High Precision Thickness Control on a Rolling Mill for Flat Rolled Metal

Abstract

The high precision rolling design utilizes a high speed hydraulic roll position control along with a high accuracy roll gap measurement. The lower work roll position is fixed and the upper work roll is positioned by a hydraulic roll force cylinder using an inner and outer control loop. The inner loop is a cylinder position control that moves the upper work roll. The outer loop uses a measurement of the work roll gap to trim the inner cylinder positioning control. Both control loops coordinate together to provide a high precision and even strip thickness tolerance to +/−0.15 mils or less. Both sides of the upper work roll are controlled separately to achieve the overall tolerance goal.

Description

CROSS REFERENCE TO RELATED APPLICATIONS

Not applicable.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not applicable.

REFERENCE TO SEQUENCE LISTING, A TABLE, OR COMPUTER PROGRAM LISTING

Not applicable.

BACKGROUND OF THE INVENTION

(1) Field of the Invention

This invention is directed to a single stand rolling mill and associated thickness control that is used to reduce the thickness of flat rolled metal stock. It is particularly directed toward methods and control that provides high tolerance thickness control when rolling metal strip that will be used in stamping out coins for common use and for specialized Numismatics coinage.

(2) Description of Related Art

Some ancient coins were “cast,” meaning the metal was melted and poured into coin shaped molds. When the metal cooled, the mold was opened and the newly minted coin was removed. A refinement of the process was to pour fine granulated metal into the coin mold, heat the mold above the melting point of the metal, and then cool it. A more accurate weight coin was the result.

In other coins, blanks (called flans) were often cast in molds or individually cut from long round bars. The blanks were then struck in a die to imprint official images on the coin.

In either case, the thickness control of these methods were poor by modern standards. Over the centuries, various methods improved the coin thickness and diameter.

In modern times, a die strikes coin blanks approximately 120 times a minute in a press to make high numbers of coins.

In more modern coin making methods, strip for coin blanks is rolled in a multi-pass rolling mill. The exit thickness of the in process strip depends largely on the amount of space between the two work rolls. Roll position control is provided by setting both rolls by using a mechanical screw and an associated indicator. The screw position sets the roll position which in turn, sets the strip thickness. As the strip is rolled, the strip work hardens as thickness reduction is taken on the strip. Coins have different thickness reduction amounts depending upon the material and final desired properties.

The in process strip thickness during rolling is dependent upon a number of factors that are difficult to control. It is difficult to grind the work rolls to a completely uniform diameter. Typical roll eccentricity is 0.5 to 1.0 mils due to grinding machine tolerances. The eccentricity will be a permanent feature in the rolls. As the material hardens, eccentricity imprints onto the strip lessen due to an increased ‘spring back’ effect in the metal.

Additionally, the work rolls will heat up during the rolling process as the rolling process is commonly done on a ‘dry’ basis, that is, without utilizing a rolling oil or water cooling. This heating can be uneven and create an additional eccentricity pattern due to varying thermal expansion.

During initial rolling, the strip enters the roll gap and immediately generates a significant striking force. The roll gap is thinner than the strip because it is desirable to reduce the thickness for the ‘head end’ of the strip. The rolling force is high enough to cause the mill housing to stretch several mils (1 mil=0.001″). The frequent strip to roll impacts eventually cause uneven roll wear.

A multi pass rolling mill compounds the problem of roll eccentricity. The roll eccentricity from a previous pass is imprinted at a fixed distance between sinusoidal peaks, and remains imprinted on a longer strip during the next pass. Three or four passes are common. The overlaid eccentricity imprinting becomes a significant thickness variance as sometimes eccentricity patterns overlap in a way that increases the strip thickness, and sometimes decreases the strip thickness. Also, each work roll has its own eccentricity pattern that imprints on to the strip.

It should be kept in mind that two rolls are used in the rolling process, and the eccentricity of each roll is independent of the other.

The US mint guarantees that certain coins (such a bullion coins) will always be a certain minimum weight, this provides incentive to improve the thickness control. The blanking process creates a highly accurate diameter and the coin to coin weight variance is largely dependent upon the metal thickness of the blank.

