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Cutting behaviour of tungsten carbide taper pin reamers
Int. J. Mach Tool Des. Res.
Vol. 15, pp. 69-76.
Pergamon Press 1975.
Printed in Great Britain.
CUTTING BEHAVIOUR OF TUNGSTEN TAPER PIN REAMERS
CARBIDE
D. M. TURLEY* (Received 28 June 1973)
Abstract--The cutting behaviour of tungsten carbide taper pin reamers used to ream holes in a composite structure of aluminium alloy and ultra-high strength steel has been investigated. The widths of the circular lands was found to be an important variable in determing the cutting efficiencyof the reamers. The maximum permissible wid~:h of the circular land for cutting to occur depends upon the hardness of the workpiece material. Cutting of both the aluminium alloy and ultra-high strength steel occurs under built-up-edge conditions. Estimates have been made of the magnitude of the radial force which acts on the circular lands during reaming of the steel. INTRODUCTION A SURVEY of the literature indicates that very little work has been reported on the cutting behaviour of taper pin reamers. This is unfortunate as taper reaming has some important applications such as the preparation of holes for interference fit fasteners in aircraft structures; the interference is achieved by drawing tapered pins into reamed tapered holes. In very highly stressed aircraft structures, where good fatigue and stress-corrosion properties are required, it is often necessary to ream by hand and without lubricant to obtain the required surface finish and hole accuracy, and to minimize the presence of surface corrodants. This paper reports the results of an investigation into the cutting behaviour of tungsten carbide taper pin reamers with 18-straight flutes used to finish ream holes in a composite structure of aluminium alloy plate overlaying a plate of ultra-high strength steel. This structure is subjected to high fluctuating stresses in service, and since the ultra-high strength steel plate is the main load-bearing member it is the reaming of this material which is the main subject of the paper. EXPERIMENTAL Prior to reaming with the tungsten carbide taper pin reamers with 18-straight flutes (taper, ¼ in/ft) (20.813 ram/m), the holes are drilled and initially reamed with a tapered (taper, ¼ in/ft) (20.82, ram/m) left-hand spiral 6-flute high-speed steel (HSS) reamer (ANSI B94.21971 Size 7). All reaming was done dry and by hand with only a small axial force being applied to the reamers. The tungsten carbide reamers, which contained approximately 6.5 ~ cobalt were made from a C-1 grade carbide [1]. The hardness of the aluminium alloy and ultra-high strength steel plate were 180 and 480 HV, respectively. Profiles of transverse sections of the reamers were obtained from casts of T a y l o r - H o b s o n plastic replicating material. These were polished metallographically, finishing on a 0~2/~m * Australian Defence Scientific Service, Department of Supply, Defence Standards Laboratories, Melbourne, Australia. 6
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diamond pad and shadowed with aluminium prior to photographing. The surface finish of the reamed holes was measured with a Talysurfinstrument. RESULTS During reaming of holes it was found that some tungsten carbide reamers cut quite satisfactorily whereas others, even when new, did not. Measurement of rake angles, clearance angles, and width of the circular lands from the plastic casts of the reamers showed that the only difference between the two types was the width of the circular lands (Fig. 1). Moreover, projecting these plastic casts at a magnification of fifty times showed that all of the circular lands were equidistant from the centre of the reamer.
Fic,. I. Plastic casts of reamers showing differencesin circular land widths. (a) Reamer with narrow lands; cuts well; (b) Reamer with broad lands; will not cut satisfactorily. (50 × ) The widths of the circular lands on reamers that cut satisfactorily were all in the range 0.0020-0.0030 in. (0.0508-0.0762 mm) whereas the widths of the circular lands on reamers that would not cut satisfactorily ranged, for any particular reamer, from 0.0030-0-0080 in. (0.0762-0.2032 mm), many of them being 0.0060-0.0080 in. (0.1524-0.2032 mm). For convenience, reamers that cut satisfactorily and those that did not are denoted as S- and U-type, respectively.
