Precision broaching fixture and broaching method for tenon and groove of small test block of aviation high-pressure turbine disk

Abstract

The present invention discloses a fixture and a broaching method for the precision broaching of the tenon and groove of a small test block of an aviation high-pressure turbine disc. The fixture includes a support frame, a chassis and a pressure plate. The support frame is plugged into the axial end face of the chassis through a solid shaft, and there is an inclined angle between the solid shaft and the axial end face of the chassis, and the pressure plate is connected to the solid shaft through a thread. When precision broaching the tenon and groove of a small test block, the small test block is first placed between the support frame and the pressure plate and clamped, and the tenon and groove size of the small test block is gradually formed by a horizontal high-speed broaching machine, and the broaching and forming process is divided into two steps: rough drawing and fine drawing. The present invention provides a fixture for drawing the tenon and groove of a small test block during the broaching process of a high-pressure turbine disc. The fixture has a simple structure, is light in weight and is easy to clamp. The use of the fixture greatly improves the production efficiency and forming quality of the high-pressure turbine disc.

Description

Technical Field

[0001] The invention belongs to the field of metal processing, and in particular relates to a design of a precision broaching fixture for a small test block tenon and groove of an aviation high-pressure turbine disk and a broaching method. Background Art

[0002] High-pressure turbine disks are typical rotating components in aircraft engines, and their quality and mechanical properties directly impact the overall performance and service life of the engine. Therefore, strict processing requirements are placed on the machining process during the forming of high-pressure turbine disks. The main processes involved in forming the disks are drawing the small test piece mortise, drawing the large test piece mortise, and drawing the full part mortise. Only when the dimensions of the small test piece mortise meet the design requirements can the large test piece mortise and full part mortise processes be carried out. Therefore, the broaching process for the small test piece mortise dimensions is extremely critical.

[0003] At present, in the actual processing of high-pressure turbine discs, a large fixture design is generally used for the fixture used to pull the tenon groove of the small test block. The large fixture includes a connecting structure at the bottom and a supporting structure at the top, and the connecting structure and the supporting structure are rigidly connected or processed as one piece. Among them, the connecting structure is generally designed as a circular disc with the same diameter as the chassis, and the supporting structure is a frustum with non-parallel upper and lower end faces. After the connecting structure and the chassis are fixed by fastening screws, the small test block is installed on the upper end of the supporting structure for broaching. Due to the large size and heavy weight of the entire fixture and the cumbersome installation, a hoist is often required to complete the installation of the small test block fixture, and a large amount of labor time and cost is consumed in hoisting, connection, fastening and adjustment during installation.

[0004] In addition, the large-sized fixture design makes installation and positioning extremely difficult, which further directly affects the production efficiency of the high-pressure turbine disc on site.

[0005] Based on the above reasons, it is urgent to design a fixture for pulling the mortise and tenon of the small test block of the high-pressure turbine disc to meet the production needs of the high-pressure turbine disc on site, improve the production efficiency and forming quality of the high-pressure turbine disc, and thus reduce the on-site production cost. Summary of the Invention

[0006] The purpose of the present invention is to provide a precision broaching fixture and broaching method for the tenon and groove of a small test block of an aviation high-pressure turbine disk, which can well realize the precision broaching and forming of the tenon and groove of a small test block of a high-pressure turbine disk, and the fixture for drawing the small test block is convenient to clamp, and the broaching and clamping of the small test block can be realized without a sling, which can effectively improve the working efficiency and forming quality of the tenon and groove broaching process of the small test block, and thus effectively improve the production efficiency and forming quality of the high-pressure turbine disk.

[0007] The technical solutions of the present invention are as follows:

[0008] Aviation high-pressure turbine disc small test block tenon groove precision broaching fixture, including,

[0009] The chassis includes a plurality of inclined grooves and an axial positioning ring. The plurality of inclined grooves are distributed on a circumferential surface of the chassis and are concentric with the chassis. The axial positioning ring is located on an axial end surface of the chassis and is coaxial with the chassis. The axial end surface where the axial positioning ring is located also has a recessed hole for a support frame and a protrusion for a high-pressure turbine disk.

