1. What Is a Cutting Tool Holder?
A cutting tool holder is the mechanical interface that connects a cutting tool — a drill, end mill, boring bar, or turning insert — to the spindle or turret of a CNC machine tool. It is far more than a simple clamp: it defines the geometric accuracy of tool positioning, transmits cutting forces, provides vibration damping, and in modern designs, channels coolant directly to the cutting zone.
In CNC turning centres and multi-tasking machines, the tool holder is mounted on a turret. The turret rotates to present different tools to the workpiece, and the holder must ensure sub-micron repeatability with every index cycle. Even a few micrometres of radial run-out can translate into visible surface defects on precision aerospace or medical components.
Modern tool holders are engineered to DIN, ISO, and machine-builder-specific standards (Mazak, FANUC, Doosan, Okuma), making interface compatibility a central procurement concern. Xiray Tools' Cutting Holder series is engineered to meet these demanding interface and accuracy requirements.
2. Types of Cutting Tool Holders
2.1 Static (Fixed) Tool Holders
Static holders do not rotate independently. The workpiece rotates (as on a lathe), and the holder presents the cutting tool at a fixed angle. They are used for OD/ID turning, grooving, threading, and parting. Static holders prioritise rigidity and clamping force over dynamic balance, and their simpler internal construction allows extremely high stiffness values.
2.2 Driven (Live) Tool Holders
Driven tool holders — also called power tool holders — contain an internal spindle driven by the machine's live tooling axis (typically the C-axis). They enable milling, drilling, and tapping on a turning centre without a separate machining centre operation, dramatically reducing part cycle time. BMT Driven Tool Holders and VDI Driven Tool Holders from Xiray represent this category at a high level of engineering.
2.3 Angle Head Holders
A specialised variant, the angle head redirects the spindle axis by 90° (or other angles) to reach features that cannot be accessed on-axis. This is critical in the machining of complex housings, pump bodies, and aerospace structural parts.
3. Interface Standards: BMT, VDI & PSC
3.1 BMT (Bolt-on Mount Turret)
The BMT standard — widely adopted by Doosan, Hyundai-Kia, and compatible machines — fastens the tool holder to the turret face using a bolt pattern rather than a central shank. This face-mount design delivers exceptionally high rigidity and is preferred for heavy interrupted cutting. BMT sizes (BMT45, BMT55, BMT65, BMT75) define the bolt-circle diameter. Xiray's BMT Driven & Static Tool Holder line covers all major size variants.
3.2 VDI (Verein Deutscher Ingenieure)
Standardised under DIN 69880, VDI holders use a cylindrical shank that locks into the turret with an eccentric clamping ring. The quick-change characteristic makes VDI the most widely used system globally. Common sizes: VDI 20, 25, 30, 40, 50, 60. Xiray's VDI Driven & Static Tool Holder range covers the full spectrum with ground shanks holding h6 tolerance.
3.3 PSC (Polygon Shank with Coolant)
PSC (ISO 26623) — commercially known as Capto® — features a hollow tapered polygon shank that provides simultaneous taper and face contact, achieving near-zero run-out and excellent torque transmission without keys. The integrated coolant channel runs axially through the centre. Xiray's PSC Tool Holder Series leverages this geometry for high-speed precision work.
| Standard | ISO/DIN Ref. | Clamping Method | Repeatability | Typical Application |
|---|---|---|---|---|
| BMT45 – BMT75 | Machine-specific | Face bolt pattern | < 3 µm | Heavy turning, driven milling |
| VDI 20 – VDI 60 | DIN 69880 | Eccentric ring | < 5 µm | General turning, live tooling |
| PSC (C3 – C8) | ISO 26623 | Polygon taper + face | < 2 µm | High-speed, multi-tasking |
4. External Coolant Supply Systems
The Xiray Cutting Holder (A-24) features an external coolant supply design — one of the most impactful technological choices in modern metal cutting.
Why Coolant Delivery Matters
Cutting generates intense localised heat at the tool-chip interface. Temperatures can exceed 900 °C during high-speed steel or titanium machining. Uncontrolled heat leads to accelerated tool wear (flank wear, crater wear), thermal workpiece distortion, poor surface finish (Ra degradation), and risk of white-layer formation in hardened steels.
External Coolant Supply: Design Principles
In an external coolant supply configuration, coolant is channelled through the body of the holder and exits through a precision-directed nozzle positioned close to the cutting insert. This approach offers several advantages over flood coolant applied at a distance:
① Thermal control — Coolant reaches the cutting zone directly, reducing insert temperature by 30–50% compared to external flood systems.
② Chip breaking — High-pressure coolant (up to 80 bar) fractures chips and clears them from the cutting zone, preventing re-cutting.
③ Tool life extension — Studies in industry literature report 30–200% tool life improvement with through-tool coolant delivery.
④ Surface finish — Lower temperatures and clean cutting zones correlate with Ra values 15–25% better than dry or flood-cooled counterparts.
Pressure Specifications
Tool holders must be rated for the coolant pressure delivered by the machine. Standard machines operate at 20–40 bar. High-pressure systems (Okuma, Mazak DONE IN ONE®, Nakamura-Tome) can reach 70–120 bar. The holder body, nozzle threads, and internal passages must maintain integrity across this range with zero leakage.
5. Key Technical Parameters & Tolerances
5.1 Dimensional Accuracy & Run-out
Tool holder radial run-out is measured at the cutting edge or tool-mounting bore using a precision dial gauge. For turning operations, run-out directly translates to dimensional error on the finished part. For driven holders, run-out also causes vibration, accelerating bearing wear. Precision-grade holders achieve radial run-out below 3 µm; standard grade is typically 5–8 µm.
5.2 Material & Heat Treatment
Body material is typically alloy steel (such as 40CrMnMo or equivalent) quench-tempered to 48–52 HRC on functional surfaces. Shank surfaces are precision ground. Internal coolant channels are honed to minimise flow resistance and prevent corrosion. Some holders feature hard chrome or DLC (diamond-like carbon) coating on internal passages for enhanced corrosion resistance.
5.3 Clamping Force & Torque Capacity
Static holders must transmit the full cutting force without tool deflection. Clamping is achieved by tightening the holder's locking mechanism to a specified torque (referenced in the holder's data sheet). Over-torquing can distort the bore; under-torquing risks tool pull-out during cutting. Driven holders additionally specify the maximum input torque at the live-tool coupling and the maximum output speed.
| Parameter | Standard Grade | Precision Grade | Unit |
|---|---|---|---|
| Radial Run-out | ≤ 8 | ≤ 3 | µm |
| Max RPM (driven) | 4,000 | 8,000+ | rev/min |
| Max Input Torque | 20 | 50+ | N·m |
| Coolant Pressure | 20–40 | 70–120 | bar |
| Body Hardness | 45–48 | 48–52 | HRC |
