workpiece. The wiper effect is mainly designed for straight line turning and face turning.
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Standard radius
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Wiper radius
Comparison of standard inserts and wiper inserts based on feed rate
Notice! All values corresponding to the standard tool nose R angle are theoretical values. The value corresponding to the wiper tip R angle is based on the test value of low alloy steel.
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1.16 mm (0.06 inch) radius values based on DNMX inserts
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External turning application tips
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Parts prone to vibration
Complete cutting in one pass (e.g. pipe fittings)
It is recommended to complete the entire cut in one pass to direct cutting forces axially in the collet/spindle direction.
Example:
Outside diameter (OD) = 25 mm (0.984 inches)
Inside diameter (ID) = 15 mm (0.590 inches)
Depth of cut ap = 4.3 mm (0.169 inches)
Resulting wall thickness = 0.7 mm (0.028 inches)
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Cutting forces can be directed axially using entering angles close to 90° (plunge angle close to 0°). This will minimize the bending forces on the part.
Cutting completed in two passes
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Simultaneous upper and lower tool holder machining will balance radial cutting forces and avoid part vibration and bending.
Slender/thin wall parts
When turning slender/thin-walled parts, the following factors should be considered:
Use an entering angle close to 90° (an entry angle close to 0°). During the machining process, even a small change (entering angle/plunge angle from 91°/-1° to 95°/-5°) will affect the cutting force direction
The depth of cut ap should be greater than the tool nose radius RE. Large depth of cut ap will increase the axial force Fz and reduce the radial cutting force Fx, thereby reducing vibration
Use inserts with sharp cutting edges and small nose radius RE to reduce cutting forces
Consider using cermet or PVD grades to ensure wear resistance and a sharp blade cutting edge, which is the first choice for this type of process
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Shoulder machining/shoulder turning
Follow steps 1-5 to avoid damaging the insert cutting edge. This method is ideal for CVD coated blades and greatly reduces blade breakage.
Steps 1-4:
Keep the distance of each step (1-4) the same as the feed rate to avoid chip jamming.
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Step 5:
The final cut is made by making a vertical cut starting from the outside diameter and working toward the inside diameter.
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If the shoulder is face turned from the inside diameter to the outside diameter, there may also be a problem with chips wrapping around the tool radius. Changing the tool path can change the chip direction and solve the problem.
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Car end face
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Start with the face (1) and chamfer (2). If possible and the geometry of the workpiece permits, priority is given to machining chamfers (3). Longitudinal cutting (4) is the last step and the insert will advance and retract smoothly during the machining process.
Facing should be the first operation that sets the reference point on the part for the next pass.
When the cutting edge leaves the workpiece, a burr forms at the end of the cut, which is often troublesome. Leaving a chamfer or fillet (flip fillet) can minimize or even avoid burr formation.
A chamfer on the part will allow smoother entry of the cutting edge (whether facing or longitudinal turning).
Intermittent cutting
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When performing interrupted cutting:
Use PVD grades to ensure edge line toughness in applications with fast interrupted cuts (e.g. hexagonal bars)
Use tough CVD grades to ensure overall toughness in large parts and heavy interrupted cutting applications
Consider using high-strength chipbreakers to fully improve chipping resistance
Turning off coolant may help avoid thermal cracks
Machining undercuts on finished parts
Using the largest possible nose radius RE for longitudinal and face turning ensures:
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High strength cutting edge, higher reliability
good surface quality
Ability to use high feeds
Do not exceed the undercut width and perform it as the last step in deburring.