[0010] The support frame is detachably connected to the chassis, and the support frame includes a solid shaft, a small test block support surface, a positioning groove and a broaching groove. The solid shaft is perpendicular to the small test block support surface, and the positioning groove and the broaching groove are located on the circumferential surface of the support frame. The inner diameter and shape of the positioning groove match the outer diameter and shape of the axial positioning ring. The broaching groove extends parallel to the axial direction of the solid shaft. The axial first end of the solid shaft is processed with an external thread, and the axial second end of the solid shaft is plugged into the concave hole of the support frame. When the axial second end of the solid shaft is plugged into the concave hole of the support frame, there is an inclination angle β between the axis of the solid shaft and the axis of the chassis, and the inclination angle β satisfies the following relationship:

[0011] , where α is the angle between the centerline of the groove of the high-pressure turbine disk and the centerline of the shaft;

[0012] A pressing plate is provided with a threaded hole and is threadedly connected to the external thread of the first axial end of the solid shaft through the threaded hole.

[0013] As an option, the support frame is detachably connected to the chassis by screws, and a weight-reducing groove or a weight-reducing hole is opened on the end surface connected to the chassis.

[0014] As an option, the maximum outer diameter of the base disc is equal to the outer diameter of the high-pressure turbine disc.

[0015] As an option, the outer diameter of the axial positioning ring is equal to the inner diameter of the high-pressure turbine disk.

[0016] As an option, the diameter of the solid shaft is equal to the inner diameter of the small test block.

[0017] As an option, the small test block supporting surface is a circular plane, and the diameter of the circular plane is equal to the diameter of the small test block.

[0018] As an option, it is characterized in that the maximum diameter D of the pressure plate y-max The following relations are satisfied:

[0019] , where D os is the distance between the bottom surface of the small test block's tenon groove and the center line of the shaft, and Δs is the installation gap between the bottom surface of the small test block's tenon groove and the pressure plate.

[0020] The method for broaching the tenon groove of a small test block of an aviation high-pressure turbine disk uses the aforementioned small test block of an aviation high-pressure turbine disk with a precision broaching fixture, and includes the following steps:

[0021] Step 1: Place the small test block on the small test block support surface of the support frame by sleeve-fitting the small test block onto the solid shaft of the support frame through the inner hole of the small test block, and tighten the small test block by screwing the pressing plate into the external thread of the solid shaft;

[0022] Step 2, rough drawing and grooving of the small test piece;

[0023] Step 3: Perform fine drawing of the mortise and tenon cross section on the small test piece.

[0024] As an option, steps 2 and 3 are performed on a horizontal high-speed broaching machine.

[0025] As an option, in steps 2 and 3, the tooth lift of the broach is determined by the following formula:

[0026] , where ΔH is the broach tooth lift, D os is the distance between the bottom surface of the small test block groove and the center line of the shaft, D s is the diameter of the small test block.

[0027] As an option,

[0028] In step 2, the feed speed during rough drawing and grooving is 1-3 m / min;

[0029] In step 3, the feed speed during the fine drawing of the mortise and tenon cross section is 1.5-2 m / min.

[0030] Compared with the existing technology, the present invention designs a mortise and tenon clamp for small specimens of high-pressure turbine disks based on the characteristics of small size and light weight of small specimens of high-pressure turbine disks. The clamp can efficiently complete the clamping and precision broaching of the mortise and tenon of small specimens of high-pressure turbine disks. It mainly includes chassis design, support frame design and pressure plate design, ensuring that the clamping efficiency and forming quality of the small specimen are greatly improved during the broaching and clamping process.

[0031] Compared with the traditional fixture for mortise and tenon broaching of small high-pressure turbine disc test blocks, the present invention omits the connecting structure part (disc structure), thereby greatly reducing the overall design size, reducing the overall weight and material usage (reducing material usage by more than 60%), making on-site use and installation extremely convenient and improving on-site work efficiency.

[0032] Compared with the traditional fixture for mortise and tenon broaching of small test blocks of high-pressure turbine discs, the present invention can quickly determine the inclination angle and thus the support surface position of the small test block through the solid shaft on the support frame. It can be quickly and accurately assembled on the chassis in conjunction with the positioning groove and axial positioning ring, concave hole and protrusion. At this time, the connecting screws only play a tightening role and no additional adjustment and positioning are required.

[0033] The small test block can be quickly pressed by the pressure plate in conjunction with the small test block support surface on the support frame and the external thread of the solid shaft so as to perform broaching.

