Boring Bar Guide: ISO Codes, Material & Selection

Boring bar codes combine shank material, diameter, length, insert shape, clearance angle and clamping system into a standardized system, letting users quickly select the right tool for internal machining. Reading the codes correctly improves bore accuracy and stability, and helps avoid vibration and tool damage in CNC internal turning.

This guide covers internal boring bars on a lathe, where the workpiece turns and the bar does not. For rotating boring bars and heads on a machining centre, see Boring Bar and Head Selection: Rigidity, Micro-Adjust and Coolant Delivery; for the four boring methods and the rigidity rule, see Boring Basics: Four Boring Methods and the Rigidity-First Rule; for why anti-vibration bars hold down chatter, see Anti-Vibration Tool Holders: Chatter, Dynamic Vibration Absorbers & Setup; for toolpaths and bar geometry limits in small bores, see Toolpath Planning for Overhung Workpieces and Small Bores: Stock Removal Passes, Zero-Point Drift and Boring Bar Geometry Limits.

1. Boring Bar Code Structure

Taking the ISO-style bar S16MSCLCR09 as an example, the code is built from fields in this order: shank material (S), bar diameter (16), bar length (M), clamping system (S), insert shape (C), entering angle (L), insert clearance angle (C), direction (R) and cutting-edge length (09). Reading each field lets you quickly identify the spec and speed up selection. Anti-vibration bars use a different order (e.g. FSCLC1008R06E2/3): the first letter F is the function (fixed type), and the shank material code comes near the end (E carbide shank, S steel shank). The same letter can mean different things in the two systems, so check that series’ code chart; the field-by-field codes for both systems are in Sections 9 and 10.

2. Shank Material

Shank material sets how far the bar bends at a given overhang, and so how far it can reach. Carbide has a higher modulus of elasticity than steel, so at the same overhang it deflects less; it can reach further than a steel bar, and the bore is less easily pushed off size by the cutting force. Anti-vibration bars add damping and reach further still. Approximate maximum overhangs (as multiples of bar diameter; manufacturer specifications for reference, check each maker’s catalogue):

3. Bar Diameter and Capability

Bar diameter affects rigidity and vibration resistance — the larger the diameter, the higher the rigidity — but it must fit within the bore (the bar has to enter the hole). Common diameters are Ø8, Ø10, Ø12, Ø16, Ø20, Ø25 and Ø32 (mm).

4. Bar Length and Overhang Ratio

The longer the overhang, the more prone the bar is to vibration. Control the overhang ratio (L/D) and avoid over-long bars to improve stability and surface quality. Overhang ratio is one of the keys to stable internal machining.

A few numbers help control overhang: clamp the bar over a length of 3 to 4 times its diameter to balance the cutting force; how far it can reach depends on shank material (see Section 2). Keep the entering angle no smaller than 75° where possible, or radial cutting force rises sharply and the bar deflects more; for small and medium bores from 6mm diameter up, a positive insert with a 7° clearance angle is the first choice, giving lower cutting force and less deflection. These multiples and angles are manufacturer specifications for reference. Note the different bases: overhang multiples for lathe boring bars are counted in bar diameters, while the machining-centre rule of switching to a dampened tool beyond 4 times overhang is counted in interface diameters, so the two cannot be swapped.

For setting tool angles on the lathe and checking a bore that grows or shrinks with depth, see Boring on the Lathe: Setting Tool Angles and Why the Bore Grows or Shrinks.

5. Insert Shape and Application

Insert shape affects the cutting angle and available space. Small bores are space-limited and favor small-angle inserts (V); general internal work commonly uses C.

6. Entering Angle and Direction

Entering angle affects cutting-force direction and chip flow. On ISO-style bars (e.g. S16MSCLCR09) the entering-angle codes are F 91°, K 75°, L 95°, Q 107°30′ and U 93°; the anti-vibration bar coding (e.g. FSCLC1008R06E2/3) adds P 117°30′ and J 142°. The same letter does not always mean the same thing in different series, so go by that series’ code chart. Direction is coded R (right-hand) or L (left-hand) and must match the workpiece machining direction.

7. Insert Clearance Angle

The clearance angle gives a gap between the insert and the bore wall. On ISO-style bars the clearance codes are N 0°, C 7°, P 11° and E 20°; the anti-vibration bar coding adds B 5° (C 7° and P 11° are the same in both). A small clearance gives higher rigidity; a large clearance gives lower cutting resistance. Choose by bore size and operation.

8. Clamping, Minimum Bore & Selection

Common clamping methods are screw, lever and double clamp, affecting insert seating and vibration resistance. Always check the "minimum bore diameter" — the bar diameter must be smaller than the bore so it can enter. Overall logic: choose shank material by overhang — a steel bar with standard inserts for short overhang, a carbide bar when steel cannot reach, and an anti-vibration bar for deep holes and long overhang (multiples in Section 2); pair high-accuracy work with a small-nose insert. The keys to optimizing internal machining are shank rigidity, overhang control and insert choice — get them right to raise accuracy, cut vibration and extend tool life. For actual cutting conditions, see the Turning Machining Formulas.

9. ISO-Style Boring Bar Codes, Field by Field

The charts below list each field in the order its letters appear in the code. The same letter can mean different things in the ISO-style and anti-vibration systems, so go by that series’ code chart when selecting.

ISO-style boring bars (covering the aluminium-machining, M, P and S types), e.g. S16MSCLCR09

Cutting-edge length codes, ISO-style bars

10. Anti-Vibration Boring Bar Codes, Field by Field

Anti-vibration bars use a different order from ISO-style bars: the first letter is the function, and the shank material code comes near the end.

Anti-vibration boring bars, e.g. FSCLC1008R06E2/3

Cutting-edge length codes, anti-vibration bars

Bar length, anti-vibration bars (carbide shanks only, mm)

For the full reading guides on this topic, see Insert Selection: A Complete Reading Guide, Vibration and Chatter: A Complete Reading Guide and Hole Accuracy: A Complete Reading Guide.

Published: 2026-03-24 | Last updated: 2026-10-22