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​DuoBore821 is a versatile rough boring tool that can be used in three different operation types: multi edge- , step- and single edge boring. The slides can be adjusted individually, both axially and radially. The system consists of adaptors with matching slides, shims and covers, making it possible to adapt the tool to various materials and conditions.


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  • Versatile with three different operation types in one tool
  • Large standard assortment of CoroTurn 107 and T-Max P inserts
  • Available as dampened tools, 821D (Silent Tools), for productive boring even at long overhangs



  • Two insert design
  • Short, rigid and compact gives maximum stability
  • Optional slide assemblies, individually adjustable, both axially and radially giving reduced inventory costs.
  • Cutting fluid through the tool for good chip evacuation



  • Dedicated geometries and grades for all materials
  • Use CoroTurn 107 screw clamping as first choice
  • Use T-Max P and CoroTurn RC for improved economy and process security in stable conditions
  • Use tool for various application types such as counterbore, through hole and interrupted cut
Multi edge boring​
Step boring​
Single-edge boring


Counterbore Through hole Interrupted cuts Blind hole


  • 90° First choice for counterbore and vibration-prone operations
  • 75° First choice for stack boring, interrupted cut, sand inclusions etc.
  • 84° RC slides, for through holes, stack boring, interrupted cut, sand inclusions etc.
  • Round insert carriers are a productive solution for machining in cladded materials, e.g. boring of Inconel 625 weld in oil and gas applications
ISO application area


CoroBore® BR20 and CoroBore® BR10

A new generation of rough boring tools is available. For our latest boring solutions within the same application area as DuoBore, see CoroBore BR20 and CoroBore BR10 (back boring).



  • Start with a roughing geometry unless a small depth of cut is to be taken.
  • Choose a medium geometry for smaller cutting depths or improved chip breaking
  • Choose CoroTurn 107 for lower cutting forces process security
  • Step boring is an effective way to increase the depth of cut without excessive vibrations
  • First choice for roughing with DuoBore 821D, dampened tools:
    • Entering angle 90/Lead angle 0
    • Recommended radius R08, otherwise try R04


Product range

DuoBore 821
Duobore 821 D
Boring range mm (inch)
(0.984-3.976 )
Hole tolerance ​IT9 ​IT9 ​IT9
Slides are available for both ceramic and solid carbide CoroTurn 107 inserts. CoroTurn RC slides from diameter 69 mm (2.717 inch) in styles CN and SN.
Overview of assembly parts:
Conventional DuoBore 821​ DuoBore 821 dampened​
​ ​

1. Slides
2. Adaptor​

1. Slides
2. Dampened adaptor

Grades and geometries


​ ​ ​ Medium Roughing Complementary

Positive inserts


CoroTurn 107 screw clamping​ ​ ​ ​ ​

-PM / GC4225​ -PR / GC4225​ -WM / GC4215​
-MM / GC2025​ -MR / GC2025​ -WM / GC2015​
-KM / GC3215​ -KR / GC3215​ -WM / GC3215​
-AL / H10​ -AL / H10​ -AL /GC1810​
-MM / GC1105 - MM / GC 1105 -UM / GC1115
Negative inserts

T-Max P, CoroTurn RC  rigid clamping

-PM / GC4225​ -PR / GC4225​ WMX / GC4215​
-MM / GC2025​ -PR / GC4225​ WMX / GC2015​
-KM / GC3215​ -KR / GC3215 WMX / GC3215​
-QM / GC1105​ -QM / GC1105​ -NM / GC1115

*H13A for Ti-based ISO S-materials
** For recommendation on fine boring in ISO H material, see technical guide, General turning.
Note: Grade recommendations are valid for average conditions
For recommendations on when an alternative grade should be used, se below:

Application Toughness First choice Wear resistance
Roughing GC4235 GC4225​ GC4215​
GC2035 GC2025 GC2015
GC4215 GC3215 GC3210/GC3205*
GC1115 H10 GC1810
GC1115/H13A GC1105 GC1105

*GC3210 Nodular cast iron, GC3205 Gray cast iron
For recommendation on fine boring in ISO H material, see technical guide, General turning.
Cutting data recommendations, see main catalogue.

​Tool holding

Bending stiffness and torque transmissions are the most important factors when choosing a tool holder for boring operations. For best stability and hole quality use Coromant Capto or the Coromant EH system.

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 adaptors can be used in different applications and machines. This makes it possible to standardize on one tooling system for the entire workshop. Remember the following:

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

The Coromant EH system delivers great tooling flexibility – choose from an extensive variety of heads, adaptors and shanks.

  • Reduced tool inventory with standardized tools minimizing machine downtime
  • For boring operations requiring long reach, as in large machining centres in the diameter range of 1–36 mm (0.039–1.42 inch). Deep pockets is a typical application where the Coromant EH system offers the required reach without limiting process security
Choice of adaptors/chucks for boring 
​ ​ Adaptor and chuck systems​
Choose coupling/chuck first,
then adaptors/chucks​​
Coromant Capto​ CoroChuck 930​
Shrink fit
Collet chuck
+++ Very good Torque transmission +++ +++ +++ +++
++ Good Run-out +++ +++ +++ +++
+ OK Balance +++ +++ +++ +++
Boring tool couplings​ ​
CoroBore 820​ Coromant Capto​ 1​
DuoBore​ Coromant Capto​ 1​
CoroBore XL​ Coromant Capto​ 1​
CoroBore 825​ 1​ 2​ 3​
Cylindrical​ 1​
EH​ 1​
Fine boring head 391.37A/B​ Coromant Capto
CoroBore 824 XS​ Coromant Capto​ 1​
CoroBore 824 XS​ EH​ 1​
1 ​ = First choice​
2​ 3​ = Complementary choice​


Coromant Capto® system - first choice


First choice should always be tools with Coromant Capto coupling, which offers excellent repeatability and the best stability characteristics on the market. ​
CoroBore 820​
Coromant Capto basic holders and adaptors​ CoroBore 825​
Fine boring head​
CoroBore XL


CoroChuck™ 930 – first choice for boring tools with cylindrical shanks. 


CoroChuck 930 precision chucks have a very good run-out precision and high torque transmission thanks to the latest Fulcrum technology. This technology delivers the best pull-out security on the market.​ ​
HD version​ CoroBore 825 Cylindrical shank​
Slender version
CoroBore 824 XS EH
Complementary adaptors


Shrink fit adaptors can be used for accessibility purposes. However, CoroBore 825 with integrated dampening mechanism is preferable if accessibility is required.​
Shrink fit adaptor​ CoroBore 825 Cylindrical shank
Collet chucks can be used together with CoroBore 825 cylindrical shanks. However, it is not the optimal solution due to lower clamping forces and larger run-out compared to HydroGrip. Furthermore, it is not individually balanced. ​
Collet chuck adaptor​ CoroBore 825 Cylindrical shank​​
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