[0034] On the premise of having sufficient rigidity to support the small test block, the support frame can adopt a weight-reducing design (for example, designing weight-reducing holes and grooves on the connection surface between the support frame and the chassis) to further reduce the deadweight of the fixture, thereby avoiding the need for assembly with a sling.

[0035] In addition, the design of the tongue-and-groove fixture for pulling small test blocks also ensures that the fixture is easy to disassemble (it has a positioning structure, an anti-rotation structure and a light weight), which directly improves the production efficiency of the high-pressure turbine disc while reducing production costs. BRIEF DESCRIPTION OF THE DRAWINGS

[0036] Figure 1 This is a schematic diagram of the parts of an aviation high-pressure turbine disk according to an embodiment of the present invention;

[0037] Figure 2 yes Figure 1 Schematic diagram of the middle AA section;

[0038] Figure 3 Schematic diagram of a small test block of an aviation high-pressure turbine disk according to an embodiment of the present invention, wherein (a) is a schematic diagram of the small test block before broaching, and (b) is a schematic diagram of the small test block after broaching a tongue and groove;

[0039] Figure 4 This is a schematic diagram of a small test block chassis of an aviation high-pressure turbine disk in an embodiment of the present invention;

[0040] Figure 5 Schematic diagram of a small test block support fixture for an aviation high-pressure turbine disk according to an embodiment of the present invention;

[0041] Figure 6 yes Figure 5 Middle F direction view;

[0042] Figure 7 This is a front view of the assembly of a small test block support frame of an aviation high-pressure turbine disk and a chassis in an embodiment of the present invention;

[0043] Figure 8 yes Figure 7 Middle G-direction view;

[0044] Figure 9 yes Figure 8Schematic diagram of the middle section AA;

[0045] Figure 10 2. It is a schematic cross-sectional view of a pressure plate of a small test block of an aviation high-pressure turbine disk according to an embodiment of the present invention;

[0046] Figure 11 Schematic diagram of the precision broaching and clamping of the tenon and groove of a small test block of an aviation high-pressure turbine disk according to an embodiment of the present invention;

[0047] Figure 12 Schematic diagram of the precision broaching of the tenon and groove of a small test block of an aviation high-pressure turbine disk according to an embodiment of the present invention;

[0048] Figure 13 Schematic diagram of the feed rate for precise broaching of a small test block of an aviation high-pressure turbine disk in an embodiment of the present invention;

[0049] In the figure, 1-nut, 2-support frame, 3-chassis, 4-pressure plate, 5-small test block. DETAILED DESCRIPTION

[0050] The present invention is further described below with reference to the accompanying drawings and specific embodiments. However, it should not be understood that the scope of the subject matter described in the present invention is limited to the following embodiments. Without departing from the above technical ideas of the present invention, various modifications, substitutions and changes made according to common technical knowledge and customary means in the field are included in the scope of the present invention.

[0051] by Figure 1 and Figure 2 Taking the aviation high-pressure turbine disk as an example, the small test block tenon-groove precision broaching fixture and broaching processing method include the following:

[0052] (1) Design of small test block tenon and groove precision broaching fixture:

[0053] like Figure 3 As shown in the figure, based on the small size, light weight and simple structure of the small test block 5, a fixture for drawing the tenon groove of the small test block is designed. The complete set of small test block tenon groove precision drawing fixture mainly includes a chassis 3, a support frame 2 and a pressure plate 4.

[0054] The structure of chassis 3 is as follows Figure 4 As shown, for the chassis 3, in order to enable the chassis 3 to be simultaneously used in the process of drawing and mortise forming of the high-pressure turbine disk, the maximum outer diameter of the chassis 3 is equal to the outer diameter of the high-pressure turbine disk, and an inclined groove is opened, and the broach forms the mortise size of the high-pressure turbine disk through the inclined groove.

[0055] The structure of the support frame 2 is as follows Figure 5 and Figure 6As shown, the support frame 2 is designed to be small in size. The small test block support surface of the support frame 2 is a circular plane surface with a diameter equal to the diameter of the small test block 5. The support frame 2 is mounted on the chassis 3 and is fastened to the chassis 3 by four fastening screws. In addition, the support frame 2 mounted on the chassis 3 is tilted at a certain angle β, and a weight-reducing groove is provided on the end surface in contact with the chassis 3, as shown in FIG. Figure 7 、 8 and 9.

