Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
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AS SINTERED COINING PROCESS
FIELD OF INVENIION
This invention relates to a process of coining sintered articles to final shape and in
particular relates to a process of precision coining sintered articles of powder metal
having a composition of between 0.3% to 2.0~ m~ng~nese, 0.2 to 0.85% carbon withthe rem~inder being iron and unavoidable impurities where the sintered articles are
coined to final shape so as to narrow the tolerance variability of the coined articles.
Background to the Invention
Powder metal technology is well known to the persons skilled in the art and generally
comprises the formation of metal powders which are compacted and then subjected
to an elevated temperature so as to produce a sintered product.
Conventional sintering occurs at a m~xi~ temperature of approximately up to
1,150 ~ C. Historically the upper temperature has been limited to this temperature by
sintering equipment availability. Therefore copper and nickel have traditionally been
used as alloying additions when sintering has been conducted at conventional
temperatures of up to 1,150~ C, as their oxides are easily reduced at these
temperatures in a generated atmosphere, of relatively high dew point cont~ining CO,
CO2 and H2. The use of copper and nickel as an alloying material is expensive.
Moreover, copper when utilized in combination with carbon as an alloying material
and sintered at high temperatures causes dimensional instability and accordingly the
use of same in a high temperature sintering process results in a more difficult process
to control the dimensional characteristics of the desired product.
M~nllf~ctllrers of metal powders utilized in powder metal technology produce
prealloyed iron powders which are generally more difficult to compact into complex
shapes, particularly at higher densities (> 7.0 g/cc). Manganese and chromium can be
incorporated into prealloyed powders provided special m~nl~f~cturing precautions are
taken to ~ e the oxygen content, for example, by oil atornization.
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Notwithst:~n~ling this, these powders still have poor compressabilities compared to
admixed powders.
Conventional means to increase the strength of powder metal articles use up to 8~c
nickel, 4% copper and 1.5~o molybdenum, in prealloyed, partially prealloyed, or
admixed powders. Furthermore double press double sintering can be used for high
performance parts as a means of increasing part density. Conventional elements are
expensive and relatively ineffective for generating mechanical properties equivalent to
wrought steel products, which comrnonly use the more effective strengthening alloying
elements m~ng~nese and chro~
Moreover, conventional technology as disclosed in United States Patent No. 2,402,120
teach pulverizing material such as mill scale to a very fine sized powder, and thereafter
reducing the mill scale powder to iron powder without melting it.
Furtherrnore, United States Patent No. 2,289,569 relates generally to powder
metallurgy and more particularly to a low melting point alloy powder and to the usage
of the low melting point alloy powders in the formation of sintered articles.
Yet another process is disclosed in United States Patent No. 2,027,763 which relates
to a process of m~kinE sintered hard metal and consists essentially of steps cormected
with the process in the production of hard metal. In particular, United States Patent
No. 2,027,763 relates to a process of m~king sintered hard metal which comprisesproducing a spray of dry, finely powdered mixture of fusible metals and a readily
fusible ~lxili~ry metal under high pressure producing a spray of adhesive agent
customary for binding hard metals under high stress, and so directing the sprays that
the spray of metallic powder and the spray of adhesive liquid will meet on their way
tn the molds! or within the latter, whereby the mold will become filled with a compact
moist mass of metallic powder and finally completing the hard metallic particle thus
formed by sintering.
United States Patent No.4,707,332 teaches a process for m~n~lf~cturing structural parts
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from intermetallic phases capable of sintering by means of special additives which
serve at the same time as sintering assists and increase the ductility of the finished
structural product.
Moreover, United States Patent No. 4,464,206 relates to a wrought powder metal
process for pre-alloyed powder. In particular, United States Patent No. 4,464,206
teaches a process comprising the steps of commllnim~ting substantially non-compactible
pre-alloyed metal powders so as to flatten the particles thereof heating the
commllninllted particles of metal powder at an elevated temperature, with the particles
adhering and forming a mass during heating, crushing the mass of metal powder,
compacting the crushed mass of metal powder, sintering the metal powder and hot
working the metal powder into a wrought product.
Finally, Goining is a process well known to those persons skilled in the art. However,
a comprehensive method of precision coining of powder metal blanks is lacking. For
example, United States Patent No. 2,757,446 teaches a method of forming articles from
metal powders which includes hot forging the article to a minimum density of 95 % of
the theoretical density wherein the entire change of shape of the article takes places in
one direction of movement and wherein the minimum internal flow of the particleswithin the article is at least 5 % and finally finishing the forged article.
