Successful Chip Control
Chip control is one of the key factors in turning, and there are 3 basic chip breaking forms:
Self breaking chips (e.g. gray cast iron)
impact tool chip breaking
Chip breaking by impacting the workpiece
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Chip breaking
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impact tool chip breaking
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Chip breaking by impacting the workpiece
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Influencing factors of chip breaking
Insert geometry: more open or more compact chip depends on flute width and micro and macro structural design
Nose radius: A small nose radius controls the chip more than a large nose radius
Entering (lead-in) angle: Depending on the entering angle, chips are directed in different directions: towards or away from the shoulder
Depth of cut: Depending on the workpiece material, greater depths of cut will affect chip breaking, resulting in greater cutting forces for chip breaking and evacuation
Feed: Higher feed will generally produce stronger chips. May aid in chip breaking and chip control in some cases
Cutting speed: Changes in cutting speed may affect chip breaking performance
Materials: Short chipping materials such as cast iron are usually easy to machine. For materials with excellent mechanical strength and creep resistance (the tendency of a material to move slowly or deform under pressure), such as Inconel, chip breaking is of greater concern
Cutting data for turning
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When choosing the correct speed and feed for turning, it is important to consider the machine tool, tool, insert and material.
Start with low feed rates to ensure insert safety and surface quality; then increase feed rates to improve chip breaking
Use a depth of cut greater than the nose radius. This minimizes radial deflection of the insert, which is important in internal machining
Setting the cutting speed too low will shorten tool life. Always use the recommended cutting speed vc m/min (ft/min)
Using Coolant to Improve Turned Part Quality
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When applied correctly, coolant will improve process 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 nozzle diameter (outlet), material being machined, depth of cut and feed
The required coolant flow depends on the pressure and the total coolant delivery area of the coolant holes
In semi-finishing and roughing applications, under coolant is recommended
For finishing operations, it is recommended to use both high-precision overcoolant and undercoolant
Meeting challenges with the correct use of coolant
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Chip Control Issues: Use Over Coolant
Dimensional issues: the cause is usually too high temperature - use both over and under coolant and the highest possible coolant pressure
Poor surface quality: use over coolant if the defect is caused by chips
Unpredictable Tool Life in Roughing Operations: Only Use Below Coolant
Unpredictable Tool Life in Finishing Operations: Using Over and Under Coolant Simultaneously
Poor chip evacuation in internal turning operations: use both over and under coolant, and the highest possible coolant pressure
How to achieve 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 (central, positive and negative rake and positive relief) affects surface quality
The choice of insert material has some effect on surface quality
If there is a tendency to vibrate, choose a smaller nose radius
Wiper blades
Wiper inserts are capable of turning parts at high feed rates - without losing the ability to produce good surface finish or chip breaking.
The general guideline is: twice the feed rate, same surface quality. Same feed rate, twice the surface quality.
Wiper inserts are designed to produce a smoother surface as the insert is fed along the workpiece. The wiper effect is primarily designed for straight line turning and facing.
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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 standard tool nose R angles are theoretical values. The value corresponding to the R angle of the wiper (wiper edge) is based on the test value of low alloy steel.
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1.16 mm (0.06 in) radius values are based on DNMX inserts
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External turning application skills
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Parts prone to vibration
Cutting in one pass (e.g. pipe fittings)
It is recommended to complete the entire cut in one pass to direct the 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 approaching 90° (leading angles approaching 0°). This will minimize the bending forces experienced by the part.
Cut in two passes
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Simultaneous machining of upper and lower turrets balances radial cutting forces and prevents vibration and bending of the part.
Slender/thin-walled parts
When turning slender/thin-walled parts, the following factors should be considered:
Use an entering angle close to 90° (leading angle close to 0°). During machining, even small changes (entering angle/lead 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. A large depth of cut ap will increase the axial force Fz and reduce the radial cutting force Fx, thus reducing vibration
Use inserts with sharp cutting edges and a small nose radius RE, thereby reducing cutting forces
Consider cermet or PVD grades to ensure wear resistance and sharp blade cutting edges, which are preferred for this type of operation
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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 inserts and can greatly reduce insert breakage.
Steps 1-4:
Keep the distance and feed rate the same for each step (1-4) to avoid chip jamming.
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Step 5:
The final cut is done by making a vertical cut from the outside diameter towards the inside diameter.
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Chip wrapping on the tool radius can also be a problem if the machining sequence is ID to OD when facing a shoulder. Changing the toolpath can change the chip direction and solve the problem.
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car end
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Start with the facing (1) and the chamfer (2). If possible and the geometry of the workpiece allows it, the chamfer (3) is given priority. Longitudinal cutting (4) is the final operation and the insert will enter and exit smoothly during the process.
Facing should be the first operation to set a reference point on the part for the next pass.
Burrs form at the end of the cut when the cutting edge leaves the workpiece, which is often troublesome. Leaving a chamfer or fillet (flipping a fillet) can minimize or even avoid burr formation.
