Machinability Report

Transcription

INDUSTRIAL
RESEARCH+DEVELOPMENT
INSTITUTE
i
649 PROSPECT BLVD., P.O. BOX 518 518
MIDLAND, ONTARIO, CANADA L4R 4L3 4L3
TEL:
FAX:
MACHINABILITY TESTING
OF MOLD STEELS
SF-2000 @ 321Bhn
SF-2000 @ 350Bhn
versus
DIN 1.2738 @ 311Bhn
Presented to:
Hoang LeHuy
Sorel Forge
By
Victor SONGMENE
Sasi Ratnasabapathy
June 30, 1999
(705) 526-2163
(705) 526-2701
Ref. # PRO 310 229
Machinability Testing of New Mold Steels
TABLE OF CONTENTS
Table of content ........................................................................................................................................ ii
List of Figures and Tables........................................................................................................................ iii
Summary ………………………………………………………..…………………………………………1
1.
Introduction...........................................................................................................................2
2.
Objective ................................................................................................................................2
3.
Background of machinability of mold steel .........................................................................2
4.
Testing Procedure .................................................................................................................4
4.1
Materials ........................................................................................................................................4
4.2
Equipment .....................................................................................................................................4
4.3
Tool life testing..............................................................................................................................5
4.4
Cutting force testing .....................................................................................................................5
4.5
Surface finish testing ....................................................................................................................6
5.
Results & Discussions...........................................................................................................7
5.1
Tool Life.........................................................................................................................................7
5.2
Cutting forces tests .......................................................................................................................7
5.3
Surface finish tests ........................................................................................................................7
5.4
Global machinability rating.........................................................................................................8
5.5
Wear pattern and Tool Life .........................................................................................................9
5.6
Taylor Model ...............................................................................................................................11
6.
Concluding Remarks ..........................................................................................................12
7.
References ...........................................................................................................................12
Appendixes start pages 13-16
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Ref. # PRO 310 229
List of figures
Figure 1: Partial Machinability indexes ...........................................................................................1
Figure 2 : Global Machinability rating .……………………......………….…..……..………….1
Figure 3: Wear progression of inserts of two series of test ...……………………..……………..10
Figure 4: Taylor Model on Cutting Speed-Tool Life relationship ..…………………………….11
LIST OF TABLES
Table 1: Effect of alloying elements on machinability
…………………………………………….3
Table 2: Chemical composition of steels, conditions & hardness
……………………...…………5
Table 3: Machinability rating data ……………………………………………………………….8
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# PRO 310 229
Summary.
Milling and drilling tests were performed to compare new SF-2000 mold steel manufactured
by Sorel Forge to that of standard DIN 1.2738. Partial machinability ratings of the three
tested steels are computed based on cutting speed of 30 minutes tool life, cutting force and
surface finish of machined parts (Fig.1). In this graph the partial machinability represents the
ratio of performance index (cutting speed for 30 minutes, force, and surface finish) of the
selected material to that of the standard.
150
136
Speed
Force
Ra
Partial machinability indexes (%)
125
117
107
100
100
100
97
100
88
74
75
50
25
0
SF-2000-321Bhn
SF-2000-350 Bhn
DIN 1.2738- 311Bhn
Materials
FIG. 1: Partial Machinability indexes
From Global machinability rating (Fig. 2), it can be noted that:
125
100
Machinability rating ( %)
100
82
78
75
50
25
0
SF-2000-321Bhn
SF-2000-350 Bhn
DIN 1.2738- 311Bhn
Materials
FIG. 2: Global Machinability Rating
The
SF-2000 even at high hard condition is easier to machine than the DIN 1.2738. The
standard SF-2000 can be machined 20% quicker than the DIN 1.2738. This was confirmed
with validation tests.
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1. INTRODUCTION
Increasingly, manufacturing companies are looking for short lead times and greater machining
efficiency. Along with the introduction of new work piece material and cutting tool, it has
become necessary to standardize the tool, to characterize work piece machinability and to search
for appropriate machining parameters ranges for each work piece/tool/operation. Each
operation/work piece/tool system has unique requirement to be tested to achieve maximum
productivity, and low unit cost.
Traditional machinability comparisons look at either tool life or power alone. A new approach
developed in this project includes surface finish addressing many machinists’ concerns,
especially for mold makers. A generalized procedure is designed to obtain global machinability
rating for a given material. The procedure includes three tests to generate individual indices for
tool life, cutting force and surface finish.
