TECHNICAL FIELD

The present subject matter relates to a method and an apparatus for assessing lubricity of cutting fluids, according to the preamble of independent claims.

BACKGROUND

Cutting fluids are essentially used for the purpose of lubrication during machining of metals. Apart from lubrication, cutting fluids carry away a portion of the heat generated during machining, promotes chip-curl and reduces the possibility of tool-chip stiction. Lubricity of cutting fluids can influence the working life of the cutting tool. Efficient lubrication reduces the friction between the cutting tool and the work piece, thereby reducing the power losses due to friction and also decreasing tool-wear. This results in increased tool- life.

Lubricity characteristics that are required for the cutting fluids may depend on the type of machining, for example, turning or grinding, heavy or light machining and long or short duration machining. The lubricity characteristics may also depend on the material to be machined or cut, and cutting speeds. For machining or cutting a work piece, a cutting fluid with good lubricity is required that minimizes friction between the cutting tool and the work piece. Measurement of the lubricity of cutting fluids is essential for efficient manufacturing practices involving metal cutting.

Typical cutting fluids are oil based liquids, pastes, gels, aerosols etc. Oil based cutting fluids are prepared through emulsification of oil(s) in water using suitable emulsifier(s). The lubricity of the cutting fluid can be controlled by altering the type and concentration of oil(s) in water, the type and concentration of emulsifiers and/or by the addition of proper lubricity additives in the cutting fluid. Knowledge of the lubricity characteristics required for a particular type of machining or cutting is essential to optimize the contents of a cutting fluid.

Conventional methods for assessing the lubricity of cutting fluids either employ a standard tribological test (rubbing test) or use a cutting test on a machine tool set-up (chip generating tests). Rubbing tests involve friction testing in a conventional tribometer between mating pairs in relative motion in the presence of a lubricant at the interface. The rubbing test takes into account the effect of the action of cutting fluids on a surface pre-coated with an oxide layer. Chip generation tests involve measurement of cutting forces during cutting with the application of a cutting fluid and estimate the coefficient of friction from the measured cutting forces, using a particular model (force-diagram model) which encapsulates the mechanics at the chip tool interface. The appropriateness of certain accepted force-diagram models in modeling the force and moment interactions at the chip-tool interface has been a subject of debate. Thus, none of these methods can be considered to be truly superior in assessing the lubricity of cutting fluids.

SUMMARY

The subject matter disclosed herein describes a method for assessing lubricity of a cutting fluid. The method comprises cutting a work piece immersed in the cutting fluid with a cutting tool. The work piece is cut to generate at least one new surface on the work piece. The method further comprises performing a friction test in-situ on the at least one new surface immersed in the cutting fluid.

The subject matter disclosed herein further describes an apparatus for assessing lubricity of a cutting fluid. The apparatus comprises a chamber filled with the cutting fluid, a work piece positioned in the chamber and immersed in the cutting fluid, a cutting tool immersed in the cutting fluid to cut the work piece and generate at least one new surface on the work piece, and a friction measuring unit to measure friction in-situ on the at least one new surface immersed in the cutting fluid.

These and other features, aspects, and advantages of the present subject matter will become better understood with reference to the following description and appended claims. This summary is provided to introduce a selection of concepts in a simplified form. This summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter.

BRIEF DESCRIPTION OF DRAWINGS

The novel features of the subject matter are set forth in the appended claims hereto. The subject matter itself, however, as well as a preferred mode of use, further objectives, and advantages thereof, will best be understood by reference to the following detailed description of illustrative embodiments when read in conjunction with the accompanying drawings.

wherein the same numbers are used throughout the drawings to reference like features, and wherein:

Figure 1 illustrates schematics of methodology for method and apparatus, according to the present subject matter.

Figure 2 shows a section of the apparatus, according to an embodiment of the present subject matter.

Figure 3 shows a comparison of friction tests performed for a cutting fluid on a nascent surface generated in a cutting fluid, according to the present subject matter, and on an oxidized surface.

Figure 4 shows friction tests results obtained using a cutting fluid under a substantially high rotation speed of the work piece, with cutting and friction testing done simultaneously, according to the present subject matter.

