TECHNICAL FIELD
[0001] The present subject matter generally relates to an abrasive
flow machining process. In particular, the present subject matter relates to an abrasive flow machining process of inner surface in an as-cast compressor housing.
BACKGROUND
[0002] Abrasive flow machining (AFM) is a process of polishing or
abrading a component, for example, a turbocharge compressor housing, by
passing an abrasive slurry having abrasive particles therein, under
pressure, over the component surface to be machined or through an orifice
extending into the component surface. The AFM technique has been
applied over a wide range of applications. For example, the AFM technique
is used for surface finishing of aerospace and medical components.
[0003] In AFM, the component may be connected to a dispensing
system, which assists the flow of abrasive slurry through the orifice extending into the component surface, where the abrasive slurry performs abrasion. The material which forms valleys on the surface of the component are ploughed by the abrasive particles present in the abrasive slurry, when the abrasive particles come in contact with the material at high pressure. The ploughed material flows along with the abrasive slurry in the direction of the motion of the abrasive particles. AFM is, thus, used to reduce the surface friction, and improve the finishing of the surface of the component.
BRIEF DESCRIPTION OF DRAWINGS
[0004] A detailed description is provided with reference to the
accompanying figures. In the figures, the left-most digit(s) of a reference number identifies the figure in which the reference number first appears. The same numbers are used throughout the drawings to reference like features and components.

[0005] Fig. 1(a) illustrates a cross sectional view of a compressor
housing with a volute, in accordance with an implementation of the present
subject matter.
[0006] Fig. 1 (b) illustrates a magnified view of the circled area in Fig.
1(a), in accordance with an implementation of the present subject matter.
[0007] Fig. 2 illustrates a sectional view of an as-cast compressor
housing, in accordance with an implementation of the present subject
matter.
[0008] Fig. 3(a) illustrates a perspective view of the as-cast
compressor housing, in accordance with an implementation of the present
subject matter.
[0009] Fig. 3(b) illustrates a cross-sectional view of the as-cast
compressor housing, in accordance with an implementation of the present
subject matter.
[0010] Fig. 3(c) illustrates a cross-sectional view of the as-cast
compressor housing, in accordance with an implementation of the present
subject matter.
[0011] Fig. 4 illustrates a method for reducing surface roughness of
the as-cast closed volute, in accordance with an implementation of the
present subject matter.
[0012] Fig. 5(a) illustrates a front view of an AFM apparatus, in
accordance with an implementation of the present subject matter.
[0013] Fig. 5(b) illustrates a cross-sectional view of the AFM
apparatus, in accordance with an implementation of the present subject
matter.
[0014] Fig. 6(a) illustrates a fish-bone diagram representing variables
affecting the outcome of AFM, in accordance with an implementation of the
present subject matter.

[0015] Fig. 6(b) is an interaction plot for average surface roughness
after AFM, in accordance with an implementation of the present subject
matter.
[0016] Fig. 7(a) is a plot of the measured average surface roughness
of an inner surface of a number of as-cast compressor housings before
AFM, in accordance with an implementation of the present subject matter
[0017] Fig. 7(b) is a scatter plot and a histogram of the average
surface roughness after AFM of the inner surface for a number of as-cast
compressor housings, in accordance with an implementation of the present
subject matter.
[0018] Fig. 8(a) illustrates a volute cavity of the as-cast compressor
housing before AFM, in accordance with an implementation of the present
subject matter.
[0019] Fig. 8(b) illustrates the volute cavity of a compressor housing
after the AFM, in accordance with an implementation of the present subject
matter.
[0020] Fig. 8(c) illustrates the results of borescope inspection of the
volute cavity after AFM, in accordance with an implementation of the
present subject matter.
[0021] Fig. 9 illustrates an exit port sealed with a plug in the as-cast
compressor housing, in accordance with an implementation of the present
subject matter.
DETAILED DESCRIPTION
[0022] Abrasive flow machining (AFM) is a technique used to reduce
the roughness of internal passages in a component. In AFM, abrasive particles suspended in a suitable slurry medium, termed as the abrasive slurry, are used to reduce the roughness. The abrasive slurry is pumped in through one opening of the internal passage and the abrasive slurry exits through another opening of the internal passage. As the abrasive slurry