1. Reamers with circular lands 0-0020-0"0030 in. (0.0508-0.0762 mm) wide (S-Type) The cutting edge of a new reamer is shown in Fig. 2. The transverse grinding marks on the circular land and the longitudinal grinding marks on the clearance face can be clearly seen. The cutting edge was not very sharp, because relatively large pieces had been broken from the edge during grinding of the reamer. During reaming of a taper hole, shown schematically in Fig. 3, cutting begins generally at the top of the aluminium alloy plate and progresses down through the hole during the course of the reaming operation. The crests of the surface undulations produced by the
Cutting Behaviour of Tungsten Carbide Taper Pin Reamers
71
Fxo. 2. Scanning electron micrograph of the cutting edge of a new reamer. (1750x) HSS reamer are cut away first until finally all of the original surface produced by the HSS reamer is removed. During the reaming of the aluminium alloy an appreciable built-up-edge (BUE) of the alloy is formed which adheres to both the circular lands and the cutting edges (Fig. 4). When the reamer begins cutting the steel plate the aluminium alloy BUE is removed from the reamer. Examination of the reamed steel surface showed that the top of the hole (first 0-010-0.020 in.) (0.254-0.508 mm) is often rougher than the surface produced further down due to the presence of large tears; the cause of the roughness appeared to be associated with the removal of the aluminium alloy BUE. Examination of the cutting edges of the reamer after reaming of the steel had been completed showed that steel was adhering in the cavities between tungsten carbide grains in the circular lands and that a BUE was present on the cutting ,edges (Fig. 5). This BUE was quite different in appearance from the aluminium alloy BUE (cf Figs. 5 and 4) and moreover, it appeared in many cases to be continuous along the cutting edge; the BUE was positively identified as steel. A longitudinal section through a partly formed chip also confirmed that a small BUE, in this case still attached to the chip, was present (Fig. 6). The steel swarf produced by this reamer was tightly curled and scroll-like in appearance (Fig. 7), the maximum width of the scrolls being approximately 0.070 in. (1.778 ram). Small tears could be clearly seen on the rake face side of the swarf at higher magnification, and examination of the reamed surface showed that small tears were also present (Fig. 8).
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Ultra-high strength steel plate FIc. 3. Schematic sketch of tapered hole in composite structure. (Scale ~ 2-5: 1)
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D . M . TURLEY
FIG. 4. Scanning electron micrograph showing large aluminium alloy BUE adhering to the circular land and cutting edge. (300 x ) FIG. 5. Scanning electron micrograph showing steel adhering in the cavities on the circular land and a small steel BUE adhering to the cutting edge. (550×)
FIG. 6. Optical micrograph of a longitudinal section through a chip showing a BUE. Arrows indicate where chip and new surface separate from the BUE. (I 350× ) FIG. 7. Scanning electron micrograph of the scroll-like swarf produced by a new reamer (S-type). (12x) FiG. 8. Scanning electron micrograph of the chip (shown in Fig. 6) prior to sectioning. Many tears can be seen on the new surface. (I 30 :~ )
Cutting Behaviour of Tungsten Carbide Taper Pin Reamers
73
The presence of tears is usually associated with the presence of a BUE at the cutting edge of the tool during cutting, because it has been shown [2] that generation of the new surface and the chip occurs at two distinct fracture points and these fracture points produce tears. The fracture point where the new surface separates from the BUE is adjacent to the cutting edge of the tool, whereas the one where the chip separates from the BUE is some distance along the rake face of the tool (see Fig. 6). A scanning electron micrograph of the chip shown in Fig. 6 prior to it being sectioned clearly showed the association of tears on the machined surfitce with the presence of a BUE (see Fig. 8). The centre-line average roughness (cla) value of the steel surface was in the range 8-10/zm. It was usually found that S-type reamers continued to cut satisfactorily for a considerable period of time. Examination of these reamers after the aluminium alloy and the steel BUE had been removed revealed very little evidence of wear on either the cutting edges or the circular lands.
