Reed pitch change is achieved by removing metal from either the base or tip of the reed. Metal removal from the tip lightens it and raises the pitch while removal from near the base increases the outer mass of moving metal and lowers the pitch.
Two particular elements of metal removal seem important:
It is well known to design engineers that a sharp change of section in a mechanical system where stresses are repeated and constantly reversed (eg. a reed, overhung rotating shaft etc.) should be avoided as this is liable to cause a ‘stress raiser’ which can result in the formation of a fracture. Alan Douglas (a senior member of the Society of Organ Builders and one time senior scientific officer with the British Iron and Steel Research Association) is in print stating “Reed tongues must be carefully contoured; any abrupt change in section causes a serious reduction in the safe range of stress”.
It is therefore felt that sharp, local scoring of the reed is to be avoided.
Tools in use to remove metal include files, emery cloth, scraper, jewellers engraving bit and hand held rotary grinder.
The selection of tool can depend on the size of the reed being dealt with and whether it is the outer or inner one. Files made specially for reeds are available from some dealers. An effective scraper can be made from a small, square sectioned needle file by grinding the tip to form four cutting edges. During grinding the file should be frequently water-cooled so that the hardness/temper of the metal is not lost. The file teeth can be removed to prevent damage to the reed slot sides.
While making these changes to the reed it is all too easy to bend and disfigure it. This will result in a false (and temporary) impression of frequency change. Sharp tools are essential as there is a likely tendency to use excessive force with a blunt or dull tool to try to achieve the desired effect. Considerable patience is required to obtain satisfactory change without disfiguration and is one of the most difficult procedures attempted by the author.
It is helpful if the reed is supported while work is being done.
To work on the inside reed its tip can be lifted out of the slot or it can be supported with simple tools:-
The above operations are carried out either when the reeds being dealt with are the outer ones or when the reed blocks are removed from the instrument. Since it is frequently required to make some final tuning adjustments with the reed blocks fitted in the instrument a means of supporting the inner reeds is required. A useful hook can be made from stiff wire (such as piano wire) so that the inner reed tip can be raised out of its slot or simply just supported while it is filed or scraped.
The valves/leathers are not shown in the above diagram.