DE102011008998B4 - Process for machining a workpiece - Google Patents

Abstract

Method for machining a workpiece which consists at least in sections of a high-temperature-resistant cast steel, wherein the workpiece is machined by means of a milling tool having at least one cutting edge, wherein the machining of the workpiece is carried out at least in sections using minimum quantity lubrication, characterized in that a VS contact is set between the workpiece and the tool during machining.

Description

The invention relates to a method for machining a workpiece made of a high-temperature cast steel according to the preamble of patent claim 1.

From the series production of motor vehicles, it is known to machine workpieces, which in particular consist of high-temperature cast steel, using a tool and to cool and lubricate the workpiece and the tool during the machining process using flood lubrication. With this flood lubrication, a large amount of cooling lubricant is supplied to a machining area, which floods the workpiece and dissipates the heat generated in the machining area. It has been shown that this machining process is associated with high tool wear and high consumption of cooling lubricant and therefore causes undesirably high costs.

The WO 2009/155909 A2 describes a multi-edge drilling tool which is suitable for use with a minimum quantity lubrication (MQL). In addition, similar tools and the use of such tools together with a minimum quantity lubrication are also known from the DE 203 07 258 U1 as well as from the US 2006/0269372 A1 fundamentally known.

In connection with other machining processes such as turning processes, reference can be made to the DE 24 44 208 A1 as well as the EN 36 87 837 T2 to get expelled.

Regarding milling, the basic principle is EN 103 46 500 A1 as well as the subsequently published EN 10 2010 000 640 A1 be pointed out.

Furthermore, the EP 1 354 984 A2 a wear protection layer for cutting tools, especially for rotating cutting tools.

For the state of the art, please refer to Max Kronenberg: Max Kronenberg: Fundamentals of Machining Theory. Theory and practice of machining for the construction and operation of Machine tools, second volume, multi-edge machining (face milling, drilling). Heidelberg : Springer-Verlag Berlin, 1963. XXXI, 362, ISBN 978-3-642-92862:8 . to get expelled.

It is therefore an object of the present invention to provide a method for machining a workpiece made of a high-temperature cast steel, which enables a more cost-effective machining of the workpiece.

This object is achieved by a method for machining a workpiece with the features of patent claim 1. Advantageous embodiments with expedient and non-trivial further developments of the invention are specified in the remaining claims.

The workpiece, which consists at least in sections of a high-temperature cast steel, is machined using minimum quantity lubrication. This means that the method according to the invention differs fundamentally from the conventional machining of high-temperature cast steel, which is carried out as so-called wet machining and in which large quantities of cooling medium are supplied to the respective machining areas of the workpiece by flood lubrication. As is known from the prior art, such flood lubrication is necessary in order to dissipate the high quantities of heat generated during conventional machining of the workpieces and thus to cool the workpieces as well as the corresponding tool.

In contrast to this conventional technical teaching, the invention provides for a minimal amount of lubricant to be supplied to the workpiece directly at the machining point and for the machining parameters to be set in such a way that - despite this small amount of lubricant - a long service life of the tools is achieved. This is possible because the method according to the invention involves heat removal from the machining area, in particular via chips that are created in the machining area during machining of the workpiece.

According to the invention, a so-called VS contact is set between the cutting edge of the milling tool and the workpiece to be machined, whereby the cutting edge, in particular the cutting corner, enters along a line. The setting of the VS contact results in particularly low wear on the tool, which leads to a long service life or a high tool life. This means that the workpiece can be machined particularly cost-effectively. With regard to the VS contact, reference can be made to the above-mentioned book by Max Kronenberg, who describes one such contact on page 8 and shows a schematic representation in table S/1 on page 10.

In the case of minimum quantity lubrication as quasi-dry machining, it can be provided in particular that the machining area is Lubricant is supplied directly to cool the workpiece and the tool, with lubricant consumption being 50 ml per hour or less. Alternatively or additionally, the lubricant can be dissolved in compressed air, creating an aerosol which then acts as a cooling medium itself and which also reaches the corresponding tool for machining even at high peripheral speeds, thus cooling the workpiece and the tool.

