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How to apply boring

How to apply boring

To achieve an optimized boring process, there are several factors to consider. Correct tool set-up, insert and cutting data must be carefully considered to achieve good chip control. Make sure to follow our tips to achieve good component quality.


Stable boring tool set-up

Bending stiffness and torque transmission are the most important factors when choosing a tool holder for boring operations. For best stability and hole quality, use Coromant Capto® or a modular system for small diameters.

Coromant Capto® coupling is the only modular tooling system designed for all metal-cutting operations, including all hole-making methods. The same cutting tools and adapters can be used in different applications and machines. This makes it possible to standardize one tooling system for the entire workshop. Remember the following:

  • Choose the shortest possible adapter
  • Choose the strongest possible adapter
  • If reduction is needed, use the tapered version, if possible
  • For long overhangs (> 4 × DCON), use dampened adapters
  • For long overhangs, ensure rigid clamping with flange contact to spindle, if possible

A modular system for small diameters, such as Coromant® EH, delivers great tooling flexibility. It is possible to choose from an extensive variety of heads, adapters and shanks. Reduced tool inventory with standardized tools minimizes machine downtime.

Suitable for:

  • Boring operations requiring long reach, as in large machining centers, in the diameter range 1–36 mm (0.039–1.42 inch)
  • Deep pockets, as it offers the required reach without limiting process security

Tips and hints

  • For maximum stability, choose the largest possible coupling size and shortest possible overhang
  • Increase dynamic stiffness by adding a dampening mechanism
  • For long overhangs (> 4 × coupling diameter), use dampened tools
  • If reductions are needed, use the tapered version, if possible
  • Coromant Capto® TailorMade reduction/extension adapters are available for optimized design, hence highest stability
  • For long overhangs, a machine tool with a flange contact machine interface coupling provides best performance, e. g. Coromant Capto® coupling, HSK and BIG-PLUS®.

Successful chip control in boring

Chip formation and chip evacuation are critical issues in boring operations, especially in blind holes.

Make sure chip breaking and evacuation are satisfactory. Chip jamming affects hole quality, reliability and tool life. Insert geometry and cutting data is crucial. Use cutting fluid to improve chip evacuation, tool life and hole geometry.

Inappropriate cutting data can lead to chips that are too short/thick and excessive cutting forces, resulting in deflection and vibration. Long chips can accumulate in the hole and cause surface finish deterioration and chip jamming, leading to insert breakdown.

Ideal chips should be in the form of defined commas or spirals that are easily evacuated from the hole. Factors that have an influence on chip breaking are:

  • The insert’s micro- and macro-geometry
  • Cutting depth, feed and cutting speed
  • Material
  • Nose radius
  • Entering angle (lead angle)

Cutting data in boring

The cutting speed must be reduced when working with longer overhangs. The charts give a general trend for how the cutting speed must be reduced at different geometries and overhangs.

L1 / D5m
Cutting speed in relation to overhang, different geometries
Blue: -PR
Red: -WM
L1 / D5m
Cutting speed in relation to overhang
Blue: Conventional adapter
Red: Dampened adapter

The information taken from these charts must be seen only as a general trend for the relationship between cutting speed and overhang/coupling size ratio.

Recommendations for the chosen insert and grade can be generally followed, but with the following exceptions:

  • Recommended starting cutting data to ensure proper chip evacuation is for roughing 200 m/min (656 ft/min) and for finishing 240 m/min (790 ft/min)
  • Recommended starting speed when using boring bars for fine boring:
    • For short steel and carbide bars with inserts: 120 m/min (395 ft/min)
    • For long steel bars: 90 m/min (295 ft/min)
    • For solid carbide bars with ground geometry: 60 m/min (195 ft/min)
  • Insufficient cutting edge engagement can increase vibration through friction during cutting, instead of a clean cutting action
  • If the tool is adjusted to the minimum possible diameter, chip evacuation will be more critical and it might be necessary to reduce cutting depth
  • Maximum feed in fine boring is limited by the desired surface

How to improve tool life in boring

The three main machining parameters in a boring process are speed, feed and depth of cut. Each has an effect on tool life. The depth of cut has the smallest effect followed by the feed rate. Cutting speed has the greatest effect on insert tool life by far.

Effects of cutting speed

Too low
  • Built-up edge
  • Dulling of edge
  • Uneconomical
Too high
  • Rapid flank wear
  • Poor finish
  • Rapid crater wear
  • Plastic deformation

Cutting speed, vc, has a significant effect on tool life.
Adjust vc for best economy. (Tool life on Y-axis)


Effects of feed rate

Too light
  • Stringers
  • Rapid flank wear
  • Built-up edge
  • Uneconomical
Too heavy
  • Less chip control
  • Poor surface finish
  • Crater wear/plastic deformation
  • High power consumption
  • Chip welding
  • Chip hammering

Feed, fn, has less effect on tool life than vc.
(Tool life on Y-axis)


Effects of depth of cut

Too small
  • Loss of chip control
  • Vibration
  • Excessive heat
  • Uneconomical
Too deep
  • High power consumption
  • Insert breakage
  • Increased cutting forces

Depth of cut, ae, has a minor effect on tool life.
(Tool life on Y-axis)


How to choose correct insert in boring

To achieve good chip control, it is important to select the correct insert size, insert shape, geometry and insert nose radius.

