US 2736224 A
Description (OCR text may contain errors)
Feb. 28, 1956 C, TRlNKLE 2,736,224
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United States Patent C) PIN BLCK Carl Trinkle, Cincinnati, Ohio, assigner to The Baldwin Piano Company, Cincinnati, Ohio, a corporation ot Ohio Application October 12, 1955, Serial No. fltlt Claims. (Cl. SLL-202) This is a continuation-in-part of my copending application of the same title Serial No. 296,861, led July 2, 1952, since abandoned My invention relates to pin blocks, sometimes called wrest-planks, in stringed musical instruments, such as the piano. The pin block is a wooden member in which threaded metal tuning pins are set, the strings of the instrument being fastened at one end to a metal frame and at the other individually to the tuning pins for tuning purposes. The pin block itself is, of course, rigidly mounted also with respect to the metal frame or plate. The tension on the strings by which they attain correct frequency characteristics is obtained by turning the tunning pins, thereby tightening or loosening the strings. It will be evident that the tuning pins must be turnable in the pin block for tuning purposes, but at the same time they must hold so firmly in the pin block that they will not be liable to rotation due to the tension on the strings, or rapid detuning will result.
It is a primary object of my invention to provide a pin block and a method of using it which will result in a readily tunable instrument less liable to loss of tuning in the course of time by loosening of the pins.
This and other objects of my invention which will be set forth hereinafter or will be apparent to one skilled in the art upon reading these specifications, I accomplish by that construction and in that mode of use of my novel pin blocks, of which I shall now set forth certain examplary embodiments. Reference is made to the accompanying drawings wherein:
Figure 1 is an exemplary cross-sectional View showing a portion of the frame or plate of a grand piano equipped with my pin block and a series of tuning pins.
Figure 2 is a chart showing the behavior of tuning pins in a pin block of ordinary construction when drilled with different sizes and conditions of drills.
Figure 3 is a similar chart showing the behavior of tun- General considerations Pin blocks are normally made of wood, and in the United States hard maple is used almost exclusively for the purpose by reason of its strength. A softer wood could not be expected to hold the pin as well against rotation. Hard maple may be replaced by comparable woods; and while I shall describe my invention hereinafter in connection with the use of hard maple, it will be understood that this does not constitute a limitation and that the term wood is inclusive of other woods suitable for pin block manufacture and having characteristics similar to those of hard maple. The teachings hereinafter set forth result 2,736,224 Fatented 28, 1956 ICC in an improvement of the torque of tuning pins irrespective of the character of the wood.
Economic considerations for the most part dictate the use of hard maple because, to all intents and purposes, it is better than other woods obtainable at roughly the same price. By way of illustration, Technical Bulletin 479 of the United States Department of Agriculture shows that sugar maple having a moisture content of 12% exhibited a compression strength parallel to the grain of 5390 pounds per square inch and a compression strength perpendicular to the grain of 1810 pounds per square inch, Whereas the comparable values for yellow poplar at the same moisture content were respectively 3550 and 580 pounds per square inch.
The failure of pin blocks to hold the pins properly is largely due to changes in the wood occurring by reason of changes in its moisture content. it has been found over the years that grand piano pin blocks are more apt to show loose tuning pins after several years service than upright pianos. In a grand piano the pin block l. is fastened to the lower surface of the piano plate 2 substantially as shown in Figure l. The plate or frame is drilled with holes 3 through which the tuning pins can extend. The plate exerts no force opposing the rotation of the pins 4, which are engaged solely in the block as shown. But while the upper surface of the block may be covered by the plate 2, the block elsewhere is xposed to atmospheric conditions so that its moisture content can change quite readily. Tests have shown that grand piano pin blocks may reach a moisture content of 14 to 16% along the Gulf Coast in summertime, while they may drop to 3% moisture content in northern States in the wintertime.
ln the case of upright pianos, the pin block is glued to the front surface of the piano back which is a rather massive wood assembly, and the piano plate then covers the entire front of the pin block. Thus in place of a pin block (usually about 13/s in. thicl in a grand piano having all but perhaps one of its faces exposed to the atmosphere, the pin block in an upright piano becomes part of a massive wood section about 8 in. wide, 41/2 in. thick and 54 in. long. Hence the moisture content change in upright piano pin blocks has been found to be not nearly so great as in grand piano pin blocks. The invention here set forth, is of course, applicable to pin blocks of either type, and will produce proportional improve* ments. lts greatest value will be achieved in any type of instrument where the circumstances of its use subject it repeatedly to wide changes in moisture content.