2. OBJECTIVE
To generate global and partial machinability indices that designates the degree of difficulty (or
ease) with which materials can be machined. The machinability rating will enable the
manufacturing engineer quickly to evaluate the manufacturing time and cost of the tested
material for any type of jobs.
The influence of the different machinability criteria is determined and recorded. The test assesses
the global machinability rating of a work piece material compared to a reference material. The
parameters used for machinability assessment are the tool life, the cutting forces and the surface
finish.
3. BACKGROUND OF MACHINABILITY OF MOLD STEEL
Machinability is a general term used to rate the ease (or difficulty) of machining. It is a work
piece property depending on thermo-mechanical, structure, and compatibility with tool material.
Caren [1] was noticing that P-20 mold steel is widely used for making plastic injection molds in
North America. It offers a good combination of hardness, machinability, and toughness, but its
June 30, 1999
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# PRO 310 229
properties are not always consistent. Frequently, the hardness is not uniform throughout a block,
which leads to machining problems.
The alloying elements in steels have a profound effect on its properties and machinability. The
following are the effects of various elements in steels in regards to machinability [2]:
•
•
•
•
•
•
•
•
•
Carbon is the principal strengthening element in steel. It can have a great effect on
numerous metallurgical properties. Its effect on machinability depends on the presence of
other alloying elements.
Manganese increases strength and toughness and improves the machinability. Higher
levels have a negative effect on weldability
Sulfur improves strength when combined to manganese, lowers impact strength and
ductility; impairs surface quality. The chips produced are small and break up easily. The
shape, orientation, distribution and concentration of manganese sulfides inclusions
(second-phase particles) formed significantly influence the machinability. Considered an
impurity, except when intentionally added to improve machinability.
Lead and Phosphor are considered as impurities, except when intentionally added to
improve the machinability
Silicon is added to steel to tie up free oxygen. It decreases the machinability while
strength, hardness and corrosion resistance
Nickel improves strength and toughness when combined with other alloying elements.
Molybdenum has a strong effect on hardenability (similar to manganese).
Copper adversely affects hot working characteristics and surface quality and reduces little
the ductility.
The presence of aluminum and silicon in steels is always harmful on machinability
because they combine with oxygen and form aluminum oxides and silicates. These
compounds are hard and abrasive but they makes the material more brittle.
The Table 1 summarizes the effect of different alloying elements on machinability.
Table 1: Effect of alloying elements on machinability [3]
Positive
Negative
June 30, 1999
Pb
Mn
Ni
Si
S
Al
Cu
P
Cr
V
Mo
C 0.3-0.6%
C>0.6% C<0.3%
3
# PRO 310 229
4. TESTING PROCEDURE
The procedure used in this project for machinability testing is the one established in the project
Tool life & Machinability Testing [4] and the results are computed using the IRDI’s TL&M
software. The test consists of milling parts and the surface finish generated. In addition cutting
forces are recorded during drilling operations for comparison purposes.
4.1
Materials
The chemical composition and properties of the mold steels materials submitted for testing are
summarized in Table 2.
Table 2: Chemical composition of steels, conditions & hardness
Materials
SF-2000
Chemical composition
Part
Hardness
number
(Bhn)
C
Mn
10807-3
321
.33
.68 .008 .65
.15 1.61 .35 .011 .11 .033
10547-1
350
.34
.82 .005 .40
.26 1.89 .48 .011 .12 .017
11258-1
311
.39 1.37 .007 .29 1.05 1.89 .18 .008 .07 .011
S
Si
Ni
Cr
Mo
V
Cu
Al
“Standard
”
SF-2000
“Hi-Hard”
DIN 1.2738
4.2
•
•
•
Equipment
Machine-Tool: FADAL VMC 6030 CNC milling machine, 22 hp, 10 000 rpm
Profilometer: Mitutoyo SURFPAK
Table dynamometer: KISTLER 3 components dynamometer 9255B
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4.3
Tool life testing
Side milling tests were used to set the partial machinability based on tool life. The cutting speed
tested ranges from 61 m/min to 122 m/min. All the tests were run at the following cutting
parameters:
Tool diameter: 38.1 mm
Coolant: Blasocut Universal
Inserts: TiN-coated carbides
(63% mineral oil; 4% water,
Feed per tooth: 0.1016mm
Additives: chlorinated paraffin;
Radial depth of cut: 12.7mm
Viscosity @40º = 39mm²/sec)
Axial depth of cut: 2.54mm
Once the tool life at different speed recorded, the Taylor exponent “n” and the constant “C” are
computed to obtain the tool-life cutting speed relationships described in Equation 1.