Consequently, there is an ongoing desire to control the metal cost of producing coins by achieving only the desired thickness and avoiding any excess thickness. It is especially important in the precious metal coinage (i.e. silver and gold). For example, the US Mint guarantees that the weight, content and purity of the following common bullion coins:

	TABLE 1
		American Eagle	American Eagle
		Gold coin	Silver coin
	Diameter	32.70 mm	40.6 mm
	Thickness	2.87 mm	2.98 mm
	Gross weight	1.0909 troy oz	1.00 troy oz
		(33.930 g)	(31.103 g)
	Face value	$50	$1

This provides high confidence to investors to purchase them, knowing the coins contain the stated amount of precious metal. Sales statistics for 2013 are in Table 2.

TABLE 2
2013 American Eagle Bullion Sales
Coin type	Pieces sold	Ounces
1	ounce Gold	758,500	758,500
½	ounce Gold	57,000	28,500
¼	ounce Gold	114,500	28,500
1/10	ounce Gold	555,000	55,500
1	ounce Silver	42,675,000	42,675,000

Annual sales in 2013 of all denominations of the American Eagle coin (1 oz, ½ oz, ¼ oz and 1/10 oz) was 856,500 ounces. For example, if the current excessive thickness is 0.25%, it would mean that the US Mint ‘gives away’ 2,141 ounces. At $1,200 per ounce for a gold coin, it equates to over $2,400,000 per year. Similarly, at $50/ounce, the excess silver for the one ounce silver eagle would be over $5,300,000.

It is desirable to achieve an improved metal thickness accuracy and precision, to a value of +/−0.15 mils (0.0038 mm) or +/−0.13% of the desired thickness for a one ounce American Eagle gold coin.

An additional complication is that it is very difficult to accurately measure the thickness of precious metals to the desired accuracy of 0.0038 mm in a dynamic rolling process. Physical contact probes, x-ray based sensors, and other methods are not practical or accurate enough for control purposes.

Additionally, measuring the thickness after the strip has left the roll bite is not a satisfactory method of controlling work roll imperfections to a fine degree. Once the strip has left the roll bite, there is no ability to take corrective action on thickness variances due to imperfections in the roll bite caused by factors previously mentioned.

Additionally, in process techniques of measuring thickness variances, such as changes in rolling force, are not accurate and stable enough for the desired high precision control. Further, these types of control methods are overly complicated and expensive in this field.

The strip thickness variance is currently determined by stamping out coin blanks and weighing individual coins in a sample lot. The coin to coin weight variance gives a correct thickness accuracy. Such thickness determining methods are clearly unsuitable for dynamic control in a rolling mill.

Consequently, it is highly desirable to improve the rolling thickness variance by at least half of the current amount. The ability to roll a precious metal strip without concern for dynamic changes in the roll gap due to roll wear, grinding imperfections, and thermal variances is important. Also there must be a satisfactory method of tightly controlling the strip thickness without the ability to directly measure the thickness of the strip during rolling process.

BRIEF SUMMARY OF THE INVENTION

The high precision rolling design utilizes a high speed hydraulic roll position control along with a highly accurate and precise roll gap measurement. The lower work roll position is fixed. The upper work roll is positioned by a hydraulic roll force cylinder using an inner and outer control loop. The inner loop is a cylinder position control that moves the upper work roll. The outer loop uses a measured roll gap to trim the inner cylinder positioning control. Both control loops coordinate together to provide a high precision and even flat metal product thickness tolerance to +/−0.15 mils or less. Both ends of the upper work roll are controlled separately to achieve the overall tolerance goal.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

FIG. 1 shows a schematic of the automatic thickness control.

FIG. 2 shows an example side view of a typical two roll stack mill design.

FIG. 3 show a simplified layout of the light sensors that measure the roll gap in two places on either side of the strip being rolled.

FIG. 4 shows a schematic of a supporting thickness control hydraulic system.

FIG. 5 shows a schematic of a simplified automatic control system.