Wood as it comes from the tree may contain as much as 250% of moisture. The moisture content of wood is defined as:
Wt. of sample-Wt. of oven dry sample Percent moisture content of sample The water in wood may exist in two forms: imbibed water in the cell walls of the wood may amount to 25% to 30% of the weight of the oven dry wood and is generally accepted as being about 30%. Free water is all water present in excess of imbibed water, and usually is water found in the cell cavities of the wood.
There is no shrinkage in the early stages of drying green wood until the free water has been substantially cornpletely driven out, that is to say, until the moisture content cf the wood has been reduced to approximately 30%. lt is mbibed water or water actually present in the cell walls which causes the swelling or shrinking of wood. Thus from the liber saturation point to a state of zero moisture content wood will shrink, and the shrinkage may be considered a straight line function of the loss of imbibed water. The shrinkage, however, is not the same in all directions.
avenant A line drawn from the center of a log to its circumference is referred to as radial, whereas a straight line for the most part parallel to the growth rings is referred to as tangential. With this terminology in mind, it will be found that if a cube of sugar maple, which is exactly 1 in. across Veach face at the fiber saturation point, be dried to a zero moisture content, it will measure .951 in. in the radial direction, .905 in. in the tangential direction, and substantially 1 in. in the remaining direction. Restated, this means that sugar maple, when carried from the fiber saturation point to oven-dry condition will shrink substantially 4.9% radially and 9.5% tangentially.
In an exemplary pin block for a grand piano as hitherto made in the art, the 1% in. thick structure has been made up of plies, a top and bottom ply each 1/s in. in thickness, and three center plies each 3/8 in. thick. The grain direction of the top, center and bottom piies is parallel to the front edge of the piano. The grain direction of the other plies (plies 2 and 4) is parallel to the length of the piano, i. e. at right angles to the grain direction in plies 1, 3 and 5 of the structure. The lumber used is quartered northern white hard maple (sugar maple). By theterrn quartered l mean that the annular rings of the wood are essentially perpendicular to the face of the plies, the width of the plies lying in the radial direction of the wood.
The pin structure is standard in the United States. The pin has a diameter of .281 in. The top end of the pin is squared or otherwise configured for engagement by a tuning wrench. A hole for the string is formed 5A; in.
from the top. The threaded length of the pin is about 1% in., and the lower pointed end is 1A in. in length, the overall length of the pin being 21/2 in. When properly driven in a pin block 1% in. thick, the lower end of the pin will extend substantially to the lower surface of the block as shown in Figure 1.
The pin blocks are drilled with holes which are undersize with respect to the tuning pins themselves in order to give the pins a relatively high torque. If, say, a tuning pin having a diameter of .281 in. were driven into a .2594 in. diameter hole in a solid maple piece of 1% in. thickness, there would be great likelihood of splitting the piece. This is the reason why the maple pin blocks hitherto used in pianos have been made of a plied construction with crossed grain directions. It is also true that alternate layers in the conventional plied block help somewhat to restrain the swelling and shrinking of adjoining layers, improving the dimensional stability of the wood assembly, although the effect is very slight.
As illustrative of the eifect of changes in moisture content in a piece or ply of maple, the following may be pointed out: if a hole .260 in. in diameter is drilled through such a piece or ply when the ply is at 6% moisture content, and the structure is then placed in a controlled humidity chamber until it reaches a moisture content of 12%, there will be little change in the .260 in. dimension in the direction of the grain; but across the grain the dimension will change to .2625 in. f
If the same piece of Wood is then dried down to a moisture content of 3%, the cross-grain dimension of the hole will become .2588 in. When the pin is assumed to be present in the hole, it will be evident that because of the relatively low crushing strength of maple perpendicular to the grain, crushing of the wood takes place when the expansion due to moisture change exceeds the elastic limit of the wood so that the holding power of the wood for the pin is less after redrying. Thus, during the cycling of moisture content in a pin block in normal use, the holding power of the tuning pins diminishes, although this occurs irregularly, and often over a period of many years.
