What are the different toolholders available for CNC machining centres?

The toolholder is the connection between the machine tool and the tool. The toolholder is a key link in concentricity and dynamic balance and should not be treated as a general component. Concentricity determines the uniformity of the cutting volume of each cutting edge part of the tool during one rotation; dynamic imbalance during spindle rotation will produce periodic vibrations.

01 There are two main categories according to the spindle taper bore

The taper of the spindle bore of a machining centre is usually divided into two main categories.

SK general purpose toolholders with a taper of 7:24

HSK vacuum shanks with a taper of 1:10

SK general purpose shanks with a taper of 7:24

7:24 means that the shank has a taper of 7:24 and is positioned with a separate tapered surface and a longer tapered shank. The tapered surface has to play two important roles at the same time, namely the precise positioning of the toolholder in relation to the spindle and the clamping of the toolholder.

Advantages.

Non-self-locking, allowing for fast tool loading and unloading; manufacturing of toolholders ensures the accuracy of the connection as long as the taper angle is machined to a high degree of precision, so toolholder costs are relatively low.

Disadvantages.

The tapered hole at the front end of the spindle will expand during high speed rotation, the size of the expansion increases with the increase in the radius of rotation and speed, the stiffness of the tapered connection will be reduced and the axial displacement of the toolholder will change under the action of the drawbar tension. The radial dimension of the toolholder will change after each tool change and there is the problem of unstable repeatable positioning accuracy.

Universal toolholders with a taper of 7:24 are usually available in five standards and specifications.

1. International Standard IS0 7388/1 (abbreviated as IV or IT)

2. Japanese Standard MAS BT (abbreviated as BT)

3. German Standard DIN 2080 type (abbreviated NT or ST)

4. American Standard ANSI/ASME (abbreviated CAT)

5. DIN 69871 type (abbreviated JT, DIN, DAT or DV)

Tensioning method.

NT type toolholders are tensioned on conventional type machines by means of a drawbar, also known as ST in China; the other four types of toolholder are tensioned on machining centres by means of a spigot at the end of the toolholder.

Versatility.

1) At present, the two most used toolholders in China are DIN 69871 type (i.e. JT) and Japanese MAS BT type.

2) The DIN 69871 type toolholders can also be fitted to machines with ANSI/ASME spindle taper bores.

3) The international standard IS0 7388/1 shank can also be fitted to machines with DIN 69871 and ANSI/ASME spindle taper, so in terms of versatility, the IS0 7388/1 shank is the best.

HSK vacuum shanks with a 1:10 taper

HSK vacuum toolholders rely on the elastic deformation of the toolholder, not only does the 1:10 taper of the toolholder come into contact with the 1:10 taper of the machine tool spindle bore, but also brings the flange face of the toolholder into close contact with the spindle face. This double-sided contact system is superior to the 7:24 universal toolholder in terms of high speed machining, joint rigidity and overlap accuracy.

HSK vacuum toolholders play an important role in high speed machining by improving the rigidity and stability of the system, product accuracy at high speeds and reducing tool change times, adapting to machine spindle speeds of up to 60,000 rpm.

HSK toolholders are available in types A, B, C, D, E and F. The A, E and F types are commonly used on machining centres (automatic tool change).

The main differences between Type A and Type E are

1. Type A has a transmission groove while Type E does not. So relatively speaking the A type transmits a higher torque and can relatively carry out some heavy cutting. The E type transmits less torque and can only make lighter cuts. 2.

2. In addition to the drive groove, there are also manual fixing holes and directional grooves on the A type shank, so the balance is relatively poor. The mechanism of the E-type and F-type is identical, the difference between them is: the same name of E-type and F-type shank (for example, E63 and F63), the F-type shank has a smaller taper part. This means that the flange diameter of both the E63 and F63 is φ63, but the taper of the F63 is only the same size as that of the E50. Therefore, the speed of the F63 is faster compared to the E63 (smaller spindle bearing).

02 Toolholder loading forms

Spring collet toolholders

Mainly used for the clamping of straight shank tools and tools such as drills, milling cutters, taps, etc., with an elastic deformation of 1 mm in the chucking spring and a clamping range of 0.5 to 32 mm in diameter.

Hydraulic chucks

A- Locking screw, tighten the locking screw using an Allen key.

B- The locking piston, which presses the hydraulic medium into the expansion chamber.

C- Expansion chamber, pressed by the fluid to generate pressure.

D- Thin expansion bushing, which centres and evenly envelops the tool-holding rod during the locking process.

E- Special seals to ensure ideal sealing and long service life.

Heating the toolholder

Induction heating technology is applied to heat the tool clamping area on the toolholder so that its diameter will expand and then the cold toolholder is placed into the hot toolholder. The heated toolholder has a high clamping force and good dynamic balance, making it suitable for high-speed machining. High repeat positioning accuracy, generally within 2μm and radial runout within 5μm; good dirt resistance and good anti-interference ability in machining. However, each specification shank can only be fitted with one shank diameter tool and a set of heating equipment is required.