2. Reamers with circular lands 0.0030-0-0080 in. (0.0762-0.2032 mm) wide (U-type) When reaming the aluminium alloy an appreciable BUE of the alloy, similar to that previously observed on S-type reamers, is formed. This BUE is similarly removed when the reamers begin cutting the steel. These rearners, however, did not cut the steel satisfactorily. Since the only difference
FIG. 9. Scanning electron micrographs of a reamer (U-type) showing, (a) some swarf in a flute adjacent to a narrow land, and (b) very little swarf in a flute adjacent to a broad land. (14×) FIG. 10. Scanning electron micrograph of a surface produced by the reamer shown in Fig. 9. There is evidence of surface smearing as indicated by the arrows. (570 × )
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between these and S-type reamers was the widths of the circular lands, it seemed reasonable to conclude that the cutting edges with broad circular lands did not cut but rubbed the surface. By stopping the reaming and carefully removing the reamer, it was confirmed that the cutting edges with the broad circular lands removed very little material by cutting but mainly rubbed the surface. Examination in the scanning electron microscope showed that there was a considerable amount of swarf in the flutes adjacent to the narrow circular lands (Fig. 9(a)) whereas in the flutes adjacent to the broad circular lands there was very little (Fig. 9(b)). Examination of the cutting edges and circular lands at higher magnification showed that, unlike the S-type reamers, the steel BUE was not continuous along the cutting edges. There was also appreciable build up of steel on the circular lands where most of it had penetrated down between the tungsten carbide grains.
FiG. I 1. Optical micrograph of a longitudinal section through the surface shown in Fig. 10. Note the extensiveplastic deformation of the surface layers. Seam-likedefects are also present in the surface as indicated by the arrow. (700x ) The rubbing behaviour of the reamer was also substantiated by examining the reamed surface. There was evidence of surface smearing (Fig. 10) and there were few tear marks which are associated with normal cutting under BUE conditions (see Fig. 8), and hence the surface finish was generally very good (cla 2-3/~m). Examination under the optical microscope of a longitudinal section through this surface showed that the rubbing had caused extensive plastic deformation of the surface layers (Fig. 11); a white-etching layer was present in the surface, the nature of which is currently being investigated. Beneath this layer plastic deformation is evident by the distortion of the base structure parallel to the surface of the reamed hole. The total depth of deformation was estimated to be 0.0010in. (0.0254 mm). When cutting with an S-type reamer, however, examination under the optical microscope of a longitudinal section of a reamed hole showed no evidence of surface deformation (see Fig. 6). Seam-like defects were also present in the metal surface (see Fig. 11), and these are obviously associated with the surface smearing indicated in Fig. 10. These surface defects are not tears; they are elongated in the direction of reaming, whereas the 'point' of a tear is directed opposite to the reaming direction (see Fig. 8). Some of the smeared
Cutting Behaviour of Tungsten Carbide Taper Pin Reamers
75
regions shown in Fig. 10 are believed to be produced by the detachment of steel, which was originally adhering to the circular lands and became welded by friction to the reamed surface. Whilst examination of U-type reamers at higher magnification showed that the swarf produced by the cutting edges with narrow circular lands was 'chip-like', it was not similar and generall:r much finer than the scroll-like swarf produced by the S-type reamers that had circular lands of similar widths (0.0020-0.0030 in.) (0.0508-0.0762 mm) (compare Fig. 9(a) and Fig. 7). The swarf produced by the cutting edges which had broad circular lands was less 'chip-like' and much of it had a granulated appearance (see Fig. 9(b)), although some obvious parts of chips were identified. It is important to note, however, that whereas the cutting edges with broad circular lands would not cut the ultra-high strength steel, they were much more efficient in cutting the softer aluminium alloy. Examination of the circular lands of U-type reamers in the scanning electron microscope after the aluminium alloy and steel BUE had been removed revealed that appreciable wear had occurred on both the narrow and broad lands. Very little of the original surface of the circular lands remained. Moreover, examination of the reamed steel surface in the electron-probe microanalyser revealed the presence of some tungsten carbide grains embedded in the surface, particularly in the smeared regions shown in Fig. 10.