It has been found, however, that if a pin block sample is subjected to conditions of 80% to 85% relative humidity until the moisture content of the wood has reached 16%, then is gradually dried until the wood reaches a 3% to 3.5% moisture content, and if this cycle is repeated three times, further cycling does not appear to cause a significant loss in the holding power of the tuning pins. This furnishes the opportunity for accelerated tests on pin blocks, especially since the loss in holding power occurring in actual service lies substantially within the limits developed in the test.
The torque of a tuning pin is the product of the force or pressure exerted on the pin by the wood multiplied Yby the coefhcient of friction, multiplied by the radius of the tuning pin. When the size of the hole in which the tuning pin is engaged in the wood member varies due to swelling and shrinking of the wood, the torque will vary. At the same time the coefficient of friction changes with alteration in the moisture content of the Wood. lt is also true that wood will withstand a higher than normal pressurefor a short time without crushing. When a pin is first driven into the wooden member, it will exhibit a very high initial torque; but this also gives rise to a problem. The initial torque (and, indeed, the'torque at any time) should not exceed about 300 inch pounds, since the ordinary tuning pins will themselves break at a torque of about 350 inch pounds. It is not practicable to solve this problem by changing the size of the tuning pins. The pins have, as is well known, a transverse hole or aperture to accept the end of the string. Therefore, the pins have a tendency to break at the position of this hole. increasing their diameter would not increase their strength usefully without such a dimensional change as would make tuning less accurate or more difficult, or both. Further, the ease of tuning the instrument must be considered; and the average piano tuner prefers a pin which has a torque substantially in the range of to 150 inch pounds at the time the piano is tuned.
Theoretically, pins having a holding power or torque of 35 inch pounds or less are regarded as loose Such pins may permit a reverse rotation when a string is pulling on them under a tension of 250 pounds, since 250 pounds multiplied by .14 in. (the radius of the pin) equals 35 inch pounds. As a matter of experimental determination, a pin having a torque of 35 inch pounds is just safe for a string under a 300 pound tension. This is because all pins exhibit a slightly higher break-away torque. Under given circumstances a pin which has a true torque of 35 inch pounds may show a break-away torque of 45 to 50 inch pounds, the torque dropping to the lower value as soon as actual starting of the pin has been accomplished. Variations in the moisture content of the woodwill affect the starting torque ditferently at different times. Actually, however, it is an object of this invention toV provide a pin block and method of using it such that the torque of any pin (including its initial torque) shall not exceed 300 inch pounds, while its torque, after many years of service vin normal variations of atmospheric humidity, shall not drop below 35 inch pounds. A continuous torque over the life of the instrument and under all'conditionsiof its :use lying in the range of 50 to 100 inch pounds is regarded as ideal.
In the past, efforts have been made to depart from the use of wo-od pin blocks. lt has been suggested that pin blocks might be made of laminated resinous construction by saturating cloth laminae with incompletely poly merized synthetic resins and pressing and curing panels from such laminae. These efforts have not been suecessful.
T he nature of the invention l have found that the holding power of a pin block as exerted on tuning pins engaged in it is dependent upon a plurality of factors, which may be listed as follows:
(a) The size of holes preformed in the pin block prior to the driving of the pins into position,
(b) The structural characteristics of the pin block itself,
as hereinafter set forth,
(c) The density in grams per cubic centimeter, or specific gravity, of the pin block, and
(d) The manner of its formation.
In view of the fact, as already described, that substantial stabilization of the tuning pin torque occurs after the conclusion of three wet-dry cycles in testing, this stabilized value may be taken as a convenient measure of the ultimate torque.
