Patent Application
20190015944
This invention is directed to a method of grinding a pair of matching rolls used in a strip rolling mill.
Steel sheet is made by pressing thick slabs of steel between large rollers. The rollers are referred to as “rolls.” The rolls are generally cylindrical and are journaled on parallel axes. Depending on the process, the rolls used to make steel strip can vary from about 2 inches in diameter to 72 inches in diameter. The space between the rolls through which the slabs pass is referred to as the “nip.” On passing through the nip, the thickness of the slab is reduced. For example, one step in making steel sheet is with the hot strip mill. Thick slabs are heated to about 2,300 degrees Fahrenheit. Typically, the slab will pass through two banks of six mill stands, one rough bank and one finish bank. A mill stand is a vertical group of rolls, typically four or six. The rolls defining the nip are backed up by backup rolls journaled on parallel axes. At each stand, the slab becomes thinner and longer. At the end of its journey, water sprays cool the strip, and it is wound into coils. A slab that started 10 inches thick may now be 0.05 to 0.5 inches thick. There are also cold strip mills. These take coils about 0.1 inch thick and convert them to coils as thin as 0.01 inch thick.
The pressure and heat in a mill stand are so high that the rolls flex slightly. This would cause the steel sheet to be slightly thicker at the center than the edges. To compensate, the rolls are formed to be slightly fatter at the center than the edges. This is called a “crown” profile. A crown might be as small as a few thousands (0.001) of an inch. Other profiles are used. In modern mills, roll profiles can be very complicated.
Of course, the surfaces of the rolls must not have surface irregularities that produce undesirable surface quality of the steel strip. Imperfections in the roll surface will cause imperfections in the surface of the rolled sheet. An uneven shape of the roll causes unevenness in the rolled product, which at the very least means a waste of rolled material. Over a period of use, rolls undergo wear and deterioration in surface quality. From time to time the rolls need to be reshaped by machining or grinding.
Other industries have similar processes that use rolls, for example, the aluminum and paper industries. There are practical limits to the accuracy of roll grinding, and improved accuracy of the nip will be of benefit primarily for very thin products, such as aluminum foil.
Rolls are shaped and reshaped by a process known as “roll grinding.” Modern off-line roll grinders comprise a headstock that journals the roll and rotates the roll about its axis. A carriage moves parallel to the roll axis supporting a grinding wheel that rotates at several hundred RPM. A stream of water cools the grinding wheel and roll. The grinding wheel axis is held by an infeed mechanism that precisely moves the grinding wheel toward the roll. On-line roll grinders are similar.
Modern roll grinders are computer controlled. Grinding involves a number of steps from coarse fast grinding to slow finish grinding. This includes carefully bringing the grinding wheel to the roll and periodic measurement. The measurements are often continuous made by a high-precision computer controlled electronic caliper mounted on the roll grinder. A computer controlled grinder automates all these activities. With the KP10 system provided by the assignee of the patent application, seventy five distinct grinding-related steps can be controlled.
It is desirable for the grinding operation to return the roll, within tolerance, to the desired profile and surface condition within the minimum time, removing the minimum amount of material from the surface of the roll. There is always a residual error up to the acceptable tolerance level, for example, several microns.
This invention is directed to a novel method of grinding a matching pair of rolls. A matching pair is two rolls that are used together to define the nip.
Roll grinding methods for single rolls are described, for example, in Mori et al. U.S. Pat. No. 6,450,861 entitled “Rolling Mill Equipped with On-line Roll Grinding System and Grinding Wheel” and Kumar et al. U.S. Pat. No. 8,029,338 entitled “Grinding Wheel for Roll Grinding Application and Method of Roll Grinding Thereof.”
Briefly, according to this invention, there is provided a method of grinding a pair of matching rolls used in a strip rolling mill in which the position of a grinding wheel relative to the rolls is computer controlled, and the surface profile of a roll is continuously measured and input to the computer.