moves over a surface with protrusions, the contact of the moving abrasive
particles in the abrasive slurry with the protrusions on the surface chips
away the protrusions and leads to removal of roughness.
[0023] The AFM can be used for compressor housings manufactured
by casting process, as the casting process lends a rough profile to an
internal surface of the compression housing. The AFM in the as-cast
compressor housing is typically carried out before further precision
machining of the compressor housing. The precision machining may be
performed for surfaces requiring close clearances. The AFM process is to
be carried out before the precision machining because clearances of the
surfaces cannot be precisely controlled during the AFM process.
[0024] Typically, a compressor housing comprises an axial opening
in which an impeller can be fixed, a volute, and a diffuser portion. During operation, the impeller may suck in and propel a fluid, such as atmospheric air. The volute includes a radial opening and a discharge port. The radial opening is designed to intake fluid discharged by the impeller through the diffuser portion. Hence, the diffuser portion bridges an exit of the impeller and the radial opening. The volute collects the fluid from the diffuser portion and may increase the pressure of the fluid. The fluid entering the radial opening of the volute flows through the volute and exits through the discharge port.
[0025] A type of the compressor housing is a closed volute type,
which may be interchangeably referred to as closed volute compressor housing. In case of the closed volute compressor housing, in the as-cast profile, before any machining is carried out, the diffuser portion is completely blocked. Therefore, when the abrasive slurry is supplied to the closed volute through the discharge port, the abrasive slurry cannot be recovered from another opening of the closed volute. Therefore, employing the AFM to reduce the roughness of the surface of the closed volute in the as-cast profile is not feasible.

[0026] The subject matter detailed herein relates to machining an
inner surface of an as-cast closed volute in an as-cast compressor housing. An abrasive slurry used for flow machining of the inner surface is pumped in through the discharge port of the as-cast closed volute. The abrasive slurry comprises a mixture of abrasive particles with a grit grade of F30 and a slurry medium. As will be understood, a significant number of abrasive particles having the grit grade of F30 have a size in the range from about 500 urn to about 710 urn. The volume fraction of the abrasive particles in the slurry medium is about 30% to about 50%. Subsequently, the abrasive slurry is collected through an exit port. The exit port is provided on an exterior surface of the as-cast closed volute, at a location of the as-cast closed volute that is away from the discharge port. The positioning of the exit port ensures that the abrasive slurry flows through a substantial length of the volute before exiting through the exit port. A subsequent injection of the abrasive slurry through the discharge port is carried out if an average surface roughness of the inner surface of the as-cast closed volute is above the target surface roughness. The average surface roughness is an average of the surface roughness values measured at a plurality of locations of the inner surface.
[0027] The present subject matter facilitates using AFM for as-cast
closed volute compressor housings. A smoothened volute enhances the performance of the compressor. Also, in the present subject matter, the optimum range of values of the pressure, size of abrasive particles, and the volume fraction of the abrasive particles in the slurry medium are determined for the AFM and used for the AFM. This further enhances the energy and process efficiency of the AFM. By using the present subject matter, closed volute with a smooth internal surface can be obtained in a closed volute compressor housing.
[0028] In the below description, the term "about" when referring to a
numerical value is intended to encompass the values resulting from variations that can occur during the normal course of performing a method.

Such variations are usually within plus or minus 10 percent of the stated numerical value.
[0029] Fig. 1(a) illustrates a cross sectional view of a compressor
housing 100 with a volute 102, in accordance with an implementation of the present subject matter. A compressor may be provided in an automobile, for example, to provide compressed air to an engine in the automobile. The compressor intakes a fluid, such as atmospheric air, compresses the fluid and outlets compressed fluid.
[0030] The compressor housing 100 includes an axial passage 104.
Through a first end106 of the axial passage 104, an impeller (not shown in figure) may be mounted. A second opening in a direction of the arrow 108 is for entry of fluid into the axial passage 104. The fluid enters the axial passage 104 when the impeller rotates, which causes suction of the fluid. The second opening is illustrated in Fig. 3(a).
[0031] The volute 102 serves as a passage for the compressed fluid
entering the volute 102 and may also further compress the fluid. The volute 102 includes a radial opening 110 for the compressed fluid to enter a volute cavity 112. The volute 102 extends in a second plane perpendicular to a first plane illustrated in Fig. 1(a), i.e., the plane on which the axial passage 104 extends. On the second plane, the volute 102 extends between a first end of the volute 102 and a second end of the volute 102 in a spiral fashion. Further, the volute cavity 112 progressively reduces in cross-sectional area from the first end to the second end. The first end of the volute acts as an outlet for compressed fluid and may be referred to as a discharge port. The discharge port is illustrated in Figs. 3(a)-3(c).
[0032] Fig. 1(b) illustrates a magnified view of the circled area 114,
in accordance with an implementation of the present subject matter. A diffuser portion 116 connects the radial opening 110 to the axial passage 104. When the compressor is operated, the fluid exiting the impeller passes through the diffuser portion 116, and is directed into the radial opening 110 of the volute 102. The fluid flowing through the diffuser portion 116 gets