DETAILED DESCRIPTION

Cutting fluids used for the purpose of machining or cutting materials should possess necessary lubricity characteristics. Lubricity induced by the cutting fluids reduces (ideally minimizes) the friction between a cutting tool and work piece, removes a portion of the heat generated during cutting and thus helps in improving the working life of the cutting tool. The lubricity characteristics of the cutting fluids that are required for machining depend on various parameters, for example, type of machining and type of material to be machined. The composition of a cutting fluid emulsion is decided based on this information. For example, for heavy and long duration machining, a cutting fluid that can maintain lubricity under such demanding situations is needed in comparison to light and short duration machining. Thus, it is essential to know the lubricity characteristics of the cutting fluids for it to be employed for a particular machining process.

Conventional methods like rubbing tests utilize friction test on standard tribological apparatus for estimating the lubricity characteristics of cutting fluids. However, they do not yield realistic results. During machining or cutting of a work piece, the cutting fluid is supplied intermittently/continuously in the cutting zone in the region between the cutting edge of the cutting tool and the work piece. As the work piece is cut, a nascent/new surface is created during the process. Lubricity of the cutting fluid need to be tested on the nascent surface(s) generated during the cutting process. If the work piece is transferred to another environment for testing, this can expose the nascent surface to air that causes oxidation of the surface. As a result, an oxide layer is formed on the surface of the work piece. The conventional rubbing test and apparatus assess the lubricity of the cutting fluids over pre- coated (oxidized) surfaces of the work pieces. Interaction of the cutting fluid with the oxidized surface is different from the interaction of the cutting fluid directly with the nascent surface. The later one is rather more realistic in machining process. Thus, there is a need for a method and apparatus that substantially assesses more realistic lubricity characteristics of the cutting fluids.
The present subject matter relates to a method and an apparatus for assessing lubricity of cutting fluids. The method and the apparatus according to the present subject matter, carry out a substantially more realistic assessment of the lubricity of the cutting fluids.
Figure 1 illustrates the schematics of methodology for the method and apparatus, according to the present subject matter. Figure 1 shows a work piece 1 immersed in a pool of a cutting fluid 2. A cutting tool 3 is engaged with the work piece 1 to cut or machine the work piece 1 to generate at least one new (nascent) surface 4. The new surface 4 generated on the work piece 1 is kept immersed in the cutting fluid 2 thereby generating a layer same as that would be formed on a nascent surface (generated during cutting) exposed to a cutting fluid 2. A friction measuring unit 5 is engaged via its pin 6 over the new surface 4 which got modified by the action of the cutting fluid 2, for performing a friction test in-situ on the new surface 4.
The pin 6 is separated from the cutting tool 3 by a predefined distance 7. The term 'predefined distance' 7 is interchangeably used as 'predefined separation' 7 in the specification. The work piece 1 is moved in a direction 8 (as shown) at a predefined speed v for cutting and measurement of friction. The friction test is performed by the pin 6 at a predefined time t after cutting the work piece 1, and the predefined time t is dependent on the predefined separation 7 and the predefined speed v. The predefined separation 7 and the predefined speed v may vary in an embodiment.
The method, according to an exemplary embodiment, includes cutting the work piece 1 immersed in the cutting fluid 2 with the cutting tool 3 to generate the at least one new surface 4 on the work piece 1, and simultaneously performing a friction test in-situ on the at least one new surface 4 immersed in the cutting fluid 2.
Further, the method, according to the exemplary embodiment, includes performing the friction test in-situ on the new surface 4 at the predefined time t after the work piece 1 is cut to generate the new surface 4. For performing the friction test the work piece 1 is not exposed to air to prevent oxidation of the new (nascent) surfaces 4. The work piece 1 is kept immersed in the cutting fluid 2 throughout, from cutting till performing the friction test. The cutting fluid 2 modifies the new surface 4. The friction test results obtained from the method according to the present subject matter reflect the lubricity of the cutting fluid 2 due to an interaction of the cutting fluid 2 with the new surfaces 4. The new surface 4 in here is not oxidized. Thus, the lubricity results are substantially more realistic in regard with the machining or cutting process.
