Understanding Machining Center Tool Holders: Types and Applications

In this article, we will be discussing the knowledge of machining center tool holders.

CNC machine tools are widely used in factories, and these machines and tools come from all over the world with different models and standards.

Do you know what 7:24 means in a machining center BT tool holder? Also, are you familiar with the standards BT, NT, JT, IT, CAT?

The tool holder is the connection between the CNC machine tool and the cutting tool, and it is a crucial link that affects concentricity and dynamic balance. It should never be treated as an ordinary component.

Concentricity is important in determining whether the cutting amount of each cutting edge part of the tool is uniform when the tool rotates one circle. Dynamic imbalance, on the other hand, will produce periodic vibration when the spindle rotates.

tool holder

Fundamentals of Tool Holders

Machining Processes

When I work with machining processes, I typically use a variety of tools such as drills, mills, and lathes. It is essential that I align these tools precisely to ensure accuracy and repeatability. That’s where tool holders come in. They provide a secure connection between the cutting tool and the machine spindle. Here are some common machining processes where using tool holders plays a significant role:

  • Milling – In milling, I often use end mills to remove material from a workpiece through cutting and rotation.
  • Drilling – Drilling involves creating holes in the workpiece, and I use twist drills for this purpose.
  • Turning – In turning operations, I use lathes to remove material, cutting along the external or internal surfaces of a workpiece.

Tool Holding Mechanisms

There are numerous tool holding mechanisms available, each with their own benefits and drawbacks. As a machinist, it’s crucial for me to understand these differences to make sure I’m choosing the right tool holder for my process. Here are some common types of tool holding mechanisms:

  1. Collet chucks – These are my go-to for holding cylindrical tools. They offer high accuracy and gripping force while maintaining low TIR (Total Indicator Reading). ER collets and R8 collets are popular types of collet chucks that I often use.
  2. End mill holders – I often use end mill holders when it comes to holding tools with straight shanks, like end mills. The main advantage of end mill holders is their rigidity, which allows me to work with larger cutting tools and apply more aggressive cutting strategies.
  3. Hydraulic chucks – I find hydraulic chucks particularly useful for high-precision and high-speed machining. They utilize hydraulic pressure to grip the cutting tool, ensuring a uniform and centered clamping force.
  4. Shrink-fit holders – Shrink-fit holders rely on a heating and cooling process, allowing the expansion and contraction of the holder to secure the cutting tool. I often use these holders for high-speed and high-torque applications due to their excellent concentricity and holding power.
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Types of Tool Holders

According to the taper of the tool hole of the machining center spindle, it is usually divided into two categories:

  • SK universal tool holder with a taper of 7:24
  • HSK vacuum tool holder with a taper of 1:10

SK universal tool holder with a taper of 7:24

7:24 means that the taper of the tool holder is 7:24, which is a single taper surface positioning with a longer taper shank.

The tapered surface serves two important functions at the same time, which are the precise positioning of the tool holder relative to the spindle and the clamping of the tool holder.

SK universal tool holder with a taper of 724
SK universal tool holder with a taper of 724


The non-self-locking design allows for quick tool loading and unloading. The cost of the tool holder is relatively low since the taper angle can be machined to a high degree of accuracy, ensuring a precise connection.


During high-speed rotation, the tapered hole at the front end of the spindle will expand. The amount of expansion increases with the increase in rotation radius and speed, which decreases the taper connection rigidity. The axial displacement of the tool holder will also change under the action of the drawbar tension. After each tool change, the radial dimension of the to

There are usually five standards and specifications for universal tool holders with a taper of 7:24:

HSK vacuum tool holder with a taper of 1:10

  • International Standard: IS0 7388/1 (abbreviated as IV or IT)
  • Japanese standard: MAS BT (abbreviated as BT)
  • German standard: DIN 2080 type (abbreviated as NT or ST)
  • American Standard: ANSI/ASME (abbreviated as CAT)
  • DIN 69871 type (abbreviated as JT, DIN, DAT or DV)

Tensioning Method.

Type NT tool holders are tightened by a drawbar on conventional machines, which is also known domestically as ST.

The other four tool holders are tightened on the machining center through a spigot at the end of the tool holder.


(1) At present, the most widely used tool holders in China are DIN 69871 (JT) and Japanese MAS BT.

2) DIN 69871 tool holders can also be mounted on machines with ANSI/ASME spindle taper bores.

(3) The international standard IS0 7388/1 tool holder can also be installed on DIN 69871, ANSI/ASME spindle tapered bore machine tools. So in terms of versatility, the IS0 7388/1 tool holder is the best.

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HSK vacuum tool holders with a taper of 1: 10

HSK vacuum tool holders rely on the elastic deformation of the tool holder, not only the tool holder with a taper of 1: 10 in contact with the 1:10 taper of the machine tool spindle bore, but also the flange face of the tool holder is in close contact with the spindle face.