Thus, in contrast to the known and conventional methods for achieving good heat dissipation, it is not necessary to supply a very large amount of lubricant to the machining area in order to keep the temperature of the workpiece and the tool low and to prevent failure or undesirable high wear of the tool. In contrast, high tool life and service life are achieved with the method according to the invention by setting the machining parameters in the manner according to the invention.

When using minimum quantity lubrication, it is possible to apply significantly higher cutting forces than with the wet machining described above. A VS contact is advantageously formed between the tool cutting edge and the workpiece, which further has a positive effect on the machining of the workpiece. If the tool advantageously enters the workpiece in a VS contact, this leads to very high tool quantities and service life, as this keeps tool wear and chipping to a minimum, especially in comparison to a so-called S contact.

• - Diese Werkstoffe sind aufgrund der in ihnen enthaltenen Chrom-Anteile und der daraus gebildeten Karbide sehr hart; dies legt den Schluss nahe, dass ein sehr stabiles Werkzeug für die spanende Bearbeitung notwendig ist, woraus ein negativer Spanwinkel sowie eine negative Schutzfase resultieren würde.
• - Andererseits weist der hochwarmfeste Stahlguss aufgrund von Nickel-Anteilen eine hohe Zähigkeit auf, was den Schluss zulassen könnte, dass ein schneidfreudiges Werkzeug vonnöten ist. Daraus würde ein stark positiver Spanwinkel resultieren, was jedoch das entsprechende Werkzeug filigran und damit verschleißanfällig werden lassen würde.

Furthermore, by using minimum quantity lubrication and setting a positive rake angle, a solution is found for the following conflicting requirements that are known to occur when machining workpieces made of high-temperature cast steel:

• - These materials are very hard due to the chromium they contain and the carbides formed from them; this suggests that a very stable tool is necessary for machining, which would result in a negative rake angle and a negative protective chamfer.
• - On the other hand, the high-temperature cast steel is very tough due to the nickel content, which could lead to the conclusion that a tool with good cutting ability is required. This would result in a strongly positive rake angle, which would, however, make the corresponding tool delicate and therefore susceptible to wear.

In wet machining, a negative or neutral rake angle is usually set to achieve a compromise. However, when using minimum quantity lubrication, it has been found that very good machining results can be achieved with a positive rake angle. The positive rake angle means that chips are removed more effectively, so that good heat can be removed from the machining area and the heat input is lower. Therefore, the flood cooling of wet machining can be dispensed with and minimum quantity lubrication can be used.

However, in contrast to conventional wet machining, with minimum quantity lubrication, the resulting machining heat is not removed by a particularly large amount of lubricant. Furthermore, particularly with workpieces made of high-temperature cast steel, the resulting machining heat remains in the workpiece for longer due to the poor thermal conductivity. This aspect is advantageously taken into account by designing a machining sequence accordingly: To achieve high machining accuracy, it is advantageous to space a pre-machining step and a fine machining step as far apart as possible in a given machining area of the workpiece. It is advantageous, for example, to form a hole, a pre-drilling is one of the first machining steps, while to form the hole with high precision, reaming of the hole is one of the last or even the last machining step. In this way, in the time between the two drilling operations, the machining area can cool down enough that the desired dimension is achieved with the second drilling operation.

A preferred embodiment of the invention relates to a method for milling a workpiece made of high-temperature cast steel, in which the workpiece is milled by means of a milling element having at least one cutting edge while adjusting a cutting speed, a feed, a rake angle and a clearance angle.

It has proven advantageous that the cutting speed is at least substantially in a range from 120 metres per minute (m/min) to 140 m/min and the tooth feed is at least substantially in a range from 0.1 millimeters per tooth (mm) up to and including 0.5 mm and the chip angle is set at least substantially greater than 0°, in particular at least substantially in a range from 5° up to and including 30°, and the clearance angle is set at least substantially greater than 0°, in particular at least substantially in a range from 5° up to and including 15°. By setting the machining parameters during milling in this way, the workpiece made of a high-temperature cast steel can be machined (milled) particularly cost-effectively, since the machining parameters result in low tool wear and thus high tool quantities and tool service lives. Advantageous embodiments of the first aspect of the invention are to be regarded as advantageous embodiments of the second aspect of the invention and vice versa.

