US 3712157 A
This invention provides a method and apparatus for the manufacture of high speed steel coinage dies, and dies made by the method.
Claims available in
Description (OCR text may contain errors)
States aient Kratz et al. 1 Jan. 23, 1973 DIE AND METHOD OF MANUFACTURE  References Cited  Inventors: Hans Otto Kratz, 44 Wells Hill Ave., Toronto, Ontario; Heinz Peter Schurman, 42] Abington Ave Mi 1,932,426 l0/l933 Stevens ..76/l07 R sissauga, Ontario; George Steiner, 68 Heathdale Rd., Toronto, On- Primary ExaminerRichard J. Herbst tario, all of Canada Assistant Examiner-R. M. Rogers  Fned: March 29, 1971 Attorney-Rogers, Bereskm & Parr  App]. No.: 129,106 57 ABSTRACT This invention provides a method and apparatus for  [1.8. CI ..76/l07 R, 72/359, 72/373 the f ct re f hi h s d steel coinage dies, and  Int. Cl ..B21k 5/20, B2 1d 22/00 dies made by the method  Field of Search... 76/107 R, DIG. 2, 101 B;
DIE AND METHOD OF MANUFACTURE This invention relates generally to the manufacture of coinage, and more specifically to coinage dies and to a method of making the dies.
The term coinage die will be used generally to describe any one of the dies used in the manufacture of coinage. Specific reference to one of the dies will be made according to terminology to be developed by describing a typical sequence of die manufacture as follows.
An oversize model of a desired coin is first made in plastiscene or clay and then a negative of the model is made by pouring plaster of Paris over the model and allowing the plaster to harden. The negative so formed is then covered by acrylic resin which when hard forms an enlarged positive impression of the desired coin. Next, after putting finishing touches to the acrylic impression, it is placed in a machine which copies the impression in steel to form a first die having an impression of the desired size. This die has a positive impression and will be referred to as a master reduction although it is also sometimes known as a Cameo.
After hardening, the master reduction is used to form a negative impression in a steel blank to form a matrix (sometimes called Intaglio). This process is known variously as forging, pressing, or bobbing For the purposes of the present application forging will be used. Next, the matrix is hardened to forge a positive impression into a working punch which after hardening is used to forge a negative impression into a working die. Coins are produced by minting with theworking die after first hardening the die.
The working dies are replaced periodically by new dies forged from the working punch. After the working punch has been used many times, it must be renewed and so on back to the master reduction. Thus each of the dies prior to the working die is made so that more dies can be made without significantly affecting the quality of the impression on the coins. I
7 One of the major expenses in making coins is the cost of replacing worn or damaged dies. The dies are conventionally made by a forging process in which the die blank is subject to a relatively high pressureto form an impression. The steel used for the die has therefore been chosen primarily for its grain flow characteristics under pressure to form the impression. v
The toughness of steels generally decreases with increasing wear resistance of the steel, which resistance is in'turn directly proportional to the hardness of the steel. Thus for good wear characteristics a relatively high hardness is desirable combined with the ability to maintain that hardness in use. As a result the chosen and temper the steel for providing a die with sufficient toughness to withstand the pressures used in makingv the dies and to wear well in use.
Many mints now use dies made from so called fcold work" steels, although hot work" steels have also been tried. These steels suffer from different disadvantages. Cold work steels mustbe heat treated-to give a compromise between hardness and toughness. However although a relatively high hardness is achieved this compromise results in dies which deteriorate relatively quickly. By contrast, the hot work steels can not be hardened to the same extent as cold work steels and are therefore less suitable than the cold work steels. Con
American and Iron Steel Institute AlSl W1 Typical analysis Carbon 1.05% Manganese 0.20% Silicon 0.20%
Although this steel has a maximum hardness of about 65 to 67 Rockwell C without tempering, the hardness falls off along a relatively smooth downward curve when the steel is tempered.
b. Medium Life Atlas cold work tool steel Keewatin AlSl Ol Typical analysis Carbon 0.90% Manganese l 20% Silicon 0.30% Chromium 0.50% Tungsten 0.50% Vanadium 0.20%
This steel is recommended for many purposes where a medium production run is required. Hardness and toughness characteristics are generally similar to those of Atlas A10 previously described with a maximum hardness of about 64 Rockwell C.