pressurized, enters the volute 102, flows through the volute cavity 112, and exits through the discharge port of the volute 102.
[0033] Fig.2 illustrates a sectional view of an as-cast compressor
housing 200, in accordance with an implementation of the present subject matter. The as-cast compressor housing 200 includes an as-cast closed volute 202. The as-cast compressor housing 200 may be subjected to various machining processes to obtain the compressor housing 100. Accordingly, upon completion of the machining processes, the volute 102 is obtained from the as-cast closed volute 202. As illustrated in Fig. 2, the diffuser portion 116 is blocked, and the radial opening 110 is sealed. During the machining processes, precision machining is performed to unblock the diffuser portion 116 with precise clearance.
[0034] An inner surface 204 of the as-cast closed volute 202 is rough
due to the casting process. For instance, the irregularities on a surface of a mould used for casting may translate as irregularities on the surface of the cast during solidification of molten metal in the mould. A rough inner surface 204 may cause friction during fluid flow through the volute cavity 112. Therefore, if the roughness is not removed, efficiency of fluid flow decreases, which, in turn, reduces performance of the compressor. Hence, the inner surface 204 is to be smoothened such that the surface roughness is equal to or less than an acceptable value. The acceptable value of surface roughness may be referred to as target surface roughness. In an example, the target surface roughness is 3 urn. In other words, the surface roughness is to be lesser than 3 urn.
[0035] To reduce the surface roughness of the inner surface 204, an
AFM process may be used. The terms "AFM process" and "AFM" may be interchangeably used. In AFM, abrasive particles suspended in a suitable slurry medium, termed as the abrasive slurry, is used to reduce the surface roughness. Typically, abrasive slurry is pumped in through one opening of an internal passage to be smoothened, and the abrasive slurry exits through another opening of the internal passage. Since, as explained earlier, the

diffuser portion 116 is blocked, the volute cavity 112 can be accessed only through the discharge port (not shown in Fig. 2) of the as-cast closed volute 202. When the abrasive slurry is supplied to the as-cast closed volute 202 through the discharge port, the abrasive slurry cannot be recovered from another opening of the as-cast closed volute 202. Therefore, employing the AFM to reduce the roughness of the inner surface 204 is not feasible. The utilization of the AFM to reduce the roughness of the inner surface 204 in accordance with the present subject matter will now be explained with reference to the subsequent figures.
[0036] Fig. 3(a) illustrates a perspective view of the as-cast
compressor housing 200, in accordance with an implementation of the
present subject matter. As illustrated, the as-cast compressor housing 200
includes the axial passage 104. As mentioned earlier, during operation, fluid
enters the axial passage 104 through the second opening 301. Further, the
discharge port, which is disposed at an end of the as-cast closed volute 202,
is represented by the reference numeral 302. The inner surface 204 of the
as-cast closed volute 202 is partially visible. The as-cast closed volute 202
spirals from the discharge port 302 around the axial passage 104 with
decreasing diameter to a second end (not shown in figure).
[0037] To facilitate performing AFM for the inner surface 204, an exit
port 304 is provided in the as-cast compressor housing 200. During the
AFM, the exit port 304 acts as an outlet through which the abrasive slurry
can flow out. Hence, the abrasive slurry entering the discharge port 302,
flows through the volute cavity 112, over the inner surface 204 and exits
through the exit port 304. Thus, the present subject matter enables the AFM
of the inner surface 204 of the as-cast closed volute 202.
[0038] In an implementation, a boss 306 is provided on an outer
surface of the as-cast closed volute 202. The boss 306 serves to provide a flat surface for providing the exit port 304. The exit port 304 may be provided on the boss 306, as shown in Fig. 3(a). The boss 306 may be positioned close to the second end. For instance, the boss 306 may be positioned

closer to the second end 308 as compared to the discharge port 302.
Accordingly, the boss 306 may be said to be disposed away from the
discharge port 302. Since the exit port 304 is provided on the boss 306, the
exit port 304 may also be said to be disposed away from the discharge port
302 in a direction of extension of the as-cast closed volute 202. The
positioning of the exit port 304 and the boss 306 will be explained below.
[0039] Fig. 3(b) illustrates a cross-sectional view of the as-cast
compressor housing 200, in accordance with an implementation of the
present subject matter. The cross-sectional view shows the inner surface
204 of the as-cast closed volute 202. As illustrated, the volute cavity 112
extends from the discharge port 302 to the second end, numbered 308, of
the as-cast closed volute 202, narrowing progressively.
[0040] The exit port 304 may be formed away from the discharge port
302. For example, in a direction in which the as-cast closed volute 202
spirals, the exit port 304 is located closer to the second end 308 as
compared to the discharge port 302. To locate the exit port 304 closer to the
second end 308, the boss 306 is provided closer to the second end 308.
Hence, when the abrasive slurry is pumped through the discharge port 302,
the abrasive slurry flows in the as-cast closed volute 202 till the second end
308 before exiting through the exit port 304, ensuring the smoothening of a
significant portion of the inner surface 204. The exit port 304 may be in the
form of a through-hole that provides a passage from the volute cavity 112
to the outer surface of the as-cast compressor housing 200.
[0041] Fig. 3(c) illustrates a cross-sectional view of the as-cast
compressor housing 200, in accordance with an implementation of the present subject matter. The arrows 310 - 320 point the direction of the flow of the abrasive slurry during the AFM process. As illustrated by the arrows 310 - 320, the abrasive slurry enters through the discharge port 302 and exits through the exit port 304. The exiting abrasive slurry may be collected from the exit port 304. The flow path of the abrasive slurry illustrated by the

