FR2509076A1 - Musical bow stick manufacturing method - involves impregnating reinforcing strands before pouring pigmented resin into mould - Google Patents

Abstract

The method manufactures a composite bow-stick for a musical instrument, determining the number of strands of fibres, and the modulus of elasticity as a function of the diameter of the print in the mould half, using a formula also taking account of the moduli of elasticity of the fibre material and of the resin, and of the cross-section of the fibre strands. The strands forming the reinforcement of the stick are produced in equipment supplying them in different lengths and impregnating them, and after coating the mould surfaces with a separation agent, part of the previously prepared and pigmented resin is poured into it. The strands are then placed in the mould prints in their lengthwise direction and fanned out at the ends, after which the remainder of the resin is poured.

Description

 For several years, professional lutherie circles have been questioning the rarity, quality and price of the wood used in the making of stringed musical instruments.

 With regard to bows requiring hardwoods with high mechanical performance, such as PERNAMBOUC, imported from Brazil, the supply problems arise in the same way. The prodigious increase, during these last decades, of the consumption for construction and furniture requires felling taking place more and more towards the heart of the warmer and rainier forest. This results in essences of trees having more and more loose fibrous structures, which gives objects less mechanical characteristics.

 In addition, shippers faced with this situation, aware of the prospect of shortages (the regeneration cycles of these tree species are counted in centuries, tend to raise their prices and institute increasingly rigid supply regulations, in particular with regard to the freedom to choose lots.

 This situation therefore encourages, in order to protect this artistic branch, to seek satisfactory alternatives.

 Thus, the new composite materials with high mechanical performance (high modulus of elasticity and high resistance for a low density) such as carbon fibers recently marketed are the only ones able to provide a solution as we have been able to verify by carrying out pro totypes according to the process described below.

 This process, which we propose to develop, is based on the technique of molding by mold in half-shells symmetrical with respect to the plane of symmetry of a traditional model baguette (plate 1/5) and that of "draping" or the "stratification" of unidirectional filamentary materials coated with polymerizing resin at room temperature.

 It consists, more precisely (plate 2/5), of placing longitudinally bundles (1) of carbon fiber wicks, previously impregnated with a pigmented resin, in the two half-cavities of the molds.

These half-shell molds are fitted with 2 centering pins ensuring the perfect position of the two elements when they are assembled by gluing or overmolding. ns are moreover provided with "cores" ensuring the reproduction of all the functional recesses of the conventional bow stick (plan ches 1/5 and 2/5), namely - the mortise of the point (plate 2/5 - 2 ) - the mortise (5) for housing the screw-nut tensioning system
horsehair - the bore (4) for the passage of the control button screw - the front range of this screw (5)
These molds can either be metallic with engraving of the imprint or made much more economically using a wooden frame filled with a silicone resin which will ensure direct overmolding of the model wand.

 The number and different lengths of carbon rods constituting the beams (Plate 2/5 - 1) on which ultimately depend the mechanical properties of the rod are defined by the calculation which takes into account the characteristics of the fiber (density, section, module of elasticity), of that of the resin (density, modulus of elasticity) and of the rigidity Ge 'one wishes to impart to the rod.

où EF est le module de la fibre de carbone
E R celui de la résine
E B celui du bois que l'on se propose de emplacer
D le diamètre de la baguette en un point considéré de la longueur de
cette dernière
SC la section de la mèche de fibre de carbone
Le système de résine destiné à l'enrobage des fibres de carbone a été choisi en fonction des critères suivants
- bonne compatibilité avec la fibre de carbone, notamment du point de vue de la mouillabilité
- condition de mise en oeuvre (polymérisation à température ambiante pour éviter les déformations après cuisson)
- bonne résilience (résistance au choc)
- aptitude à la pigmentation
Le système retenu est du type "résine époxyde assouplie polymérisant à "froid" qui lui confère les propriétés attendues, c'est-à-dire
- un module d'élasticité de 2000 YPa et
- un allongement à rupture de 8 à tO %
Une baguette d'archet de violoncelle a été réalisée suivant le présent procédé à titre de démonstration.The mathematical expression giving the number N of loaves as a function of section D (variable along the baguette) is by a zemi-imprint

