Method for hard turning a work piece

Abstract: A method for cutting or hard turning the surface of a workpiece comprising using a cutting tool in which a cooling fluid is supplied to the cutting area by means of at least one jet nozzle. The cooling fluid is supplied as a jet with high pressure leaving the jet nozzle (16) with a pressure above 60 bar.

Description: The present invention relates to a method for hard turning the surface of a workpiece in which a cooling fluid is supplied to the cutting area. More particularly, the invention relates to a method for hard turning of workpieces with a Rockwell hardness of 50 HRC or above with ceramic or CBN (cubic boron nitride) tools with a cutting edge having a Vickers hardness of 1700 HV or above. In this description the expression "hard turning" means turning of carbon steel through or surface hardened to a Rockwell hardness of > 50 HRC and/or with martensitic or bainitic micro structure. As an example the invention may be used for cutting a race surface of a bearing which is in rolling contact with rolling elements. Today, all Hard Turning with ceramic or CBN tools is performed dry or with low-pressure flood cooling. All known recommendations say that it is very important to keep a high temperature in the cutting zone in order to achieve a good tool life and production economy. Such a tool is known from US-A-5,882,777. The research work performed according to the invention shows that this is not really true. The primary object of the invention has, thus, been to provide a method of hard cutting by which the tool life is increased, higher cutting speed and a better control of the chip shape are obtained. According to the present invention it has surprisingly shown that when applying a high-pressure jet of cooling fluid just into the gap between the clearance face of a tool and the workpiece material, a number of important advantages are obtained as will be described below. US-A-5,148,728 discloses a method for high pressure lubricating and cooling of a workpiece of metal during machining thereof. This publication does not refer to hard machining or a cutting edge of cubic boron nitride. The method generally entails using high pressures, the cooling fluid just before leaving the jet nozzle having a pressure from 60, preferably 120 to about 400 bar. The method according to the invention includes hard turning of workpieces with a Rockwell-hardness of 50 HRC or above using a cutting tool with a cutting edge comprising CBN (cubic boron nitride) with a Vicker hardness above 1700 HV, preferably above 2800 HV. As a cooling fluid water or e.g. carbon dioxide may be used. According to another aspect of the invention the cooling fluid is applied simultaneously to the gap between the chip and a rake face of the tool and between the clearance face of the tool and the work material. The above and other advantages of this invention will be appreciated from the following description in connection with the drawings in which: Fig 1 is a schematic sectioned view of a part of the workpiece and tool in hard turning with the method according to the invention. Fig 2 is a schematic sectional view similar to Fig 1 of a second embodiment of the method according to the invention. A method is provided for hard turning a workpiece which is a relatively new and very powerful method, which if used correctly, can lead to significant increase of productivity and production economy. As stated above hard turning is a machining operation with a geometrically defined cutting edge on materials that have undergone a hardening treatment. All materials (ferrous and non ferrous) which contain sufficient alloy elements (carbon etcetera) that can be hardened via a heat treatment process, giving a microstructure which consists of Martensite and/or Bainite with a hardness >50 HRC, this includes through hardening, air hardening and surface hardening for example by induction and carburizing treatments, can be hard turned. For ferrous materials mostly CBN or PCBN tools are applied, for non ferrous materials diamond is used. Figure 1 represents the method according to the invention and shows a workpiece 10 such as a bearing race which is hard turned. The workpiece 10 is moving with the speed Vo indicated by the arrow in relation to a tool 12 cutting a piece of the workpiece material such as a chip 14 with the tool tip. According to this invention a high-pressure jet is by means of a nozzle 16 applied into the gap between the clearance face of the tool insert and the workpiece material. A fluid wedge 18 is formed between the clearance face of the tool and the workpiece. As an example it has shown that when applying a high-pressure jet just into the gap between the clearance face of a CBN 10-insert and the workpiece material, the tool life is increased by more than 100%. The jet must be applied as perpendicularly as possible to the cutting edge. It has also been observed that the residual stresses on the workpiece are slightly more comprehensive that in conventional low-pressure cooling. The pressures used shall be in the range between 6 and 40 MPa (that is between 60 and 400 bar). The highest increase of the tool life (about 110%) was observed for the higher pressure. The jet was created using a nozzle with a diameter of 0,7 mm. A conventional synthetic cutting fluid was used. The solution was about 7%. According to a second embodiment of the method according to the invention the high-pressure jet is also applied into the gap between the chip and the tool rake surface as shown in Figure 2, in which parts which correspond to the parts of the first embodiment have the same numerals. Thus, a nozzle 16 applies a high-pressure jet between the chip 14 and the tool 12 rake surface forming a fluid wedge 18 between the chip and tool. It has been observed that when applying a second jet to the tool rake surface, the chip control was improved. Instead of long and continuous chips short, "more convenient" chips were produced. It is especially important in turning of large rings. As seen from the above the method according to the invention comprising directing a high-pressure jet in hard turning using CBN-inserts leads to a number of important advantages such as: Decrease of the edge temperature (30 -50%) Significant decrease of the tool abrasive wear Possibility to use higher cutting speed Increased chip forming possibility Higher quality of product due to lower wear of the tool and lower temperature Possibility to control the residual stresses on and below the workpiece surface

Claims: A method for hard turning the surface of a workpiece (10) of hardened steel using a cutting tool (12) incorporating a cutting edge formed of cubic boron nitride comprising supplying a cooling fluid to a cutting area, wherein the cooling fluid is supplied as a jet (18) with high pressure leaving the jet nozzle (16) with a pressure above 60 bar. A method as claimed in claim 1, characterized in that the cooling fluid jet (18) has a pressure above 120 bar. A method as claimed in claim 1 or 2, characterized in that the workpieces (10) have a Rockwell hardness of 50 HRC or above and the cutting edge has a Vickers hardness of 1700 HV or above. A method as claimed in claim 1, characterized in that the cubic boron nitride has a Vickers hardness above 1700 HV, preferably above 2800 HV. A method as claimed in anyone of claims 1 - 4, characterized in that water is used as a cooling fluid. A method as claimed in anyone of claims 1 - 4, characterized in that carbon dioxide is used as a cooling fluid. A method as claimed in anyone of claims 1 - 6, characterized in that the cooling fluid (18) is applied between a clearance face of the tool (12) and the workpiece (10). A method as claimed in anyone of claims 1 - 6, characterized in that the cooling fluid (18) is applied to the gap between the chip(14) and a rake face of the tool (12). A method as claimed in anyone of claims 1 - 4, characterized in that the cooling fluid (18) is applied to the gap between the chip and a rake face of the tool (12) and a clearance face of the tool (12) and the workpiece (10) simultaneously.

Inventors: KAMINSKI JACEK, KAMINSKI, JACEK
Applicants: SKF SVERIGE AB
Application year: 2001
Main classification: B23Q11
IPC codes: B23Q11, B23B1, B23B27