ISO 26623-1 Polygonal Taper Interface with Flange Contact Surface — Shank Dimensions and Designation

ISO 26623-1:2020 specifies the dimensions and designation system for polygonal taper shanks with flange contact surface — commonly known as the PSC (Polygon Shank Connection) or Capto interface. Unlike round-taper systems that rely on friction for torque transmission, the PSC system uses a precision-ground polygonal (non-round) cross-section that transmits torque through positive form lock, providing exceptional torsional rigidity and positioning repeatability.

The PSC polygon profile is mathematically defined by a precise parametric equation featuring eccentricity and angular terms — the ground curve differs from the theoretical by only +0 to −0.007 mm at the sectioned areas, a testament to modern CNC grinding capabilities.

Shank Size Range and Key Characteristics

Size	d1 — Shank dia (mm)	d4 — Thread	b1 — Flange width (mm)	Clamping force (kN)	Dm — Mean dia (mm)
32	32	M12×1.5	28.3	15	22
40	40	M14×1.5	35.3	20	22
50	50	M16×1.5	44.4	25	22
63	63	M20×2	55.8	30	22
80	80	M20×2	71.1	40	22
100	100	M24×2	88.7	60	22

Engineering Design Insights

The Polygon Geometry Advantage

The defining innovation of the PSC system is the polygonal taper profile defined by the parametric equations in Figure 7 of the standard. Unlike a simple circle or triangle, this optimized polygon provides: (1) self-centering capability — the polygon geometry automatically centers the shank in the receiver when clamped; (2) high torque transmission through form lock rather than friction, eliminating the slip risk that limits round-taper systems under heavy cutting loads; (3) multiple repeatable angular positions — the shank can be indexed at three positions 120° apart, providing flexibility for tool presentation. The eccentricity parameter (e = 0.7 mm for all sizes from 32 to 100) is the single most important geometric constant — it determines the polygon’s torque capacity and self-centering behavior.

Never mix PSC polygon components from different manufacturers without verifying the polygon profile. While the standard defines the theoretical curve, subtle variations in grinding wheel profile and CNC compensation can produce interfaces that fit statically but fail under cutting loads.

Torque Transmission Through Form Lock

In round-taper systems (such as ISO 26622-1 KMTS), torque is transmitted primarily by friction at the taper interface and by driving keys. The maximum transmissible torque is proportional to the clamping force times the coefficient of friction. In the PSC system, torque is transmitted directly through the polygon form, with the clamping force primarily dedicated to maintaining face contact (which determines axial and bending stiffness). This decoupling of torque transmission from clamping force is a fundamental advantage — it means the PSC can transmit higher torques for a given clamping force, and the system stiffness is less sensitive to clamping force variations.

Medium-Transfer Unit Integration

One notable addition in the 2020 third edition is the integration of medium-transfer units per ISO 22402-2. These units enable through-tool coolant delivery at high pressures (up to 80 bar / 8 MPa), essential for modern high-performance machining. The medium-transfer unit mounts inside the hollow shank and interfaces with the coolant supply through the receiver. Proper sealing at this interface is critical — a 0.5 mm misalignment can reduce coolant pressure by 50 %, negating the benefits of high-pressure coolant for chip evacuation and thermal management.

For turning applications with stationary tools, the standard provides alternative dimensions in Annex B that allow extra room for clamping devices. This flexibility — one shank design adapted for both rotating and static use — makes the PSC system unusually versatile across the full spectrum of machining operations.

FAQs

Q1: How does the PSC (Capto) system compare to HSK or KMTS?
PSC offers superior torque transmission and repeatability at the cost of higher manufacturing complexity. HSK is more economical for general-purpose machining. KMTS (ball track) offers faster tool changes. PSC is preferred for heavy roughing and high-precision finishing on multi-tasking machines.

Q2: What surface hardness is required for the polygon taper?
Minimum HRC 42 for the tapered polygon, internal clamping groove, and axial contact surface. The external gripper groove area requires minimum HRC 51. Through-hardening or case hardening is acceptable depending on application.

Q3: Can PSC shanks be dynamically balanced?
Yes — Annex A provides specific dimensions for dynamical balancing by design, including chip hole configurations and balancing material removal locations. This is essential for high-speed (＞ 10,000 RPM) applications.

Q4: What does the eccentricity parameter (e = 0.7 mm) represent?
The eccentricity is a constant in the polygon parametric equations that defines the deviation from a circular profile. It determines the torque capacity and self-centering characteristics of the interface. The same value (0.7 mm) applies across all sizes from 32 to 100 mm.