where EF is the modulus of carbon fiber
ER that of resin
EB that of the wood that we propose to replace
D the diameter of the rod at a point considered the length of
this last
SC section of carbon fiber wick
The resin system for coating carbon fibers was chosen according to the following criteria
- good compatibility with carbon fiber, in particular from the point of view of wettability
- processing conditions (polymerization at room temperature to avoid deformation after baking)
- good resilience (impact resistance)
- aptitude for pigmentation
The system chosen is of the “softened“ cold ”polymerizing epoxy resin type which gives it the expected properties, that is to say
- a modulus of elasticity of 2000 YPa and
- an elongation at break of 8 to tO%
A cello bow wand was made using this process for demonstration.

 Its definition was established on the basis of the weight and elasticity characteristics of a traditional PERNAMBOUC wooden stick. The diagram on board 3/5 allows you to judge the result obtained.

 The bundles of wicks are prepared on a suitable work surface illustrated in Plate 4/5. This device makes it possible to group the operations of induction flow rate of the wicks t of making up the bundles at their desired length.

 After despite a separating agent (release) in the mold cavities, a certain quantity of the pre-mixed and pigmented resin is poured so as to constitute the external "skin" of the parts.

 The draping of the bundles in the molds is carried out according to the configuration shown diagrammatically by the board 2/5, starting with the main bundle, extending by opening out in the point (6). A few reinforcing wicks (7) of the mortise (2) are arranged perpendicular to the trajectories of the main beam.

 After the removal of the other bundles, a central profiled core (board 2/5 - 8) made of a low density material (balsa or expanded polystyrene) is inserted and wedged in the axis of the two half-elements.

 After observing the time required for polymerization at room temperature, the mold is removed from the mold, the two elements of the strip are thickened, then their provisional assembly by double-sided adhesive to allow mass and position of the center of gravity.

 Although there are not yet specifications relating to the position of the center of gravity of the bow rods, it would be possible, at this stage of the process, to make corrections either at the tip by trimming or ballasting, or by ballasting in the extension of the front range of the button screw (plate 2/5 - 5).

 The final assembly of the two elements of the rod is then ensured by pressure bonding, at room temperature, using the same resin system described above, which will thus ensure perfect mechanical homogeneity of the assembly.

 The correct maintenance in their relative position of the two elements during the hardening of the adhesive is obtained by their placement in their respective imprints and provided with their core. After closing the mold, mechanical pressure is applied so as to expel the excess glue.

 The last finishing operations before the classic assembly of the bow consist of deburring the excess glue, sanding and polishing the stick.

 The control of the elasticity characteristic of the rod is carried out by the connection test of the type at a recessed end having the diagram accompanying the diagram of the board 3/5. This simple test can be easily carried out using the apparatus, the principle of which is illustrated in Plate 5/5. I1 comprises a frame (1) supporting a clamping system (2) of the upward end of the rod ensuring the desired embedding.

This clamping system is adjustable in position in the vertical plane of the frame and lateral of the vice (2) to take account of the various shapes and dimensions according to the types of rods. To the right of the point of the bow, the frame supports the system of application of the force constituted by r - a screw (3) screwing in the stirrup (4) linked to the btti and acting on the trap
(5) ensures the compression of the calibrated spring (6). The part (4) is load-bearing
two indexes (7) and (8) allowing the reading of part of the displacements
do the point of the bow on the graduated strip (9) fixed to the frame and, on the other
F part of the applied forces read on the strip (10).

The advantages arising from the use of the present method for producing bow sticks in composite material, compared with the current state of the art, are numerous. Indeed, in addition to the fact that it constitutes a palliative satisfying any deficiencies of natural materials, this process has the following advantages A simple, industrial implementation, requiring only little shaping
with the tool, compared to that of traditional wooden sticks
requiring exceptional manual skill or even with other professionals
yielded (*) by circumferential winding of fibers then requiring
special winding machines.

- Precise and continuous adjustment of the mechanical properties of rigidity and
mass of the rod allowing rapid optimization of the defined
tion of a bow according to the personal wishes of the artist.

Respect for the current general form specific to each type of instrument,
this form resulting from a long evolution through several centuries.