[0056] like Figure 10 The figure shows the structural design of the pressing plate 4. In order to effectively press the small test block 5, the pressing plate 4 needs to be designed with sufficient rigidity and strength.

[0057] In the above solution, the maximum outer diameter of the chassis 3 can be determined as follows:

[0058]

[0059] Among them, D p-max is the maximum diameter of chassis 3, D f is the outer diameter of the high-pressure turbine disc.

[0060] In the above solution, in order to ensure that the chassis 3 can be used in the broaching process of the high-pressure wheel tenon groove in the later stage, an axial positioning ring is designed on the chassis 3. At the same time, the axial positioning ring can also serve as the axial positioning dimension of the support frame 2 and match the positioning groove on the support frame 2. The outer diameter of the axial positioning ring on the chassis 3 is calculated as follows:

[0061]

[0062] Among them, d p is the outer diameter of the axial locating ring of chassis 3, d f is the inner diameter of the high-pressure turbine disk.

[0063] In the above solution, the mounting surface of the chassis 3 (the axial end surface connected to the support frame 2) is designed with a recessed hole and a protrusion. The recessed hole is designed to prevent the support frame 2 from rotating during installation, and the protrusion is designed to prevent the high-pressure turbine disk from rotating during installation. At the same time, the protrusion is also used to locate the first mortise position of the turbine disk.

[0064] In the above solution, the inclination angle β of the support frame 2 is calculated as follows:

[0065] ;

[0066] Where α is the angle between the centerline of the groove of the high-pressure turbine disk and the centerline of the shaft.

[0067] In the above scheme, a solid shaft is designed in the middle of the support frame 2. The upper end of the solid shaft is processed with an external thread. The nut 1 is connected to the external thread to tighten the pressure plate 4, thereby pressing the small test block 5. The lower end of the solid shaft can extend into the recessed hole of the chassis 3, which can effectively prevent the support frame 2 from rotating when installed. In addition, to ensure that the solid shaft can pass through the inner hole of the small test block 5, the diameter of the solid shaft of the support frame 2 can be calculated as follows:

[0068] ;

[0069] Where, d is the diameter of the solid shaft of the support frame 2, d s is the inner diameter of the small test block 5.

[0070] In the above scheme, in order to prevent the broach from interfering with the pressing plate 4 when the small test block 5 is broached, the maximum diameter of the pressing plate 4 can be calculated as follows:

[0071] ;

[0072] Among them, D y-max is the maximum diameter of the pressure plate 4, D os It is the distance between the bottom surface of the small test block's tenon groove and the center line of the shaft, and Δs is the installation gap between the bottom surface of the small test block's tenon groove and the pressure plate 4, which is generally 3~5mm.

[0073] In the above solution, the designed chassis 3, support frame 2 and pressure plate 4 all meet the strength and rigidity of the materials used.

[0074] (2) Precision broaching method for small test block mortise and tenon:

[0075] According to the small test block tenon groove precision broaching fixture designed in (1), the small test block 5 is placed on the small test block support surface of the support frame 2, the pressure plate 4 presses the small test block 5, and the pressure plate 4 is fastened by fastening the nut 1 to the external thread of the support frame 2, thereby fixing the small test block 5 on the support frame 2, as shown in FIG. Figure 11 shown.

[0076] The broaching process is carried out on a horizontal high-speed broaching machine. In the precision broaching process of the small test piece mortise and tenon, broaching is carried out in two steps. The first step is the rough drawing and grooving of the small test piece 5, and the mortise and tenon are gradually formed by the rough broach. The second step is the fine drawing and grooving of the small test piece 5, and the cross-sectional dimensions of the mortise and tenon of the small test piece are gradually formed by the fine broaching tooth shape. Figure 12 When the mortising of the small test piece is completed, the broaching turntable is withdrawn and the small test piece 5 and the support frame 2 are removed.

[0077] In the above solution, the solid shaft of the small test block tenon and groove precision broaching fixture passes through the small test block 5 and the pressure plate 4, and the nut 1 is connected to the external thread of the solid shaft of the support frame 2.

[0078] In the above scheme, the tooth lift of the broach can be determined by the following formula:

[0079] ;

[0080] Where ΔH is the broach tooth lift, D os is the distance between the bottom surface of the small test block groove and the center line of the shaft, D s is the diameter of the small test block.

