US 3651604 A
A buffing process for improving the surface finish of clad aluminum-alloy sheets. Aluminum sheet stock is burnished according to a specific process program to provide uniform surface appearance and improved corrosion resistance, and the process provides controlled removal of a thin surface layer of cladding used in aircraft-skin aluminum sheets.
Description (OCR text may contain errors)
United States Patent Queyrel et al.
PROCESS FOR IMPROVING SURFACE FINISH ON CLAD ALUMINUM SHEETS Paul J. Queyrel, Newport Beach; Jerome O. Koenig, Culver City, both of Calif.
Mill Polishing Corporation, Huntington Park, Calif.
Filed: Nov. 25, 1970 Appl. No.: 92,562
Field ofSearch ..51/3l73l8, 328, 51/283, 45,142, 51, 38, 358
[ 51 Mar. 28, 1972  References Cited UNITED STATES PATENTS 2,293,515 8/1942 Littwin ..5 H45 2,436,466 2/1948 Wilson.... ....51/45 3,553,901 l/1971 Burt ..51/142 Primary Examiner-Donald G. Kelly Attorney-Christie, Parker & Hale  ABSTRACT A buffing process for improving the surface finish of clad aluminum-alloy sheets. Aluminum sheet stock is burnished according to a specific process program to provide uniform surface appearance and improved corrosion resistance, and the process provides controlled removal of a thin surface layer of cladding used in aircraft-skin aluminum sheets.
6 Claims, 4 Drawing Figures Patented March 28, 1972 3,651,604
m ii iiiiiii lllll 1w '1" filiililllll mnziq 39 1 num W INVENTORS.
PROCESS FOR IMPROVING SURFACE FINISH ON CLAD ALUMINUM SHEETS BACKGROUND OF THE INVENTION Clad aluminum sheets such as alloy 2024T3 are in wide use in aircraft construction, and are formed as an aluminum-alloy core or base sheet to which is metallurgically bonded a protective coating or cladding of high-purity aluminum or an alloy of aluminum. The base-sheet alloy is selected to provide the desired mechanical properties in the overall sheet, and the cladding alloy is selected to provide corrosion protection for the underlying base sheet.
Ordinary mill run clad-aluminum sheets are generally not suitable for airframe construction as these sheets lack a uniform finely finished surface which is desired for aircraft exterior surfaces. Airframe manufacturers have instead tended to select premium-finish sheets which receive additional mill treatment with finish cold-rolling equipment and other processes. This premium-finish treatment adds appreciably to the price of the aluminum stock as the production yield of satisfactory sheets is relatively low, and the additional processing is a significant increase in factory cost.
Buffing processes have been used in the past on aircraft aluminum stock, but these skin materials have typically been heavy sheets or plates (sometimes chemically milled for weight reduction) with thick cladding which could withstand heavy abrasive treatment without exposing the base alloy. Thin clad-aluminum sheet is demanded for construction of modern aircraft, and this stock may have a cladding thickness of only a few thousandths of an inch. Conventional buffing techniques have proved unsatisfactory with these thin sheets due to excessive removal of cladding which may expose the base material and in any event provides inadequate corrosion resistance.
We have developed a buffing or burnishing process which achieves aircraft-quality finishes on mill run sheets with a reduction in cladding thickness of only about 0.0003 inch. Sheets processed according to the invention exhibit a superior surface finish of uniform color, and minor scuffs, scratches, and blemishes such as oil stains are removed from the mill run stock. The processed sheets further exhibit improved corrosion resistance which is believed due to a micro-levelling and compaction of the cladding which results from the buffing process. Yield of satisfactory sheets from the process is high and uniform, and aircraft specifications are met at a cost substantially lower than that of sheets finished by premium coldrolling treatment.
SUMMARY OF THE INVENTION The process of this invention first involves positioning a clad-aluminum sheet in a buffing machine, the machine having a rotatable motor-driven cloth buff of an axial length adequate to span the sheet. The buff is rotated to maintain a buff surface speed in the range of 3,750 to 4,450 surface feet per minute. The buff is moved against the sheet while maintaining relative motion of the sheet and buff in a direction normal to the buff rotation axis to pass the buff across the sheet at a substantially constant lineal feed rate. A slight oscillatory relative motion of the buff and sheet is also maintained during buffing, and this oscillatory motion is about one cycle per second in a direction generally parallel to the buff rotation axis. A liquid buffing compound is applied to the buff and sheet throughout the buffing process, and dwell time of the buff on any portion of the sheet is limited to about 6 to 7 seconds.
