Establishing the Fundamental Line of Symmetry.
On a Medlin horn, the fundamental line of symmetry occurs at the mid plane of the bell tail. That is to say, if you were to stand the bell up on its end and slice exactly down the middle, you would cut along the mid plane of the bell. Once this plane is established, other geometry is revealed, for example, a symmetrical relationship between the valve angle, the mid plane and the tuning slide pair. A good mid plane reference also helps ensure that the connection between the bell tail and first branch is not skewed.
Practically, the taper of the bell tail makes finding and measuring the mid plane very difficult. A layout jig is used to keep the bell in line so that the valves and first branch can be installed in line with the mid plane. By referencing from a single surface, geometries can be measured and documented much more precisely. My first layout jig used a series of spacers to elevate the bell from the table and locate the valve placement. However, as new models emerged and geometries evolved, it became clear that a more responsive and universal setup jig was necessary. It was most clear when I built a traditional single Bb for the first time. The design was simple enough, but without a proper layout jig, getting an aesthetically pleasing geometry was extremely difficult and time consuming.
To make any sort of jig or fixture, the first thing required is some sort of flat surface to reference off of. Granite and ground steel are often the best, but they are heavy and extremely expensive at a larger size. They are additionally impractical to modify to fit the specific needs of a horn maker. A perfectly serviceable flat surface can be built of wood using a torsion box design. The result is relatively lightweight, but very stable and doesn't change much due to heat or moisture variations. I built a large scale version of one in 2017 as a flat, rigid base for my CNC machine. For this project, the parts fit nicely into the working dimensions of the machine so I built the base in CAD and then cut it out using the machine.
Once it was cut, assembled and flattened (again on the machine). It was flat to about .005 over 18 inches. That’s about equal to commercial aluminum surface plates, more than enough for this application. The advantage is that this one can be flattened many times if ever needed, and attaching fixtures or modifying it is simple.
After crafting the flat reference surface, taper clamps were fashioned to hold the bell such that the mid plane of the bell tail is parallel to the table. With the bell ring on the table, the clamps hold the bell at the correct height. Even if I use a different bell taper or bell bend, the clamps will still work. From there, it is a simple matter to reference placement of other parts. The valves get their own fixture and a series of risers place the centerline of the first branch in line with the bell mid plane. The result is a horn that looks crisp and professional, things I expect in an elite hand crafted horn.