[0081] In the above scheme, the feed rate of the small test block mortise and tenon broaching is planned in two parts, such as Figure 13 As shown, in the rough drawing stage t1 of the small test piece, the feed speed is v s1 , in the small test piece tenon groove fine drawing forming stage t2, the feed speed is v s2 .

[0082] The following takes a small test block 5 for an aviation high-pressure turbine disk as an example to further illustrate the design and processing method of the precision broaching fixture for the tenon and groove of a small test block for an aviation high-pressure turbine disk of the present invention.

[0083] The relevant dimensions of the high-pressure turbine disc are: d f =108.9mm, D f =429.66mm, T f = 61.6mm, T Z =28.6mm, α=18.7°. The relevant dimensions of the small test block 5 of the high-pressure turbine disc are: D s =245.3mm,,d s =44mm,H s =26.4mm, D os =95.65mm. Therefore, the specific steps of the design and processing method of the precision broaching fixture for the tenon and groove of a small test block of an aviation high-pressure turbine disk are as follows:

[0084] (1) Design of small test block tenon and groove precision broaching fixture:

[0085] According to the characteristics of small size, light weight and simple structure of the small test block 5, Figure 3 As shown in the figure, the design of the processing fixture for drawing the tenon groove of the small test block is carried out. The complete set of small test block tenon groove precision broaching fixture mainly includes the chassis 3, the support frame 2 and the pressure plate 4. For the chassis 3, in order to make the chassis 3 simultaneously used in the process of drawing the tenon groove of the high-pressure turbine disk, the maximum outer diameter of the chassis 3 is equal to the outer diameter of the high-pressure turbine disk, and an inclined groove is opened. The broach forms the tenon groove size of the high-pressure turbine disk through the inclined groove, as shown in FIG. Figure 4 As shown;

[0086] The relevant dimensions of the designed chassis 3 are: D p-max =429.66mm, d p =108.9mm.

[0087] The support frame 2 is designed to be small in size. The small test block support surface of the support frame 2 is a circular plane. The diameter of the circle is equal to the diameter of the small test block 5. The support frame 2 is installed on the axial end surface of the chassis 3 and is fastened to the chassis 3 by 4 fastening screws. Figure 5 、 6 , 7. In addition, the support frame 2 mounted on the chassis 3 is tilted at a certain angle β, as shown in FIG. Figure 7 、 8 and 9; the relevant dimensions of the designed support frame 2 are: D = 245.3mm, d = 44mm, β = 71.3°, H = 20mm.

[0088] For the pressing plate 4, in order to effectively press the small test piece 5, the design of the pressing plate 4 needs to have sufficient rigidity and strength, such as Figure 10 As shown, the relevant dimensions of the designed pressure plate 4 are: D y-max =195.66mm, d y =44mm,H y =30°, D y =165.66mm,δ=45°.

[0089] (2) Small test block mortise and tenon broaching forming method:

[0090] Using the fixture designed in (1), the small test block 5 is placed on the support frame 2, the pressure plate 4 presses the small test block 5, and the pressure plate 4 is fastened by connecting the fastening nut 1 with the external thread of the support frame 2, thereby fixing the small test block 5 on the support frame 2, as shown in FIG. Figure 11 As shown. The broaching process is carried out on a horizontal high-speed broaching machine. In the process of broaching the small test block mortise and tenon, broaching is carried out in two steps, and the total tooth lift of the broach is ΔH = 27mm. The first step is the rough drawing and grooving of the small test block 5, and the mortise and tenon are gradually formed by the rough broach. The second step is the fine drawing and grooving of the small test block 5, and the cross-sectional dimensions of the small test block mortise and tenon are gradually formed by the fine broaching tooth shape, as shown in FIG. Figure 12 When the mortising of the small test block 5 is completed, the broaching turntable is withdrawn and the small test block 5 and the support frame 2 are removed.

[0091] In addition, in the broaching process, the feed rate of the small test block tenon and groove precision broaching is planned in two parts, such as Figure 13 As shown, in the rough drawing stage t1 of the small test piece, the feed speed v s1 The feed speed is 1~3m / min. In the small test piece tenon groove fine drawing forming stage t2, the feed speed v s2 1.5~2m / min.