In the prior manufacture and use of pin blocks, it was found that there is generally an optimum drill feed for each drill speed for drilling a block. The hole should be clean-cut and neither visibly torn nor scorched. it has been a common practice to touch the llute of a new drill with a piece of emery cloth for a distance of from 1/2 in. to l in. from the drill tip in the belief that this would give a tighter pin. lt was also a practice to set aside worn drills for drilling the tuning pin holes in the pin blocks of critical pianos. It was not clear just what happened under these circumstances since the holes had apparently the same internal diameter as holes drilled with new or untreated drills. However, when the matter was checked by drilling test holes in samples of pin blocks of normal density and thickness, weighing the block before and after drilling and computing the wood removed as well as measuring the apparent diameter of the hole, it was found, by way of example, that a new 6.6 millimeter drill (.2594 in. diameter) actually removed wood equivalent in weight to a .259AI in. diameter hole. An old drill of .2594 in. diameter removed wood equivalent to that occupying a space only .230 in. in diameter and then compressed the wood until the actual hole was .2594 in. in diameter. The compression of the wood exerts a considerable effect on the torque of the pin, making initially for a very high torque but eventually, after three wet-dry cycles, for a very low torque. Compression of the wood by an old or treated drill was, therefore, found to be harmful and to tend away from the results sought by me.
Superior results were secured with clean, well-cut holes; but the matter of the initial hole size as compared with the outside diameter of the tuning pin was found to be a matter of very substantial importance. As has already been said, the tuning pins are driven into undersized holes in the pin block, so that the pins themselves compress the wood. The eifect of this is substantially different, however, from the effect of that type of compression which occurs when a worn drill is employed. In the tests to which this specification relates, the holes in all of the pin blocks were pinned with standard Amsco tuning pins having a diameter of .281 in. i .001 in. The pins were driven in with a pneumatic hammer to the standard depth employed in commercial grand pianos i. e. the tip of the pin was driven to a point 1% in. below the top surface of the pin block so that the threaded portion of the pin contacts a length of 1%@ in. of the internal surface of the hole. Variations in the thickness of the pin blocks as such under these circumstances do not affect the torque of the pins. All pins were torqued one hour after driving and seating Seating is a term employed to indicate the practice of turning a pin one-half turn in the clockwise direction, then backing it t turn, then advancing it Mt turn, during which procedure the coils of the string are lifted, in ordinary practice.
Figure 2 shows the typical behavior of standard pin blocks when drilled and pinned with various size holes and before and after cycling. These blocks corresponded in structure to conventional blocks as set forth above, namely each block was formed of two face pieces of 1/s in. maple and three interior pieces of in. maple. in the chart, torque is measured on the X axis. T he Y axis is divided into sections indicating the use of different size drills. The rst horizontal section shows the result of employing a worn 6.6 millimeter (.2594 in.) drill. The
length of the horizontal lines on the chart illustrates the spread of values as determined for different pins, while the cross mark intermediate each line indicates the average value. lt will be seen that with the named drill size the average torque of the pins as driven was very high, namely 240 inch pounds as indicated at 5. When the moisture content of the pin blocks had been raised to 16%, the torque averaged about 170 inch pounds, as shown at 6. When the pin block was again dried to a moisture content about 3.5%, the torque had fallen to about 55 inch pounds, as shown at 7. After three wetdry cycles, the torque had become stabilized at a relatively low value of about 2O inch pounds, as shown at 8.
The use of a new, clean-cutting 6.6 millimeter (.2594 in.) drill produced a lowering of the initial torque, as at 9; bu the final torque 1l) had risen from about 2O to about 40 inch pounds. The use of a new .266 in. drill, as shown in the third section of the chart at 11, reduced the initial torque to 150 inch pounds but maintained the final torque at approximately the same value as that produced by the smaller drill, namely a value of about 40 inch pounds as shown at 12. Finally, the use of a still larger drill having a diameter of .270 in., lowered the starting torque to a little over 100 inch pounds, as at t3, but also produced a lowering of the final torque to about 25 inch pounds as shown at 14. It is evident that new drills are preferable to old or dulled drills and that a relatively larger drill size (.266 in.) produced a better result than a smaller drill (.2594 in.) in maintaining final torque while diminishing initial torque, although a still larger drill (.270) lowered final torque.