The apparatus, according to an exemplary embodiment of the present subject matter, employ the methodology illustrated in figure 1. The apparatus includes a pool of cutting fluid 2, the work piece 1 immersed in the cutting fluid 2, the cutting tool 3 immersed in the cutting fluid 2 to cut the work piece 1 and generate the at least one new surface 4 on the work piece 1, and the friction measuring unit 5 to measure friction in-situ on the at least one new surface 4 immersed in the cutting fluid 2.
In the apparatus, according to the exemplary embodiment, the friction measuring unit 5 measures the friction in-situ of the new surfaces 4 immersed in the cutting fluid 2 at the predefined time t after the cutting tool 3 generates the new surface 4. The work piece 1 is kept immersed in the cutting fluid 2 throughout, from cutting to friction measurement, without exposing the new surfaces 4 to air. The apparatus has a same advantage as the one stated above.
Figure 2 shows a portion of the apparatus, according to an embodiment of the present subject matter. Same reference numerals in the figures refer to similar elements. As shown in figure 2, the apparatus includes a chamber 10 filled with the cutting fluid 2. The work piece 1 is positioned in the chamber 10 and immersed in the cutting fluid 2. In an embodiment, the work piece 1 can be a disc-shaped work piece 1 with a surface 11, to be machined of cut, immersed in the cutting fluid 2.
The apparatus further includes a rotating unit 12 to rotate the work piece 1 at a predefined rotational speed Vr. The rotating unit 12 includes a shaft 13 and a holder 14 to hold the work piece 1. The holder 14 is coupled to the shaft 13 which in turn rotates the work piece 1. The work piece 1 is held onto the holder 14 through fasteners 15. In an embodiment, the fasteners 15 can be screws. Further, in an embodiment, the rotating unit 12 rotates the work piece 1 about a vertical axis 16.
The apparatus further includes the cutting tool 3 with its cutting edge engaged with the work piece 1 to cut the work piece 1 and generate at least one new surface (not shown in figure 2) on the work piece 1. The cutting tool 3 is held on a tool post 17. In an embodiment, the new surface can be in form of a cut track, cut on the work piece 1 upon its rotation. The cut track may be a partial circle or a complete circle.
The apparatus further includes the friction measuring unit 5 to measure the friction in- situ on the at least one new surface immersed in the cutting fluid 2. The working principle of the friction measuring unit 5 is similar to the one known conventionally. The friction measuring unit 5 can be a friction force measuring arm pivoted on a bearing. The friction measuring unit 5 includes a friction measuring sensor and the pin 6, configured in such a way that the pin 6 engages with the new surface, i.e. the cut track, created by the cutting tool 3. In an embodiment, the friction measuring sensor can be a force measuring device like a load cell.
Further, in an embodiment, the pin 6 and the cutting edge of the cutting tool 3 are positioned at same radii on the disc-shaped work piece 1. The pin 6 is kept at a predefined separation, particularly a predefined angular separation 0 about the vertical axis 16, from the cutting tool 3. The resistance to surface traction experienced by the pin 6 due to the friction at the new surface is measured by the friction measuring sensor.
In the apparatus, according to the exemplary embodiment, the friction measuring unit 5 measures the friction in-situ of the new surfaces immersed in the cutting fluid 2 at the predefined time t after the cutting tool 3 generates the new surface. The work piece 2 is kept immersed in the cutting fluid 2 throughout, from cutting to friction measurement, without exposing the new surfaces to air. In the embodiment described above, the predefined time t depends on the predefined rotational speed v, and the predefined angular separation 0.
In an embodiment, the least value of the angular separation 0 is dictated by the relative sizes of the tool post 17 and the friction measuring arm, so that there is a reasonable amount of separation (gap) between them. In an embodiment, the angular separation 0 is from about 30° to 180°. In an embodiment, the predefined time t is from about 0.15 seconds to 2 seconds. In an embodiment, the rotational speed Vr is from about 15 rpm (revolutions per minute) to 500 rpm, preferably from about 100 rpm to about 200 rpm. In an embodiment, the rotational speed Vr is about 150 rpm. Further, in an embodiment, the rotational speed Vr has no upper limit. In the embodiment, the angular separation 0, the rotational speed v, and the predefined time t may be variable.
Further, in an embodiment, the apparatus includes a first heating unit (not shown) to heat the cutting fluid 2 to a predefined fluid temperature Tl. Maximum value of the fluid temperature Tl depends on the boiling point of the cutting fluid 2, which is less than 100 °C for the type of oil-in-water emulsions tested. The cutting fluid 2 is heated and maintained at the predefined fluid temperature Tl throughout the cutting of the work piece 1 and performing the friction tests. In an embodiment, the predefined fluid temperature Tl may be varied. This heating of cutting fluid 2 helps in testing the performance of the cutting fluids 2 at various temperatures.