A further aspect of the invention relates to a method for machining a workpiece, in which the workpiece is machined in at least one machining area by means of a tool and cooled at least in the machining area by means of minimum quantity lubrication, a cooling medium being supplied to the machining area. According to the invention, the cooling medium is supplied at least essentially directly to the machining area via a channel that runs at least partially inside the tool and through which the cooling medium can flow. This internal and direct supply of the cooling medium of the minimum quantity lubrication to the point of action means that wear on the tool can be kept low as a result of particularly efficient heat removal from the point of action, which results in high tool life and long tool life. This keeps the tool costs per workpiece and investment costs in machine tools low, so that the workpiece can be machined more cost-effectively. Advantageous embodiments of the first three aspects of the invention are to be regarded as advantageous embodiments of the fourth aspect of the invention and vice versa.

Chips that are produced are preferably removed from the workpiece by blowing them out, for example by blowing air onto the workpiece. This prevents the formation of internal chip nests.

Further advantages, features and details of the invention emerge from the following description of preferred embodiments and from the drawing. The features and combinations of features mentioned above in the description as well as the features and combinations of features mentioned below in the description of the figures and/or shown alone in the figures can be used not only in the combination specified in each case, but also in other combinations or on their own, without departing from the scope of the invention.

• 1 gemäß dem Stand der Technik eine schematische Schnittansicht eines Bohrwerkzeugs zur spanenden Bearbeitung eines Werkstücks unter Einsatz von Minimalmengenschmierung, wobei ein Schmiermedium in Druckluft gelöst als Aerosol einer Wirkstelle über eine externe Zuführung zugeführt wird;
• 2 gemäß dem Stand der Technik eine schematische Schnittansicht eines Bohrwerkzeugs gemäß 1, wobei das in Druckluft gelöste Schmiermittel als Aerosol der Wirkstelle intern zugeführt wird;
• 3 eine schematische Perspektivansicht eines Abgaskrümmeranschlusses, welcher mittels eines Fräsers spanend zu bearbeiten ist; und
• 4 ausschnittsweise eine schematische Querschnittsansicht eines mittels des Fräsers zu bearbeitenden Bereichs des Abgaskrümmeranschlusses gemäß 3, wobei ein Eintritt einer Schneide entlang einer Linie unter Ausbildung eines VS-Kontakts erfolgt.

The drawing shows in:

• 1 according to the prior art, a schematic sectional view of a drilling tool for machining a workpiece using minimum quantity lubrication, wherein a lubricating medium dissolved in compressed air is supplied as an aerosol to an active point via an external supply;
• 2 according to the prior art, a schematic sectional view of a drilling tool according to 1 , whereby the lubricant dissolved in compressed air is supplied internally to the site of action as an aerosol;
• 3 a schematic perspective view of an exhaust manifold connection which is to be machined by means of a milling cutter; and
• 4 a schematic cross-sectional view of a portion of the exhaust manifold connection to be machined by means of the milling cutter according to 3 , whereby an entry of a cutting edge occurs along a line forming a VS contact.

The 1 shows a drilling tool 10 with a spindle 12, which has a drill chuck 14. A drill 16 of the drilling tool is clamped into the drill chuck 14 and is thereby connected in a rotationally fixed manner to the spindle, which can rotate about an axis of rotation 18. To drive the spindle 12 and the drill 16, the spindle 12 is connected, for example, to an electric motor.

The drill 16 has a cutting area 20 with a geometrically defined cutting edge, by means of which a 1 A workpiece (not shown) is to be drilled at an effective point 22.

The drilling tool 10 is assigned a line 24 which is fluidically connected to a nozzle 26. A liquid lubricant dissolved in compressed air can flow through the line 24 and the nozzle 26 and can be supplied to the effective point 22 by appropriate alignment of the nozzle 26. By dissolving the lubricant in the compressed air, which for example has a pressure of at least essentially 3 bar up to and including 6 bar, an aerosol is formed which itself represents a cooling medium and by means of which the workpiece and the drill 10 can be cooled at the effective point 22. By means of the line 24, the nozzle 26 and the aerosol, during the cutting When machining the workpiece, a so-called minimum quantity lubrication is carried out, whereby, for example, the consumption of lubricating medium (the first lubricating medium) is at least substantially 50 ml.