DISCUSSION The widtll of the circular land is an important variable in determining the cutting efficiency of taper reamers. The results suggest that in order for a reamer to begin cutting the circular lands must be able to indent the reamed surface so that a depth of cut can be taken. The radial force which acts on the circular land, and causes indentation, is largely developed by the reaction between the reamer and the reamed surface as the reamer moves down into the hole during the early stages. It can be shown that, for a new reamer with a circular land width of 0.0025 in. (0.0635 mm), the radial force required to plastically deform and indent the renamed steel surface is approximately 120 lbf (534 N) for a hardness of 480 HV and a chip width of 0.070 in. (1.778 mm) (measured from Fig. 7). Moreover, if during cutting it is assumed that only the BUE has contact with the reamed surface and the radial force is transmitted through this BUE, then from Fig. 5 it can be calculated that the required radial force is now approximately only 9 lbf (40 N~. It is apparent from Fig. 5, however, that the circ~'ular lands have some contact with the surface of the reamed hole as steel is present on the circular lands embedded between the tungsten carbide grains. The radial forces of 120 lbf (534 N) and 9 lbf (40 N) therefore represents the limiting values of the radial force and, in practice, the actual force would be expected :to lie between those two values. This does not necessarily mean that there is a high initial indentation force required to commence cutting and produce a chip 0-070 in. (1.778 mm) wide which then drops after a BUE is formed. The formation of the BUE along the cutting edges is in fact a continuous process, beginning during the initial stages of cutting when the crests of the surface undulations produced by the HSS reamer are removed. For a broad circular land of 0.0080 in. (0.2032 mm), the calculated radial force required to indent the', surface over a distance of 0.070 in. (1.778 mm) is greater than 380 lbf (1690 N). The radial force would be expected to be near this value, because in this case the BUE was less continuous and appreciable build up occurred on the circular lands mainly between the tungsten carbide grains. However, in order to develop a radial force of approximately
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380 lbf (1690 N) appreciable rubbing occurs between the reamed surface and the circular lands, and this causes plastic deformation and work hardening of the surface layers so that the required radial force to plastically deform and indent the surface is in fact much greater than 380 lbf (1690 N). Hence the broad circular lands do not produce scroll-like swarf similar to that shown in Fig. 7, because the forces required to do so are too high. The results clearly showed that the maximum permissible width of the circular land for cutting to occur depends upon the hardness of the workpiece material. Circular lands 0.0080 in. (0.2032 ram) wide cut the aluminium alloy whereas they would not cut the ultrahigh strength steel. Also the narrow circular lands of U-type reamers removed some material by cutting, but they did not produce scroll-like swarf similar to S-type reamers which had all narrow circular lands. The swarf was much narrower, indicating that a greater radial force per unit area was required to indent the reamed surface. This increase in force per unit area can be attributed to the significant increase in hardness of the surface layers caused by plastic deformation due to rubbing by the broad lands. In fact when an S-type reamer that originally cut very well was used to ream a hole that had previously been reamed with a U-type reamer, the S-type reamer would now not cut satisfactorily and produce scroll-like swarf as in Fig. 7. For S-type reamers there was very little evidence of wear on the circular lands and the cutting edges. It is most likely that for these reamers the circular lands do not have continual contact with the reamed surface. This indicates that under BUE conditions separation of the new surface from the BUE can occur below the level of the circular land. The presence of tear marks and the absence of any evidence of severe rubbing on the reamed surface supports this conclusion. For U-type reamers, however, both the narrow and broad circular lands showed appreciable wear. The wear on the broad lands is caused by the near continual rubbing between the land and the reamed surface. The wear on the narrow circular lands indicates that cutting by these lands is intermittent and appreciable rubbing also occurs. Moreover, the wear processes on both types of land are enhanced by the increased hardness of the surface layers caused by plastic deformation during rubbing, and the high radial and shearing forces generated between the lands and the reamed surface. A more detailed study is being made on the wear of these reamers. Acknowledgements--The author would like to thank Mr L. M. Bland of Aeronautical Research Laboratories
for supplying the reamers and materials and for helpful discussions. REFERENCES [1] Metals Handbook, 8th Ed. p 316, Vol. 3. ASM, Ohio (1967). [2] J. J. W~LL~AMSand E. C. ROLLASON,J. Inst. Metals 98, 144 (1970).