VxTn = C
(1)
From the Taylor model, the cutting speed for 30 min. tool life is predicted and used for
comparison.
4.4
Cutting force testing
Drilling operations were chose to evaluate the penetration force on each of the tested material.
The test consisted of making holes with uncoated High Speed Steel (HSS) drill and recording the
thrust forces. The following drilling parameters were used:
Drill diameter: 9.92mm
Spindle speed: 225 rpm
Feed rate: 57.15 mm/min
Hole depth: 12.7mm
Coolant: Blasocut Universal (63% mineral oil; 4% water,
Additives: chlorinated paraffin,
Viscosity @ 40º = 39mm²/sec)
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4.5
Surface finish testing
The surface finish tests consisted of a face-milling operation at constant cutting parameters on
each of the material tested. Then the surface texture is recorded and analyzed using a
profilometer. The cutting parameters used for racing the parts are the followings:
Cutting tool diameter: 38.1 mm
Inserts: uncoated carbides
Cutting speed: 140.5 m/min
Feed per tooth: 0.1016 mm
Radial depth of cut: 38.1 mm
Axial depth of cut: 0.508 mm
Coolant: Blasocut Universal (63 % mineral oil; 4% water)
There are many parameters defining the surface texture but the more used parameter is the
arithmetical mean deviation Ra. We retained Ra for Machinability rating calculations but
others parameters were also recorded and can be found in Appendix C. They are:
Ra: arithmetical mean deviation of the
profile
Rz: Ten point height of irregularities
Rt: Total height of the profile
Dq: Root-mean square of the profile
Sk: skewness of the profile
Rk: Core roughness depth
June 30, 1999
Rpk: Reduced peak height
Rvk: Reduced valley depth
Mrl: Material ratio 1 (Upper limit of
bearing length ratio)
Mr2: Material ratio 2 (Lower limit of
bearing length ratio)
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5. RESULTS & DISCUSSIONS
5.1
Tool Life
The tool life was set using the standard [4,5] value of flank wear (VB = 0.3mm). Two failure
modes were recorded: catastrophic failure due to depth of cut notch wear and flank wear. The
first wear mechanism (notch or flank wear) that reached the maximum limit admissible ended the
life of the tool. The Table 3 summarized the tool life obtained at different cutting speeds, the
Taylor’s model exponents and the machinability indexes.
A first set of tests with uncoated carbide showed that the standard SF-2000 (321 Bhn) is easier to
machine than the Hi-Hard SF-2000 (350 Bhn). However, the tool life was so short for the 350
Bhn material that we changed for TiN-coated carbide inserts for others tests.
The SF-2000 (321 BHN) was then choose as standard reference material for comparing the other
molds steels while using TiN-coated carbides tools. The standard SF-2000 material has the
highest speed (141.4 m/min) for 30 minutes tool life, followed by the Hi-Hard SF-2000 (104.6
m/min). The Din 1.2738 has the lowest cutting speed (91.8 m/min) for the same tool life (Table
3).
5.2
Cutting forces tests
An example of drilling tests is showed in Appendix 1 where the thrust force is recorded in
function of the time. From this data, we determine the mean cutting forces in drilling operations.
The Hi-Hard SF-2000 material and the DIN 1.2738 mold steels required respectively 3423 and
3737 Newtons of forces during drilling tests while the standard SF-2000 required only 3205
Newtons (Appendix 2). The same tendency were observed for drilling torque. Less energy
(power) is required to cut the standard steel.
The chip thickness ratio (feed /deformed chip thickness) confirmed that the softer material
deforms better than the harder.
5.3
Surface finish tests
After milling with the same parameters, the same arithmetic roughness 60 microns was recorded
on standard SF-2000 (321 BHN) and on DIN 1.2738 materials while the High Hard SF-2000 was
30 microns higher (Table 3).
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Table 3: Machinability rating data
Materials
Parameters
SF-2000
321 BHN
Tool Life (min)
Cutting Speeds
61 m/min
91.4 m/min
122 m/min
Failure mode
146
42
Flank wear
SF-2000
350 BHN
1.2738
311 BHN
52
45
21.75
Notch wear
85
21
18
Notch wear
Taylor’s Model of Tool Life Exponents & Constants
Exponent “n”
0.528
0.827
0.424
Constant “C”
853
1744
390
Cutting speed for 30 minutes tool life 141.4 m/min 104.6 m/min 91.8 m/min
Other Machinability indexes
Thrust forces (N)
3205
Torque (N-m)
9.7
Chip thickness ratio
.29
Surface finish Ra (µm)
67
5.4
3423
10
.34
91
3736.6
12.6
.35
60
Partial and Global Machinability rating
Speed
100%
74%
Thrust forces
100%
107%
Surface finish Ra
100%
136%
Global
100%
82%
97%
117%
88%
78%
Global machinability rating
The global machinability rating given in Figures 1 and 2 takes into account the metal removal
rate through the cutting speed, the cutting force and surface finish ratios “RCv”, “RCF”, and
“RCRa” respectively.