Further, in an embodiment, the apparatus includes a second heating unit (not shown) to heat the pin 6 to a predefined pin temperature T2. Maximum value of the pin temperature T2 is also governed by the constraint mentioned for the fluid temperature Tl, i.e., it depends on the boiling point of the cutting fluid 2, which is slightly less than 100 °C for the oil-in- water emulsions tested. This heating of the pin 6 helps in performing friction tests at the desired pin temperatures.
Further, in an embodiment, the apparatus includes a first temperature sensor (not shown) to measure the temperature of the cutting fluid 2. In an embodiment, the first temperature sensor can be a resistance-temperature detector (RTD).
Further, in an embodiment, the apparatus includes a second temperature sensor (not shown) to measure the temperature of the interface of the pin 6 and the new surface 4. In an embodiment, the second temperature sensor can be a thermocouple.
Further, in an embodiment, the apparatus includes a speed detector (not shown) to measiu-e the rotational speed of the work piece 1. In an embodiment, the speed detector is a tachometer, preferably a conventional digital tachometer.
In the method, according to an embodiment of the present subject matter, the work piece 1 is cut by rotating the work piece 1 at the predefined rotational speed Vr, engaging the cutting tool 3 against the work piece 1, and generating at least one cut track with the cutting tool 3 on the work piece 1 as the new surface 4.
Further, in the method, according to the present subject matter, the friction test is performed in-situ by engaging the pin 6 of the friction measuring unit 5 against the new surface 4. The friction is monitored between the new surface 4 and the pin 6, and recorded. Further, the friction test is carried out in-situ on the new surface 4 at the predefined time t after cutting the work piece 1. As stated above, the predefined time t is dependent on the rotational speed Vr of the work piece 1 and on the predefined separation 7, 0 between the cutting tool 3 and the pin 6.
Further, the method, according to the present subject matter, includes heating the cutting fluid 2 to the predefined fluid temperature Tl.
Further, the method, according to the present subject matter, includes heating the pin 6 of the friction measuring unit 5 to the predefined pin temperature T2.
Figure 3 shows a comparison of friction tests performed for a cutting fluid 2 on the new (nascent) surface 4 generated in the cutting fluid 2, according to the present subject matter, and on an oxidized surface. For this, the cutting fluid 2 was made of an aromatic oil (0.5% v/v) in water (pH: 10.2) using PEG 860 ester as an emulsifier. The rotational speed v, of the work piece 1 was 15 rpm (sliding speed 3 cm/sec). In the friction tests, coefficient of friction is measured as a function of time. In figure 3, curve A is for the coefficient of friction measured in the case where the work piece 1 is cut in the cutting fluid 2 to generate a new surface 4, then taken out, cleaned chemically and exposed to air to form a oxide layer on the surface 4, and then friction tested. In figure 3, curve B is for the coefficient of friction measured in the case where the work piece 1 is cut in the cutting fluid 2 and friction tested in- situ, according to an embodiment of the present subject matter. Curve B is obtained for a case that resembles to a substantially more realistic cutting process where the cutting fluid 2 acts on the new (nascent) surface 4 rather than on an oxidized surface. Curve A resembles the friction test results obtained through the conventional rubbing method or apparatus. Curve A is off by about 53% from a more realistic result shown by curve B.
Figure 4 shows friction tests results obtained for a cutting fluid 2 under a substantially high rotation speed Vr of the work piece 1, with the cutting and friction measurement done simultaneously, according to an embodiment the present subject matter. In figure 4, the graph shows the friction recorded during a sequence of cutting tests, with the peaked structures in the graph corresponding to the friction measured during each cutting operation. For this, the cutting fluid 2 was made of naphthenic base oil (0.5% v/v) in water (neutral pH) using PEG 860 ester as an emulsifier. The rotational speed v, of the work piece 1, preferably the disc, was 150 rpm (sliding speed of 30 cm/sec). The high rotation speed v, reduces the time t between cutting the work piece 1 and performing the friction test. By the reduction of time t, the time lag between cutting and measurement reduces and approaches to that of a real-time measurement of the friction experienced during the cutting process.
In the embodiment described above, the cutting tool 3 is kept stationary and the work piece 1 is rotated. In an alternate embodiment, the work piece 1 may be kept stationary and the cutting tool 3 is rotated to cut the track on the work piece 1. Further, m an embodiment, any one can be rotated to make the work piece 1 rotate relative to the cutting tool 3. Further in an embodiment, the work piece 1 is moved (translation motion) relative to the cutting tool 3 for the purpose of cutting.