  • Select the largest possible nose angle on the insert for strength and economy
  • Select the largest possible nose radius for insert strength
  • Select a smaller nose radius if there is a tendency for vibration

l = cutting edge length (insert size)

RE = nose radius

Nose angle


See Rough boring and Finish boring for more information.

Boring insert shape

​The insert shape and point angle for boring vary considerably, from triangular-shaped inserts with small point angles to round inserts.

Each shape has unique properties. Some provide the highest roughing strength while others give the best accessibility for fine boring.

Each shape also has its unique limitations. For example, high edge accessibility during machining leads to a weaker cutting edge.

Cutting edge strength (large point angle)
  • Stronger cutting edge
  • Higher feed rates
  • Increased cutting forces
  • Increased vibration
Less vibration tendency (small point angle)
  • Weaker cutting edge
  • Increased accessibility
  • Decreased cutting forces
  • Decreased vibration

Round inserts

Round inserts have stronger cutting edges and are hence more resistant to large depth of cuts than triangular-shaped inserts.

Round inserts are a productive solution for machining in cladded materials, e.g. boring Inconel 625 weld in oil and gas applications.

Insert nose radius and depth of cut

The nose radius, RE, is a key factor in boring operations. The selection of nose radius depends on depth of cut and feed, and influences the surface finish, chip breaking and insert strength.

Small nose radius
  • Ideal for small cutting depth
  • Reduces vibration
  • Weak cutting edge
Large nose radius
  • Heavy feed rates
  • Large depths of cut
  • Strong edge security
  • Increased radial pressures

The relationship between nose radius and depth of cut affects vibration tendencies. It is often an advantage to choose a nose radius that is smaller than the depth of cut.

At small depths of cut, the resultant cutting force is radial, trying to push the insert away from the bore surface. When depth of cut is increased, the resultant cutting force is changed to an axial force.

As a general rule of thumb, the depth of cut should be no less than 2/3 of the nose radius. Avoid depths of cut smaller than 1/3 of the nose radius when finishing at small depths of cut.

With a small nose radius, the radial cutting forces can be kept to a minimum, while utilizing the advantages of a larger nose radius leads to a stronger cutting edge, better surface texture and more even pressure on the cutting edge.

The generated surface finish will be directly influenced by the combination of nose radius and feed rate. Read more about how to achieve good surface finish.​

Wiper insert

Use wiper inserts for improved surface finish with standard cutting data, or maintained surface finish at a substantially higher feed rate.

Wiper inserts are not recommended for unstable conditions and long overhangs.


Cutting fluid and coolant in boring

Chip evacuation, cooling and lubrication between the tool and the workpiece material are primary functions of cutting fluid. This affects the hole quality and tool life. Use an internal cutting fluid supply to get the fluid as close as possible to the cutting edge.

  • Use a mixture of 5–8% soluble oil
  • Higher pressure and volume improves chip evacuation
  • Mist cutting fluid or minimal lubrication can be used, especially in aluminum
  • Dry boring can be performed in short-chipping materials, preferably in horizontal or through hole applications
    • Tool life will be reduced
    • Compressed air will improve chip evacuation greatly
  • Dry machining is never recommended in stainless steel (ISO M) and HRSA (ISO S) materials
  • External cutting fluid supply is acceptable in short-chipping materials, but must continuously be directed to the cutting zone. If this is not possible, try dry boring

Assemble a boring tool

Assembled boring tools often have to be built from several items to suit an operation:

  • Slides or cartridges
  • Boring adapters
  • Reductions
  • Extensions
  • Basic holders

It is always vital that recommended torque values are followed for the assembly and that a suitable fixture and a good tool pre-setter are used.

Torque wrench

To get the best performance out of the boring tools, a torque wrench should be used to get the correct tightening of the assembled boring tool and insert. Torque that is too high will affect the performance of the tool negatively and cause insert, washer, cup spring and screw breakage. Torque that is too low will cause slide or insert movement, vibration and degradation of the cutting result.

See the rotating tools catalog for the recommended tightening torque for your boring tool.


Tool maintenance

​Check insert seats regularly to ensure that they have not been damaged during machining or handling. Make sure that insert seats are free from dirt or metal chips from machining.

Replace worn or exhausted screws, washers and cup springs. Ensure that you have a torque wrench to obtain correct screw-tightening.

To get the best performance, we recommend cleaning all male and female parts and lubricating them with oil at least once a year. Lubricant should be applied when needed to the screw thread as well as to the screw head face.

  • Clean all assembly items before assembly
  • Use Molykote for screws
  • Lubricate all assembly items with oil at least once a year
  • Clean and lubricate fine boring heads and face grooving heads with oil. At a minimum, once a year; if in permanent use, once a month
  • Use acid-free light machine oil type
    • Mobil Vactra Oil No. 2
    • BP Energol HLP-D32
    • Klueber Isoflex PDP 94

Lubrication is carried out by depressing the spring-loaded ball and adding a few drops of light oil. Due to the centrifugal force, the oil is forced outwards, which stops dirt from getting into the adapter.

Lubrication of fine boring head​s


Boring in different materials

See general turning for information about turning in different materials; the same recommendations are also valid for boring.



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