Preferably, the lineal feet rate of the sheet is maintained in a range of 25 to 35 feet per minute, and the buff is urged against the sheet with a force which produces a drive-motor load of about one-quarter horsepower per inch of buff-sheet contact in a direction parallel to the buff rotation axis. This force fiattens the buff to produce a contact surface of about 3-inches width between buff and sheet across the full width of the sheet. The buff preferably has a diameter in the range of about 16 to 24 inches, and is formed of alternating disks of cloth and paper mounted between fiber spacers on a spindle, the paper disks having a diameter less than the minimum diameter of the buff to remain out of contact with the sheet.
DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view of a buffing machine suitable for use according to the method of this invention;
FIG. 2 is an end view of the machine taken from the left side of FIG. 1;
FIG. 3 is a side view of a buff-wheel disk; and 1 FIG. 4 is a sectional elevation of a portion of a buff-wheel assembly.
DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to the drawings, a burnishing or buffing machine 10 includes a table assembly 11 supported by a plurality of wheels 12 on a pair of rails 13 secured to a level fioor 14. A steel plate 15 forms the top surface of the table assembly, and the plate is stiffened by lateral cross members 16 and longitudinal members (not shown) to insure a level top surface. The table assembly must be quite large to accommodate clad-aluminum sheets as used in aircraft construction, and plate 15 is typically about 44 feet long and 10 feet wide.
As is conventional in buffing machines for sheet stock, a drive means (not shown) is provided to drive the table assembly along the supporting rails. A satisfactory drive means is a rotary hydraulic motor secured to the floor between the rails and having an output cog engaged with a drive chain or rack secured to the table. Electric motors or other drive systems of conventional types can also be used to drive the table at a desired speed of about 25 to 35 feet per minute during sheet buffing.
A conventional friction clamp 18 extends across the width of one end of plate 15, and a clad-aluminum sheet 19 to be buffed is secured at one edge in the clamp. A slightly resilient pad 20 ispositioned between the undersurface of sheet 19 and the top of plate 15 to protect the bottom of the sheet and to provide a slight cushion during buffing. Preferably, pad 20 is an inexpensive expendable material such as a compressed wood-fiber sheet known in the construction industry as Upson Board.
A buff-rake assembly 23 is secured to an end of plate 15 remote from clamp 18, and is used to clean and condition the buff (described below) prior to finishing of sheet 19. Assembly 23 includes a coarse rake 24 formed as a plurality of upstanding pins extending across the full width of plate 15, a serrated curry comb 25 used for intermediate dressing, and a fine rake 26 (preferably 24-grit open-coat aluminum oxide abrasive) for final dressing of the buff.
A buffing-head base 30 includes a pair of generally U- shaped portions 31 on opposite sides of table assembly 11 and each having upstanding members 32 and 33. Portions 31 are secured to floor l4, and the upper ends of members 32 are tied together by a cross beam 34. A buffing-head frame 35 is pivotally mounted to base 30 by a hinge 36 extending along the length of cross beam 34. Hinge 36 has several inches of axial freedom so the buffing-head frame can be oscillated sideways during buffing as described below.
A hydraulic cylinder 39 is secured to each member 33 of the base, and a piston rod 40 extending from each cylinder is secured to buffing-head frame 35 by a limited-motion fitting (not shown) which permits the sideways oscillatory motion of the frame mentioned above. The frame is raised or lowered by actuating cylinders 39 to pivot the frame around hinge 36. A pair of adjustable stops 41 are secured to opposite sides of frame 35, and a ball 42 at the lower end of each stop rests and rolls freely on a level upper surface of pedestallike member 33 to permit oscillatory motion of the frame. A screw jack (not shown) or similar adjustable means is provided within stop 41 to vary the maximum downward travel of frame 35 toward the upper surface of members 33. A motor 43 is secured to frame 35 adjacent hinge 36, and an output cam (not shown) on the motor bears on one of members 32 to oscillate the frame sideways or laterally with respect to the direction of motion of table assembly 11. This oscillatory motion is preferably about one cycle per second at a peak-to-peak amplitude of about 1 inch.