The processes as described in the prior art above present a relatively less cost effective
process to achieve the desired mechanical properties of the sintered product.
An aspect of the invention relates to a coining process for blending, sintering, and
coining powder metal articles, that coining process comprising: blending carbon, ferro
manganese and lubricant with compressible iron powder to form a blended mixture;pressing said blended mixture to form said articles; sintering said articles in a reducing
atmosphere at a temperature of at least 1250C; and coining said sintered articles to a
final shape.
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It is another aspect of this invention to provide a process of precision coining a sintered
article of powder metal comprising: selecting iron powder; determining the desired
properties of said sintered article and selecting; a quantity of carbon; and a quantity of
ferro manganese to produce an article having a composition of between 0.3% to 2.0%
manganese, 0.2% to 0.85% carbon with the remainder being iron and unavoidable
impurities; grinding separately said ferro manganese to a mean particle size of
approximately 8 to 12 microns and substantially all of said ferro manganese having a
particle size of less than 25 microns; introducing a lubricant while blending said
carbon, and ferro manganese with said iron powder; pressing said mixture to form said
article; sintering said article at a temperature between 1,250~C and 1,350~C in a
vacuum or reducing atmosphere of 90% blended nitrogen and 10% hydrogen to
produce said sintered article of powdered metal; coining said sintered article to a final
shape to narrow the dimensional tolerance variability of coined articles and
substantially elimin~te secondary operations.
It is a further aspect of the invention to provide a process of gas quenching coined
articles of powder metal, that process comprising: blending carbon, ferro m~ng~nese,
ferro molybdenum, ferro chromium and lubricant with compressible iron powder;
pressing said blended mixture to form said articles; sintering said articles at a
temperature in the range of 1250 C to 1350 C in a reducing atmosphere; coining said
articles to a final form; and gas quenching said articles.
It is another aspect of the invention to provide an as-sintered ferrous metal product
comprising a compacted and sintered mass composed of a blend of iron powder, carbon
and ferro manganese alloy having a mean particle size of approximately 8 to 12
microns, subjected to a high temperature sinter so as to result in an as-sintered mass
having between 0. 3 to 2. 5 % manganese and between 0.2 to 0. 85 % carbon composition
where said product is machined or coined to final dimensional requirements.
It is another aspect of the invention to provide an as-sintered ferrous metal product
comprising a compacted and sintered mass composed of a blend of iron powder, carbon
and ferro manganese alloy having a mean particle size of approximately 8 to 12
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microns, subjected to a high temperature sinter so as to result in an as-sintered mass
having between 0. 3 to 2 . 5 % manganese and between 0.2 to 0. 85 % carbon composition
where said product is machined or coined to final dimensional requirements.
It is another aspect of the invention to provide an as-sintered ferrous metal product
comprising a compacted and sintered mass composed of a blend of iron powder, carbon
and ferro manganese alloy having a mean particle size of approximately 8 to 12
microns, subjected to a high temperature sinter so as to result in an as-sintered mass
having between 0.5 to 2.0% manganese and between 0.2 to 0. 85 % carbon composition
where said product is machined or coined to final dimensional requirements.
It is another aspect of the invention to provide an as-sintered ferrous metal product
comprising a compacted and sintered mass composed of a blend of iron powder, carbon
and ferro manganese alloy having a mean particle size of approximately 8 to 12
microns, subjected to a high temperature sinter so as to result in an as-sintered mass
having 1.5% manganese and .8% carbon composition where said product is machined
or coined to final dimensional requirements.
Description of Drawings
These and other features and objections of the invention will now be described in
relation to the following drawings:
Figure 1 is a drawing of the prior art mixture of iron alloy.
Figure 2 is a drawing of a mixture of elemental iron, and ferro alloy in accordance
with the invention described herein.
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Figure 3 is a graph showing the distribution of particle size in accordance with the
invention herein.
Figure 4 is representative drawing of a jet mill utilized to produce the particle size of
the ferro alloy.
Figure S is a stress strain graph.
Figure 6 illustrates a coin~d part such as a clutch backing plate made in accordance
with the invention.
Figure 7 is a dimensional stability graph.