DETAILED DESCRIPTION OF THE INVENTION

It has become understood that use of a slow response electro-mechanical screw method in positioning the upper work roll provides an inadequate correction speed when attempting to control the gap between the work rolls to a high tolerance. In particular, work roll eccentricity issues are impractical to control if the response of the roll positioning system does not match changes in the roll gap. It has been found by experimentation that measuring the roll gap and controlling it with a system that has a response of 20 milliseconds for a 0.001″ roll position change is adequate to achieve the desired thickness control. This response works well for a rolling speed of the strip at approximately 50 fpm and work rolls approximately 8″ in diameter with a 10″ roll face.

The current inventive design was discovered after other methods of attempting fine thickness control were unsuccessful. Attempts to control a fixed roll gap opening by setting the position of a roll force cylinder was unable to obtain the desired thickness tolerance. Mill stretch and roll eccentricity were particularly difficult compensate to achieve a very small variance in the roll gap. Also the strip being rolled increased in hardness during the three to four rolling passes, which added an additional complexity by stretching the mill stand differently in each pass. Use of a constant rolling force control, did not provide the needed fine tolerance as the control did not compensate well enough.

The control of the roll gap position must be maintained across the roll face, so the roll gap correction must be applied to both ends of the work rolls through the roll chocks. It is important that the roll gap is even across the width of the roll during the rolling process. Otherwise, the thickness will be uneven across the strip width.

FIG. 1 shows a schematic of the automatic thickness control system with an inner roll force cylinder position control loop and outer roll gap position control loop. A pair of work rolls 100a,b are used to roll a metal piece that begins as approximately 3.5″×0.19″×6 feet long. A light emitter 101 shines a uniform collimated beam to a high resolution light receiver sensor 102 through the work roll gap opening to determine the gap on either side of the metal strip. The roll gap sensor is able to measure at an accuracy of +/−0.08 mils. A thickness signal 103 is sent to a mathematic calculation 105 to compare against a roll gap setpoint 104. The resulting calculation provides a roll gap error signal 111 that is fed into a trim PID controller 107. The trim is applied to the roll force cylinder position and will adjust the cylinder position output from the trip PID controller. A roll force cylinder position PID controller 109 receives feedback from a cylinder position indicator 108. The PID output then controls a hydraulic control valve 110, preferably a servovalve, which positions the roll force cylinder 112 which in turn, positions the work roll chock/bearing to move the roll on one side. A second roll force cylinder 113 is on the drive side of the work roll and is used to position the drive side work roll chock/bearing.

FIG. 2 shows an example side view of a typical two roll stack single stand mill design where the upper work roll is movable for metal thickness control. A roll force cylinder 201 and associated cylinder position sensor 202 are located within a mill housing 203. An upper work roll bearing chock 204 is attached to the roll force cylinder 201. A lower work bearing chock 205 is fixed against the mill housing 203 through a roll removal slide 206 and base 207. A runout table 208 provides support for the metal strip during rolling.

In an alternate embodiment, the bottom work roll is movable and the upper work roll is fixed by a stop when a strip is being rolled. In this case, the roll force cylinder is located under the bottom work roll chocks. The control design is the same with inner and outer control loops.

FIG. 3 show a preferred layout of the roll gap sensors that measure both sides of the roll gap. Two light beam emitters 303a,b are directed between the upper work roll 301 and the lower work roll 302 gap to two receivers 304a,b. The light emitters and receivers are on either side of the strip 305 being rolled. To simplify the figure, the two work rolls are shown without bearings or bearing chocks.

In an alternate embodiment, only a single light beam is used for gap feedback. In this alternate embodiment, the gap measurement is used to control the position of both roll force cylinders. One design method used to ‘level the mill,’ i.e. setting the roll gap equally across the width, is done by applying an even rolling force across the work roll face and noting any difference in roll force cylinder position between the two sides of the work roll. This offset is manually used in the cylinder setpoint.