MRR = 50 * RCV +
June 30, 1999
10
40
+
RCF
RCRa
(2)
8
# PRO 310 229
RCv =
Vx
Vo
The “constant tool life cutting speed ratio”. (The higher this
coefficient the easier to machine the tested material).
RCF =
Fx
Fo
The “specific cutting force ratio”. (The higher this coefficient the
lower the machinability of the tested material)
RCRa =
Rax
Rao
The “surface roughness ratio”. (The higher this coefficient the
lower the machinability of the tested material)
Based on the tool life, cutting forces and finish results, it comes that the SF-2000 even at high
hard condition is easier to machine than the DIN 1.2738. The standard SF-2000 can be
machined 20% quicker than the DIN 1.2738. The results show:
•
As expected the lower hardness of SF-2000 (321Bhn) machines better than the harder SF2000 (350 Bhn).
•
The machinability of DIN 1.2738 (311 Bhn) is practically equal to that of Hi-hard SF-2000,
although the later is harder (350 Bhn).
•
The cutting speed used to machine the standard SF-2000 is equivalent to those used to
machine the DIN 1.2738. On the other hand, the standard SF-2000 (321 Bhn) can be
machined 26% lower than Hi-Hard SF-2000 (350 Bhn).
•
The specific cutting forces required to cut the standard SF-2000 samples is lower than that
required for Hi-hard SF-2000 and DIN 1.2738. The Hi- hard SF-2000 required 7 % higher
force while the DIN 1.2738 was 17% higher.
•
The finishes, during the milling operations, were slightly different among the materials. The
DIN 1.2738 shows better machinability (12% better), while higher hardness SF-2000 (350
BHN) shows lowest machinability (36% lower.) compared to the standard material SF-2000.
5.5
Wear pattern and Tool Life
The milling inserts experienced flank and depth of cut notch wear (Fig. 3). Most of cutting tools
experienced progressive flank wear up to 0.2 mm. After this level, notch wear took place and
progressed faster than flank wear.
June 30, 1999
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# PRO 310 229
The depth of cut notch wears lead to catastrophic failure while the flank wears usually progressed
up to the wear limit. The notch wear can be seen in the picture of an insert shown in Fig. 3 for
Hi-hard SF-2000 (350 Bhn). This wear mechanism is very dependent on the material
homogeneity and uniformity of hardness. The failure modes obtained for each steel are
summarized in Table 3. The Hi-hard SF-2000 and DIN 1.2738 failed by notch wear, while the
standard SF-2000 steel failed by progressive flank wear mode.
The tool life was set using the standard value of flank wear (VB = 0.3 mm). The first wear
mechanism (notch or flank wear) that reached its maximum limit admissible ended the life of the
tool. More information on wear progression at different cutting speeds can be found in Appendix
3.
Inserts1 @
61 m/min
Inserts2 @
61 m/min
@ 66.3 Minutes
Inserts1 @
Insert2 @
91.4 m/min
91.4 m/min
Standard SF-2000 - 321 Bhn
Inserts1 @
122 m/min
@ 41 Minutes
Insert 2 @
122 m/min
@ 15 Minutes
Hi-Hard SF-2000 - 350 Bhn
@ 60 Minutes
@ 39 Minutes
@ 23 Minutes
Figure 3: Wear progression of inserts of two series of test
The figure 3 depicts the inserts at different times and for speeds of 61, 91.4 and 122 m/min.
•
•
The inserts used in High Hard SF-2000 steel wear faster and differently than the inserts used
in standard SF-2000 samples.
Hi-Hard SF-2000 inserts reached notch wear limits at 60, 39,and 23 minutes at speeds of 61,
91.4 and 122 m/min respectively. This gives short tool life.
June 30, 1999
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# PRO 310 229
5.6
Taylor Model
The figure 4 displays the tool life cutting speed relationships established based on Taylor model.