[0092] Any matters not described in detail in the present specification are prior art known to those skilled in the art. Although the above description of the present invention is based on illustrative embodiments to facilitate understanding of the present invention by those skilled in the art, it should be understood that the present invention is not limited to the scope of the specific embodiments. As long as various modifications are within the spirit and scope of the present invention as defined and determined by the appended claims, such modifications will be obvious to those skilled in the art, and all inventions and creations utilizing the concepts of the present invention are protected.

Claims

1. Precision broaching fixture for small specimens of aviation high-pressure turbine disks, characterized by: include, A chassis (3), the chassis (3) comprising an inclined groove and an axial positioning ring, wherein the plurality of inclined grooves are distributed on a circumferential surface of the chassis (3) and are concentric with the chassis (3), the axial positioning ring is located on an axial end face of the chassis (3) and is coaxial with the chassis (3), and a concave hole for a support frame and a convex block for a high-pressure turbine disc are also provided on the axial end face where the axial positioning ring is located; A support frame (2), wherein the support frame (2) is detachably connected to the chassis (3), and the support frame (2) includes a solid shaft, a small test block support surface, a positioning groove, and a broaching groove. The solid shaft is perpendicular to the small test block support surface, and the positioning groove and the broaching groove are located on the circumferential surface of the support frame (2). The inner diameter and shape of the positioning groove match the outer diameter and shape of the axial positioning ring. The broaching groove extends parallel to the axial direction of the solid shaft. An external thread is machined on the axial first end of the solid shaft, and an axial second end of the solid shaft is plugged into a concave hole for the support frame. When the axial second end of the solid shaft is plugged into the concave hole for the support frame, there is an inclination angle β between the axis of the solid shaft and the axis of the chassis (3), and the inclination angle β satisfies the following relationship: , where α is the angle between the centerline of the groove of the high-pressure turbine disk and the centerline of the shaft; A pressing plate (4), wherein a threaded hole is formed on the pressing plate (4), and the pressing plate (4) is threadedly connected to the external thread of the first axial end of the solid shaft through the threaded hole; The support frame (2) is detachably connected to the chassis (3) by screws, and a weight-reducing groove or a weight-reducing hole is provided on the end surface connected to the chassis (3); The maximum outer diameter of the chassis (3) is equal to the outer diameter of the high-pressure turbine disc; The outer diameter of the axial positioning ring is equal to the inner diameter of the high-pressure turbine disk; The diameter of the solid shaft is equal to the inner diameter of the small test block (5); The small test block support surface is a circular plane, and the diameter of the circular plane is equal to the diameter of the small test block (5).

2. The precision broaching fixture for the tenon and groove of a small test block of an aviation high-pressure turbine disk according to claim 1 is characterized by: The maximum diameter D of the pressure plate (4) y-max The following relations are satisfied: , where D os is the distance between the bottom surface of the small test block's mortise and tenon groove and the axis centerline, and Δs is the installation gap between the bottom surface of the small test block's mortise and tenon groove and the pressure plate (4).

3. A method for broaching the tenon and groove of a small test block of an aviation high-pressure turbine disk, characterized by: The apparatus for precision broaching of a small specimen of an aviation high-pressure turbine disk according to claim 1 is used, and includes the following steps: Step 1, the small test block (5) is placed on the small test block support surface of the support frame (2) by sleeve-fitting the inner hole of the small test block (5) onto the solid shaft of the support frame (2), and the small test block (5) is pressed tightly by screwing the pressing plate (4) into the external thread of the solid shaft; Step 2, rough drawing and grooving the small test piece (5); Step 3, performing fine drawing of the mortise and tenon cross section on the small test block (5).

4. The method for broaching a small specimen of an aviation high-pressure turbine disk according to claim 3, characterized in that: In steps 2 and 3, the tooth lift of the broach is determined by the following formula: , where ΔH is the broach tooth lift, D os is the distance between the bottom surface of the small test block groove and the center line of the shaft, D s is the diameter of the small test piece (5).

5. The method for broaching a small specimen of an aviation high-pressure turbine disk according to claim 3, characterized in that: In step 2, the feed speed during rough drawing and grooving is 1 to 3 m / min; In step 3, the feed speed during the fine drawing of the mortise and tenon cross section is 1.5 to 2 m / min.

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