- Greater overall mechanical resistance for a mass equivalent to
traditional bows, thanks to the longitudinal arrangement of the beams fi
breux parallel to the neutral fiber of the rod, compared to at least
very provisions, or, in the case of wooden sticks whose texture is
correctly oriented, but randomly interrupted by the cutting tool.

- This aspect is even more interesting in the critical zone of the point where,
thanks to the extension of the wicks trajectories of the main beam of the
baguette and their distribution throughout the extent of the tip
(plate 2/5 - 6), we obtain here a solidity clearly superior to the ba
wooden lookouts.

(Xs Patent of invention N01515007 G1OD PV N077682 Improvements to ar
fall of musical instruments.

- Better stability over time to aging and variations
of temperature and humidity of the ambient atmosphere. Carbon does
does not age and its coefficient of thermal expansion is practically
negligible.

The process by molding the bow sticks also ensures
interchangeability with added accessories (Rise and buttons)
which would be produced using a molding process. Currently, the in
traceability of these accessories is made impossible by the making
craft.

- Finally, the choice of the longitudinal arrangement of the fiber bundles
provides, to a certain extent, an aesthetic appearance reminiscent of that
of the grain of the wood, since the orientation of the texture of this last point
cides with that of the carbon wick fibers of the present process.

Claims (8)

 1. Method for making bow sticks of composite materials characterized by the following @@ apes s  - determination of the quantity of strands of fibers to obtain the desired elastic modulus according to the mathematical expression below giving the number N of strands as a function of the diameter D of the rod at the current point for a half mold imprint,

 - finishing operations and deburring of the glue, general sanding: and polishing  - final assembly of the two constituent elements of the baguette by cold gluing under pressure using the same resin system having ribbed during molding  adjusting the weariness of the rod and the position of its center of gravity by trimming or ballasting at the tip or ballasting in the cavity provided for this purpose (plate 2/5 - 5)  - provisional assembly of these two elements to make the following adjustments  - after polymerization: surfacing and deburring of the two symmetrical elements of the rod thus obtained  - addition of resin residue  - development of the main bundle wicks in the tip area (plate 2/5 - 6)  - longitudinal arrangement of the bundles of wicks in the mold cavities (parallel to their plane of symmetry)  - casting in the ioules of a part of the resin previously prepared and pigmented  - deposit of a separating agent on the surface of the fingerprints of the ioules  - making of the bundles of dryers constituting the frame of the rod using a device which ensures the flow of the different lengths of the dryers and their impregnation (plate 4/5)  that of the resin @C the section of the wick of fibers  4 that of fiber  EB being the desired modulus of elasticity  2. A method of making the reinforcement of the point of the rod according to claim 1 characterized by the arrangement of carbon fibers oriented perpendicular to the paths of the wicks of the main bundle.  3. Method for producing functional recesses according to claims 1 or 2 characterized by the seat in place of the board cores 2/5 - 2 - 3 - 4 and 5).  4. A method of lightening the strip according to claim 1 @, 2  or 3 characterized by the insertion of the central core made of a material  of the balsa or expanded polystyrene type (plate 2/5 - 8) -  5. Method for checking the conformity of physical characteristics  mechanics of the bow wand obtained by applying the method according to  one of the preceding claims, which is broken down as follows:  . mass control  . control of the position of the center of gravity  . elasticity check  6.Adjusting the position of the center of gravity of the bow rod  according to claim 5 characterized in that it is carried out by the-  tage of a cavity located in the extension of the range of the adjustment screw  of the tension in the bow designs (plate 2/5 - 5) and / or by lightening gold  ballasting of the point of the stick.  7. Apparatus for implementing the method according to claim 5  and allowing to measure the elasticity of the bow rod (plate 5/5)  by subjecting the part to a bending test of the simple embedding type  and which is characterized by the following elements  . a clamping system (2) ensuring the embedding  . a force application system (3, 4, 5s 6, 7 and 8)  . a bar for reading the effort  , an arrow reading strip (9)  8. Bow wand obtained by applying the method according to one of claims 1 to 6 and characterized by the following 2 materials  - high performance carbon fiber ensuring the resistant structure  - a flexible low temperature appay resin system  (polymerizes at room temperature) ensuring the role of binder  opposite fiber back.