0. Long Life 'Atlas cold work tool steel FNS-Fm AlSl D2S Typical analysis Carbon 1.50% Manganese 0.50% Silicon 0.30% Chromium l 1.50% Molybdenum 0.80% Vanadium 0.85% Sulphur 0. l 0% This steel includes elements which provide a relatively high degree of stability in hardening and relatively high wear resistance. It is therefore suitable for longer production runs and lasts longer than the previously discussed steels. However, unlike the X10 and Keewatin steels,-the hardness initially falls off steeply with tempering temperature and thenrises again to a' peak before again falling steeply. It is recommended by Atlas Steels that for an optimum compromise between hardness and toughness, the tempering temperature should be 900 F (i.e. at the aforementioned peak), resulting in a final hardness of about 58 to 60 Rockwell C.
Although dies made from cold work steels are relatively hard when first made, these nevertheless deteriorate quite rapidly in use. It would therefore be preferable to have a working die which is both harder than a new cold work steel die and which endures longer in use.
Up to the present, the steels recommended for coinage dies have excluded high speed tool steels characterized by their relative hardness and ability to retain their hardness in use. These steels are recommended for use in making tools such as reamers, broaches, gear cutters, milling cutters, lathe tools, twist drills and the like. High speed steels are not recommended for applications where grain flow is an essential property of the steel to be used. However, some of the high speed steels can be hardened to about 70 Rockwell C apparently making them suitable for coinage dies if an economic method of making the dies could be found.
High speed steels are characterized from other steels (a) by their tungsten and molybdenum content (b) by their capacity for hardening and (c) their ability to retain this hardness under working conditions. However, high speed steels are also noted for their tendency to resist grain flow making impression difficult.
Typically, high speed steels include tungsten and molybdenum to an equivalent of between about 14 and 18.5 per cent depending upon the other elements included and the characteristics desired of a steel. Tungsten and molybdenum are to some extent complementary, five-ninths of one per cent of molybdenum being generally equivalent to one per cent of tungsten. However, for the steel to be a high speed steel tungsten will be present with or without molybdenum to a percentage equivalent to about 14 to 18.5 per cent.
Apart from carbon which is always present in high speed steels, the elements commonly included are Manganese up to about 0.25%
Silicon between about 0.10% and 0.30%
Chromium between about 3.75% and 4.25%
Vanadium between about 1% and 4% Cobalt may also be added.
The percentage of vanadium is increased to add wear resistance and cobalt is added if greater hot hardness is required.
It is an object of the present invention to provide a method of making coinage dies from a high speed steel to produce a die having improved wear characteristics.
It is a further object of the present invention to provide coinage dies of high speed steel and having a hardness suitable for relatively long production runs.
These and other objects of the invention will be better understood with reference to the drawings, wherein:
FIG. I is a diagrammatic representation of a preferred method of manufacturing dies and coinage according to the invention; and
FIGS. 2 and 3 are sectional side views of a preferred apparatus for use in the method of making the dies.
Reference is first made to FIG. 1 in which an acrylic model AM is prepared conventionally (as previously described) and placed together with a blank B1 in a reduction machine R. The blank B1 is of a high speed steel denoted as AISI M42 and having the following typical analysis.
Carbon 1.06% Manganese 0.25% Silicon 0.30% Chromium 4.00% Tungsten 1.45% Vanadium 1.15% Molybdenum 9.50% Cobalt 8.00%
This steel is recommended by steel makers for use in machining prehardened steel, titanium alloys and the like. After master reduction MR is made, the reduction is preheated to 1500/ 1550C FF and then heated to 2150l2160F in a salt bath. The reduction MR is then interrupt quenched in salt at 1000l1 F before cooling in air. The resulting reduction will be relatively hard and too brittle for use in forging a matrix. Toughness is then improved by quadruple drawing at 1050F to give a hardness of 64-66 on the Rockwell C scale. The master reduction MR is then ground to produce a taper 10 for reasons which will be explained.
Master reduction MR is next used to press a negative impression in high speed steel blank B2 in forging operation Fl, resulting in matrix MA. The blank B2 (and all other die blanks) are preferably heat treated before forging to improve grain flow.
Blank B2 preferably has the following typical analy- SIS.
Carbon 0.83% Manganese 0.23% Silicon 0.30% Chromium 3.90% Tungsten 1.65% Molybdenum 8.75% Vanadium 1.20%
This steel is recommended by steel makers for use in making twist drills, reamers, gear cutters and the like. Grain flow is promoted by forging according to a method to be described with reference to FIGS. 2 and 3.