arrows indicate that substantial portion of the as-cast closed volute is subjected to the AFM process.
[0042] The dimension of the exit port 304 may be provided large
enough to ensure exit of the abrasive slurry, while small enough to ensure easy sealing after the AFM. In an example, the exit port 304 has a diameter of about 12 mm.
[0043] Fig. 4 illustrates a method 400 for reducing surface roughness
of the as-cast closed volute 202, in accordance with an implementation of the present subject matter. The method 400 of the present subject matter is performed by an AFM apparatus, which will be described in detail with respect to Fig. 5(a) and 5(b). The order in which the method blocks are described is not included to be construed as a limitation, and some of the described method blocks can be combined in any order to implement the method 400, or an alternative method. Additionally, some of the individual blocks may be deleted from the method 400 without departing from the scope of the subject matter described herein.
[0044] At block 402, the exit port 304 is provided on a surface of the
as-cast closed volute 202, at a location of the as-cast closed volute 202 that is away from the discharge port 302. The exit port 304 may be provided, for example, by drilling. The exit port 304 may be provided in the form of a through-hole that provides a passage from the volute cavity 112 to the outer surface of the as-cast compressor housing 200. The outer surface may also be referred to as the exterior surface.
[0045] In an example, the location may be close to the second end
308 of the as-cast closed volute 202. For instance, the boss 306 may be provided closer to the second end 308 and farther from the discharge port 302 in a direction of extension of the as-cast closed volute 202. The exit port 304 may be provided on to the boss 306.
[0046] At block 404, the method comprises injecting abrasive slurry
through the discharge port 302. The injection may be performed to reduce a surface roughness of the inner surface 204 to less than a target surface

roughness. The abrasive slurry may comprise a mixture of abrasive particles and a slurry medium. The abrasive particles may have a size in a range from about 500 urn to about 710 urn. A volume fraction of the abrasive particles in the slurry medium may be about 30% to about 50%. The roughness of a surface may vary over a range of values at different places on the inner surface 204.
[0047] In an implementation, the size range of abrasive particles is
determined based on an average surface roughness of the inner surface
204 prior to the AFM. The average surface roughness may be the average
of the surface roughness values measured at various locations of the inner
surface 204. The average surface roughness prior to the AFM may be
referred to as initial average surface roughness. For instance, the initial
average surface roughness may be determined prior to injecting abrasive
slurry. A profilometer may be used to determine the surface roughness. The
initial average surface roughness of the inner surface 204 may be
determined using a known technique, such as linear roughness
measurement technique or area roughness measurement technique.
[0048] The surface roughness that is to be achieved by the AFM may
be referred to as the target surface roughness. In an implementation, a size range of abrasive particles may be selected based on the initial average surface roughness of the inner surface 204, a target surface roughness of the inner surface 204, or both. For example, the size range is determined based on a look-up table, which may include different values of initial surface roughness and target surface roughness and the corresponding size ranges of the abrasive particles. In an example, the look-up table may be utilized by a computer numerical control (CNC) machine to determine the size range of the abrasive particles based on the initial average surface roughness and/or a target surface roughness of the inner surface 204. In an example, the abrasive particles selected have a size in the range of about 500 urn to about 710 urn. Further, the abrasive particles are selected from

a group consisting of: aluminium oxide, boron carbide, silicon carbide, and titanium carbide.
[0049] In an implementation, the method 400 comprises selecting a
type and amount of slurry medium based on the initial average surface roughness of inner surface 204, the target surface roughness of inner surface 204, or both. The slurry medium may have rheological characteristics (the response of a material to applied stress) ranging from that of viscous flowing to a semi-solid depending on pressure applied during injection. In an implementation, the slurry medium may have thixotropic characteristics, whereby the slurry medium exhibits progressively increasing viscous flowing rheological characteristics with increasing pressure. The slurry medium may have the consistency of putty. The abrasive slurry may comprise a mixture of the abrasive particles as determined above and the slurry medium. In an example, the amount of slurry medium may be selected such that the abrasive slurry has a volume fraction of the abrasive particles of about 30% to about 50% in the slurry medium.
[0050] The abrasive slurry, which is injected through the discharge
port 302, is formulated using the abrasive particles and slurry medium. In an implementation, the method comprises injecting the abrasive slurry through the volute cavity 112 by applying an injection pressure in a range of about 60 bar to about 80 bar. Such a pressure range ensures effective AFM process and enables the ejection of the abrasive slurry through the narrow opening of the exit port 304.
[0051] At block 406, the abrasive slurry exiting through the exit port
304 is collected. Upon collection of the slurry medium, it may be checked whether the determined average surface roughness is below the target surface roughness.
[0052] In an implementation, the injection (at block 404) and
collection (at block 406) of the abrasive slurry may be repeated for a plurality of times, until the inner surface 204 is smoothened to the target surface

