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The versatile 391.37A/B system can be used for both fine boring and face grooving applications. It allows precision adjustment in microns in order to achieve close hole tolerances. Build modular assemblies with Coromant Capto® or HSK for increased tooling flexibility.


Fine-adjustable heads and sleeves


  • Choose between conventional (.37A) and high speed (.37B) fine-adjustable head
  • Flexible solution for a broad diameter range
  • Excellent surface finish with wiper inserts
  • Close hole tolerances of IT6


  • Diameter adjustment by 0.002 mm (0.00008 inch) for high accuracy
  • The high speed fine boring heads have an adjustable counterweight, which enables runs at 20 000 rev/min
  • Reduction sleeves available from size 20 to 16
  • Internal coolant supply/Cutting fluid through the tool


  • Fine boring for small diameters from 3 mm (0.118 inch)
  • For conventional fine boring and face grooving
ISO application area


R429U boring and face grooving bars 

R429U bars ​Boring ​Face grooving
Diameter range 3–36 mm (0.118–1.42 inch)​14–50 mm (0.551–1.97 inch)
Insert choice3–11 mm solid carbide bars            
8–36 mm CoroTurn 107 indexable inserts
​CoroCut MB-09FA
  • ​Flexible solution for a broad diameter range.
  • Three length ratios available in a clear and structured assortment based on CoroTurn®107
  • Boring bars with increased performance dedicated to its length/diameter ratio
  • ​Optimized bar length for each diameter for excellent chip evacuation.
  • Excellent for small O-ring grooves, for example found on water- and hydraulic pumps



  • Choose a light-cutting insert with a positive geometry for best surface finish and close hole tolerance.
    • Recommended first choice is the light cutting knife edge geometry (L-K) with nose radius 0.2 mm (0.008 inch)
  • Choose a small nose radius to minimize vibrations and cutting forces.
  • Choose a nose radius smaller than depth of cut
  • Wiper inserts can be used with small L/D ratios at high feeds under stable conditions to increase workpiece quality and productivity levels.

Product range

 With adjustable counterweightSolid carbide barSteel barCarbide bar with brazed steel head
Boring range mm (inch)14–50
Hole toleranceIT6IT6IT6IT6



Precision adjustment
Single-edge fine boring tools have adjustment possibilities to accurately
pre-set the cutting edge within microns.
  • One full turn of the scale will move the insert 0.25 mm (0.010 inch)radially.
  • Adjustment with nonius scale (0.002 mm (.00008 inch) on dia.)
  • One full turn of the scale will change dia. by 0.5 mm (0.020 inch)
  • Each division of the dial has 50 divisions, representing 0.010 mm (0.0004 inch)/ diameter. The vernier has 5 divisions, making it possible to adjust the diameter by 0.002 mm (0.00008 inch).





  • Sleeves are available in order to decrease the diameter range from 20 to 16 mm (0.787 to 0.630 inch)
    • Example: From diameter 16 mm (0.630 inch) bars in heads to dia
      20 mm (0.787 inch) bars

Start recommendations fine boring 

Material Designation Vc
Ra 0.8 µm (31 µinch) Ra 1.6 µm (63 µinch)
Insert ap
Insert ap
TCEX 110302-L-F 11250.2
TCMT 110304-PF 43250.4
TCMT 110304-PF 43250.4
Super DuplexF55130
TCMT 110304-MF 11250.4
TCMT 110304-MF 11250.4
TCGX 110204-ALH100.4
TCGX 110204-ALH100.4
TCGX 110204-ALH100.4
TCGX 110204-ALH100.4
Cast ironK3.2.C.UT225
TCMT 110304-PF 43250.4
TCMT 110304-PF 43250.4

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​
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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