The Study of Minimum Quantity Lubrication (MQL) in High Speed Milling of NAK80 Mould Steel
Date Issued
2007
Date
2007
Author(s)
Lin, Hsien-Mou
DOI
zh-TW
Abstract
The influence of the minimum quantity lubrication (MQL) in high speed milling (HSM) of NAK80 mould steel was studied in this thesis. The TiAlN coated carbide tool and the lower viscous oil were adopted. It is found that cutting under flood cooling condition results in the shortest tool life due to severe thermal cracks. While the use of MQL, in comparison with dry cutting, is beneficial to tool life and surface funish both in the lower speed cutting and the higher speed cutting conditions. The oil with a higher viscosity is preferable to increase the tool life in the lower speed cutting. In the higher cutting speed, the less viscous oil containing a higher fraction of low molecular weight components that could volatilize more easily is essential so that cooling effect can be effective and welding of chips can be delayed. It in turn leads to the enhancement of tool life. The oil droplets are proved to increase the cooling effect of MQL so as to improve the tool life. However, the tool life will be decreased due to cracks resulted from the large fluctuation of temperature on tool face when there is too much flow rate of oil in MQL. MQL can provide extra oxygen to activate the oxidation of the chip and TiAlN coating, as a result to generate the oxide layer. The cutting tool can be protected by this layer, which in turn leads to an increase of tool life.
Based on this study, the mechanism dominating the tool life of HSM of NAK80 mould steel with MQL is proposed below. Firstly, the cooling effect by MQL can retain the strength and wear resistance of cutting tool, so as to enhance the tool life. Secondly, thermal fatigue induced by the excessive supply of oils would reduce the tool life. Thirdly, the formation of the protective layer can prolong the tool life. The tool life is determined by the combined effect of the aforementioned three factors. Hence, it is concluded that the tool life can be significantly improved due to the appropriate cooling effect by MQL together with the protection of oxide layer in HSM of mould steels with the use of TiAlN coating tool and lower viscous lubricant when cutting speed and flow rate of oil are chosen properly.
Subjects
模具鋼
最少量潤滑
氧化物層
刀具壽命
表面光度
high speed milling
mould steel
MQL
oxide layer
tool life
surface finish
Type
thesis