A chamfer on the part will allow for a smoother entry of the cutting edge (whether facing or longitudinal turning).
Interrupted cutting
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When performing interrupted cutting:
Use PVD grades for edge-line toughness in applications with fast interrupted cuts (e.g. hex bar)
Use tough CVD grades to ensure overall toughness in large parts and heavy interrupted cut applications
Consider using a high-strength chipbreaker to maximize chipping resistance
Turning off coolant may be beneficial to avoid hot cracks
Machining undercuts on finished parts
Use the largest possible nose radius RE for longitudinal turning and facing, thereby ensuring:
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High strength cutting edge, higher reliability
good surface quality
Able to use high feed
Do not exceed the undercut width and perform it as the last step in deburring.
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Internal turning application skills
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Choose 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 evacuation
Ensure the cutting parameters used are conducive to 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 shall not be less than 3 times the diameter of the boring bar
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Use dampened boring bars when machining vibration-sensitive parts
Choose an entering angle as close to 90° (and a lead angle as close to 0°) as possible to direct cutting forces along the boring bar. The entering angle shall not be less than 75° (the cutting angle shall not be greater than 15°)
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As a first choice, indexable inserts should have a positive base shape and a positive insert geometry to minimize tool deflection
Choose an insert nose radius that is smaller than the depth of cut
Insufficient cutting edge engagement can increase vibration caused by friction during cutting. Choose a cutting edge engagement larger than the 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 lightly coated inserts generally generate lower cutting forces than thickly coated inserts. This becomes especially important when the length-to-diameter ratio is large. Sharp cutting edges generally minimize the tendency to vibrate, improving hole quality
For internal turning, geometries with open chipbreakers are often more beneficial
In some operations an insert grade with a higher level of toughness can be considered as it can handle any risk of chip jamming or tendency to vibrate
If improved chip formation is required, consider modifying the toolpath
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Hard Part Turning Application Tips
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In addition to general turning recommendations, there are some key considerations for hard part turning (such as the production process including part preparation in the soft turning phase prior to hardening):
avoid glitches
maintain tight dimensional tolerances,
Chamfering and machining radii before heat treatment
Do not suddenly enter or withdraw the knife
Cut in or out by arcing in or out
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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 essential
As a general guideline, for workpieces supported at only one end, it is generally recommended that the length-to-diameter ratio of the workpiece not exceed 2:1. L/D ratio can be increased if additional tailstock support is present
Note that the thermally symmetrical design of the deck 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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Insert Micro Geometry
Two typical edge-passivated CBN inserts are S-type and T-type.
Type S: has the best edge strength. Provides microchipping resistance for consistent surface quality.
Type T: Enables optimum surface quality in continuous cutting and minimizes burr formation in interrupted cutting. Cutting force is low.
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Tool tip geometry
In stable conditions, always use a wiper geometry to ensure the best surface quality.
When higher productivity is required, inserts with small entering angles are used.
When stability is poor (slender workpieces, etc.), regular radius inserts should be used.
wet or dry processing
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Hard part turning without the use of coolant is ideal and quite possible. Both CBN and ceramic inserts can withstand higher cutting temperatures, thus eliminating the cost issues and difficulties associated with coolant.
Certain 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 operation.
Typically, the heat generated during machining is distributed to the chip (80%), the workpiece (10%) and the insert (10%). This shows the importance of chip evacuation from the cutting edge area.
Cutting parameters and wear
High heat in the cutting edge area reduces cutting forces. Therefore, cutting speeds that are too low generate less heat and may cause the insert to break.
Crater wear gradually affects insert strength, but does not affect surface quality to the same degree. Conversely, flank wear gradually affects dimensional tolerances.
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Wear ratio that determines tool life
*) flank wear **) crater wear
Tool Change Guidelines
Predetermined surface quality (B) is a common 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.
For an optimized and more stable machining process, a predetermined number of parts (A) is set as the standard for tool changes. This value should be 10-20% less than the average part count, the exact value depends 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 cut strategy
One-cut "metal removal" strategies are available for both external and internal operations. In internal turning, stable setup is very important and the tool overhang should not exceed the boring bar diameter (1×D). For good machining results, lightly honed inserts with chamfering and moderate cutting speeds and feeds are recommended.
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advantage
Fastest possible processing times
a pocket
shortcoming
Difficulty meeting tight dimensional tolerances
Shorter tool life (compared to secondary cutting)
Dimensional deviations due to relatively rapid wear
two cut strategy
The two-cut strategy can be used in unmanned production to process high surface quality. Roughing inserts with a radius of 1.2 mm (0.047 in) and T-shaped finishing inserts with only one chamfer are recommended. Both inserts should have a wiper geometry.
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advantage
Tools optimized for roughing and finishing
Greater security, tighter tolerances and potentially longer tool change intervals
shortcoming
two blades required
two tool positions
one tool change