A buff-wheel assembly 45 is mounted on a spindle 46 by a pair of bearing supports 47 secured at opposite sides of frame 35. A drive motor 48 is mounted on a pad 49 on frame 35, and is coupled to the buff-wheel assembly by a plurality of drive belts 50.
A rail 52 is secured across the front of frame 35, and a spray-head assembly 53 is movably mounted on the rail to move across the surface of buff-wheel assembly 45. A liquid buffing compound is delivered to the spray head by a line 54, and the head is reciprocated across rail 52 during a buffing operation by a conventional drive mechanism (not shown) to spray the liquid compound across the buff.
Buff-wheel assembly 45 is shown in detail in FIGS. 3 and 4. The assembly comprises a plurality of buffing disks 60 having a central opening 61 with opposed keyways 62. Spindle 46 fits through opening 61 in each disk, and keys (not shown) which extend the length of the spindle engage keyways 62. Adjacent buffing disks 60 as mounted on spindle 46 are separated by hard fiber rings or spacers 63 of about one-sixteenth inch thickness. The spacers allow the buff disks to spread slightly to impart a desirable softness to the buff surface.
Each buffing disk 60 is preferably made of 16 plys of 86/93 cotton buff-cloth disks 65 (FIG. 3) having a weight of about 2.5 ounces per yard. The individual cloth plys are separated by a 40-pound kraft-paper disk 66, and the cloth and paper disks are secured together by three rows of stitching 67. The cloth disks have a typical outside diameter of 24 inches, and a central opening of about -inch diameter. The kraft-paper disks and fiber spacers have an outside diameter of about inches to remain well below the buff surface as the overall diameter of the buff-wheel assembly decreases due to wear.
Bufflng disks 60 have a thickness of about one-fourth inch, and 300 to 400 disks are needed to make a buff-wheel assembly of say 10 feet in length to span the large aluminum sheets used in aircraft construction. The buffing disks and fiber spacers are stacked individually on spindle 46, and are compressed tightly together and locked in place by clamps (not shown) at the ends ofthe spindle.
In use of the process, buff-wheel assembly 45 is first raked, smoothed and dressed by passing buff-rake assembly 23 beneath the buffing wheel. The table-drive system is then actuated to feed sheet 19 beneath the buffing-wheel assembly, We have found that the lineal feed rate of the sheet should be maintained substantially constant in the range of 25 to 35 feet per minute. Excessive sheet heating may occur with lineal feed rates less than 25 feet per minute, and an inferior sheet surface may result from feed rates in excess of 35 feet per minute due to string lines arising from loose threads in the buff.
As the buffing operation is commenced, a water-based liquid buffing compound is delivered to spray head 53 for application to the buff-wheel assembly. Preferably, this compound is an unfused aluminum oxide of 320 grit or finer in a tallow vehicle. A satisfactory compound is available under the designation M-ll buffing compound from M & T Chemical Company, Rahway, New Jersey. The amount of compound applied is controlled by the machine operator, and compound flow may be increased to provide extra buffing action to remove oil stains or other surface irregularities.
The speed of drive motor 48 is controlled to maintain the surface speed of the buff-wheel assembly in the range of3,750 to 4,450 surface feet per minute. Surface speeds less than the lower end of this range result in inferior buffing because the edges of the individual buffing disks tend to bend and deflect, and to lose their burnishing action. Surface speeds above the upper end of the range tend to harden the buff, preventing following of slight surface irregularities in the sheet and causing excessive removal of clad material from the surface being bufi'ed. The speed of drive motor 48 is adjusted as the buff wears to maintain the surface speed in the desired range. A substantial variation in motor speed may be needed, as a buff having an original diameter of 24 inches may be used down to a diameter of about 16 inches before it must be discarded.