The roll gap sensors have a measuring accuracy of +/−0.08 mils. The sensor captures 16,000 samples per second and a maximum 30 mm gap can be measured. The sensors are preferably mounted on each side of the roll, as close to the edge of the roll face as possible. They are positioned so as to be protected from errant strip tracking during the rolling process. The sensor mounting must also be stable, and free of vibration.

It was found that when the roll gap sensor is used in an inner/outer loop control as described, the desired tight rolling thickness accuracy was achieved to an acceptable, commercial level when 95% of the strip length was within the desired thickness range. When a roll gap sensor was not used, roll position alone did not provide the desired thickness tolerance. The hydraulic control valve and hydraulic system were designed to move the roll force cylinder quickly, and provide for a 20 millisecond response when making a 0.001″ roll gap correction. The servovalve design flow rate was 2.5 gpm. This high response design provides for gap corrections at up to 125 times per roll rotation when rolling at 50 fpm.

FIG. 4 shows a high pressure hydraulic system that provides the needed pressure and flow rate to support the hydraulic servovalve. A pump 401 is mounted near a tank 408 and supplies pressurized hydraulic fluid to a filter 402. The pressurized fluid line is connected to a dump valve 403 which is used to drop the pressure in the hydraulic system. The dump valve is useful for rolling emergencies, startup sequence, and when doing maintenance on the mill stand to ensure that hydraulic pressure is shut off. On the pressurized line a servovalve enable valve 404 is used to turn off the pressure to the servovalve for safety and operational reasons. An accumulator 405 provides immediate fluid for the high response servovalve 406 which is connected to the thickness control system as mentioned in FIG. 1. A roll force cylinder 407 is used to develop the needed rolling force in the mill stand. A connection 409 indicates that there is a replicate hydraulic control system (items 404-407) for the second roll force cylinder.

Not shown in FIG. 4 are various other operational and maintenance valves for roll changing and small flow restricting valves to smooth the operation of the illustrated hydraulic valves.

FIG. 5 shows a design schematic of a simplified automatic control system with two substantially replicated control loops. The controls are a simplified version of FIG. 1. A work roll gap measuring sensor outputs a signal 501 which represents the roll gap on one side. That signal inputs to a work roll gap PID controller 502. The operator inputs a roll gap setpoint 503 to set the exit thickness of the rolling pass of a metal strip. The setpoint will vary with each rolling pass. The PID controller 502 controls a hydraulic control valve 504, which is preferably a servovalve. The hydraulic control valve then moves the roll force cylinder 505 which in turn positions the upper work roll. As shown, a parallel, replicated control loop moves the second sides of the upper work roll in order to position it. Not shown are needed hydraulic and control components which will move the work rolls for maintenance, various operational reasons, etc. unrelated to thickness control.

Also illustrated are components of the system 506 that drives the work roll rotation which includes a motor, gearbox with two output shafts, and two spindles which connect to the upper and lower work rolls to the gearbox.

The stock used in rolling is relatively small—approximately 3.5 inches wide×0.28 inches thick×6 feet long. In one embodiment, a 40% thickness reduction is accomplished in 3 or 4 rolling passes. The strip increases in length by approximately 2½ times. Other thickness reductions and number of rolling passes are possible. However, productivity improves if the number of passes is reduced by suitably increasing the thickness reduction on each pass.

The development of hydraulic pressure in the roll force cylinders is accomplished by a supporting high pressure hydraulic system and a high response servovalve. A 6 gpm hydraulic pump at 2,500 psi provides suitable pressure and hydraulic supply for both roll force cylinders. The roll force cylinders were a 160 mm bore and a 65 mm stroke. A position sensor is mounted on each roll force cylinder with 0.1 micron resolution. These values are not the only ones possible. The cylinder position is measured for position control feedback. The hydraulic system may also be utilized for support systems such as equipment used when changing rolls.

The outer work roll gap control loop preferably updates at 5 millisecond intervals, and the inner hydraulic cylinder positioning loop updates at 1 millisecond intervals. However, these values are not the only possible values. It was found that setting the outer loop control to a 50 ms update produced acceptable, high tolerance results. Overall, when considering the fast response of the hydraulic control valve and associated hydraulic system, it is preferable for the design to provide at least a 50 millisecond response when moving the work roll 0.001 inches.