Further, the rotation, as shown, is about a vertical axis 16. However, the rotation axis is not restricted only to the vertical axis 16, and in an embodiment, the work piece 1 may be rotated about an axis in any direction.
Further, in an embodiment, the new surface can be created in any form. The new surface is not restricted to a cut track or circular cut track.
Further, in an embodiment, the friction measuring unit 5 includes a ball instead of the pin 6 to measure the friction on the new surface 4.
Further, in an embodiment, the separation 7 and the angular separation 0 may be varied and pre-selected.
Further, in an embodiment, the rotational speed v, may be varied and pre-selected.
Further, in an embodiment, speed v of relative motion of work piece 1 with respect to the cutting tool 3 may be varied and pre-selected, and not restricted to any particular value.
Further, in an embodiment, the time / between cutting and performing the friction test may be varied and pre-selected, and not restricted to any particular value.
Further, in an embodiment, the predefined fluid temperature Tl and the predefined pin temperature T2 may be varied and pre-selected, and not restricted to any particular value.
Further, in an embodiment, the work piece 1 can be in any form and shape and not restricted to any particular form and shape.
Further, in an embodiment, the cutting fluid 2 can be an oil or oil-based emulsion in a solvent including, but not restricted to, plain oils, soluble oil emulsions, synthetic fluids and emulsions, and emulsions which come under the category of bio-lubricants, environmental friendly lubricants and green lubricants.
Further, in an embodiment, the cutting tool 3 can be made of a hard material and can be any type of conventional tool used for cutting or machining.
Further, in an embodiment, the friction measuring unit 5 can be any of type without being restricted to any particular type.
Further, in an embodiment, the rotating unit 12 can be of any type that enables a relative rotational motion of work piece 1 with respect to the cutting tool 3.
Further, in an embodiment, the work piece 1 and/or the cutting tool 3 can be coupled to the rotating unit 12 to cause a relative rotational motion of the work piece 1 with respect to the cutting tool 3.
Further, in an embodiment, the work piece 1 can be held on the holder 14 by a holding means, not restricted to fasteners such as screws.
Further, in an alternate embodiment, the cutting fluid 2 can be exposed to a predefined pressure, which can be varied by suitable modifications to elevate the boiling point of the cutting fluid 2 by pressurization, so as to perform the friction tests at conditions other than permitted by the ambient.
Further, in an alternate embodiment, a free surface 18 of the cutting fluid 2 can be exposed to an atmosphere, which can be varied by encasing the chamber 10 in a controlled atmosphere. The atmosphere can be controlled by a gas, for example argon.
The essential advantage of the method and apparatus of the present subject matter is that the lubricity characteristics obtained for the cutting fluids are substantially more realistic than those obtained through the conventional rubbing tests. The environment simulated for assessing the lubricity characteristics is rather more realistic. In other words, the lubricity characteristics of the cutting fluids are obtained in a more realistic environment. According to the present subject matter, the lubricity offered by the cutting fluid while interacting with the nascent surfaces of the work piece is measured and assessed. This is substantially more realistic than the conventionally measured and assessed lubricity of the cutting fluid while interacting with the oxidized surfaces of the work piece. The results shows that lubricities assessed in both conditions were off by about 53%. The lubricity, expressed using coefficient of friction found from friction tests, is not dependent on a particular force-diagram model representing the forces and moments resulting from the chip-tool interaction. Because of these factors, as pointed above, the apparatus is used as a suitable tribological equipment to assess the lubricity of cutting fluids.
Other advantages of the inventive method and apparatus for assessing lubricity of a cutting fluid will become better understood from the description and claims of an exemplary embodiment of such a unit.
The inventive method and apparatus for assessing lubricity of a cutting fluid of the present subject matter is not restricted to the embodiments that are mentioned above in the description.
Although the subject matter has been described with reference to specific embodiments, this description is not meant to be construed in a limiting sense. Various modifications of the disclosed embodiments, as well as alternate embodiments of the subject matter, will become apparent to persons skilled in the art upon reference to the description of the subject matter. It is therefore contemplated that such modifications can be made without departing from the spirit or scope of the present subject matter as defined.