Figure 8 graphically illustrates the narrow variability tolerance of the coined parts.
DESCRIP IION OF THE INVENTION
Sintered Powder Metal Method
Figure 1 is a representative view of a rni~cture of powder metal utilized in the prior art
which consists of particles of ferro alloy in powder metal technology.
In particular, copper and nickel may be used as the alloying materials, particularly if
the powder metal is subjected to conventional temperature of up to 1150 ~ C during the
sintering process.
Moreover, other alloying materials such as manganese, chromium, and molybdenum
which were alloyed with iron could be added by means of a master alloy although such
elements were tied together in the prior art. For example a comrnon master alloyconsists of 22% of m~n~nese, 22~o of chromium and 225~o of molybdenum, with the
balance consisting of iron and carbon. The utilization of the elements in a tied form
made it difficult to tailor the mechanical properties of the final sintered product for
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specific applications. Also the cost of the master alloy is very high and uneconomic.
By lltili7.ing ferro alloys which consist of ferro m~nganese, or ferro chromium or ferro
molybdenum or ferro vanadium, separately from one another rather than l]tili7in~ a
ferro alloy which consists of a combination of iron, with manganese, chromium,
molybdenum or vanadium tied together a more accurate control on the desired
properties of the finished product may be accomplished so as to produce a methodhaving more flexibility than accomplished by the prior art as well as being more cost
effective.
Figure 2 is a representative drawing of the invention to be described herein, which
consists of iron particles, Fe having a mixture of ferro alloys 2.
The ferro alloy 2 can be selected from the following groups:
Name Symbol Approx. % of Alloy
Element
ferro manganese FeMn 78%
ferro chromium FeCr 65%
ferro molybdenum FeMo 71%
ferro vanadium FeVa 75%
ferro silicon FeSi 75%
ferro boron FeB 17.5%
The ferro alloys available in the market place may also contain carbon as well as
unavoidable impurities which is well known to those people skilled in the art.
Chromium molybdenum and v~n~(lillm are added to increase the strength of the
finished product particularly when the product is subjected to heat treatment after
sintering. Moreover, manganese is added to increase the strength of the finished
CA 02104607 1998-11-26
product, particularly if one is not heat treating the product after the sintering stage.
The reason for this is manganese is a powerful ferrite strengthener (up to 4 times more
effective than nickel).
Particularly good results are achieved in the method described herein by grinding the
ferro alloys so as to have a Dso or mean particle size of 8 to 12 microns and a Dloo of
up to 25 microns where subst~nti~lly all particles of the ferro alloys are less than 25
microns as shown in Figure 3. For certain application a finer distribution may be
desirable. For example a D50 of 4 to 8 microns and a Dloo of 15 microns.
Many of the processes used in the prior art have previously used a D50 of 15 microns
as illustrated by the dotted lines of Figure 3. It has been found that by finely grinding
the of the ferro alloy to a fine particle size in an inert atmosphere as described herein
a better balance of mechanical properties may be achieved having improved sintered
pore morphology. In other words the porosity is smaller and more rounded and more
evenly distributed throughout the mass which enhances strength characteristics of the
finished product. In particular, powder metal products are produced which are much
tougher than have been achieved heretofore.
The ferro alloy powders may be ground by a variety of means so long as the mean
particle size is between 8 and 12 microns. For example, the ferro alloy powders may
be ground in a ball mill, or an attritor, provided precautions are taken to prevent
oxidation of the ground particles and to control the grinding to obtain the desired
particle size distribution.
Particularly good results in controlling the particle size as described herein are
achieved by utili~ing the jet mill illustrated in Figure 4. In particular, an inert gas such
as cyclohexane, nitrogen or argon is introduced into the grinding chamber via nozzles
4 which fluidize and impart high energy to the particles of ferro alloys 6 upward and
causes the ferro alloy particles to break up against each other. As the ferro alloy
particles grind up against each other and reduce in size they are lifted higher up the
chamber by the gas flow and into a classifier wheel 10 which is set at a particular
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RPM. The particles of ferro alloy enter the classifier wheel 10 where the ferro alloy
particles which are too big are returned into the chamber 8 for further grinding while
particles which are small enough namely those particles of ferro alloy having a particle
size of less than 25 microns pass through the wheel 10 and collect in the collecting
zone 12. The grinding of the ferro alloy material is conducted in an inert gas
atmosphere as described above in order to prevent oxidization of the ferro alloymaterial. Accordingly, the grinding mill shown in Figure 4 is a totally enclosedsystem. The jet mill which is utilized accurately controls the size of the particles
which are ground and produces a distribution of ground particles which are narrowly
centralized as shown in Figure 3. The classifier wheel speed is set to obtain a D50 of
8 to 10 microns. The speed will vary with different ferro alloys being ground.