It was found that the mill design stretched about 5 mils or so when the strip entered the roll bite. The sudden increase in force opened the roll gap and caused off thickness above an acceptable amount. The control system then quickly corrects by increasing the rolling force, and the desired thickness quickly settles out. Approximate 4″ of the beginning strip ‘head end’ as measured on the final rolled length, was found to be at an undesirable tolerance and will be cut off before proceeding to create blanks for minting coins. There was no ‘tail end’ loss.

Future efforts may be utilized to reduce or eliminate the small length of head end metal that is out of specification.

The overall rolling system design also incorporates a record keeping data logger that is useful for recording rolling values, and for future records and troubleshooting.

The operator interfaces with the rolling mill through work roll gap setpoints and rolling force cylinder position setpoints on both sides of the mill. The roll force cylinder setpoint is set to an estimated value where the cylinder position will be when rolling. The operator sets the roll gap based on the desired exit strip thickness. The roll force cylinder position setpoint is primarily used for maintenance functions, such as roll change, and also as a back-up thickness control system when the roll gap sensor fails. When rolling, the outer loop design will, in effect, take over the control of roll force cylinder position. The inner loop roll force cylinder position setpoint becomes a starting reference and the outer control loop will quickly take over the cylinder position.

The rolling design is based on a dry method, that is, no rolling oils or cooling water is used. This is preferable for the finished product, and also for overall mill stand cleanliness that will keep the light sensors used to measure the rolling gap clean. Additionally, if fluid is allowed on the roll body surfaces, the variances in fluid thickness will cause inaccurate gap measurement.

It was not found necessary to utilize work roll bearings with a tight rolling tolerance. The ability to directly measure the work roll gap compensates for any bearing issues.

The overall rolling control system is designed to level the mill (i.e. create the same exit thickness across the strip width) as part of the thickness control system. It is also unnecessary to include a mill stretch calculation in the thickness control system. The effects of mill stretch are taken care of by the direct roll gap measurement with the light sensors.

Both work rolls are driven by a single A/C motor through a gearing box with two exit shafts. The exit shafts are in turn are connected to the two work rolls through spindles. The spindles allow for roll changing and also for the upper work roll to move up and down. The work rolls are preferably chrome plated to reduce wear, and preferably have a smooth finish.

If a roll gap sensor fails, or if the operator needs to intervene due to a control issue, the operator can switch to direct roll force cylinder position control. This allows the operator to complete a pass even though the final tolerance will not be as tight. In this case, a bumpless transfer between direct roll gap control and cylinder position control is utilized. This provides a ‘fail safe’ control design.

Use of the term ‘strip’ should not be restrictive as to the potential material dimensions that is rolled by the teachings of this invention. Flat rolled metal product such as bar, plate, and sheet dimensions are equally rolled to a high precision tolerance.

While various embodiments of the present invention have been described, the invention may be modified and adapted to various operational methods to those skilled in the art. Therefore, this invention is not limited to the description and figure shown herein, and includes all such embodiments, changes, and modifications that are encompassed by the scope of the claims.

Claims

1. A single stand rolling mill with a thickness control system designed to roll a flat metal product comprising:

A) a pair of work rolls, wherein one work roll is designed to be separately movable on each side of said rolling mill for metal thickness control purposes,

B) wherein said thickness control system is designed to position said movable work roll on both sides of said rolling mill,

C) wherein said thickness control system comprises the following features on each side of said movable work roll: a) a roll force cylinder designed to position said movable work roll, b) a roll gap sensor, wherein said roll gap sensor has an accuracy of +/−0.08 mils or less, c) an outer roll gap position control loop, wherein said outer roll gap position control loop is connected to said roll gap sensor, d) wherein said outer roll gap position control loop is additionally connected to an inner roll force cylinder position control loop, e) wherein said an inner roll force cylinder position control loop is connected to a hydraulic control valve, f) wherein said hydraulic control valve is connected to said roll force cylinder, and g) said hydraulic control valve and associated hydraulic system is designed to move said movable work roll 0.001 inches in 50 milliseconds or less,

D) whereby said thickness control system is designed to roll said flat metal product within +/−0.15 mil of a desired thickness.