Vol. 15, pp. 69-76.
Pergamon Press 1975.
Printed in Great Britain.
CUTTING BEHAVIOUR OF TUNGSTEN TAPER PIN REAMERS
CARBIDE
D. M. TURLEY* (Received 28 June 1973)
Abstract--The cutting behaviour of tungsten carbide taper pin reamers used to ream holes in a composite structure of aluminium alloy and ultra-high strength steel has been investigated. The widths of the circular lands was found to be an important variable in determing the cutting efficiencyof the reamers. The maximum permissible wid~:h of the circular land for cutting to occur depends upon the hardness of the workpiece material. Cutting of both the aluminium alloy and ultra-high strength steel occurs under built-up-edge conditions. Estimates have been made of the magnitude of the radial force which acts on the circular lands during reaming of the steel. INTRODUCTION A SURVEY of the literature indicates that very little work has been reported on the cutting behaviour of taper pin reamers. This is unfortunate as taper reaming has some important applications such as the preparation of holes for interference fit fasteners in aircraft structures; the interference is achieved by drawing tapered pins into reamed tapered holes. In very highly stressed aircraft structures, where good fatigue and stress-corrosion properties are required, it is often necessary to ream by hand and without lubricant to obtain the required surface finish and hole accuracy, and to minimize the presence of surface corrodants. This paper reports the results of an investigation into the cutting behaviour of tungsten carbide taper pin reamers with 18-straight flutes used to finish ream holes in a composite structure of aluminium alloy plate overlaying a plate of ultra-high strength steel. This structure is subjected to high fluctuating stresses in service, and since the ultra-high strength steel plate is the main load-bearing member it is the reaming of this material which is the main subject of the paper. EXPERIMENTAL Prior to reaming with the tungsten carbide taper pin reamers with 18-straight flutes (taper, ¼ in/ft) (20.813 ram/m), the holes are drilled and initially reamed with a tapered (taper, ¼ in/ft) (20.82, ram/m) left-hand spiral 6-flute high-speed steel (HSS) reamer (ANSI B94.21971 Size 7). All reaming was done dry and by hand with only a small axial force being applied to the reamers. The tungsten carbide reamers, which contained approximately 6.5 ~ cobalt were made from a C-1 grade carbide [1]. The hardness of the aluminium alloy and ultra-high strength steel plate were 180 and 480 HV, respectively. Profiles of transverse sections of the reamers were obtained from casts of T a y l o r - H o b s o n plastic replicating material. These were polished metallographically, finishing on a 0~2/~m * Australian Defence Scientific Service, Department of Supply, Defence Standards Laboratories, Melbourne, Australia. 6
69
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D.M. TURLEY
diamond pad and shadowed with aluminium prior to photographing. The surface finish of the reamed holes was measured with a Talysurfinstrument. RESULTS During reaming of holes it was found that some tungsten carbide reamers cut quite satisfactorily whereas others, even when new, did not. Measurement of rake angles, clearance angles, and width of the circular lands from the plastic casts of the reamers showed that the only difference between the two types was the width of the circular lands (Fig. 1). Moreover, projecting these plastic casts at a magnification of fifty times showed that all of the circular lands were equidistant from the centre of the reamer.
Fic,. I. Plastic casts of reamers showing differencesin circular land widths. (a) Reamer with narrow lands; cuts well; (b) Reamer with broad lands; will not cut satisfactorily. (50 × ) The widths of the circular lands on reamers that cut satisfactorily were all in the range 0.0020-0.0030 in. (0.0508-0.0762 mm) whereas the widths of the circular lands on reamers that would not cut satisfactorily ranged, for any particular reamer, from 0.0030-0-0080 in. (0.0762-0.2032 mm), many of them being 0.0060-0.0080 in. (0.1524-0.2032 mm). For convenience, reamers that cut satisfactorily and those that did not are denoted as S- and U-type, respectively.