roughness or lesser than the target surface roughness. For instance, after collecting the abrasive slurry, an average surface roughness of the inner surface 204 may be determined. Further, it may be checked whether the determined average surface roughness is equal to or below the target surface roughness. If the determined average surface roughness is same as or below the target surface roughness, no more injection of the abrasive slurry is to be performed. However, if the determined average surface roughness is above the target surface roughness, a subsequent injection and collection is performed by repeating the steps at blocks 404 and 406. Accordingly, the steps at blocks 404 and 406 may be performed repeatedly until the determined average surface roughness is equal to or below the target surface roughness.
[0053] At block 408, the method 400 comprises sealing the exit port
304 with a plug. In an example, the plug may be a steel plug. After sealing, a leak test may be carried out to ensure that leakage of fluid does not occur during the operation of the compressor.
[0054] Fig. 5(a) illustrates a front view of the AFM apparatus 500, in
accordance with an implementation of the present subject matter. The AFM
apparatus 500 may be used to implement the steps at blocks 404 and 406.
[0055] The AFM apparatus 500 comprises a reservoir 504, a control
unit (not shown in Fig. 5(a)), a cylinder 506, and a nozzle plate 508. The reservoir 504 may be used to store the abrasive slurry used for the AFM process. The cylinder 506 may be used to pump the abrasive slurry from the reservoir 504. The AFM apparatus 500 further comprises a holder 510 to detachably couple the as-cast compressor housing 200 to the AFM apparatus 500. For example, the holder 510 may include a pair of clamps for clamping the second end of the nozzle plate 508 and the discharge port 302 together.
[0056] A first end of the nozzle plate 508 is connected to the reservoir
504 and a second end of nozzle plate 508 is connected to the discharge port 302 of the as-cast closed volute 202 by the holder 510. Thus, the nozzle

plate 508 connects the reservoir 504 to the discharge port 302, to enable
transfer of abrasive slurry from the reservoir 504 to the discharge port 302.
Accordingly, the pumped abrasive slurry flows from the reservoir 504 to the
as-cast closed volute 202 through the nozzle plate 508. In an example, an
opening of the nozzle plate 508 is large at the first end of the nozzle plate
508 and progressively decreases in cross-sectional area to a small opening
at the second end of the nozzle plate 508. Therefore, when the abrasive
slurry from the reservoir 504 is transferred to the discharge port 302, it
passes through a decreasing cross-sectional area. This causes the velocity
of the abrasive slurry to increase as it flows through the nozzle plate 508.
[0057] To pump the abrasive slurry from the reservoir 504, the
cylinder 506 comprises a hydraulic piston 516 and ports 518-1 and 518-2. The hydraulic piston 516 may be actuated by pumping a hydraulic fluid in and out of the cylinder 506 through the ports 518-1 and 518-2. The actuation of the hydraulic piston 516 facilitates the pumping of the abrasive slurry from the reservoir 504 to the discharge port 302, as will be explained with respect to Fig. 5(b).
[0058] Fig. 5(b) illustrates a cross-sectional view of the AFM
apparatus 500, in accordance with an implementation of the present subject matter.
[0059] The actuation of the hydraulic piston 516 facilitates the
pumping of the abrasive slurry from the reservoir 504 to the discharge port 302. The actuation of the hydraulic piston 516 refers to moving the hydraulic piston 516 towards and away from the reservoir 504. To cause movement of the abrasive slurry in the reservoir 504 due to the actuation of the hydraulic piston 516, a ram plate 520 is attached to the hydraulic piston 516. The movement of the hydraulic piston 516 also moves the ram plate 520. The ram plate 520 is disposed in the reservoir 504, to move in the reservoir 504 in response to movement of the hydraulic piston 516, to pump the abrasive slurry into the as-cast closed volute 202.

[0060] As mentioned earlier, the movement of the hydraulic piston
516 is facilitated by pumping hydraulic fluid in and out of the cylinder 506
through the ports 518-1 and 518-2. For instance, when the hydraulic fluid is
pumped in through the port 518-1 and pumped out through the port 518-2,
the hydraulic piston 516 is displaced towards the reservoir 504. The
hydraulic piston is displaced from an initial position to a final position.
Conversely, when the hydraulic fluid is pumped in through the port 518-2
and pumped out through the port 518-1, the hydraulic piston 516 is
displaced away from the reservoir 504. Thus, the hydraulic piston 516 is
displaced from the final position back to the initial position. Since the ram
plate 520 is attached to the hydraulic piston 516, the ram plate 520 moves
along with the hydraulic piston 516. When the hydraulic piston 516 is
displaced towards the reservoir 504, the ram plate 520 moves in the
reservoir 504 and consequently, the abrasive slurry in the reservoir 504 is
pressurised. Further, the pressurization of the abrasive slurry in the
reservoir 504, by the hydraulic piston 516, forces the abrasive slurry into the
as-cast closed volute 202 through the discharge port 302.
[0061] When the hydraulic fluid is pumped through a directional
control valve (not shown in the Fig. 5(b)) into the cylinder 506 through the port 518-1, the extent to which the directional control valve is opened determines the amount of pressure applied on the abrasive slurry in the reservoir 504 by the ram plate 520. Thus, the directional control valve may be opened to an extent that corresponds to a desired pressure range at which the abrasive slurry is to be injected. In an example, the desired pressure range may be about 60 bar to 80 bar. In this manner, the directional control valve enables the control of flow of abrasive slurry from the reservoir 504.
[0062] To control the flow of the abrasive slurry flowing from the
reservoir 504 through the as-cast closed volute 202 and to the exit port 304, the AFM apparatus 500 further comprises a control unit, such as a programmable logic controller (PLC) unit. The AFM apparatus 500 may also