150
DIN 1.2738 311Bhn:VT^0.424=390
140
130
SF-2000- 321Bhn: VT^ 0.537=853
Cutting time or Tool life (min)
120
SF-2000 321 Bhn
110
SF-2000-350 Bhn: VT^0.827=1040.6
100
90
80
DIN 1.2738 311Bhn
70
60
50
40
30
SF-2000-350 Bhn
20
10
0
60
65
70
75
80
85
90
95
100
Cutting Speed (m/min)
105
110
115
120
125
Fig.4: Taylor Model on Cutting Speed-Tool Life relationship.
From this graph the following conclusions can be made:
•
•
•
In the studied range of cutting speeds 60-120 m/min, the tool life of standard SF-2000
samples is more than twice that of Hi-Hard SF-2000.
At higher cutting speeds (above 90 m/min), the tool life obtained for Hi-Hard SF-2000 (350
Bhn) is better than DIN 1.2738 (311 Bhn).
DIN 1.2738 (311 Bhn) steels performs better at speed below 90 m/min.
.
The tool life is dependent on the maximum wear limit allowed as shown in Appendix 4. As the
wear limit decreases, the tool life decreases
June 30, 1999
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6. CONCLUDING REMARKS
•
•
•
•
•
•
Based on the tool life, cutting forces and surface finish results, standard and Hi-Hard SF-2000 are
easier to machine than the DIN 1.2738 (311 Bhn)
There is a difference of 22 % machinability rating between standard SF-2000 and DIN 1.2738.
The machinability of DIN 1.2738 (311 Bhn) is practically equal to that of Hi-hard SF-2000,
although the later is harder (350 Bhn).
The cutting speed used to machine the Hi-Hard SF-2000 (350 Bhn) can be 25 % lower than standard
SF-2000 (321 Bhn) or Din 1.2738 (311 Bhn).
The texture obtained after machining standard SF-2000 (321 Bhn) and DIN 1.2738 (311 Bhn) is
better compare to Hi Hard SF-2000 (350 BHN).
For Hi-Hard SF-2000, we recommend to use cutting speeds between 60 and 90 m/min during end
milling of with coated carbide. At higher cutting speed, the tool life is shorter.
7. REFERENCES
1. Caren, S., “Prehardened mold steels offer machinability and weldability”, reprint from (PM&E Plastics Machinery & Equipment, October 1993, pp.1-4.
2. Improved Tool Steels for Injection molds, Advanced Materials & Processes, June 1992.
3. Sandvik Coromant, Modern Metal Cutting: A practical Handbook, AB Sandvik Coromant,
Sweden, 199.
4. International Standard ISO 8688-1, Tool life testing in milling - part1: face milling, first edition,
1989.
5. International Standard ISO 8688-2, Tool life testing in milling - part2: End milling, first edition,
1989
6. Songméné V., Stefan, I., Stefan, M., Yan, D., Hirholzer, J, Tool Life & Machinability Testing -Phase
1- Testing Procedure &database, Report of project, IRDI, November 1996.
7. Songméné V., Machinability Testing of Mold steels, Report of project, IRDI, 1996
June 30, 1999
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Ref. # PRO 310 229
Machinability Testing of New Mold steels
Appendix 1
Cutting Force obtained in drilling
4000
3500
3000
SF-2000 (350 BHN)
mean = 3423 N
Force (N)
2500
2000
1500
1000
500
0
0
5
10
15
20
time (sec.)
June 30, 1999
13
Ref. # PRO 310 229
Appendix 2
Average Cutting Force
3800
3736.6
3700
3600
3500
Force (N)
3423
3400
3300
3203
3200
3100
3000
2900
SF-2000-321Bhn
SF-2000-350Bhn
DIN 1.2738-311Bhn
Material
June 30, 1999
14
Ref. # PRO 310 229
Appendix 3:
Tool wear of High Hard SF-2000 (350 Bhn)
0.8
61 m/min
0.7
91 m/min
0.6
Flank wear (mm)
122 m/min
0.5
0.4
0.3
0.2
0.1
0
0
10
20
30
40
50
60
70
80
90
Cutting Time (min)
June 30, 1999
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Ref. # PRO 310 229
Appendix 4
Tool life as function of Cutting Speed
160
61 m/min
91.4 m/min
122 m/min
146
140
Tool Life (min)
120
100
85
80
60
52
42
45
40
22
20
25
18
0
SF-2000-321Bhn
SF-2000-350Bhn
DIN 1.2738-311Bhn
Material
June 30, 1999
16

Machinability Report

Transcription

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