1. Reamers with circular lands 0-0020-0"0030 in. (0.0508-0.0762 mm) wide (S-Type) The cutting edge of a new reamer is shown in Fig. 2. The transverse grinding marks on the circular land and the longitudinal grinding marks on the clearance face can be clearly seen. The cutting edge was not very sharp, because relatively large pieces had been broken from the edge during grinding of the reamer. During reaming of a taper hole, shown schematically in Fig. 3, cutting begins generally at the top of the aluminium alloy plate and progresses down through the hole during the course of the reaming operation. The crests of the surface undulations produced by the
Cutting Behaviour of Tungsten Carbide Taper Pin Reamers
71
Fxo. 2. Scanning electron micrograph of the cutting edge of a new reamer. (1750x) HSS reamer are cut away first until finally all of the original surface produced by the HSS reamer is removed. During the reaming of the aluminium alloy an appreciable built-up-edge (BUE) of the alloy is formed which adheres to both the circular lands and the cutting edges (Fig. 4). When the reamer begins cutting the steel plate the aluminium alloy BUE is removed from the reamer. Examination of the reamed steel surface showed that the top of the hole (first 0-010-0.020 in.) (0.254-0.508 mm) is often rougher than the surface produced further down due to the presence of large tears; the cause of the roughness appeared to be associated with the removal of the aluminium alloy BUE. Examination of the cutting edges of the reamer after reaming of the steel had been completed showed that steel was adhering in the cavities between tungsten carbide grains in the circular lands and that a BUE was present on the cutting ,edges (Fig. 5). This BUE was quite different in appearance from the aluminium alloy BUE (cf Figs. 5 and 4) and moreover, it appeared in many cases to be continuous along the cutting edge; the BUE was positively identified as steel. A longitudinal section through a partly formed chip also confirmed that a small BUE, in this case still attached to the chip, was present (Fig. 6). The steel swarf produced by this reamer was tightly curled and scroll-like in appearance (Fig. 7), the maximum width of the scrolls being approximately 0.070 in. (1.778 ram). Small tears could be clearly seen on the rake face side of the swarf at higher magnification, and examination of the reamed surface showed that small tears were also present (Fig. 8).
\ T\(2Por831/4m'rnn'/f; At-AlLoy plo,e
Ultra-high strength steel plate FIc. 3. Schematic sketch of tapered hole in composite structure. (Scale ~ 2-5: 1)
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FIG. 4. Scanning electron micrograph showing large aluminium alloy BUE adhering to the circular land and cutting edge. (300 x ) FIG. 5. Scanning electron micrograph showing steel adhering in the cavities on the circular land and a small steel BUE adhering to the cutting edge. (550×)
FIG. 6. Optical micrograph of a longitudinal section through a chip showing a BUE. Arrows indicate where chip and new surface separate from the BUE. (I 350× ) FIG. 7. Scanning electron micrograph of the scroll-like swarf produced by a new reamer (S-type). (12x) FiG. 8. Scanning electron micrograph of the chip (shown in Fig. 6) prior to sectioning. Many tears can be seen on the new surface. (I 30 :~ )
Cutting Behaviour of Tungsten Carbide Taper Pin Reamers
73
The presence of tears is usually associated with the presence of a BUE at the cutting edge of the tool during cutting, because it has been shown [2] that generation of the new surface and the chip occurs at two distinct fracture points and these fracture points produce tears. The fracture point where the new surface separates from the BUE is adjacent to the cutting edge of the tool, whereas the one where the chip separates from the BUE is some distance along the rake face of the tool (see Fig. 6). A scanning electron micrograph of the chip shown in Fig. 6 prior to it being sectioned clearly showed the association of tears on the machined surfitce with the presence of a BUE (see Fig. 8). The centre-line average roughness (cla) value of the steel surface was in the range 8-10/zm. It was usually found that S-type reamers continued to cut satisfactorily for a considerable period of time. Examination of these reamers after the aluminium alloy and the steel BUE had been removed revealed very little evidence of wear on either the cutting edges or the circular lands.