By means of the drilling tool 10 and the described minimum quantity lubrication, a workpiece made of a high-temperature cast steel is to be provided with a lubricant, whereby, in contrast to wet machining, the effective point 22 is supplied with a particularly small quantity of lubricant. In order to machine the workpiece particularly cost-effectively and to keep the wear of the drill 16 to a minimum, the machining parameters of the drilling must be set accordingly and in particular adapted to the high-temperature cast steel.

The 2 shows an alternative embodiment of the drilling tool 10 according to 1 , which is designated 10. The drilling tool 10 according to 2 comprises a mixing head 28 in which a mixture of the liquid lubricant and the compressed air takes place, so that the lubricant is dissolved in the compressed air and the aerosol is formed, which in the 2 is designated 30.

Again 2 As can be seen, both the spindle 12 and the drill 16 each have a channel 32 and 34, which are integrated into the spindle 12 and the drill 16 respectively and run within the spindle 12 and the drill 16 respectively and are fluidically connected to one another. The channel 32 is fluidically connected to the mixing head 28 via a rotary feedthrough 36, so that the aerosol from the mixing head 28 can be transferred to the channel 34 despite the rotation of the spindle 12. In other words, the aerosol can flow from the mixing head 28 via the rotary feedthrough 36 into the channel 32 and further into the channel 34.

The drill 16 has an outlet opening on a front side 38 facing the workpiece, through which the aerosol can exit the channel 34 and directly cool the drill 16 and the workpiece. In this way, the aerosol and thus the lubricating medium can be fed internally and directly to the working point 22 in order to keep the wear on the drill 16 to a minimum. This is accompanied by a long service life and a high service life of the drilling tool 10. Heat is dissipated from the working point 22 by chips resulting from the machining of the workpiece. The workpiece can be, for example, a housing part of an exhaust gas turbocharger made of a high-temperature cast steel that is machined.

The 3 and 4 show an exhaust manifold connection 40 made of high-temperature cast steel, which can be machined and thus milled using a milling cutter. For this purpose, the milling cutter has a cutting edge with at least one cutting corner.

As in particular the 4 As can be seen, a so-called VS contact is advantageously set between the cutting edge and an area of the exhaust manifold connection 40 to be machined during milling, whereby the cutting edge, in particular the cutting corner, enters along a line. The setting of the VS contact under which the milling cutter enters the exhaust manifold 40 results in particularly low wear of the milling cutter, which leads to a long service life or a high service life of the milling cutter. This means that the exhaust manifold connection 40 can be machined particularly cost-effectively. In series production with corresponding machining of the exhaust manifold connection 40, a particularly high number of exhaust manifold connections 40 can be machined particularly cost-effectively due to economies of scale.

Claims (3)

Method for machining a workpiece which consists at least in sections of a high-temperature-resistant cast steel, wherein the workpiece is machined by means of a milling tool having at least one cutting edge, wherein the machining of the workpiece is carried out at least in sections using minimum quantity lubrication, characterized in that a VS contact is set between the workpiece and the tool during machining. Procedure according to Claim 1 , in which the workpiece is milled by means of a milling tool having at least one cutting edge by setting a cutting speed, a feed, a rake angle and a clearance angle, characterized in that - the cutting speed is at least substantially in a range from 120 m/min to 140 m/min inclusive and - the tooth feed is at least substantially in a range from 0.1 mm to 0.25 mm inclusive and - the rake angle is at least substantially greater than 0°, in particular at least substantially in a range from 5 to 30° inclusive, and - the clearance angle is at least substantially greater than 0°, in particular at least substantially in a range from 5° to 15° inclusive. Method (532, 534) according to one of the preceding claims, for machining a workpiece, in which the workpiece is machined in at least one machining area (22) by means of a tool (10) and is cooled at least in the machining area (22) by means of minimum quantity lubrication, a cooling medium (34) being supplied to the machining area (22), characterized in that the cooling medium (34) is supplied at least substantially directly to the machining area (22) via a channel (32, 34) which runs at least partially within the tool (10) and through which the cooling medium (34) can flow.

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