The computer has a stored program that is executed to perform the steps of: inputting to the computer the desired profile and acceptable tolerance of a first roll of the matched pair; measuring and inputting to the computer the actual profile of the first roll; grinding the first roll to bring the profile of the first roll within the acceptable tolerance; inputting to the computer the residual shape error profile of the first roll; inputting to the computer the desired profile and acceptable tolerance of a second roll; modifying the desired profile of the second roll of the matched pair adjusted by the residual shape error profile of the first roll to establish a uniform gap at axial positions between the pair of rolls; and grinding the second roll to bring the profile of the second roll within acceptable tolerance based on the modified desirable profile.
According to a preferred embodiment, the computer calculates the difference between the measured roll profile and the desired roll profile and adjusts the grinding conditions to reduce the difference.
In a still further preferred embodiment, the profile tolerance of the first roll is relaxed, so as to reduce grinding time while still improving the accuracy of the nip.
In yet another embodiment, the profile tolerances of the first and second rolls are relaxed to further reduce grinding time and achieve a nip accuracy comparable to one obtained without this method.
Further features and other objects and advantages will become apparent from the following detailed description made with reference to the drawings in which,
Referring to
A measuring caliper 20 is mounted on a carriage 22 to move parallel to the axis in order to measure the profile of the roll. A computer 24 receives profile information from the caliper 20. The computer 24 also receives position information from the carriage 16. A stored computer program controls the position of the carriage 16 and the infeed mechanism 18 such that with controlled grinding the profile of the roll is brought into tolerance. The computer is provided with manual input devices 26 such as a keyboard and a display device 28.
Modern automatic roll grinding machines already have the necessary control and measuring equipment and the required data collection and processing facilities for practice of the invention. The method can also be implemented in suitably equipped on-line grinding installations.
The standard procedure for grinding a single roll is as follows:
(1) Mount roll in machine.
(2) Acquire (from database) or specify target parameters:
(a) Diameter, this may be an absolute number, such as when it needs to match a previously ground matching roll or it may be relative, such as when we specify a minimum amount of stock removal to remove the work hardened layer; (b) ideal (target) profile and tolerance (max allowable deviation from ideal shape); (c) max allowable out of roundness; (d) desired surface roughness, typically an admissible range of Ra values; (e) desired surface quality, typically a subjective, visual assessment of (absence of) surface features (feed lines, chatter, grit marks, and shadows).
(3) Measure actual roll profile and alignment. If necessary, correct alignment mechanically.
(4) Residual horizontal misalignment will be compensated for electronically.
(5) Roughing: Optional phase to remove minimum stock.
(6) Shape correction grinding. In this phase measuring takes place ‘on the fly’ and grinding continues until roll diameter and shape agree to within tolerance to target diameter and shape.
(7) Finish grinding: Successive passes to achieve roll surface quality, without changing roll shape.
(8) Remove roll from machine. The final measuring pass of the shape correction phase provides the values for out of roundness, the actual profile after grinding and the residual error profile.
The new method disclosed herein concerns the phase of shape correction grinding for roll pairs. It assumes that the difference between the two roll profiles defusing the gap, matters more than the individual shape profiles. The standard, existing procedure for grinding roll pairs simply applies the procedure for grinding a single roll twice. Typically, the target diameter for the second roll is set to match the diameter of the first roll.
Referring now to
For the second roll of the pair, the target profile in the shape correction phase is modified, adjusting it by the residual error of the first roll.
This results in a pair of rolls where the error in the gap between the two rolls no longer follows the sum of two error profiles, but the single error profile of the second roll.
Thus, in the same grinding time, a tighter tolerance for the gap than possible, by grinding the rolls independently, can be achieved.
Alternatively, one can shorten the shape correction grinding phase by relaxing the shape tolerance. One can do this for both rolls or one can relax the tolerance only on the first roll. The final error in the gap between the rolls will still correspond to the error profile of the last grind.
Having thus described the invention with the detail and particularity required by the Patent Laws, what is desired protected by Letters Patent is set forth in the following claims.
