By Chris Abell
with technical help from Clifford Tretick

A Little History

Ever since woodwind instruments evolved to include extra keywork, pads have been needed to seal the holes covered by the keys. The pads and the method for fastening the pads into the cups have gone through as varied and interesting an evolution in materials and design as the acoustical changes in these instruments; and the search for the best solution to the problems in padding is still a major part of woodwind-manufacture and repair. Early wind instruments had relatively small tone-holes requiring small pads and cups, and these were usually padded with leather pads held in the cups with hide glue or shellac. With a small tone-hole, it was enough to stuff a ball of leather into a salt spoon cup and spring it hard enough to have the leather pad seal the tone-hole. As the tone-hole sizes were later enlarged to help remedy acoustical problems in intonation and volume, new problems were encountered in pad design and in the method for attaching the pads into the cups. Larger tone-holes meant a larger margin of error in getting the pads to seal perfectly. The early Boehm system flutes often borrowed from the previous “technology” of floating the pad into the cup by heating shellac or a similar substance. With the pad in the cup and the keywork on the flute, the cup was heated until the shellac melted and the pad “seated” against the tone-hole providing the needed seal. This method had one very good quality; it immediately indicated the inside surface of the cup with the tone-hole seat giving the pad a stable base parallel to the tone-hole seat. The melted shellac filled the inside surface of the cup taking up any discrepancies in either the shape or the levelness of the cup. This method also had one very bad drawback; if the pad needed to be removed for any reason, it was usually ruined and unable to be reused. Eventually, flute-making evolved to dispense with the floating method of attaching pads into the cups and began using mechanical methods: bushings in the French cups and pad screws in the closed cups which allowed for the easy removal and reuse of the pad. However, the method for insuring the positive seat of the pad on the tone-hole now required the use of thin shims placed behind the pad to build the stable, parallel base.

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The Problem

One of the most important aspects of a properly made flute, whether it be a student model or the highest quality hand-made professional model, is how well the pads seal the tone-holes. Of course, the flute must have a well-made mechanism in proper adjustment with a useable scale and a good head-joint, well-made pads, and flat, level tone-holes. A perfectly padded student flute will play and sound better than a poorly padded hand-made flute. The purpose of this essay is to describe a new technique in the padding process that will not only save time, but will ultimately produce a more stable padding job. The technique will be beneficial for both manufacturers at all levels of flute-making and for repair technicians working on any type of flute from band instruments to hand-made flutes; and the type of pad used in manufacturing or repairing flutes is not affected by this process. In the process of manufacturing a modern flute, keywork is assembled and mounted to a body tube which has the tone-holes in it. The keywork is mounted on steel rods running through posts that are mounted to ribs attached to the body. The placement of the rib and post assembly is crucial as it determines the relationship of the keywork to the tone-holes. The keywork is comprised primarily of the touch pieces and pad cups used to cover the tone-holes in playing the flute. The pad cups and touch-pieces are brazed to mechanism tubing through which the steel rods run between the posts. In the keywork assembly process, the pad cups are centered over their respective tone-holes in three dimensions: side-to-side, front-to-back, and height off the tone-hole seat. All three dimensions can be indicated fairly closely by using a plug in the tone-hole to center the pad cup before brazing the cup and arm assembly to its mechanism tube. When the keywork is finished, the pads are attached to the inside surface of the cups by the use of bushings or screws with washers. Although it is the most widely used and preferred method, there is one critical problem in manufacturing the keywork assembly by the above method of using a plug in the tone-hole to center the cup. The plug does not usually indicate the inside surface of the cup with the tone-hole seat and even if it does, there is no assurance that the entire inside surface of the cup will remain parallel to the tone-hole seat during the process of assembling the keywork. Since it is this inside surface that the pad will be seating against, it is preferable to have this surface as parallel to the tone-hole seat as possible before beginning the padding process. The usual method for assuring the proper sealing of the pad on the tone-hole seat is to place shims of various shapes and thickness behind the pad. This shimming process is essential in correcting for any discrepancies in either the flatness of the pad itself or for the inside cup surface not being parallel to the tone-hole seat.