Figure 2 should be contrasted with Figure 3, which shows the behavior of a block pinned under similar conditions but constructed (as hereinafter explained) in accordance with the teachings of this invention. Again, we nd that as a change is made from old or worn drills to new drills of the same size, and as the drill size increases with respect to the diameter of the pin, the initial torque steadily diminishes. At the same time, the final torques are very substantially improved. Even under the most adverse conditions, the final torque averaged over 55 inch pounds, and it increased to a value of nearly inch pounds when a new .266 in. drill was used. It should be noted that the final torques in all instances are definitely higher, none of them approaching the 35 inch pound minimum, though in the case of the chart, Figure 2, two of the values were below that minimum, and the remainder only slightly above it. It can be seen also that a substantial improvement in iinal torque was attained by changing from a new .2594 in. drill to a new .266 in. drill in Figure 3 accompanied by a very great diminution in the initial torque. The blocks of Figure 3 when pinned, after drilling with .266 in. and .270 in. drills, closely approach the ideal situation. Larger drills may be employed, as hereinafter set forth.
A primary diderence lies in the physical structure of the blocks. T he structure tested in Figure 3 was a structure 1.248 in. in thickness made up of 39 layers or laminae of sugar maple each 1/28 in. in thickness, the laminae being arranged cross grained, adhered together with a phenolic resin adhesive under hot pressing conditions. As will hereinafter be discussed more at length, I have found that when a pin block is made up of relatively thin laminae of resin-bonded wood within certain limits, the new and surprising results exemplified by Figure 3 are attained. I have investigated ply thicknesses of .375 in., .250 in., l25 in., .050 in., and .036 in.
in the charts of Figures 4 and 5, I have plotted the initial and final torques of tuning pins in a series of differently constructed pin blocks. Along the horizontal axis of each chart, the primary thickness of the layers in the pin blocks is charted in terms of their thickness. Torque in inch pounds is shown in the vertical axis. The blocks marked .375 were blocks of standard construction, the .375 inch figure designating the 1%; in. thickness of effacent the three main plies of the block. The block, however, had face plies top` and bottom 1%: in. in thickness as is conventional in the art. Charts of Figures 4 and 5, moreover, are charts for the behaviors of the respective blocks when drilled with new .266 in. drills and pinned in the manner set forth above.
In the chart of Figure 4 the blocks were hot pressed. The .375-in. figure designates blocks having the conventional construction, and an average density of 0.723. The .250 in. ligure designates a block made up of five laminae of 1/4 in. sugar maple with two face laminae 1/0 in. in thickness. These blocks averaged 0.718 in density.
The .125 in. figure in Figure 4 designates blocks made up of eleven pieces of 1A; in. maple. These blocks had a average density of 0.732. The .050 in. and .036 in. iigure designate respectively blocks made up of twenty-seven pieces of 1/20 in. sliced maple and thirty-nine pieces of 1/23 in. sliced maple respectively. The density of the iirst of these blocks was 0.829, the density of the second averaged 0.851.
As to the chart of Figure 5, the block. were similarly constructed in the thickness and arrangement of their plies excepting that they were cold pressed, as hereina ter described. The density of the standard blocks designated as .375 in. averaged 0.691. The density ofthe .250 blocks averaged 0.726. The density of the .125 blocks averaged 0.730. The density of the .050 blocks averaged 0.788, and the density of the .036 blocks averaged 0.777.
It will be noted in both Figures 4 and 5 that decreasing the thickness of the laminae from 3/8 in. to 1A in. had little effect on the holding power of the pins. The initial torque decreased somewhat in both instances. The final torque-in the case of hot pressed blocks remained approximately the same, while in the case of cold pressed blocks, it increased from a figure slightly below inch pounds to one slightly above that ligure. Decreasing the thickness of the plies or laminate to Vs in. or lower, how ever, had a curious effect on the torque of the tuning pins. While the initial torque tended to increase somewhat, the final torque tended to increase considerably and usefully. It is interesting to compare the charts of Figures 4 and 5 with the chart of Figure 2. lt will be evident that with pin blocks of standard or conventional construction, conditions which produce extremely high initial torques are entirely compatible with conditions producing very low final torques. Throughout the range of resin-bonded pin blocks having lamine as thin as or thinner than about in., the tendency is toward acceptable initial torques accompanied by higher final torques. Wood plies substantially thinner than .036 in. are not of commercial `interest for two primary reasons: first, because they are uneconomical and difficult to handle and, second, because eXtreme thinness makes for a structure in which the Y `density because ofthe resin content is too high.
All other factors being equal, l have found that the optimum ply thickness for hot pressed blocks is approximately .125 in. or thinner, while for cold bonded blocks a thickness of about .050 in. or thinner appears to be optimum.