We claim:

1. A method for assessing lubricity of a cutting fluid, the method comprising:
cutting a work piece immersed in the cutting fluid with a cutting tool, wherein the cutting the work piece comprises:

generating at least one new surface on the work piece; and performing a friction test in-situ on the at least one new surface immersed in the cutting fluid.

2. The method as claimed in claim 1, wherein the cutting the work piece comprises:
relatively moving the work piece with respect to the cutting tool at a predefined speed (v);

engaging the cutting tool against the work piece; and

generating at least one cut track with the cutting tool on the work piece as the new surface.

3. The method as claimed in any of the preceding claims, wherein the performing the friction test in-situ comprises:

engaging a pin of a friction measuring unit against the new surface;

monitoring friction between the new surface and the pin; and recording the friction.

4. The method as claimed in any of the preceding claims, wherein the performing the friction test in-situ on the new surface is carried out at a predefined time (t) after the cutting the work piece.

5. The method as claimed in claim 5, wherein the predefined time (t) is dependent on the predefined speed (v) and on a predefined separation between the cutting tool and the pin.

6. The method as claimed in any of the preceding claims, wherein the method comprises:

heating the cutting fluid to a predefined fluid temperature (Tl).

7. The method as claimed in any of the preceding claims, wherein the method comprises:

heating the pin of the friction measuring unit to a predefined pin temperature
(T2).

8. An apparatus for assessing lubricity of a cutting fluid (2), the apparatus comprising:

a chamber (10) filled with the cutting fluid (2);

a work piece (1) positioned in the chamber (10) and immersed in the cutting fluid (2);

a cutting tool (3) immersed in the cutting fluid (2) to cut the work piece (1) and generate
at least one new surface (4) on the work piece (1); and

a friction measuring unit (5) to measure friction in-situ on the at least one new surface (4) immersed in the cutting fluid (2).

9. The apparatus as claimed in claim 8, wherein the work piece (1) is relatively moved with respect to the cutting tool (3) at a predefined speed (v).

10. The apparatus as claimed in claim 8, wherein the apparatus comprises:
a rotating unit (12) that relatively rotates the work piece (1) with respect to the cutting tool (3) immersed in the cutting fluid (2) at a predefined rotational speed (v,).

11. The apparatus as claimed in any of the claims 8 to 10, wherein the cutting tool (3) is engaged against the work piece (1) to generate at least one cut track as the new surface (4) on the work piece (1) immersed in the cutting fluid (2).

12. The apparatus as claimed in any of the claims 8 to 11, wherein the friction
measuring unit (5) comprises:

a force measuring sensor; and

a pin (6) engaged against the new surface (4) to measure the friction in-situ using the force measuring sensor on the new surface (4) immersed in the cutting fluid (2).

13. The apparatus as claimed in claim 12, wherein the pin (6) is positioned at a predefined separation (7, θ) from the cutting tool (3).

14. The apparatus as claimed in any of the claims 8 to 13, wherein the friction measuring unit (5) measures the friction in-situ of the new surface (4) immersed in the cutting fluid (2) at a predefined time (t) after the cutting tool (3) generates the new surface (4).

15. The apparatus as claimed in claim 14, wherein the predefined time {t) is dependent on the predefined speed (v, Vr) and on the predefined separation (7, θ).

16. The apparatus as claimed in any of the claims 8 to 15, wherein the apparatus comprises:

a first heating unit to heat the cutting fluid (2), in which the work piece (1) is immersed and cut, to a predefined fluid temperature (Tl).

17. The apparatus as claimed in any of the claims 12 to 16, wherein the apparatus comprises:

a second heating unit to heat the pin (6) to a predefined pin temperature (T2).

18. The apparatus as claimed in any of the claims 10 to 17, wherein the predefined rotational speed (vr) is from about 15 rpm to about 500 rpm.