Hi List Newton Hunt said, (regarding bass string scale design) >Designing a decent scale is not hard as long as you have some minor >understanding of tension, power, inharmonicity, computers, math and >general engineering principles. The main problem is measuring and >data entry, the rest is just patience and asking, "what if?" Well said Newton! The following extract outlines the procedure we use for setting the cover length terminations of a redesigned bass scale. Manufacturers rarely take care to ensure that the copper wrap terminates at a uniform and close distance from the agraffes and bridge pins. During the late 1980s, we started marking adjusted winding termination points (with masking tape) on the original bass strings prior to destringing. The tape was placed as a guide for the string maker. When the original string scale is retained, this works very well. However, when a bass section is rescaled. the wrap terminations will shift in response to any change in the percentage of breaking strain of the core wire. For some years now, whenever rescaling basses, we mark the original strings with a corrected cover termination point. The adjusted termination points are calculated by entering the original and revised scales onto a spreadsheet. The spreadsheet, which uses the modulus of elasticity of piano wire, calculates the core wire stretch of the new and old scales. The difference is used to adjust the tape-mark for wrap termination adjustment. Using this method, the cover ends of the new strings will line up at the intended distance from the agraffes. Our preferred wrap termination points are achieved when they terminate 10mm from the agraffes and the bridges pins. I notice that there was some debate on the PTG list, regarding the actual modulus of elasticity of plain wire, ie. earlier this year. The formula we use is; Wire stretch in mm =([string length in mm]*[string tension in Kg])/((((([string diameter in mm])/2)^2)*PI())*21500) * If the above formula is copied to an excel spreadsheet in the form; =(A3*B3)/(((((C3)/2)^2)*PI())*21500) [copy into cell D3 - include the equal sign] To avoid errors when translating the formula to excel, select the formula in this email, then copy it across to the excel spreadsheet. Insert the variables into line 3 as follows cell A3 - the string length in mm cell B3 - the string tension in Kg cellC3 - the string diameter in mm The core wire stretch will now appear in cell D3 in millimetres, with the string at pitch. The above formula applies to Roslau wire only. I have heard that the US manufactured piano wire is softer than Roslau, so it is possible that the modulus of elasticity might be somewhat lower for the US wire. However, I would be very interested to recieve an email on the modulus of elasticity of piano wire from US maunfacturers. The figures we use for Roslau wire (21.5 Kg applied to a metre length of wire with a sectional area of 1 sq. mm will stretch it 1mm) results in accurate terminations for the copper wrap on our rebuilt instruments. Ron E Overs Overs Pianos Sydney Australia Email: ron@overspianos.com.au Web site: www.overspianos.com.au
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