VAs set forth above, the size of the hole into which the pins are driven is of importance with relation to the diameter of the tuning pin. It will be obvious that different size holes will be found best with pins of different diameters; but since tuning pins in this country are substantially standardized at a diameter of .281 in., it is convenient to-express the ratio in terms of drill sizes. In the practice of my invention l have found, contrary to the fact with convenitional pin blocks, that the drill size may be varied from, say, .260 in. to .274 in. or from a ratio of drill diameter to pin diameter of about .925 to about .975, without seriously affecting the final torque. lf a drill as small as about .260 in. is used, there is some In most instancesa .266 in. drill appears to be optimum but the f8 tolerance is relatively wide. Good results Tare Obtained with .270 to .274 in. drills, which is inotzithef'QaSeWthth standard pin block.
lf the coating of resin between the pliesYremainS-ithe same, the density of the pin block will increase as the number of plies in a given thickness is increased. It is advantageous to vary the hole sizes within the substantial lirnits set forth-.260 to .ZM-inversely to the ply thickness within the limits set forth-Vs to about V28 inch--and directly with the density within the limits hereinafter set forth. The density is the importantfactor, since it does not necessarily vary with ply thickness.
Density in grams per cubic centimeter, i. e. specific gravity, is governed primarily by the glue-to-wood ratio, secondarily by the temperature of bonding and the pressure applied. The blocks taught herein were made with a pressure of 200 p. s. i., and in the case of hotpressing, with a 300 F. platen temperature. Under these conditions, and when the resin was applied as hereinafter speciiically described, the hot pressed blocks using%0in. and 1/28 in. veneer had densities of .83 and '.85 respectively. Below .83 the blocks exhibited typical characteristics'of conventional pin blocks in that the torque upon drying after a humidity cycle is definitely lower than the torque at high moisture content. Above .83 the torque upon drying after the initial high moisture treatment has a tendency to approach or even to be slightly higher than thetorque when the wood has a high moisture content. The crossing of these curves at around density .83 has led me to believe that the optimum density of a hot pressed pin block of my new construction is in the vicinity of this Value, namely around .83. If the density greatly exceeds this value, the structure begins to behave like a resinous laminate n the ordinary sense, and becomes unsatisfactory as a pin block.
As indicated, density is largely controlled by the glueto-wood ratio which, in turn, will usually vary withthe number of laminae in a given block thickness, assuming the same weight of glue per lamina is used. Consequently it is difficult to establish a density range without regard to other factors; but l have achieved good results with densities varying from about .72 to about .'87.
It may be remarked parenthetically that whereas the wet-dry cycle to which I have referred gave the results shown in the gures of this application, I have found that entirely comparable results may beattained by submerging the blocks in water for 88 hours (at the end of which time they averaged 40% moisture content), drying Ithem for several days under room conditions until they reached a 10% moisture content, and finally placing them'in an oven at 15% relative humidity until their moisture content had dropped to 31/2%. This soak and dry test can be completed in 10 to 14 days whereas the highlow humidity cycle test as previously described requires about days. The type of correspondence of results referred to may be illustrated by pointing out that with hot-pressed blocks drilled with new .266 in. drills, the final torque at the end of a soak-and-dry test multiplied by the factor 1.4 was the same as the final torque determined at the end of a high-low humidity test.
Mode of manufacture Where herein I have referred to glue or adhesive I mean a resinous type of adhesive capable of--settng up under proper conditions to a hard infusible and insoluble form. The adhesive itself need not consist solely of a settable resin but may include other substances and, as required may be dissolved or suspended in a solvent. 'In my work I have employed two general types of resinous adhesives, the iirst being an incompletely polymerized resin capable of setting up under heat and pressure to'the final form and the second a resinous adhesive capable of setting up in the presence of a suitable catalyst atroom temperature.
In the manufacture of hot pressed blocks I use a thermosetting resin such as, but not necessarily limited tofa 9 phenol-formaldehyde resin. One such resin is sold under the trade name Amberlite I. R. 14. This is a phenolformaldehyde condensate carried to the point of an A- stage resin soluble or miscible in water or alcohol or acetone. Other similar resins are sold under the trade names Bakelite B. C. 17875 and Monsanto Lauxite P. F. 2600.
