Turning Tenons

Turning tenons is the process of turning down the cylindrical projection at the end of the mouthpiece (stem) to the diameter that will fit the mortise bored into the shank. This is the single surface that joins the pipe's two parts together, which is why it's considered the least forgiving stage of pipe making. Even a deviation of one percent of an inch in tenon diameter changes whether the fit ends up loose or forced. For its place in the overall process, see the how a pipe is made entry.

Tolerance and Equipment

This is not a scale that can be held to with a wood lathe's dial; the work is mostly done on a metal lathe. What the metal lathe brings isn't so much precision as repeatability: a second part machined at the same setting matches the dimension of the first. The Taig Micro Lathe II, common in small workshops, is described in manufacturer and distributor documentation as capable of roughly 0.0005-inch working precision. Since the same lathe is also used in the workshop for drilling on the lathe, the mortise and tenon are often prepared on a single machine and with a single sense of axis.

The Mortise Sets the Size

Although there's a rough target diameter kept in mind for the tenon, the final size isn't set on paper — it's set in that particular pipe's own mortise; every briar block reacts slightly differently to drilling. The test is done without removing the mouthpiece from the lathe. Once a part is taken out of the chuck it never sits back in its old axis exactly; getting a round surface on the new axis requires taking off a considerable amount of material, and if the target diameter has already been approached, that allowance no longer exists. The result is an off-axis tenon.

The fit is meant to be tight at the first assembly, because it's known that the joint loosens somewhat as the pipe is used; the reason for that loosening hasn't been fully clarified in the community either. On the other hand, there's a limit to how much force thin shanks can take: pressing an overly tight tenon into the mortise can crack the shank. Nor is it mandatory for the tenon to be cut from the mouthpiece itself; see the entries on using a separate Delrin tenon, and on the spacers used at the shank-mouthpiece joint, mouthpiece spacers.

Cutting Tool, Speed and Surface

A clean surface largely depends on the sharpness of the cutting tool: a dull tool tears the material as it cuts, leaving a mark that then has to be removed with sandpaper. Makers working with a high-speed steel (HSS) tool are reported to resharpen it for every mouthpiece. As for speed, turning behaves the opposite of drilling; a higher speed gives a smoother surface.

Diameter is monitored with calipers as the work proceeds. Common practice is to stop a few thousandths of an inch above the target diameter, leaving the remaining allowance to fine sandpaper and then to a light wax polish. It's also noted that if the cutting tool is sharp enough, that leftover allowance can be reduced.