include a flow sensor, a pressure sensor, a proximity sensor, and the
directional control valve. A foot pedal (not shown in the Fig. 5b) may be
provided to activate the hydraulic piston 516. The foot pedal is connected
to the PLC and may be manually operated. When a user presses the foot
pedal, the hydraulic piston 516 is activated, and the ram plate 520 pushes
the abrasive media in the reservoir 504 through the nozzle plate 508.
[0063] In an example, to control the extent to which the hydraulic
piston 516 moves, the proximity sensors of the AFM apparatus 500 may be utilized. The proximity sensors may be placed at two positions, which correspond to the positions between which the hydraulic piston 516 is to move. The two positions may be referred to as the initial and final positions of the hydraulic piston 516. When the hydraulic piston 516 reaches the final position, the proximity sensor at the final position detects the hydraulic piston 516, conveys to the control unit, to enable the control unit to revert the hydraulic piston 516 to the initial position. In this manner, when the hydraulic piston 516 reaches the final position, the control unit may, for example, direct the directional control valve to switch the pumping of hydraulic oil into the port 518-2. Thereafter, the hydraulic piston 516 and the ram plate 520 travel in a reverse direction, i.e., travels away from the reservoir 504. In the reverse direction, when the hydraulic piston 516 reaches the initial position, the proximity sensor at the initial position detects the same, and conveys to the control unit. Subsequently, the control unit may halt the movement of the hydraulic piston 516 and the movement of the ram plate 520, for example, by controlling the directional control valve to halt the flow of hydraulic oil into the port 518-2, thereby causing the hydraulic piston 516 to revert to the initial position. As a result, the pressurisation of the abrasive slurry in the reservoir 504 may be halted and hence the flow of abrasive slurry into the discharge port 302 may be stopped. In the figure, the dotted outline of the ram plate 520 denotes the initial position of the hydraulic piston 516 (and the ram plate 520), while the bold outline denotes the final position of the hydraulic piston 516 (and the

ram plate 520). When the hydraulic piston 516 and the ram plate 520 revert to an initial position, the foot pedal may be pressed again. This causes the control unit to activate the directional control valve to pump hydraulic oil into the port 518-1. Subsequently, the hydraulic piston 516 and the ram plate 520 are actuated to push the abrasive media in the reservoir 504 through the nozzle plate 508. Thus, the control unit controls the flow of the abrasive slurry.
[0064] The abrasive slurry entering through the discharge port 302,
flows in the as-cast volute 202 and exits through the exit port 304.
Examples
[0065] In order to ensure efficient operation of the AFM to achieve a
target surface roughness, the variables affecting the outcome of AFM need
to be identified. A fish bone diagram is a visualizing tool for categorizing and
representing the variables which can influence a result.
[0066] Fig. 6(a) illustrates a fish-bone diagram representing variables
affecting the outcome of AFM, in accordance with an implementation of the present subject matter. As illustrated, the variables affecting the outcome of AFM, can be broadly categorized into three categories: abrasive slurry, AFM parameters, and work piece. In the present subject matter, the work piece is the as-cast closed volute 202.
[0067] In an implementation, three variables, namely grit grade,
volume fraction of the slurry medium, and pressure (circled in Fig.6(a)) are selected as variables to be controlled during the AFM for the following reasons. The grit grade signifies the size of the abrasive particles. The size of the abrasive particles influences the AFM by affecting the flowability of the abrasive slurry. As the grit size decreases, the reduction achieved in roughness during AFM increases. Also, the volume fraction of the abrasive particles in the abrasive slurry and the size of the abrasive particles are parameters affecting the reduction of roughness of the inner surface 204. Since the flow characteristics of the slurry medium is a function of pressure,