2. Reamers with circular lands 0.0030-0-0080 in. (0.0762-0.2032 mm) wide (U-type) When reaming the aluminium alloy an appreciable BUE of the alloy, similar to that previously observed on S-type reamers, is formed. This BUE is similarly removed when the reamers begin cutting the steel. These rearners, however, did not cut the steel satisfactorily. Since the only difference
FIG. 9. Scanning electron micrographs of a reamer (U-type) showing, (a) some swarf in a flute adjacent to a narrow land, and (b) very little swarf in a flute adjacent to a broad land. (14×) FIG. 10. Scanning electron micrograph of a surface produced by the reamer shown in Fig. 9. There is evidence of surface smearing as indicated by the arrows. (570 × )
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between these and S-type reamers was the widths of the circular lands, it seemed reasonable to conclude that the cutting edges with broad circular lands did not cut but rubbed the surface. By stopping the reaming and carefully removing the reamer, it was confirmed that the cutting edges with the broad circular lands removed very little material by cutting but mainly rubbed the surface. Examination in the scanning electron microscope showed that there was a considerable amount of swarf in the flutes adjacent to the narrow circular lands (Fig. 9(a)) whereas in the flutes adjacent to the broad circular lands there was very little (Fig. 9(b)). Examination of the cutting edges and circular lands at higher magnification showed that, unlike the S-type reamers, the steel BUE was not continuous along the cutting edges. There was also appreciable build up of steel on the circular lands where most of it had penetrated down between the tungsten carbide grains.
FiG. I 1. Optical micrograph of a longitudinal section through the surface shown in Fig. 10. Note the extensiveplastic deformation of the surface layers. Seam-likedefects are also present in the surface as indicated by the arrow. (700x ) The rubbing behaviour of the reamer was also substantiated by examining the reamed surface. There was evidence of surface smearing (Fig. 10) and there were few tear marks which are associated with normal cutting under BUE conditions (see Fig. 8), and hence the surface finish was generally very good (cla 2-3/~m). Examination under the optical microscope of a longitudinal section through this surface showed that the rubbing had caused extensive plastic deformation of the surface layers (Fig. 11); a white-etching layer was present in the surface, the nature of which is currently being investigated. Beneath this layer plastic deformation is evident by the distortion of the base structure parallel to the surface of the reamed hole. The total depth of deformation was estimated to be 0.0010in. (0.0254 mm). When cutting with an S-type reamer, however, examination under the optical microscope of a longitudinal section of a reamed hole showed no evidence of surface deformation (see Fig. 6). Seam-like defects were also present in the metal surface (see Fig. 11), and these are obviously associated with the surface smearing indicated in Fig. 10. These surface defects are not tears; they are elongated in the direction of reaming, whereas the 'point' of a tear is directed opposite to the reaming direction (see Fig. 8). Some of the smeared
Cutting Behaviour of Tungsten Carbide Taper Pin Reamers
75
regions shown in Fig. 10 are believed to be produced by the detachment of steel, which was originally adhering to the circular lands and became welded by friction to the reamed surface. Whilst examination of U-type reamers at higher magnification showed that the swarf produced by the cutting edges with narrow circular lands was 'chip-like', it was not similar and generall:r much finer than the scroll-like swarf produced by the S-type reamers that had circular lands of similar widths (0.0020-0.0030 in.) (0.0508-0.0762 mm) (compare Fig. 9(a) and Fig. 7). The swarf produced by the cutting edges which had broad circular lands was less 'chip-like' and much of it had a granulated appearance (see Fig. 9(b)), although some obvious parts of chips were identified. It is important to note, however, that whereas the cutting edges with broad circular lands would not cut the ultra-high strength steel, they were much more efficient in cutting the softer aluminium alloy. Examination of the circular lands of U-type reamers in the scanning electron microscope after the aluminium alloy and steel BUE had been removed revealed that appreciable wear had occurred on both the narrow and broad lands. Very little of the original surface of the circular lands remained. Moreover, examination of the reamed steel surface in the electron-probe microanalyser revealed the presence of some tungsten carbide grains embedded in the surface, particularly in the smeared regions shown in Fig. 10.