The mechanical properties of a produced powder metal product may be accurately
controlled by: .
(a) selecting elemental iron powder;
(b) determining the desired properties of the sintered article and selecting:
(i) a quantity of carbon; and
(ii) the ferro alloy(s) from the group of ferro manganese, ferro
chromium, ferro molybdenum, and ferro vanadium and selecting
the quantity of same;
(c) grinding separately the ferro alloy(s) to a mean particle size of
approximately 8 to 12 microns, which grinding may take place in a jet
mill as described herein;
(d) introducing a lubricant while blending the carbon and ferro alloy(s) with
the elemental iron powder;
(e) pressing the mixture to form the article; and
CA 02104607 1998-11-26
(f~ subjecting the article to a high temperature sintering at a temperature of
between 1,250 ~ C and 1,350 ~ C in a reducing atmosphere of, for example
90% nitrogen and 10% hydrogen.
The lubricant is added in a manner well known to those persons skilled in the art so
as to assist in the binding of the powder as well as assisting in the ejecting of the
product after pressing. The article is formed by pressing the mixture into shape by
utilizing the appropriate pressure of, for example, 25 to 50 tonnes per square inch.
The invention disclosed herein utilizes high temperature sintering of 1,250~C to1,350~C and a reducing atmosphere of, for example nitrogen and hydrogen in a
90/10% ratio, or in vacuum. Moreover, the reducing atmosphere in combination with
the high sintering temperature reduces or cleans off the surface oxides allowing the
particles to form good bonds and the compacted article to develop the appl-opliate
strength. A higher temperature is lltili7e~l in order to create the low dew point
necessary to reduce the oxides of manganese and chromium which are difficult to
reduce. The conventional practice of sintering at 1150~C does not create a sintering
regime with the right combination of low enough dew point and high enough
temperature to reduce the oxides of chromium, manganese, vanadium and silicon.
Secondary operations such as machining or the like may be introduced after the
sintering stage. Moreover, heat treating stages may be introduced after the sintering
stage.
Advantages have been realized by utili7ing the invention as described herein. For
example, manganese, chromium and molybdenum ferro alloys are utili7e~1 to strengthen
the iron which in combination or singly are less expensive than the copper and nickel
alloys which have heretofore been used in the prior art. Moreover, manganese appears
to be four times more effective in strengthening iron than nickel as 1 % of manganese
is approximately equivalent to 4% nickel, and accordingly a cost advantage has been
realized.
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Furthermore sintered steels with molybdenum, chromium, manganese and vanadium
are dimensionally more stable during sintering at high temperatures described herein
than are iron-copper-carbon steels (ie. conventional powder metal (P/M) steels).Process control is therefore easier and more cost effective than with conventional P/M
alloys.
Furthermore, the microstructure of the finished product are improved as they exhibit:
(a) well rounded pores;
(b) a homogenous structure;
(c) structure having a much smaller grain size; and
(d) a product that is more similar to wrought and cast steels in composition
than conventional powder metal steels.
The process described herein allows one to control or tailor the materials which are
desired for a particular application.
( 1 ) sinter hardening grades
(2) gas quenched grades
(3) as sintered grades
(4) high strength grades
(5) high ductility grades
The following chart provides examples of the five grades referred to above as well as
the range of compositions that may be lltili7e~l in accordance with the procedure
outlined herein.