The nature of the resinous settable adhesive is not a limitation on my invention. That some resins are preferable due to inherent characteristics is of course true; but the teachings of this application will produce improvements in pin blocks bonded with any resinous adhesive capable of holding plies of wood together lirmly. Thus I may employ urea-formaldehyde resins, or resins of the acrylonitrile or melamine types.
When making blocks by the cold pressed procedure I use a type of resin which, when compounded for use, will set up to the hard infusible form without the use of heat. These resins are for the most part condensation products resorcinol and formaldehyde. To some of them a catalyst should be added before use while others contain catalytic materials which become active when the resins are dissolved or suspended in a suitable vehicle. With still others of these resins it is the usual practice to mix two compositions neither of which is complete in itself so far as the condensation reaction is concerned. Thus I may employ a resin sold under the trade name Amberlite P. R. 115 which contains the resorcinol and a portion of the formaldehyde. For use this will be mixed with a material known as Catalyst P. 117 which contains the remaining formaldehyde and a ller such as wood iiour. Other resins may be emploped such as for example those sold under the trade names Penacolite and Cascophen R. S.2l6 with catalyst FM-60.
In forming pin blocks by the cold bonded procedure, the wood laminae are suitably coated with resinous adhesive, which may be done by brushing, spraying, dipping, or otherwise. They are then laid up in a stack and subjected to pressure, say 200 p. s. i. in a suitable press of any desired type. Curing of the resin occurs at room temperature, and it is merely necessary to leave the block in the press for a suitable length of time to accomplish proper curing.
In the manufacture of hot pressed blocks, the adhesive may be applied in a similar manner and the plies stacked and pressed; but in order tc obtain a proper cure, suliicient heat for a suiiicient length of time must accompany the pressure. With the designated hot pressing adhesive a cure at 285 F. (glue line temperature) for three minutes will be found sufficient; but this gives rise to a problem in that the heat being applied through press platens or other pressing members must penetrate to all parts of the structure being treated so as to achieve the proper cure even in those parts which are furthest from the source of heat. Moreover, the heated members should not be at so high a temperature as adversely to affect either the wood or the resin.
By way of example, in making up a pin block using laminae of sugar maple 1/28 in. in thickness, I may follow the practice of coating and stacking, say, nineteen plies of the wood and resin and pressing the structure between platens heated to a temperature of substantially 300 F. to 310 F. for twenty-tive minutes. Two such composite structures, after cooling, may then be bonded together by inserting between them a 1/25 in. ply of the maple coated with the resorcinol-formaldehyde room temperature adhesive and catalyst, and permitting this to set at room temperature. It is, of course, not beyond the scope of my invention to produce a hot pressed slab or pin block in a single operation, although this ties up equipment for a greater length of time. Hot pressed blocks, as will be evident from the drawings of this case, have normally greater torques than cold pressed blocks.
In an exemplary procedure for certain styles of grand pianos I make up a pin block slab 1% in. thick by 16 in.
10 wide and 60 in. long. I may use hard northern maple sliced 1/28 in. thick. The long grain sheets of veneer are of course 16 in. wide by 60 in. long. In forming the cross grain sheets the wood is jointed, and the composite trimmed to the same dimensions.
As the glue, I may employ 20 pounds of Amberlite P. R. 14 cut back with 131/2 pounds of water and 4 pounds of methanol. In spreading this adhesive on the veneer sheet I prefer to employ a roll spreader consisting of two applicator rolls between which the veneer sheets may be passed. Each applicator roll is provided with an adjustable doctor roll; and the resin or resinous varnish is maintained in the pinch between the doctor roll and the adjacent applicator roll. The doctor roll meters the resin onto the applicator roll which in turn transfers the relatively thin iilm to the veneer sheets. In a typical operation I coat the long grain sheets only, applying to their surfaces from 38 to 40 pounds of the above mix per 1,000 square feet. The coated sheets are placed on perforated spacers in aconditioning room at 50% relative humidity and at a ternperature of 75 to 80 F. for from twenty to forty-eight hours. The uncoated cross grain sheets are preferably similarly conditioned. In assembling the laminae for pressing, an uncoated cross grain lamina is interleaved between two coated long grain laminae throughout the structure.
