A STUDY OF THE AIR GAP BETWEEN BLADES OF A REED


By Thomas H. Palmer, Stafford, England


(The following article is printed at the request of the author and is taken from one of his two interesting books on bassoon reed-making, "The Bassoon Reed Profile and Measurement." The other is, "Bassoon Reed Profiling Machine and Reed Making " Both books are available directly from the author: Mr. T. H. Palmer, 4 Chartwell Road, Stafford, England.

Mr. Palmer was born in 1915 and educated at Bryanston School, Blandford, Dorset, England. He is an inventor/engineer/bassoonist-reed maker.

After studying mining engineering before and during World War II, he became self-employed for 9 years, specializing in coil winders and electromechanical devices, and gas detectors. In 1955, he was employed by the English Electric Co., Ltd., Stafford, as a development engineer designing special purpose machines. Since 1971 he has been employed as a technician with the North Staffordshire Polytechnic.

As an amateur bassoonist for the past 14 years, he enjoys with great pleasure playing with local orchestras and various wind quintets. He is also teaching part-time as a bassoon instructor for the Staffordshire County Music Department. He is author of the two reed making books described above (see To The World's Bassoonists, Vol. III, No. 1 and Vol. III, No. 2. )

Method:

The reed is placed centrally in a container so that the tip is facing upwards. A wire is bent back on itself and pushed into the stem of a reed and shaped to form a stand. This stand should hold the stem of the reed above the bottom of the container to allow air to pass freely through the reed. Araldite is then prepared and poured into the container, taking care not to pour between the blades of the reed. As the Araldite rises in the container it will gradually fill up the reed through the stem until it finally levels out at the tip. If the Araldite were to be poured directly through the blades, large air pockets would be formed which cannot escape. It is advisable to mark pencil lines across one blade to indicate distances from the tip of the reed before encapsulating the reed.

When the Araldite is completely set hard, the pencil marks will be clearly seen where to part off sections. The parting off is best done by means of a saw, because grinding produces too high a temperature for the Araldite.

Each section is cut off and polished to the pencil line using wet carborundum paper on a flat surface, so that a clean edge is produced along each section of cane. Each section is now photographed through a microscope which can be adjusted to a desired magnification. The magnification chosen in this case is eight times full size. Direct measurement can now be taken from the photograph using a scale calibrated in 1/100 of an inch which must be divided by eight (8) to give the actual measurement.

Measurement of air gap

Two reeds were selected for cutting into slices so that the relationship between the air gap and the thickness along the edges (left edge: LE and right edge RE) could be investigated; one reed having a flat throat and the other having a round throat.

These reeds are illustrated in plate III(3): one made by Dominic Weir (London), which has a round throat, and the other made by Schober (W. Germany) having a flat throat. These profiles and measurements are recorded in section 3 using the measuring device illustrated in plate I: (Editor's note: A dial indicator set-up was used for measuring reed cane thicknesses. All plates and charts pertaining to the air gap measurement are included in this excerpted article from the larger work, "The Bassoon Reed Profile and Measurement," referred to above.)

Photographs of the sections for a reed having a flat throat are illustrated in plate VII (7) and of the sections for a reed having a round throat in plate VIII (8). These sections represent the tip and every quarter inch (1/4 inch) from the tip of the reed also at the first wire position. The relative positions of LE/ RE are indicated with identification of each blade (i.e., No. I wire twist side is at the top of each section).

Measurements taken from these photographs for a reed having a flat throat are given in Fig. 35A at points through a vertical centre line shown in Fig. 35B. The thickness of cane down the centre line of the reed is shown in columns A and D, and the air gap between blades in columns B and C. The air gap in this case has been split into two readings so that a section can be plotted and drawn. Readings B and C (Fig. 35B) are measured from a line drawn between the 2 points where the edges LE/RE meet so that a true representation of the air gap can be seen. Since the photographs are eight times full size, each measurement must be divided by 8 to produce the actual measurement. From this table the air gap and cane profile can be drawn as illustrated in Fig. 36 - for a reed having a flat throat. Fig. 37 A and B gives a similar table from which the air gap and cane profile is drawn for a reed having a round throat, which is illustrated in Fig. 38. These results can only show comparisons between the air gap along the centre line of each reed in relation to the profile of the cane.