2. The single stand rolling mill according to claim 1 wherein said pair of work rolls are designed to roll said flat metal product in dry conditions.

3. The single stand rolling mill according to claim 2 wherein said thickness control system design requires multiple rolling passes.

4. The single stand rolling mill according to claim 2 wherein said thickness control system is designed to roll said flat metal product used for coins.

5. The single stand rolling mill according to claim 4, wherein said flat metal product comprises gold, silver, and platinum products useful for said coins.

6. The single stand rolling mill according to claim 1 wherein said movable work roll is an upper work roll.

7. The single stand rolling mill according to claim 1 wherein said roll gap sensor uses to measure a gap between said pair of work rolls.

8. A single stand rolling mill with a thickness control system designed to roll a flat metal product comprising:

A) a pair of work rolls, wherein one work roll is designed to be separately movable on each side of said rolling mill for metal thickness control purposes,

B) wherein said thickness control system is designed to position said movable work roll on both sides of said rolling mill,

C) a roll gap sensor, wherein said roll gap sensor has an accuracy of +/−0.08 mils or less,

D) wherein said thickness control system comprises the following features on each side of said movable work roll: a) a roll force cylinder designed to position said movable work roll, b) an outer roll gap position control loop, wherein said outer roll gap position control loop is connected to said roll gap sensor, c) wherein said outer roll gap position control loop is additionally connected to an inner roll force cylinder position control loop, d) wherein said an inner roll force cylinder position control loop is connected to a hydraulic control valve, e) wherein said hydraulic control valve is connected to said roll force cylinder, and f) said hydraulic control valve and associated hydraulic system is designed to move said movable work roll 0.001 inches in 50 milliseconds or less,

E) whereby said thickness control system is designed to roll said flat metal product within +/−0.15 mil of a desired thickness.

9. The single stand rolling mill according to claim 8 wherein said pair of work rolls are designed to roll a flat metal product in dry conditions.

10. The single stand rolling mill according to claim 9 wherein said rolling mill thickness reduction design requires multiple passes.

11. The single stand rolling mill according to claim 9 wherein said thickness control system is designed to roll said flat metal product used for coins.

12. The single stand rolling mill according to claim 11, wherein said flat metal product comprises metals that are gold, silver, and platinum.

13. The single stand rolling mill according to claim 8 wherein said movable work roll is an upper work roll.

14. A single stand rolling mill with a thickness control system comprising:

A) a pair of work rolls, wherein one work roll is designed to be separately movable on each side of said rolling mill for metal thickness control purposes,

B) wherein said thickness control system comprises the following features on each side of said movable work roll: a) a roll force cylinder designed to position said movable work roll, b) a roll gap sensor, wherein said roll gap sensor has an accuracy of +/−0.08 mils or less, c) an inner roll force cylinder position control loop, d) an outer roll gap position control loop, and e) a hydraulic servovalve designed to move said movable work roll 0.001 inches in 50 milliseconds or less,

C) wherein said thickness control system is designed to roll a flat metal product used for coins, and

D) whereby said thickness control system is designed to roll said flat metal product within +/−0.15 mil of a desired thickness.

15. A single stand rolling mill with a thickness control system comprising:

A) a pair of work rolls, wherein one work roll is designed to be separately movable on each side of said rolling mill for metal thickness control purposes,

B) wherein said thickness control system comprises the following features on each side of said movable work roll: a) a roll force cylinder designed to position said movable work roll, b) a roll gap sensor, wherein said roll gap sensor has an accuracy of +/−0.08 mils or less, c) a roll gap position control loop, and d) a hydraulic servovalve designed to move said movable work roll 0.001 inches in 50 milliseconds or less,

C) wherein said thickness control system is designed to roll a flat metal product used for coins, and

D) whereby said thickness control system is designed to roll said flat metal product to within +/−0.15 mil of a desired thickness.