This double-sided contact system is superior to a 7:24 universal tool holder in terms of high-speed machining, connection rigidity and overlap accuracy.

HSK vacuum tool holders with a taper of 1 10
HSK vacuum tool holders with a taper of 1 10

The HSK vacuum tool holder can improve the rigidity, stability, and product accuracy during high-speed machining, and also shorten the time of tool replacement, which is essential for high-speed machining. It is suitable for machine tool spindle speeds of up to 60,000 rpm. The HSK tool system is being widely used in the aerospace, automotive, and precision mold industries, among others.

HSK tool holders are available in types A, B, C, D, E, and F, with types A, E, and F being commonly used on machining centers with an automatic tool change (ATC) process.

The biggest difference between Type A and Type E:

(1) Type A has a transmission groove, but Type E does not. Therefore, Type A has a relatively larger transfer torque, which can handle heavy cutting. The torque transmitted by Type E is relatively small, so it can only handle light cutting.

(2) Type A tool holder has manual fixing holes and direction grooves, in addition to the transmission groove, resulting in relatively poor balance. Type E does not have these features, making it more suitable for high-speed processing.

The mechanisms of Type E and Type F are identical. The difference between them is that for handles with the same name (such as E63 and F63), the taper of the Type F handle is one size smaller. This means that both E63 and F63 have a flange diameter of φ63, but the F63 taper is only the same size as the E50. Therefore, the F63 will rotate faster (with a smaller spindle bearing) compared to the E63.

Tool Clamping Form of the Tool Holder

Spring cartridge tool holder

It is mainly used for straight tool holders such as drills, milling cutters and taps, or tool clamping.

The elastic deformation of the circlip is 1mm, and the clamping range is 0.5~32mm in diameter.

Spring cartridge tool holder

Hydraulic chuck

  • A- Locking screw, which use an Allen key to tighten the locking screw;
  • B- Locking piston, which presses the hydraulic medium into the expansion chamber;
  • C- Expansion chamber, which pressurized by liquid to generate pressure;
  • D-Thin expansion bushing, which enables the center of the tool clamping rod to be positioned and evenly enveloped during the locking process.
  • E- Special seals, which ensure ideal sealing and long service life.
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Hydraulic chuck

Heated tool holder

Sensing heating technology is used to heat the tool clamping part of the tool holder, causing it to expand in diameter. The cold shank is then inserted into the hot tool holder. This results in a high clamping force and good dynamic balance, making it suitable for high-speed machining.

The technology also provides high repeatability accuracy within 2μm and radial runout within 5μm, and has good resistance to stains and interference during machining.

However, only one tool of a specific shank diameter can be installed for each tool holder specification, and a set of heating equipment is also required.

The principle of pyrocondensational tool holder clamping:

The principle of pyrocondensational tool holder clamping
The principle of pyrocondensational tool holder clamping

Comprehensive evaluation and comparison of tool holders

   EvaluationSpring clamp typeHydraulic typePyrocondensational type
Structure DiagramSpring clamp typeHydraulic typePyrocondensational type
Versatilitybe used in all processes; highly versatilelimited for high-speed machining; high maintenance costsexcellent performance in a wide range of high-speed machining applications
Toolholder beatingquality spring clip <10µm>5µmabout 3µm
Rigidgood good
Dynamic Equilibriumgoodgeneralgood
Vibrationsno advantagecan absorb vibrationsno advantage
Convenienceaccuracy depends on the operatorclamping structure is easily damagedstandardized operation
CostgeneralexpensiveCheaper than Hydraulic type

Other types of tool holders

Other types of tool holders

Tool Holder Selection and Maintenance

Factors Influencing Selection

When choosing a tool holder, some key factors influence your decision:

  1. Compatibility: You need to ensure that the tool holder fits your machine spindle.
  2. Tool type and size: You must choose the appropriate holder for the specific tool required.
  3. Material: Different materials like steel, aluminum, or plastic affect the holder’s durability and performance.
  4. Balance: For high-speed applications, look for tool holders with better balance and minimum runout.

Proper Care and Handling

In order to prolong the life of your tool holders and maintain their performance, here are the steps to follow:

  • Storage: keep my tool holders in a dry and clean environment, away from contaminants and moisture.
  • Cleaning: Before and after usage, clean tool holders with a soft cloth to remove debris.
  • Inspection: Inspect my tool holders regularly for signs of wear, damage, or corrosion. Replace them if necessary.
  • Lubrication: When needed, apply lubricants to prevent rust and ensure smooth operation.
  • Handling: To avoid damage, handle tool holders with care, avoiding dropping or knocking them against hard surfaces.

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