Stem Inserts

Stem inserts are thin slices of material glued at the end of the mouthpiece where it meets the shank. They serve no structural function; they insert a thin band of color where the mouthpiece and shank meet, making the joint line visible. The usual arrangement consists of a somewhat thicker cap piece that sits directly against the face of the shank, with one or more thin slices behind it. The whole stack is threaded onto the pin that connects the mouthpiece to the shank and bonded into a single body with the mouthpiece using epoxy.

Insert Material

Ebonite, acrylic, horn, and hardwoods are used for the slices. The choice is not determined by color harmony alone; how the materials change over the years is also taken into account. Ebonite, the classic mouthpiece material, is a hard rubber obtained by long vulcanization of natural rubber, and the sulfur content in its composition can range from 25% to 80% by weight. Ultraviolet light oxidizes this surface; free sulfur exposed to moisture converts to sulfates and sulfuric acid, and the surface shifts first to a grayish green, then to brown. So the initial color contrast should not be assumed permanent. A detailed comparison of materials can be found in the mouthpiece materials entry.

Flat Surface and Common Axis

Two things determine the outcome of the job: that the faces of the slices are truly flat, and that their holes line up on the same axis. For flatness, the slices are thinned by rubbing their two faces in turn against sandpaper resting on a hard, flat surface (thick glass, granite, or a marble slab works); measurements taken with calipers show the two faces remain parallel. The eye alone is not relied on here, because the smallest gap will show up as a thin line once the epoxy has cured. The cap piece, by contrast, is deliberately left thick and its outer face is left untouched; that face is only worked, from the front, after gluing is finished, while the mouthpiece is still on the lathe. This way, the plane meeting the shank comes out in a single pass, and the cap's final thickness is also brought down to the design's required measure at that stage.

For the common axis, the center hole of the slices is drilled in a single pass, held together as a stack, before the pieces are glued. The hole is kept somewhat larger than the pin's diameter; workshop practice gives an allowance of around 1/64 inch. Without this gap the slices bind on the pin and do not sit flat. For the general logic of drilling arrangement, see the drilling on the lathe entry.

If the pin is turned from mouthpiece material, it needs to be left extra long by the total thickness of the layers; otherwise there will not be enough length left to seat fully at the bottom of the socket. If a ready-made pin is used, the common choice is Delrin. This acetal resin, which DuPont filed for patent protection in 1956 and began producing after completing its Parkersburg facility in 1960, is suited to this job thanks to its low coefficient of friction on steel — measured between 0.31 and 0.37 — its dimensional stability, and its machinability. Workshops mostly report using 5/16-inch (roughly 7.9 mm) rod.

The Epoxy's Job

Because epoxy adhesives bond mechanically to the surface to some degree, the portion of the Delrin rod to be embedded and the inside of the socket in the mouthpiece are roughened beforehand. After the parts are dry-fitted and the fit is checked, the resin is mixed and applied as a thin, even film across all contact surfaces; the parts are pressed together and turned slightly to work out trapped air. Excess that gets onto the pin is wiped off right away, because while excess epoxy that squeezes out can be removed easily once cured, any residue stuck to the pin will spoil the fit into the socket. If working with the mouthpiece still mounted on the lathe, the traveling tailstock acts as a clamp, holding the parts in place through curing. The waiting time is not shortened, because an epoxy network that has not finished polymerizing will not gain its full mechanical, chemical, and thermal resistance, so the stack is not worked until it has fully hardened.

Finishing on the Lathe

Once curing is finished, the layers are first rounded by hand — with a coarse file or a fine saw — enough that they will not catch while spinning; the lathe is then run and a sharp scraper and sandpaper bring them down to the same surface as the mouthpiece body. The cap is then brought to the desired thickness and flattened from the front. Fitting the pin fully into the socket and drilling and countersinking the air channel are left for last; the mouthpiece is only removed from the chuck once these are complete. Black-pigmented epoxy hides small gaps from view, but it can wick capillary-fashion into porous insert material and cause staining; for this reason it is considered safer to make the fit flawless and work with clear epoxy. For lathe equipment, see the metal lathe entry; for the whole process, see how a pipe is made.