As the process is commenced, hydraulic cylinders 39 are actuated to retract piston rods 40 and thereby lower hinged frame 35 to urge the buff-wheel assembly against sheet 19. Stops 41 are adjusted in height until the surface of the buffwheel assembly which contacts the aluminum sheet is flat tened or formed as a chord of the otherwise circular buff over a distance of about 3 inches. That is, the buff is urged against the sheet with a force sufficient to cause simultaneous contact of the buff-wheel assembly with a 3-inch length of the sheet across the full span of the sheet. This flattened surface of the buff should be maintained in a range of about two and onehalf to three and one-half inches. The load on drive motor 48 increases as the extent of buff flattening is increased, and monitoring of motor load using conventional gages is a convenient way of adjusting this parameter. We have found that motor loads of 0.22 to 028 horsepower per inch of sheet width produce the proper degree of buff pressure and hence buff flattening.
The degree of buff flattening and the lineal feed rate of the aluminum sheet set the dwell time of the buff on any given portion of the sheet during each pass of table assembly 11. [t is important that this contact or dwell time be limited to a total of about 6 to 7 seconds during the buffing operation, and this maximum limit is typically achieved in about 5 back-and-forth passes of the sheet beneath the buff-wheel assembly. Total dwell times beyond this limit may produce an excessive removal of clad material which cannot be tolerated in thinly clad aluminum sheets used in modern aircraft construction.
Lateral oscillator motor 43 is operated throughout the buffing process to provide a relative lateral oscillatory motion of the sheet and buff-wheel assembly. This motion has a double amplitude of about 1 inch peak-to-peak, and a frequency of about one cycle per second. Substantially lower oscillation frequency should be avoided to prevent lateral deflection of the buff disks, and substantially higher frequencies may produce excessive vibration.
The sheet is passed beneath the buff wheel assembly a sufficient number of times to achieve the desired finish, while still observing the maximum dwell time of about 6 to 7 seconds. We have found that removal of cladding can be held to about 0.0003 inch if the above-described process parameters are carefully observed. The finished sheet has superior surface characteristics with uniform color, and oil stains and other surface blemishes are removed. These factors are all of importance in aircraft construction where adjoining sheets in the air frame should have the same surface appearance to avoid a checkerboard pattern. 1
There has been described a process for burnishing or buffing mill run clad-aluminum sheets. The process produces a premium finish on the sheets, and achieves this objective with minimum removal of protective cladding from the sheet. We have also found that sheets buffed according to the process of this invention exhibit an improved resistance to corrosion arising from exposure to sea-coast air or other cor rosive atmospheres. It is believed that the buffing process tightens or compacts the clad surface, and reduces the number of pockets or tiny pores in which corrosion typically begins.
What is claimed is:
1. A process for burnishing a clad surface of a clad-aluminum sheet to achieve uniform surface appearance and improved corrosion resistance with minimum clad removal, comprising the steps of:
a. positioning the sheet in a buffing machine having a rotatable motor-driven cloth buff of an axial length adequate to span the sheet and overlap the sheet edges;
b. rotating the buff to maintain a buff surface speed of about 4,000 surface feet per minute during the performance of the remaining process steps;
c. moving the buff against the sheet while maintaining relative motion of the sheet and buff in a direction normal to the buff rotation axis to pass the buff across the sheet at a substantially constant lineal feed rate, and while maintaining a slight oscillatory relative motion of the buff and sheet in a direction generally parallel to the buff rotation axis; I
d. maintaining a liquid buffing compound at an interface of the buff and sheet throughout the buffing;
e. limiting dwell time of the buff on any portion of the sheet to about 6 to 7 seconds.
2. The process defined in claim 1 in which the buff surface speed is held within a range of 3,750 to 4,450 surface feet per minute.
3. The process defined in claim 2 in which the lineal feed rate is held within a range of 25 to 35 feet per minute, and separate buffing passes are repeated to achieve said dwell time.
4. The process defined in claim 3 in which the buff is urged against the sheet with a force which produces a buff-driving motor load of about one-quarter horsepower per inch of buffsheet contact in a direction parallel to the buff rotation axis.
5. The process defined in claim 3 in which the buff has a diameter in the range of about 16 to 24 inches and is formed of alternating disks of cloth and paper mounted on a spindle, the paper disks having a diameter less than the minimum diameter of the buff to remain out of contact with the sheet, and in which the buff is urged against the sheet with sufiicient force to produce simultaneous contact of the buff and sheet over a distance of about 3 inches in the direction of lineal feed.
6. The process defined in claim 5 in which the relative oscillatory motion of the buff and sheet is about one cycle per second at about one inch peak-to-peak displacement.