If the width of air gap is now considered, this will take into consideration the overall shape of a reed on any one section; and if the height is divided into the width this will give a ratio of air gap. A completely round throat would therefore have a ratio of 1: 1. The relationship between the air gap ratio and the thickness along the edges LE/RE can be investigated by plotting curves.

The tables in Figs. 39 and 40 give measurements for reeds having a flat and round throat respectively. These measurements are taken from the photographs (see plates VII and VIII) which show the air gap ratio with the thickness along the edges LE/RE for given sections.

Figs. 41 and 42 are curves plotted from the tables in Figs. 39 and 40, in which the air gap ratio is shown in relation to all the measurements along the edges LE/RE. The curves are drawn between these points to give one reading for any value of air gap ratio. For a given air gap ratio, the thickness along the edges (LE/RE) is indicated with the distance from the tip. For example (see Fig. 41) at a ratio of 3.5 where this meets the curve, a thickness of 0.0195 inches at a distance 1/2 inch from the tip is indicated for a reed having a round throat - and in Fig. 42 at a ratio of 5.28 there is a thickness LE/RE of 0.015 inches at a distance of 1/2 inch from the tip for a reed having a flat throat. If these curves are drawn together as shown in Fig. 43 it is easy to see that for any given ratio, the thickness LE/RE is greater for a reed having a flat throat, and for any given thickness LE/RE the ratio is greater for a flat throat.

For example, a ratio of 3.85 at 3/4 inch from the tip on the top curve (flat throat) has a thickness of 0.0205 inches and 0.018 inches on the bottom curve (round throat).

On the other hand, at distances of 1/4 inch from the tip to 1 and 3/16ths on the reed having a flat throat, the thickness LE/RE is less than at the same distances on the reed having a round throat, with one exception at the tip, which is shown to be greater.

Fig. 44 shows the air gap ratio plotted against distance from the tip. Taking points along the top curve for a flat throat at a given ratio, it will be seen that the same ratio is nearer the tip of the reed for a reed having a round throat. For example, at 3/4 inch from the tip of the reed, the ratio is 3.85 on the curve for a flat throat, but this same ratio is at 7/16ths from the tip on the bottom curve for a reed having a round throat. If we now refer to Fig. 43, at a ratio of 3.85 (3/4 inch from the tip) on the top curve for a flat throat there is a thickness of 0.0205 inches, and at the same ratio on the curve for a round throat a thickness of 0.018 inches.

The meaning of these curves is best illustrated diagramatically as shown in Fig. 45.

The thickness along the edges LE/RE taken from the curves in Figs. 41 and 42 are shown in relation to the distance from the tip for the two reeds having a flat and a round throat.

The positions of the air gap ratios along the two reeds (taken from the curve in Fig. 44) can now be readily understood. This also illustrates the effect of squeezing No. 1 wire at the sides, which will cause the air gap ratios to decrease towards the tip.

Since the air gap ratios in any reed are decreasing towards the throat, the inside radius is also decreasing towards the throat. These radii bear no relationship to the original radius of the gouged cane which is constant throughout the whole length. The important mechanical factors in a reed are the air gap ratios in relation to the profile at points along the blades.

Whichever radius is determined for the gouged cane, the same radius must be adhered to when profiling by machine, because the cane is clamped to a base having the same radius to form a datum position from which the profile is set. This does not apply when profiling by hand, as measurements are simply taken between inside and outside points along the cane, in which case the radius can differ from cane to cane.

In order to determine a tolerance for the edges LE/RE it is necessary to plot the distance from the tip against LE/RE for each reed. This is shown in Fig. 46. The points are taken from the curves in Figs. 41 and 42 and represent the profile at the edge of the blades. If a straight line is drawn between these points for each reed, it will be seen that any point is only a maximum of 0.001 of an inch from the line in each case. This means that the profile can be considered to be linear for the full length of the blade. The reed having a flat throat compared with the reed having a round throat is thinner along the edges LE/RE from 8/8 to 7/32 from the tip where it becomes equal; it then increases in thickness to the tip. If a line is drawn between the highest point for the reed having a round throat and the lowest point for the reed having a flat throat, this can be termed the high limit to profile the edges LE/RE. This line is shown dotted, and from this line any thickness for a given distance can be observed. Convenient measurements are shown to be as follows:

Tip:     0.008''
2/8      0.014
4/8      0.020
6/8:     0.026
8/8:     0.032

This means that if the edges are first profiled to these dimensions, further adjustment by scraping can be made to suit a reed having a round or flat throat.

















Table of Contents