Alloy Type Composition Typical Mechanical
Properties
Ultimate Tensile Stren~th Im~act
UTS (ksi) ~L/Ib
As Sintered Mn: 0.3 - 2.5%90 25
C: 0.2 - 0.85%
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Sinter Hardening Mn: 1.0 - 2.0% 120 LS
C: 0.5 - 0.85%
Mo: 0 -1.0%
Gas Quenched Mn: 0.5 - 2.0% 150 15
Mo: 0.5- 1.5%
C: 0 - 0.8%
Cr: 0 -1.0%
High Strength Mn: 0.5 - 2.0% 200 8
Cr: 0.5 - 2.0%
Mo: 0 -1.0%
C: 0.1 - 0.6%
High Ductility Cr: 0.5 - 2.0% 80 15
Mo: 0- 1.0%
C: 0.1 - 0.6%
Particularly good results were achieved with the as sintered grade with 1.5% Mn and
Q.Q,C'c_; UTS of 90ksi and imp_cL strength of 2û ft lbs. Otlqer C 01-.~inati5nS e~f 2Iloyirg
are possible to produce articles with specifically tailored baIance of properties such as
high tollghness and ware resistance.
Moreover good results were achieved with:
(a) sinter hardening grade with 1.5% Mn, 0.5% Mo, and 0.850~G C;
(b) gas quenching grade
(i) with 1.5% Mn, 0.5% Mo, and 0.5% C
(ii) with 0.5% Cr, 1.0% Mn, and 0.5% C
(c) high strength grade
(i) with 1.0% Mn, 0.5% C, 0.5% Cr, 0.5% Mo
(ii) with 1.5% Cr, 0.6~o C, 1.0% Mn,
Rollable Grade
Moreover, the method described herein may be utilized to produce a sixth grade
identified as a rollable grade having the following composition:
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Rollable Grade Cr: O.S - 2.0% 80 15
Mo: 0- 1.0C~o
C: 0.1 - 0.6%
Mn: 0 to 0.6~c
Coinin~
.,
It has been found that the method of producing the as sintered grade as described
above is particularly useful when used in combination with a coining operation so as
to produce precision coining as sintered parts which substantially elimin~te thesecondary-operations such as grinding, cutting or the like.
In another embodiment, the method of producing the gas quenched grade as described
above is also particularly useful when used in combination ~vith said coining operation
so as to produce precision coining gas quenched particles which substantially eliminate
t'ne seco~nu~-~ operations such as grinding, cuttir.g or the lilce. ~n particul~r if h~ b~r
found that articles which have a gas quenched composition described herein with
relatively small sections do not require molybdenum while heavier parts require the
molybdenum.
In particular, it has been found that parts such as clutch backing plates illustrated as
30 in Figure 6, or geo rotors (not shown) may be consistently, accurately manufactured
within narrow tolerance variabilities by coining the sintered product.
In particular, the process of precision coining of a sintered article of powder metal
consists of the steps of:
1. selecting the elemental iron powder;
2. determining the desired properties of the sintered articles and
selecting:
(a) a quantity of carbon, and;
(b) a quantity of ferro m~ng~nese;
to produce an article having a composition of between 0.3~o to 2.5C~c
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,.
m~ng~nese, 0.2~o to 0.85% carbon with the remainder being iron and
unavoidable impurities;
3 ~inding the ferro all.o~ to a mean particle ~i~e of approYi~m~ate!y
8 to 12 microns and substantially all of the ferro alloy having a particle size of
less than 25 microns;
4. introducing a lubricant while blending the carbon and ferro alloy
with the elemental iron powder; and
5. pressing the ~ ur~ to form the article;
6. high temperature sintering the article at a temperature between
1,250~C 1,35Q~C in a reducing atmosphere of for example 90% blended
nitrogen and 10~ hydrogen so as produce the sintered article of powdered
metal; and
7. then coining the sintered article to a final shape so as to narrow
the tolerance variability of coined articles and substantially eliminate secondary
operations.
Another embodiment of the invention conl~ises:
1. selecting the elemental iron powder;
2. determining the desired properties of the gas quenched grade
articles and selecting:
(a) a quantity of carbon, and
(b) a quantity of ferro alloys from the group of ferro manganese,
ferro molybdenum and ferro chromium
so as to produce a sintered blank resulting in a mass having between 0.5 to
2.0% m~n~,~nese, 0.5% to 1.5% molybdenum, between 0 to l.OYo chromium,
and between 0 to 0.8% carbon;
3. grinding the ferro alloy to a mean particle size of approximate!y
8 to 12 microns and substantially all of the ferro alloy having a particle size of
less than 25 microns;
4. introducing a lubricant while blending the carbon and ferro alloy
with the elemental iron powder; and
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5. pressing the rnLl~ture to form the article;
6. high temperature sintering the article at a temperature between
1,250~C 1,350~C in a reducing atmosphere of for example 90~o blended
nitrogen and 10% hydrogen so as produce the sintered article of powdered
metal; and
7. then coining the sintered article to a final shape so as to narrow
the tolerance variability of coined articles and substantially eliminate secondary
operations.