Matrix MA has a negative impression with respect to the required coin, and after forging, it is in a relatively soft condition. The matrix is next heat treated at HT2 as follows. First the matrix is pre he ated to 155071600 F and then heated to 2060/2090F in a salt bath. The matrix is then interrupt quenched at l000/1100F in salt before air cooling. The hard matrix is finally toughened by double drawing at 10l0l1025F to give a hardness of 61 to 63 on the Rockwell C scale.
Matrix MA is used as a die to forge an impression in blank B3 at forging operation F2 to make working punch WP. Blank B3 is of the same typical analysis as blank B2 and is heat treated at HT3 in the same manner as was Matrix MA. 1
Working dies WD are made from working punch WP using the same high speed steel as was used for the matrix MA and the working punch WP after heat treatment HT4 working die WD is ready for use at a minting press MP to mint coins C from blanks B5. Heat treatment HT4 is similar to heat treatments HT2 and HT3.
The relatively hard high speed steel M81 M42 can be used for the master reduction because the impression in this die is created by machining in the reduction process R. This steel is less suitable for blanks B2, B3, B4 because it does not exhibit the same grain flow characteristics as does AlSI Ml used for these blanks, (as will be seen from test results to be described).
Turning now to the mechanical process of making coinage dies from the master reduction, because the grain flow of high speed tool steel is relatively poor under impact, a method according to the invention was devised to obtain the required grain flow while avoiding fracture of the die used in forming an impression in a blank.
Beginning with the matrix, a blank is formed by pressing the hardened master reduction into a matrix blank in the soft condition.
The master reduction MR is brought towards the matrix blank B2 and made to impact with a moderate force commonly of the order of 5 to 50 metric tons (where 1 metric ton 1000 kilograms) depending upon the size of the blank B2. The optimum magnitude of this force is found by testing and will be referred to as the change-over impact force. The grain flow commences on impact and the force is then increased to a maximum which will be referred to as the sinking force. This force is limited by the pressure which a hardened die can withstand in forging a blank and is calculated by multiplying the cross-sectional area of the die by a maximum sinking pressure for the high speed steel being used. Typically the maximum sinking pressure is about 0.35 metric tons per square millimeter. For instance, a high speed steel die of 18.0 millimeters diameter can withstand a maximum sinking force of about 89 metric tons. The die is forged into the blank at a predetermined speed known as the pressing speed. Although good results have been obtained with pressing speeds in excess of 4.00 millimeters per second, in most cases a pressing speed in the range 0.01 to 0.30 millimeters per second is preferred. If the impression is not completed when the maximum sinking force is reached, the blank must be heat treated to soften it and a further impression made. However, it has been found that the present method produces impressions in a single forging operation where conventional methods are more likely to require several forging operations.
In summarizing the mechanical process thus far, a relatively low change-over impact force is first applied followed by an increase in force at a controlled pressing speed until the force is equal to the sinking force. This contrasts with present methods where the main consideration is the sinking force, the change-over impact force being incidental to the forging process. The present method requires control of the change-over impact force to promote initial grain flow followed by a controlled pressing speed developing a maximum forging force equal to a predetermined sinking force. However, it is essential for good results that the high speed steel blanks be contained 'against excessive radial distortion as will be explained with reference to FIGS. 2 and 3.
The high speed steel blanks for the matrix, working punch and working die should preferably have a hardness of about 200230 Brinell and are preferably roofed from 5 up to depending upon the relief required in the impression.
Turning now to the size of the blanks, the diameter of each blank can be substantially equal to that of the die being pressed into the blank. However, the blank diameter is preferably about percent larger than the diameter of the face of the die. If the blank is more than per cent larger in diameter than the die face there is a tendency for increased side flow with, in most cases, a limited impression. Once a die blank has been forged and heat treated it is then ground to produce taper 10 (FIG. 1) so that the face of the die is reduced to give the desired proportion relative to the face of the next blank.
Reference is now made to FIG. 2 to describe apparatus used in the method. The apparatus can best be described as a precision pre-stressed guiding system for absorbing radial stress without permanent distortion.
As seen in FIG. 2, a blank 20 of high speed steel is positioned on a forging table 22 inside an opening 23 in a compound prestressed sleeve 24. Opening 23 is axially longer than blank 20 so that a lower end portion of a die 26 is located in opening 23 for guiding a lower end 28 of the die into engagement with an upper end 30 of the blank 20.