the pressure applied during pumping of the abrasive slurry through the discharge port 302 also plays a crucial role.
[0068] To obtain the optimum combination of values of the selected
variables, design of experiments (DoE) analysis was carried out. By manipulating multiple variables at the same time, DoE analysis can capture interactions between variables that may be missed when experimenting with one variable at a time.
[0069] A parameter set of the variables is taken as a pressure of 60
and 80 bar, abrasive particle with a grit grade F20 and F30 and volume fraction of 30% and 50%. In order to find the optimum combination for the values of variables, within the selected parameter set, DoE analysis is conducted.
[0070] Fig 6(b) is an interaction plot for average surface roughness
after AFM, in accordance with an implementation of the present subject
matter. From the DoE analysis, it was determined that a combination of (i)
a pressure of 80 bar, (ii) abrasive particles of grit grade F30, and (iii) volume
fraction of 50% was an optimum combination. As will be understood, a
significant number of abrasive particles having the grit grade of F30 have a
size in the range from about 500 urn to about 710 urn.
[0071] Fig. 7(a) is a plot of the measured average surface roughness
of the inner surface of a number of as-cast compressor housings before AFM, in accordance with an implementation of the present subject matter. The initial average surface roughness of the inner surface 204 of 30 as-cast compressor housings was estimated. In Fig. 7(a), the x-axis depicts the number of the as-cast compressor housing, from 1 to 30. The y-axis depicts the corresponding initial average surface roughness of the cast in urn. As illustrated, the surface roughness was found to vary from 6 to 12.4 urn. The target surface roughness that is required was 3 urn. Using the optimized values of the parameters as detailed above, the AFM was carried out for a set of as-cast compressor housings. The surface roughness after the AFM

was measured for the inner surface of these as-cast compressor housings. The results are given in the Table 1.
Table 1

[0072] For every as-cast compressor housing, six readings of surface
roughness on different locations on the inner surface 204 were taken. The
average of these readings, for each as-cast compressor housing, is
estimated as the average surface roughness after AFM.
[0073] Fig. 7(b) is a scatter plot 700 and a histogram 750 of the
average surface roughness after AFM of the inner surface for a number of as-cast compressor housings, in accordance with an implementation of the present subject matter. The x-axis of the scatter plot 700 depicts the number of the as-cast compressor housing, from 1 to 24. The y-axis depicts the corresponding average surface roughness of the as-cast compressor housing after AFM in urn. The x-axis of the histogram 750 depicts the average surface roughness in urn and the y-axis depicting the corresponding frequency. The data from 24 as-cast compressor housings give a mean surface roughness value of 0.88 urn and a standard deviation

of 0.1663. A normal distribution curve 752 with the mean value and standard deviation value is superimposed on the histogram 750. The average surface roughness value after AFM was well within the target surface roughness of 3um with the mean value being 0.88 urn.
[0074] Fig. 8(a) illustrates the volute cavity of an as-cast compressor
housing before the AFM, in accordance with an implementation of the
present subject matter. The inner surface 204, before AFM was found to
have surface roughness in the range of 6 - 12.4 urn.
[0075] Fig. 8(b) illustrates the volute cavity of a compressor housing
after the AFM, in accordance with an implementation of the present subject matter. The inner surface 204, after AFM was found to have surface roughness in the range of 0.4 - 1.7 urn. The volute cavity 112 examined after AFM using a borescope was found to be clear of any abrasive slurry inside.
[0076] Fig. 8(c) illustrates the results of borescope inspection of the
volute cavity 112 after AFM. The results are shown for inspection at six sections of the volute cavity 112, the position of which are indicated on the as-cast compressor housing 200, namely first section 802 to sixth section 812. For each section 802 - 812, the corresponding result images 814 - 824 are shown. The result images 814 - 824 indicate that the volute cavity 112 is clear of any abrasive media.
[0077] Fig. 9 illustrates an exit port sealed with a plug 904 in the as-
cast compressor housing, in accordance with an implementation of the present subject matter. A magnified view of the circled portion shows a much clearer view of the plug 904. By conducting a leak test, the robustness of the plug 904 was ensured. From the Fig. 9, it can be observed that the plug 904 effectively covers the exit port 304.
[0078] The present subject matter facilitates using AFM for as-cast
closed volute compressor housings. A smoothened volute enhances the performance of the compressor. Also, in the present subject matter, the optimum range of values of the pressure, size of abrasive particles, and the

volume fraction of the abrasive particles in the slurry medium are determined for the AFM and used for the AFM. This further enhances the energy and process efficiency of the AFM. By using the present subject matter, closed volute with a smooth internal surface can be obtained in a closed volute compressor housing.
[0079] Further, in the present subject matter, the optimum values of
several variables that influence the outcome of the AFM process are determined. Further, the determined optimum values are used to perform the AFM process. Thus, a highly smooth surface is achieved as a result of the AFM process. Further, by virtue of the values determined, the AFM process may be power-efficient.
[0080] Although implementations have been described in language
specific to structural features, it is to be understood that the present subject matter is not necessarily limited to the specific features described. Rather, the specific features are disclosed as example implementations.