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Internal turning application tips
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Select the largest possible boring bar diameter, but at the same time ensure that there is enough space between the boring bar and the hole for chip removal
Ensure that the cutting parameters used facilitate adequate chip evacuation and produce the correct chip type
Choose the smallest possible overhang, but at the same time ensure that the boring bar length allows for the recommended clamping length. The clamping length must not be less than 3 times the diameter of the boring bar
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Use vibration-damped boring bars when machining vibration-sensitive parts
Choose an entering angle as close to 90° as possible (plunge angle close to 0°) to direct cutting forces along the boring bar. The main deflection angle shall not be less than 75° (the entry angle shall not be greater than 15°)
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As a first choice, indexable inserts should have a positive rake base shape and a positive insert geometry to minimize tool deflection
Select an insert nose radius smaller than the depth of cut
Insufficient cutting edge engagement may increase vibration caused by friction during cutting. Choose a cutting edge engagement that is larger than the tool nose radius to ensure good cutting action
Excessive cutting edge engagement (large depth of cut and/or feed) may increase vibration caused by tool deflection
Uncoated or thinly coated inserts generally produce less cutting force than thickly coated inserts. This becomes particularly important when the aspect ratio is large. Sharp cutting edges often minimize the tendency to vibrate, thereby improving hole quality
For internal turning, geometries with open chipbreakers are often more advantageous
In some operations, insert grades with higher levels of toughness may be considered as they can cope with any risk of chip clogging or vibration tendencies
If you need to improve chip formation, consider modifying the tool path
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Hard Part Turning Application Tips
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In addition to general turning advice, there are some key considerations for hard part turning (such as the production process including part preparation during the soft turning stage prior to hardening):
avoid burrs
Maintain tight dimensional tolerances,
Chamfer and machine radii before heat treatment
Do not suddenly advance or retract the knife
Approach or retract by cutting in or out through an arc
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Surface measurement
X-axis: characteristic length
Y axis: diameter deviation
Clamping
Good machine stability, correct clamping and positioning of the workpiece are crucial
The general guideline is that for workpieces that are supported on only one end, it is generally recommended that the workpiece length-to-diameter ratio does not exceed 2:1. Aspect ratio can be increased if additional tailstock support is present
Please note that the thermally symmetrical design of the cassette and tailstock will further increase dimensional stability
Using the Coromant Capto® System
Minimize all overhangs to maximize system rigidity
For internal turning, consider carbide-shank boring bars and Silent Tools™
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Blade micro-geometry
Two typical edge blunted CBN inserts are S-type and T-type.
S type: has the best edge strength. Resistant to micro chipping to ensure consistent surface quality.
T-type: Able to achieve the best surface quality in continuous cutting and minimize burr formation in interrupted cutting. Cutting forces are lower.
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Tool tip geometry
If conditions are stable, be sure to use a wiper geometry to ensure optimal surface quality.
When higher productivity requirements are required, small entering angle inserts are used.
When stability is poor (slender workpieces, etc.), regular radius inserts should be used.
Wet or dry processing
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Turning hard parts without coolant is ideal and completely feasible. Both CBN and ceramic inserts can withstand higher cutting temperatures, thus eliminating the cost issues and headaches associated with coolant.
Some applications may require coolant, for example to control the thermal stability of the workpiece. In these cases, ensure a continuous flow of coolant throughout the turning process.
Typically, the heat generated during machining is distributed between the chip (80%), the workpiece (10%), and the insert (10%). This shows the importance of removing chips from the cutting edge area.
Cutting parameters and wear
High heat in the cutting edge area reduces cutting forces. Therefore, cutting at too low a speed will generate less heat and may cause the insert to crack.
Crater wear gradually affects blade strength but does not affect surface quality to the same extent. Conversely, flank wear gradually affects dimensional tolerances.
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Wear ratio that determines tool life
*) Flank wear **) Crater wear
Tool changing guidelines
Predetermined surface quality (B) is a commonly used and practical tool change criterion. The surface quality is automatically measured at a separate station and a specified value for the surface quality is given.
In order to achieve an optimized and more stable machining process, a predetermined number of parts (A) is set as the tool change criterion. This value should be 10-20% less than the average number of parts, the exact value will depend on the specific situation.
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A: Number of scheduled parts
B: Predetermined surface quality
X-axis: number of parts
Y axis: surface quality
Blue line: Blade wear
Red line: Maximum Ra/Rz value
One-time cutting strategy
One-cut "metal removal" strategies are feasible for both OD and ID operations. In internal turning, it is important to have stable setup and the tool overhang should not exceed the boring bar diameter (1×D). To obtain good machining results, it is recommended to use lightly honed inserts with chamfered edges and moderate cutting speeds and feeds.
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advantage
Fastest possible processing times
a tool position
shortcoming
Difficulty meeting small dimensional tolerances
Shorter tool life (compared to secondary cuts)
Dimensional deviations due to relatively rapid wear
Two cutting strategies
The two-cut strategy can be used in unmanned production to produce high surface quality. It is recommended to use roughing inserts with a radius of 1.2 mm (0.047 inches) and T-shaped finishing inserts with only one chamfer. Both inserts should have wiper geometry.
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advantage
Tools optimized for roughing and finishing
Greater safety, tighter tolerances and potentially longer tool change intervals
shortcoming
Requires two blades
Two tool positions
One tool change