Prestressed sleeve 24 has an inner ring 32 pressed into an outer ring 34 such that the ring 32 is subject to compressive stresses. Ring 32 is hardened about opening 23 and stressed sufficiently that it is not per manently deformed by the forging operation.
An upper end of die 26 is engaged by a movable upper member 36 of the forge and the forging process is carried out using the forces and feeds previously described until the maximum sinking force for die 26 is reached. At this point the forge force is released and member 36 moved upwardly leaving die 26 located in the prestressed sleeve 24 with its lower end in contact with an impression formed in the upper end 30 of the blank 20. Because of the forces involved and the relatively soft condition of the blank 20, the blank tends to spread radially applying forces on the ring 32 which oppose or cancel the compressive stresses created by prestressing the ring 32. However, the degree of prestressing is chosen such that the resultant stresses in the ring 32 after forging are not sufficient to cause permanent distortion of ring 32. Once the maximum sinking force has been applied the blank 20 will be frictionally locked in the collar 24.
Reference isnow made to FIG. 3 which illustrates apparatus used in removing impressed blank 20 from sleeve 24. After forging, the sleeve together with blank 20 and die 26 is placed on a tubular support 40 having a peripheral lip 42 for locating sleeve 24 concentrically and a central opening 44 which is larger in diameter than blank 20. A drift 46 is engaged at its upper end by forge member 36 to apply a downward force on blank 20 and die 26. The drift has an enlarged end 50 on an elongated portion 52 on which an annular stop 54 is free to move. The portion 52 is adapted to move freely in opening 23 under force applied by forge member 36 with a lip 56 locating about sleeve 24.
Blank 20 is removed by activating the forge to apply a relatively small downward force in member 36 so that drift 46 pushes the die 26 and blank 20 right through opening 23 and into opening 44. The drift 46 is proportioned so that when the blank is clear of opening 23, the stop 54 engages the top of sleeve 24 and drift end 50 engages the stop thereby preventing further movement which could damage the die 26 and block 20. Die 26 is proportioned to slide freely out of opening 23 so that when sleeve 24 is lifted off support 40 both the die and the blank can then be removed.
If the maximum sinking force used produced a suitable impression the blank can be finished by hardening and grinding to give a die face of the desired diameter. If the force was insufficient the blank has to be machined and softened before again forging to complete the impression.
The use of the apparatus shown in FIG. 2 contrasts with apparatus used in conventional methods where an oversize blank was used followed by machining to reduce the blank to a preferred size.
The present method requires that the high speed steel blank should be contained in a sleeve and a high speed steel die used to forge an impression in the blank. The die is fed to the blank to impact with a predetermined change-over impact force and then the forging pressure is increased such that the die proceeds downwardly at a controlled pressing speed until the maximum sinking force is reached. For proper forging according to the present method the change-over impact force pressing speed and sinking force are predetermined for a given die and blank, and then controlled in a restraining sleeve during forging. This method orientates the grain in the die so that dies made according to the present method are of high speed steel with the impressions defined by relatively smooth continuous and orientated lines of grain. The dies are as hard or harder than those made from cold work steels and retain their hardness over relatively long periods of use. Also the cost of minting coins is reduced because the cost of making dies of high speed steel compares with that of using cold work steels whereas the high speed steel dies wear better.
Various high speed steels can be used for making coinage dies according to the present process. However, some steels are more suitable than others as evidenced by the following table of typical test results.
The materials used in the table have the following typical analyses:
Ml M2 M3 M4 M7 Tl CO.83% C0.84% C 1.03% C 1.30% C 1.00% C 0.72% Mn 0.25% Mn 0.25% Mn 0.30% Mn 0.25% Mn 0.30%Mn 0.25% Si 0.30% Si 0.30% Si 0.30% Si 0.30% Si 0.30% Si 0.30% Cr 3.90% Cr 4.00% Cr 4.25% Cr 4.25% Cr 4.00% Cr 4.00% W 1.65% W 6.50% W 6.25% W6.00% W 1.75% W 18.00% Mo 8.75% M 5.00% Mo 6.00% 5.00% Mo 8.70% V 1.10%
V 1.20% V 1.90% V 2.50% V 4.00% V 1.90%
M42 C 1.06% Mn 0.25% Si 0.30% Cr 4.00% W 1.45% M0 9.50% V 1.15% Co 8.00%
M42 which although particularly suitable as a master reduction did not forge well enough to give the results desired.