I/We Claim:
1. A method for reducing surface roughness of an inner surface (204)
in an as-cast compressor housing (200), the as-cast compressor housing
(200) comprising an axial passage (104); an as-cast closed volute (202);
and a diffuser portion (116), the axial passage (104) having a first end for
mounting an impeller and a second opening for entry of a fluid, the as-cast
closed volute (202) having a discharge port (302) at an end of the as-cast
closed volute (202) and a radial opening (110), the diffuser portion (116)
connecting the radial opening (110) and the axial passage (104), wherein
the diffuser portion (116) is blocked, the method comprising:
providing an exit port (304) on a surface of the as-cast closed volute at a location of the as-cast closed volute (202) that is away from the discharge port (302);
injecting abrasive slurry through the discharge port (302) to reduce the surface roughness of the inner surface (204) to less than a target surface roughness, wherein the abrasive slurry comprises a mixture of:
abrasive particles having a size in a range from about 500 urn
to about 710 urn; and
a slurry medium, wherein a volume fraction of the abrasive
particles in the slurry medium is about 30% to about 50%;
collecting the abrasive slurry exiting through the exit port (304); and
sealing the exit port (304) with a plug.
2. The method as claimed in claim 1, wherein, subsequent to collecting
the abrasive slurry exiting through the exit port and prior to sealing the exit
port, the method comprises:
carrying out a subsequent injection of the abrasive slurry through the discharge port if an average surface roughness of the inner surface (204) is above the target surface roughness, the average surface roughness being an average of the surface

roughness values measured at a plurality of locations of the inner surface (204); and
collecting the abrasive slurry that is subsequently injected.
3. The method of claim 2, comprising sealing the exit port with the plug in response to the surface roughness of the inner surface (204) reducing to lesser than or equal to the target surface roughness.
4. The method as claimed in claim 1, comprising:
determining an initial average surface roughness of the inner surface (204) prior to injecting the abrasive slurry, the initial average surface roughness being an average of the surface roughness values measured at a plurality of locations of the inner surface (204); and
selecting a size range of the abrasive particles based on at least one of: the initial average surface roughness of the inner surface (204) and the target surface roughness of the inner surface (204).
5. The method as claimed in claim 4, wherein the abrasive particles are selected from a group consisting of: aluminium oxide particles, boron carbide particles, silicon carbide particles, and titanium carbide particles.
6. The method as claimed in claim 1, comprising:
determining an initial average surface roughness of the inner surface (204) prior to injecting the abrasive slurry, the initial average surface roughness being an average of the surface roughness values measured at a plurality of locations of the inner surface (204); and
selecting a type and amount of the slurry medium based on at least one of: the initial average surface roughness and the target surface roughness.
7. The method as claimed in claim 6, wherein the slurry medium has
thixotropic rheological characteristics.

8. The method as claimed in claim 1, wherein the injecting of the abrasive slurry comprises applying an injection pressure at a pressure ranging from about 60 bar to about 80 bar.
9. The method as claimed in claim 1, wherein providing the exit port (304) comprises:
providing a boss (306) on the outer surface of the as-cast closed volute (202) away from the discharge port (302); and providing the exit port (304) on the boss (306).
10. The method as claimed in claim 9, wherein providing the boss (306) away from the discharge port (302) comprises providing the boss (306) closer to a second end (308) of as-cast closed volute (202) as compared to the discharge port (302).
11. An abrasive flow machining (AFM) apparatus (500) for reducing surface roughness of an inner surface (204) of an as-cast closed volute (202), the AFM apparatus (500) comprising:
a reservoir (504) to hold an abrasive slurry to be pumped into a discharge port (302) of the as-cast closed volute (202);
a cylinder (506) comprising a hydraulic piston (516) to pressurize the abrasive slurry in the reservoir (504) to pump the abrasive slurry into the as-cast closed volute (202); and
a holder (510) to detachably couple the AFM apparatus (500) to the as-cast closed volute (202).
12. The AFM apparatus (500) as claimed in claim 11, comprising:
a ram plate (520) attached to the hydraulic piston (516) and disposed in the reservoir (504), to move in the reservoir (504) in response to movement of the hydraulic piston (516) to pump the abrasive slurry into the as-cast closed volute (202).
13. The AFM apparatus (500) as claimed in claim 11, comprising:
a nozzle plate (508), comprising a first end connected to the reservoir (504) and a second end to be connected to the discharge port (302), to

enable transfer of abrasive slurry from the reservoir (504) to the discharge port (302), wherein the holder (510) is to clamp the second end of the nozzle plate (508) and a discharge port (302) of the as-cast closed volute (202) together.
14. The AFM apparatus (500) as claimed in claim 11, comprising a control unit to control the flow of the abrasive slurry from the reservoir (504), a flow sensor, a pressure sensor, and a directional control valve to enable control of the flow of the abrasive slurry from the reservoir (504).
15. The AFM apparatus (500) as claimed in claim 11, comprising:
a foot-pedal system, connected to the control unit to actuate the hydraulic piston (516); and
a proximity sensor to detect the hydraulic piston (516) at a final position to enable the control unit to revert the hydraulic piston (516) to its initial position.