Boring Bars

An ISO boring bar is a precision cutting tool used on a lathe or turning centre to machine the internal diameter (bore) of a pre-drilled or cast hole. It holds a replaceable ISO indexable turning insert at its tip, which performs the actual cutting.

ISO boring bars follow internationally standardised designations for shank size, insert style, clamp method and approach angle which makes it straightforward to identify compatible inserts and match bars across different brands.

External, turning tool holders sit in the tool post and cut the outside diameter of a workpiece, whereas boring bars are designed to enter a hole and cut from the inside - enlarging, finishing, or correcting the bore diameter & surface finish.

Boring bars are typically longer relative to their shank diameter and must be selected carefully to avoid deflection and vibration inside the bore.

ISO boring bar part numbers follow a standardised coding system.

For example, in a designation like S20R-SCLCR09:

• S - Shank type (S = cylindrical/round shank; E = square shank)

• 20 - Shank diameter in mm

• R - Right-hand cutting direction (L = left-hand)

• S - Clamping method (S = screw clamp; C = lever lock)

• CL - Insert style (C = 80° rhombic; D = 55° rhombic; T = triangular; V = 35° rhombic; W = trigon, etc.)

• CR - Approach/lead angle and clearance angle

• 09 - Insert size (IC = inscribed circle in mm)

Understanding the designation allows you to quickly identify the correct replacement insert and find compatible alternatives.

The insert style is encoded in the boring bar designation.

Common insert options include:

• CC / DC (80° rhombic) – strong insert with good versatility; suitable for general-purpose turning and profiling

• TC (60° triangular) – economical with three usable edges; good for roughing and general internal turning

• VC / SC (35° rhombic) – sharp, pointed geometry ideal for fine finishing, profiling, and reaching into tight corners

• DN / TN / SN (negative rake variants) – stronger edge for heavier cuts and harder materials

• WN (80° trigon, negative) – very strong, multi-edge insert for heavy-duty roughing

• VB / DP (positive rake variants) – reduced cutting forces; often preferred for finishing and lighter machines

• Screw clamp (S-style) - the insert is secured directly by a torx or hex screw through the centre hole. Simple, compact and very rigid - ideal for boring bars where space is limited inside a bore. This is the most common clamping method for smaller boring bars.

• Lever lock (P-style / C-style) - a lever mechanism grips the insert from below via its centre hole, providing excellent repeatability and fast insert indexing. Often used on larger bars where there is more room for the mechanism and where frequent insert changes are required.

The shank diameter of your boring bar must be selected based on:

• Bore diameter - the bar shank must fit inside the hole with sufficient clearance for chips and coolant. A general rule is that the bar diameter should be no more than 70–80% of the bore diameter

• Bore depth - deeper bores require longer bars, which increases the risk of deflection. Longer overhangs require stiffer (larger diameter) shanks or anti-vibration bars

• Toolholder bore - the bar must fit securely into your lathe's boring bar holder or VDI/BMT turret block.

We stock shank diameters from 8mm to 50mm which covers the full range of bore sizes.

Overhang (the length of bar protruding from the holder) is critical to boring performance.

As a general guideline you can follow the below:

• Up to 3× diameter - standard steel shank bars perform well with minimal vibration

• 3–5× diameter - consider heavy metal (tungsten carbide) shank bars for improved rigidity

• 5–7× diameter - anti-vibration boring bars (damped bars) are strongly recommended

• Over 7× diameter - specialist anti-vibration systems are essential

Excessive overhang relative to diameter leads to chatter, poor surface finish, insert chipping, and reduced tool life.

You should aways aim to minimise overhang as much as the application allows!

• Right-hand (R) boring bars cut when the insert faces to the right, with the tool feeding from right to left (towards the headstock). This is the standard for most conventional boring operations

• Left-hand (L) boring bars are used when boring in the opposite direction - feeding away from the headstock or when the spindle rotation is reversed. Left-hand bars are also used in back-boring operations and on machines with live tooling or gang tool setups.

Check your lathe's spindle rotation and feed direction before selecting right- or left-hand orientation.

Insert size is typically expressed as the inscribed circle (IC) in millimetres and is encoded in the part number (e.g. 06, 09T3, 11T3, 1204, 1604).

Smaller inserts (06, 09T3) are used in smaller boring bars for tight bores, while larger inserts (1204, 1604) are used in larger diameter bars for heavier cuts.

Always match the insert size to the bar designation - our filters allow you to search by insert size for quick cross-referencing.

Yes, you can.

One of the key advantages of the ISO system is interoperability. Provided the insert shape, size, and tolerance class match, the same insert can often be used in both external turning holders and boring bars - this reduces the number of insert grades you need to stock and simplifies purchasing.

ISO boring bars, fitted with the correct insert grade and geometry can be used across the full range of workpiece materials including:

• Carbon & alloy steels

• Stainless and duplex steels

• Cast iron

• Aluminium and non-ferrous alloys

• Hardened steels (with CBN or ultra-hard carbide inserts)

• Heat-resistant super alloys (Inconel, titanium, Hastelloy)

The key is selecting the correct insert grade, chipbreaker geometry and cutting parameters for the material - our product listings include material suitability guidance and our engineers are happy to advise.