Although you deal with tool holders every day, do you know what 7∶24 means in BT tool holders? What are the standards of BT, NT, JT, IT, and CAT? Today, let's talk about the knowledge of tool holders for machining centers.
The tool holder is the connector between the machine tool and the tool. The tool holder is a key link that affects concentricity and dynamic balance. It must not be regarded as a general component. Concentricity can determine whether the cutting amount of each cutting edge part is uniform when the tool rotates one circle; dynamic imbalance will produce periodic vibration when the spindle rotates.
01 Two major categories based on the spindle taper
According to the taper of the tool hole of the machining center spindle, it is usually divided into two categories: 1) SK universal tool holder with a taper of 7∶24 2) 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 separate taper surface positioning, and the taper handle is longer. The cone surface plays two important roles at the same time, namely the precise positioning of the tool holder relative to the spindle and the clamping of the tool holder.
Advantages: No self-locking, can achieve fast loading and unloading of tools; manufacturing tool holders only need to process the taper angle to high precision to ensure the connection accuracy, so the tool holder cost is relatively low.
Disadvantages: The taper hole at the front end of the spindle will expand when rotating at high speed. The amount of expansion increases with the increase of the rotation radius and the rotation speed. The taper connection stiffness will decrease. Under the action of the pull rod tension, the axial displacement of the tool holder will also change. The radial size of the tool holder will change after each tool change, and there is a problem of unstable repeat positioning accuracy.
There are usually five standards and specifications for universal toolholders with a taper of 7:24: 1) International standard IS0 7388/1 (abbreviated as IV or IT) 2) Japanese standard MAS BT (abbreviated as BT) 3) German standard DIN 2080 (abbreviated as NT or ST) 4) American standard ANSI/ASME (abbreviated as CAT) 5) DIN 69871 (abbreviated as JT, DIN, DAT or DV)
Tensioning method: NT toolholder is tightened by a pull rod on a traditional machine tool, also known as ST in China; the other four toolholders are tightened by the pull pin at the end of the toolholder on the machining center.
Universality: 1) At present, the two most commonly used tool holders in China are DIN 69871 (i.e. JT) and Japanese MAS BT; 2) DIN 69871 tool holders can also be installed on ANSI/ASME spindle taper hole machine tools; 3) International standard IS0 7388/1 tool holders can also be installed on DIN 69871 and ANSI/ASME spindle taper hole machine tools, so in terms of universality, IS0 7388/1 tool holders are the best.
HSK vacuum tool holder with a taper of 1:10
The HSK vacuum tool holder relies on the elastic deformation of the tool holder. Not only does the 1:10 taper surface of the tool holder contact the 1:10 taper surface of the machine tool spindle hole, but also the flange surface of the tool holder is in close contact with the spindle surface. This double-sided contact system is superior to the 7:24 general tool holder in high-speed machining, connection rigidity and coincidence accuracy.
HSK vacuum toolholders can improve the rigidity and stability of the system and the product precision during high-speed machining, and shorten the tool replacement time. They play an important role in high-speed machining. They are suitable for machine tool spindle speeds of up to 60,000 rpm. HSK tool systems are being widely used in manufacturing industries such as aerospace, automobiles, and precision molds.
HSK toolholders have various specifications such as A, B, C, D, E, and F. Among them, A, E, and F are commonly used in machining centers (automatic tool change).
The biggest difference between A and E:
1) A has a transmission groove while E does not. Therefore, A transmits a larger torque and can perform some heavy cutting. The torque transmitted by E is relatively small, and can only perform some light cutting.
2) In addition to the transmission groove, the A-type toolholder also has a manual fixing hole, a direction groove, etc., so the balance is relatively poor. The E type does not, so the E type is more suitable for high-speed machining. The mechanisms of type E and type F are exactly the same. The difference between them is that for the same-named type E and type F toolholders (such as E63 and F63), the taper of type F toolholder is one size smaller. That is to say, the flange diameters of E63 and F63 are both φ63, but the taper size of F63 is only the same as that of E50. Therefore, compared with E63, F63 will have a faster speed (smaller spindle bearing).
02 Toolholder mounting form
Spring chuck toolholder
Mainly used for clamping straight shank tools and tools such as drills, milling cutters, and taps. The elastic deformation of the retaining spring is 1mm, and the clamping range is 0.5~32mm in diameter.
Hydraulic chuck
A-locking screw, tighten the locking screw with an Allen wrench; B-locking piston, press the hydraulic medium into the expansion chamber; C-expansion chamber, which generates pressure due to the extrusion of the liquid; D-thin expansion bushing, which positions the center of the tool clamping rod and evenly envelops it during the locking process. E--Special seals ensure ideal sealing and long service life.
Heated toolholder
The tool holder is heated by induction heating technology to expand its diameter, and then the cold tool bar is placed in the hot toolholder. The heated toolholder has a large clamping force and good dynamic balance, which is suitable for high-speed processing. The repeated positioning accuracy is high, generally within 2μm, and the radial runout is within 5μm; it has good anti-fouling ability and good anti-interference ability during processing. However, each specification of toolholder is only suitable for installing tools with one handle diameter, and a set of heating equipment is required.
