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简体中文 End mill is the core cutting tool for CNC milling and cemented carbide machining. Tool holders and collets are key tooling parts that connect machine tool spindles and end mills. These three components are often confused in actual production. This article systematically introduces the definition, classification, matching rules and applicable working conditions of the three parts, and sorts out the complete tool clamping system for end mill processing.
Overall Logic of Clamping System
A complete cutting and clamping system consists of four fixed assembled layers with non-interchangeable parts. The four layers in sequence are machine tool spindle, spindle tool holder, collet and tool shank of end mill. The tool shanks of end mills are mainly divided into two types: cylindrical shank and Morse taper shank.
Spindle tool holder
The spindle tool holder is installed on the end face of the machine tool spindle and serves as the basic bearing base. One end fits and locks with the taper hole of the machine tool spindle, and the other end reserves a cavity for installing various collets. It acts as a transitional connecting part between machine tools and clamping accessories.
Collet
The collet is installed inside the cavity of the spindle tool holder. It is an independent part that directly contacts and locks the end mill. Collets with different structures have different clamping accuracy and bearing torque, and are suitable for different machining scenarios.
Tool shank
The tool shank is an integrated rear handle of the cemented carbide end mill and cannot be disassembled or replaced separately. It follows two independent standard systems: DIN6535 standard cylindrical shank and Morse taper shank. The structure of the tool shank directly determines the matching collet types.
Classification of Spindle Tool Holders
According to machine tool interface standards, mainstream spindle tool holders in the industry include BT, HSK and MT series, which adapt to different processing equipment and production requirements respectively.
BT tool holders
BT tool holders are universal models for vertical machining centers in China. Equipped with ATC manipulator clamping grooves, they support automatic tool change of equipment. They adopt a 7:24 taper, with a standard basic length of 24 mm and a diameter variation of 7 mm. Short for Bottle grip taper, BT tool holders feature an ATC arm clamping part and a drawbolt, and the shape of drawbolts varies by manufacturer.
BT tool holders have good rigidity and are suitable for heavy cutting. They transmit cutting torque through conical surface friction, but are prone to tool slipping under high-speed machining conditions.
HSK hollow tool holders
HSK hollow tool holders are specially designed for high-speed finish machining with a 1:10 taper. They adopt a dual positioning structure with both conical surface and end face fitting. Under high-speed rotation, the taper sleeve produces elastic deformation due to centrifugal force. This dual positioning structure provides extremely high centering accuracy, and its rigidity and stability are better than BT tool holders. It is mostly used for high-speed cutting of aerospace parts and precision molds.
MT Morse taper tool holders
MT Morse taper tool holders comply with DIN228 standard, equivalent to China’s national standard GB/T 1443. They have stable centering performance and are mostly applied to radial drilling machines and ordinary old-fashioned milling machines for drill bit processing and simple end mill machining. Divided into flat tail type and thread tension type, they complete basic positioning through self-locking taper surfaces and cannot adapt to high-speed machining centers with automatic tool change functions.
Classification of Collets
As locking components inside spindle tool holders, collets have six mature structural types widely used in the industry.
ER collets
Tapered spring collets, also known as ER collets, are the most widely used universal collets with high accuracy and strong compatibility. Common specifications include ER16, ER20 and ER32. They only match DIN6535 standard HA smooth cylindrical shank end mills. By tightening nuts to squeeze the collet, they can clamp the tool 360 degrees evenly, suitable for finish machining and medium-load cutting of steel and aluminum parts.
Side-lock collets
Side-lock collets are special accessories for Weldon flat shank tools. Built with locking jackscrews, they are exclusively adapted to HB single-flat and HE double-flat end mill shanks. The jackscrew tightly presses the tool flat position for mechanical locking, which completely avoids tool slipping and axial pulling in heavy cutting. They are the preferred accessories for large-depth grooving of titanium alloy and stainless steel.
Hydraulic collets
Hydraulic collets wrap the tool shank evenly through internal hydraulic pressure, with a radial runout accuracy ranging from 0.005 mm to 0.015 mm and gap-free clamping. They only fit HA smooth cylindrical shank end mills and are suitable for finish machining requiring ultra-high surface finish. Their service life is easily affected by oil temperature changes, internal oil leakage and collet wall thickness.
Shrink-fit collets
Shrink-fit collets lock tools by thermal expansion and cold contraction. With higher clamping force than ER spring collets of the same specification, they have a slim overall shape and are suitable for deep cavity machining with small-diameter end mills. They can only clamp flat-free HA cylindrical shank tools. Needle roller collets achieve clamping through elastic deformation caused by needle roller extrusion, with medium clamping force for conventional medium-load cutting, but their overall versatility is weaker than ER spring collets.
Two Standard Systems of End Mill Shanks
The rear shanks of cemented carbide end mills follow two independent standard systems: DIN6535 standard cylindrical shank and Morse taper shank. The matching accessories of the two systems are completely non-universal.
DIN6535 Standard Cylindrical Shank (Mainstream for CNC Machining Centers)
With an integral cylindrical structure, it is divided into four shank types. The shank structure directly determines the applicable collet types.
HA straight shank
HA standard straight shank has no clamping flat position and a smooth full shank. It can match all friction collets including ER, hydraulic and shrink-fit collets. It is suitable for finish machining and light cutting with no processing eccentricity and optimal radial runout control effect.
HB single Weldon flat shank
HB single Weldon flat shank is designed with a locking flat on one side and only matches side-lock collets. As a special structure for heavy cutting and hard material processing, it prevents axial pulling of large helix angle end mills. Unilateral stress causes slight eccentricity, leading to ordinary radial runout accuracy.
HE double Weldon flat shank
HE double Weldon flat shank has two symmetrical locking flats, with a much higher torque bearing capacity than HB single flat shank. It is suitable for large-diameter heavy-duty end mills and super-large torque roughing machining with higher processing stability.
HF full flat shank
HF full flat shank is designed with full axial flat positions on the shank. It allows free adjustment of tool extension length and fast tool change, and a single tool can complete standard-length cutting and extended deep cavity cutting.
Morse Taper Shank (Special for Old-fashioned Universal Machine Tools)
Adopting a self-locking small taper structure, it is divided into flat tail Type A and thread tension Type B. It must be assembled with a Morse reducing sleeve, only adapts to ordinary milling machines and radial drilling machines, and cannot be matched with cylindrical clamping systems such as ER and side-lock collets. It is mostly used for long-edge tools and large-residual heavy roughing processing.
概要
The spindle tool holder is the transitional base connecting the machine tool, the collet is a detachable independent locking sleeve, and the end mill shank is the self-contained clamping base of the cutting tool.
For finish machining, HA smooth straight shank with ER, hydraulic or shrink-fit collets is the preferred combination. For heavy cutting of hard materials, HB and HE Weldon flat shanks matched with side-lock collets are adopted.
For old-fashioned universal milling machines, Morse taper shanks paired with Morse reducing sleeves are used.
Strict matching of tool holders and collets in accordance with standards can stabilize machining accuracy, extend tool service life, and avoid production failures such as tool breakage and workpiece scrapping.