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A Solution

With the addition of one step in the padding process, the inside cup surface is positively indicated to the tone-hole seat providing a more stable base for the pad and an easier process of padding. For this process to be executed properly, the tone-hole seat must be flat and level. The materials needed are as follows: a plastic plug (Delrin? recommended) for each size pad cup, thin discs of hot melt glue for each size cup, shims of various thickness for each size cup, an alcohol lamp, and butyl acetate/mineral spirit thinner. The plastic plugs must be slightly smaller than the inside diameter of the pad cups allowing the plug to float freely. When the keywork is ready for the padding process, place a thin disc of hot-melt glue behind a shim inside the pad cup. The thickness of both the plastic plug and the shim will be dependant on the thickness of the pad and the desired protrusion of the pad from the cup. Place the plastic plug inside the cup and put the keywork on the flute. Gently heat the cup with an alcohol lamp; and just as the glue begins to soften, lightly press the cup until the plug has seated flat on the tone-hole seat and gives the needed protrusion for the pad. Be sure to heat the cup sparingly so that the glue does not flow out beyond the shim and only fills the cup behind the shim. By using a feeler, the plug can be checked for level against the tone-hole seat with the same method as checking a pad. When properly executed, the shim is now perfectly parallel with the tone-hole seat and at the correct depth for the thickness of the pad. The pad can now be put into the cup and the only partial shims needed will be to take up any discrepancies in the flatness of the pad itself. Any discrepancies in the inside cup surface to tone-hole seat are now remedied with the glue. In the event that the shim or hot-melt glue need to be removed, butyl acetate/mineral spirit thinner can be used for a neat and complete job. This technique accomplishes the pad-seat to tone-hole seat indication needed for perfect pad covering in less time and with a more stable base for the pad; and it still allows for the safe removal and adjustment of the pads.
c. Chris Abell 9/1999

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Chris Abell is the owner of The Abell Flute Co., a flute manufacturing company specializing in Boehm system wooden flutes and head-joints hand-made from Grenadilla and Sterling silver. His many years of training with Brannen Brothers Flutemakers as a keywork technician gave him the necessary skills and knowledge to start his own flute company in 1989 with the sole purpose of re-introducing a professional model wooden flute to the market. He lives in Asheville, NC with his partner, flutist Kate Steinbeck and their two children, Galen and Lucy.

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  • Cup – The cup is the round metal disc over each tone-hole that houses the pad. The pad is held into the cup either by shellac or a similar substance in the case of the trill and C# pads or by one of two mechanical means: with a bushing in the French (Open-hole) cups or with a screw and washer in the closed-hole cups.
  • Bedding Adhesive Washer – The bedding adhesive washer is a thin disc of “hot melt glue” in the form of a washer placed in the bottom of the cup and covered with a paper shim. When heated to the proper temperature, the bedding adhesive washer melts and conforms to the cup forming the parallel surface indicated from the tone-hole seat.
  • Shim – The shim is a very thin disc made of paper or plastic. Shims come in various thickness (from .0005″ to .012″ or thicker) and are glued behind the pad to take up any discrepancies in cup to tone-hole indication, pad manufacture, or flatness of the tone-hole seat. Partial shims are small pieces cut from the whole disc used to refine the process of getting the whole pad to seat completely around its entire circumference.
  • Pad – The pad is the material inside the cup that actually makes an airtight seal when closed on the tone-hole. Early flutes and piccolos used various thickness of leather (or cork) for the pad. Modern makers and repairers use manufactured pads of various designs usually comprised of paper or plastic backing behind wool felt or similar man-made materials covered with one or two layers of very thin calf stomach lining.
  • Pad retaining washer – The pad retaining washer is a small disc of metal or plastic used to hold the pad in place in the Plateau (closed-hole) cups.
  • Pad screw – The pad screw with the pad retaining washer is screwed into a “spud” (a small threaded piece of metal soldered into the cup) and fastens the pad into the closed-hole cups.
  • Bushing – The bushing is a small round tube made of metal or plastic which is pressed over the chimney which protrudes inside a French cup (thus forming the hole in the cup) and mechanically fastens the pad into the cup.
  • Tone-hole – The tone-hole is the hole cut into the side of the flute tube. Its placement and size determine the scale of the instrument. Each tone- hole needs to have an airtight seal from its pad for the flute to function properly.
  • Tone-hole seat – The tone-hole seat is the flat surface at the top of the tone-hole that the pads seals against.
  • Braze – Brazing is the method of heating the as yet unattached keywork parts and adding a filler material which melts between the parts to provide a solid mechanical joint. Soldering is a similar method using less heat and different filler materials.
  • Delrin – Delrin is a plastic manufactured by Du Pont which has excellent machining characteristics and heat resistance.

For detailed technical information and supplies for performing the above padding technique, please contact us.