Instead of forming two half-sections and joining them to form a slab as described above, I may first form a slab of half thickness and, after cooling, apply additional coated and uncoated sheets to each side of the half slab until the final thickness is attained. The structure is again placed in a press and hot-cured as described above. This procedure works very well because the central portion of the full-thickness slab will previously have been bonded and the heat in the central portion does not have to be brought up to full value.
After completion, the slabs may be sawn lengthwise into pin blocks of the required dimensions.
In the test procedures illustrated in the charts forming the figures of this application, after determination of the initial torque upon driving and seating, the blocks were placed in a humidity chamber at to 85% relative humidity until their moisture content had reached 16%. The pins in the blocks were then torqued again which gave the intermediate values in which the spreads were indicated by dotted lines (see 6 in Figure 2).
The blocks were allowed to dry out gradually under normal atmospheric conditions until they reached a moisture content of about 8% or 10%, whereupon they were placed in a drying chamber at 15% relative humidity until their moisture content had dropped to 3.5%. The pins were again torqued (see 7 in Figure 2).
The steps thus far outlined make up the first high-low humidity cycle. Two more identical humidity cycles were performed on the blocks, and the pins torqued again to give the final readings. However, after the final high-low humidity cycle, the pins were re-seated as described above before the final reading. The purpose of this was to break away any slight rust film which might have formed between the pins and the surrounding wood, since I have found that re-seating the pins under these circumstances gives a more normal reading.
Modifications may be made in my invention without departing from the spirit of it. Having thus described my invention in certain exemplary embodiments, what I claim as new and desire to secure by Letters Patent is:
1. A pin block for the tuning pins of a stringed musical instrument, said block having its full pin-receiving depth made up of cross-grain plies of hard wood bonded with a set resin, said plies having a thickness not substantially greater than 1A; in. and not substantially less than /g in., the said pin block in the said pin-receiving depth having a density in the range of substantially .72 to .87 grams per cubic centimeter, in which pins are driven in holes, and in ata-36,224
l 1 WhiQh'theratio of holediameter topin diameter lies substantiallyi-n'therangeof .925 to .975.
2. Thefstructure claimed in claim 1,` saidpins having a diameter of .2,81 in., saidpins being set in preformed holes in the pin block having afdiameter of substantially .260V
in. to .i274iin.
v3. The structure claimed in claim r1 wherein the wood plies are resin-bonded :with a resin vself-,setting at room temperature under pressure.
4. lA hot-bondedipin block for the tuning pins of a stringed musical instrument, said block consisting vof cross-grain plies of hard -.,wood ybonded with a setting resin, said plies having a thickness lying substantially between .125 i-n. and .036 in., the said block having a density substantially between .72 and .87 grams'v per cubic centimeter, -in which pins are drivenvinjholes and in Which ther-atie of hole diameter to pin diameter lies substantially in the range of .925 to .975 in.
5. Ajplied pin block for thetuning pins of a stringed musical instrument, said block consisting Yof cross-grain plies of hard Wood bonded with a -cold-setting-resin, the plies-of said plied .structure having Aa thickness substantially between .O50 in vand -.036 in., the plied structure having a density substantially lbetween .72 and .87 grams Perfcubic centimeter, in which pins are driven in holes, and in which the ratio of hole diameter to pin diameter lies substantially in the range of .925 to .975.
6. The structure claimed in claim 1 wherein the wood plies are resin-bonded under a pressure of substantially 20() pounds per square inch with a resin which is selfsetting at room temperature.
7. lThe structure claimed in claim 1 wherein the-wood plies are resin-bonded at a pressure of substantially 200 pounds per square inch and at a glue line temperature of substantially 285 F. with a thermosetting resin.
8. The structure claimed in claim 2 in which the hole size varies with the density.
9. The structure claimed in claim 4 wherein the said setting resin is cured at a glue line temperatureiof vsurbstantially 285 F. and at a pressure of substantially 200 pounds per square inch for at least about 3 minutes.
10. The structure claimed in claim 5 in which the said 'resin is set in the plied structure at la pressure of ,substantially 200 pounds per square inch, substantiallyat room temperature.
No references cited.