The steels which were particularly satisfactory had a combined tungsten and molybdenum content in the range 15 (M4) to 18 percent (T1). (For the purposes of this description 1% M0 is equivalent to 9/5%W). Within this range W varied from 1.65 (M1) to 18 percent (T1) with corresponding variations in M0 from substantially zero (T1) to 8.75 percent Ml. However, although these figures represent a preferred group of high speed steels, the method can be applied to all high speed steels with variations in results. As new steels are developed it is possible the present method will be satisfactory in producing minting dies.
The foregoing description has been concerned with the forging of a coinage die to be used in minting one side of a coin. However, it will be appreciated that a similar method is used for forging a further coinage die for minting the reverse side of the coin.
1. A method of making a coinage die from a blank of un-hardened high speed steel, the blank having a predetermined external diameter and the method comprising the steps:
preparing an oversize model of an impression to appear in the coin; reducing the oversize model to prepare a corresponding impression in an end of an unhardened master reduction of high speed steel;
heat treating the master reduction to harden and toughen the master reduction, and reducing the master reduction to said predetermined external diameter; placing said blank in a retaining sleeve having an opening of diameter substantially equal to said external diameter with the lower end of the blank on a stationary pressure plate;
placing a lower portion of the master reduction in the retaining sleeve with said corresponding impression adjacent an upper end of said blank;
driving the master reduction into contact with said end of the blank using a predeterminedchange over impact force;
continuing to drive the master reduction downwardly at a predetermined pressing speeduntil the force reaches a predetermined sinking force whereby a FORGING TESTS OF HIGH meii'mms" Material Supplied by Atlas Steels Company of Welland, Ontario, Canada.
Material of blank M2 M1 M1 M1 M1 M3 M4 M7 T1 M1 M1 M42 Change-over impact force (metric tons) 15 15 15 15 15 15 15 15 15 15 15 15 Sinking force (metric tons) 89 71 75 74 74 75 75 74 Presslngspedmge mi111mmrS/m- 1,91 1&1 311 1 191 if; 391 "5.9% 19% 691 if; 6 i Roof on blank (angular degrees) 7 16 10 7 10 10 10 10 10 25 15 7 Results 1 (d) (c) (c) (b) (a) (e) 1 (a) Excellent; (b) Very good; (0) Good; ((1) Satisfactory; (e) Unsatisfactory.
In general it WASYB'und that most high speed $56K could be used to produce dies with results varying from satisfactory to excellent. An exception to this is AlSl negative of the said impression is formed in said end of the blank; releasing the force;
removing the blank and master reduction from the retaining sleeve; and
heat treating the blank to harden and toughen the blank suitable for minting impressions in coinage .blanksh 2. Themethod as claimed in claim 1 wherein the heat treatment comprises the steps:
preheating the blank to between l550 and [600 Fahrenheit;
heating the preheated blank to between 2060 and 2090 Fahrenheit in a salt bath;
interrupt quenching the blank in salt at between l000 and 1 100 Fahrenheit; and
double drawing the blank at between lOl and l025 Fahrenheit.
3. The method as claimed in claim 1 wherein said pressing speed is in the range 0.01 to 4.00 millimeters per second.
4. The method as claimed in claim 2 wherein said pressing speed is in the range 0.0l to 4.00 millimeters per second.
5. The method as claimed in claim 1 wherein the high speed steel has a combined equivalent tungsten and molybdenum content in the range 15 to 18 per unit inclusive the tungsten content being in the range 1.65 to l8 per cent inclusive and the molybdenum content being in the range zero to 8.75 percent.
6. The method as claimed in claim 5 in which the heat treatment comprises the steps:
preheating the blank to between l550 and l600 Fahrenheit;
heating the preheated blank to between 2060 and 2090 Fahrenheit in a salt bath;
interrupt quenching the blank in salt at between l000 and 1 Fahrenheit; and
double drawing the blank at between l0l0 and l025 Fahrenheit.
7. The method as claimed in claim 5 wherein said pressing speed is in the range 0.0l to 4.00 millimeters per second.
8. The method as claimed in claim 6 wherein said pressing speed is in the range 0.01 to 4.00 millimeters per second.