In particular, Figure 5 illustrates the stress strain diagram of coining sintered articles
having the prior art composition of FeCuC as well as the lower graph which illustrates
the stress strain relationship of an article produced in accordance with the method
described herein having a composition of between 0.3% to 2.5% manganese, 0.2~ to0.85% carbon, with the remainder being iron and unavoidable impurities. The stress
strain diagram of the composition described herein illustrates the plastic zone 32 which
allows the sintered blank to move upon coining to its final shape. The as sintered size
change variability is less than in conventional PM materials, on coining this variability
is further reduced.
More particularly, Figure 7 illustrates two dimensions which have an acceptable
tolerance level of between 140.00 to 139.70 as well as a second part having an
acceptable tolerance of between 1.51.20 and 1.51.00. The upper portion of the graph
in Figure 7 illustrates that a coined article made from a prior art composition of
FeCuC (0.1% to 3~o Cu and 0.5% to 0.8% Carbon) has dimensional variability
between 139.820 and 139.940 which peaks approximately between said levels. The
tolerance variability of the parts produced with a composition of Fe 0.3 to 2.5~ Mn
and 0.2 to 0.85~ C is more acceptable since the tolerance variability ranges from
139.840 to 139.880 peaking at 139.860, and since the tolerance variation lies in the
middle of the acceptable tolerance range. In other words if the CPK as illustrated in
Figure 8 lies in the middle of the acceptable tolerance range a and b, such tolerance
variability is desirable particularly since the variation peaks in the middle which takes
up approximately one-third of the tolerance.
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More particularly, it has been found that the CPK of the coined as sintered article
having a composition of Fe 0.3 to 2.5% Mn and 0.2 to 0.85% C has the desirable
CPK of greater than or equal to 1.33. If the CPK shifts from this position, it is less
desirable. In other words, the CPK illustrated in Figure 7 relating to a composition
of Fe 0.3 to 2.5% Mn and 0.2 to 0.85% C is more desirable than the CPK illustrated
in the composition of Fe 0.1% to 3% Cu and 0.5% to 0.8% C. (Although the
tolerance variability is still acceptable, it does not lie toward the middle range of the
acceptable tolerance level).
It has been found that after producing the sintered article by the sintered grade
method described above, one can expect a sintered grade of 0.5. However, upon
coining of a part made in accordance with the as sintered grade, one can expect to
obtain a CPK of greater than or equal to 1.33 which is highly desirable as the coined
sintered powder metal parts will be more uniform in dimensional size thus
substantially eliminating secondary operations such as grinding or the like.
CP relates to the "Process Capability Index" and is def1ned as
CP - Total Tolerance
Process Variation
The higher the CP the less variation there is in a process. In other words CP
measures the tightness of the spread in the dimensions produced by the process
against the acceptable tolerance. The bigger the spread the lower the CP.
The CPK is the combined measure of variation in process and relationship of process
average to specification limit (ie. upper and lower limit).
The higher the CPK the more capable a process is to specification. In other words
CPK measures the tightness of the spread as ,well as the position of the spread within
the acceptable tolerance. A high CPK translates to parts having a narrow tolerance
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spread positioned in the middle of the acceptable tolerance. The CPK can be changed
by ch~n~in~ the tooling or process.
Sintered powder metal parts such as clutch backing plates, geo rotors or the like
normally require grinding which increases the cost of same and increases the tolerance
variability of successively m~n~lf~ctllred parts. By utilizing the process as described
herein one is able to tighten down the tolerances of coined as sintered powder metal
parts thereby f~çjlitAting the design of more efficient pumps for example due to the
tightening of the tolerance levels.
The as sintered powder metal parts produced in accordance with the invention
described herein make it possible for sintered material to flow to final dimensional
size as the coining takes place in the plastic zone 32 described above.
Aithou~h the preferred e~bodiment as well as the operation and use have ~een
specifically described in relation to the drawings, it should be understood thatvariations in the preferred embodiment could be achieved by a person skilled in the
trade ~,vithout departing from the spirit of the invention as claimed herein.
