Today, the knowledge of machining center tool holders would be discussed.
Nowadays, CNC machine tools are widely used in factories.
These machines and the tools used are 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? And do you know what standards BT, NT, JT, IT, CAT are?
The tool holder is the connection between the CNC machine tool and the cutting tool, which is a key link that affects concentricity and dynamic balance, and it should never be treated as an ordinary component.
Concentricity can determine whether the cutting amount of each cutting edge part of the tool is uniform when the tool rotates one circle; when the spindle rotates, the dynamic imbalance will produce periodic vibration.
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.
Non-self-locking, which allows for quick tool loading and unloading.
The cost of the tool holder is relatively low as the taper angle can be machined to a high degree of accuracy to ensure a precise connection.
The tapered hole at the front end of the spindle will expand during high-speed rotation.
The amount of expansion increases with the increase in the rotation radius and speed, but the taper connection rigidity will decrease.
The axial displacement of the tool holder will also change under the action of the drawbar tension.
The radial dimension 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 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)
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.
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 holder can not only improve the rigidity and stability of the system and the product accuracy during high-speed machining but also shorten the time of tool replacement, which plays an important role in high-speed machining.
It is suitable for machine tool spindle speed up to 60,000 rpm.
The HSK tool system is being widely used in aerospace, automotive, precision mold and other manufacturing industries.
HSK tool holders are available in A, B, C, D, E, and F types, among which A, E, and F types are commonly used on machining centers (ATC process).
The biggest difference between Type A and Type E:
1) Type A has a transmission groove but Type E does not.
So relatively speaking, Type A has a relatively larger transfer torque, which is relatively capable of some heavy cutting.
The torque transmitted of Type E is relatively small so that it can only perform some light cutting.
2) In addition to the transmission groove on Type A tool holder, there are manual fixing holes, direction grooves, etc., so the balance is relatively poor.
But Type E does not, which is more suitable for high-speed processing.
The mechanism of Type E and Type F is exactly the same. The difference between them is that for the same-named E-type and F-type handles (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.
So the F63 will rotate faster (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.
- 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.
Heated tool holder
Sensing heating technology is applied to heat the tool clamping part of the tool holder so that it expands in its diameter, and then the cold shank is inserted into the hot tool holder.
The heated tool holder has a high clamping force and good dynamic balance, making it suitable for high-speed machining.
It has high repeatability accuracy within 2μm and radial runout within 5μm; good stain resistance and anti-interference ability during machining.
However, only one tool of one shank diameter can be installed for each specification of tool holder, a set of heating equipment is also required.
The principle of pyrocondensational tool holder clamping:
Comprehensive evaluation and comparison of tool holders
|Evaluation||Spring clamp type||Hydraulic type||Pyrocondensational type|
|Versatility||be used in all processes; highly versatile||limited for high-speed machining; high maintenance costs||excellent performance in a wide range of high-speed machining applications|
|Toolholder beating||quality spring clip <10µm||>5µm||about 3µm|
|Vibrations||no advantage||can absorb vibrations||no advantage|
|Convenience||accuracy depends on the operator||clamping structure is easily damaged||standardized operation|
|Cost||general||expensive||Cheaper than Hydraulic type|
Other types of tool holders