DISCUSSION The widtll of the circular land is an important variable in determining the cutting efficiency of taper reamers. The results suggest that in order for a reamer to begin cutting the circular lands must be able to indent the reamed surface so that a depth of cut can be taken. The radial force which acts on the circular land, and causes indentation, is largely developed by the reaction between the reamer and the reamed surface as the reamer moves down into the hole during the early stages. It can be shown that, for a new reamer with a circular land width of 0.0025 in. (0.0635 mm), the radial force required to plastically deform and indent the renamed steel surface is approximately 120 lbf (534 N) for a hardness of 480 HV and a chip width of 0.070 in. (1.778 mm) (measured from Fig. 7). Moreover, if during cutting it is assumed that only the BUE has contact with the reamed surface and the radial force is transmitted through this BUE, then from Fig. 5 it can be calculated that the required radial force is now approximately only 9 lbf (40 N~. It is apparent from Fig. 5, however, that the circ~'ular lands have some contact with the surface of the reamed hole as steel is present on the circular lands embedded between the tungsten carbide grains. The radial forces of 120 lbf (534 N) and 9 lbf (40 N) therefore represents the limiting values of the radial force and, in practice, the actual force would be expected :to lie between those two values. This does not necessarily mean that there is a high initial indentation force required to commence cutting and produce a chip 0-070 in. (1.778 mm) wide which then drops after a BUE is formed. The formation of the BUE along the cutting edges is in fact a continuous process, beginning during the initial stages of cutting when the crests of the surface undulations produced by the HSS reamer are removed. For a broad circular land of 0.0080 in. (0.2032 mm), the calculated radial force required to indent the', surface over a distance of 0.070 in. (1.778 mm) is greater than 380 lbf (1690 N). The radial force would be expected to be near this value, because in this case the BUE was less continuous and appreciable build up occurred on the circular lands mainly between the tungsten carbide grains. However, in order to develop a radial force of approximately
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D.M. TURLEY
380 lbf (1690 N) appreciable rubbing occurs between the reamed surface and the circular lands, and this causes plastic deformation and work hardening of the surface layers so that the required radial force to plastically deform and indent the surface is in fact much greater than 380 lbf (1690 N). Hence the broad circular lands do not produce scroll-like swarf similar to that shown in Fig. 7, because the forces required to do so are too high. The results clearly showed that the maximum permissible width of the circular land for cutting to occur depends upon the hardness of the workpiece material. Circular lands 0.0080 in. (0.2032 ram) wide cut the aluminium alloy whereas they would not cut the ultrahigh strength steel. Also the narrow circular lands of U-type reamers removed some material by cutting, but they did not produce scroll-like swarf similar to S-type reamers which had all narrow circular lands. The swarf was much narrower, indicating that a greater radial force per unit area was required to indent the reamed surface. This increase in force per unit area can be attributed to the significant increase in hardness of the surface layers caused by plastic deformation due to rubbing by the broad lands. In fact when an S-type reamer that originally cut very well was used to ream a hole that had previously been reamed with a U-type reamer, the S-type reamer would now not cut satisfactorily and produce scroll-like swarf as in Fig. 7. For S-type reamers there was very little evidence of wear on the circular lands and the cutting edges. It is most likely that for these reamers the circular lands do not have continual contact with the reamed surface. This indicates that under BUE conditions separation of the new surface from the BUE can occur below the level of the circular land. The presence of tear marks and the absence of any evidence of severe rubbing on the reamed surface supports this conclusion. For U-type reamers, however, both the narrow and broad circular lands showed appreciable wear. The wear on the broad lands is caused by the near continual rubbing between the land and the reamed surface. The wear on the narrow circular lands indicates that cutting by these lands is intermittent and appreciable rubbing also occurs. Moreover, the wear processes on both types of land are enhanced by the increased hardness of the surface layers caused by plastic deformation during rubbing, and the high radial and shearing forces generated between the lands and the reamed surface. A more detailed study is being made on the wear of these reamers. Acknowledgements--The author would like to thank Mr L. M. Bland of Aeronautical Research Laboratories
for supplying the reamers and materials and for helpful discussions. REFERENCES [1] Metals Handbook, 8th Ed. p 316, Vol. 3. ASM, Ohio (1967). [2] J. J. W~LL~AMSand E. C. ROLLASON,J. Inst. Metals 98, 144 (1970).