Much time and effort has been spent here mastering the art of building perfect slides. A perfect slide is parallel and in one single plane (no twist in the legs) and brings out the best resonance and articulation qualities of a horn. The standard here is that all slides should be parallel and in plane to .001 of an inch over the length of the slide. Here is a video of my fixtures to produce this level of accuracy:
Slides in many shops are not treated as precision parts and this is a mistake. While many makers agree that stress free horns are more responsive and have clearer articulations, many also ignore the basic principal of building parallel slides. As any technician who works on trombone hand slides will tell you, creating parallel slides is extremely difficult and requires patience and skill to execute well.
The series below shows what happens to the horn when an out of parallel slide is inserted into the horn:
As the slide approaches the outer slide stocking, (fig. 1) it is clear that the slide is not parallel to the horn.
When the slide enters the stocking (fig. 2), a large amount of force is exerted on the outer stocking as they are pulled toward each other
As the slide reaches the bottom of the outer stockings (fig. 3), the force pulling the slides inwards is moved to the bottom of the outer stocking pair and an additional force is introduced, pushing the legs apart at the top.
Slides out of parallel introduce a large amount of tension into the horn and reduce pitch stability and articulation clarity. Some makers try to reduce this tension by building without braces in the crooks. This makes the slide appear to run more smoothly, but it does not remove the tension in the crook. Additionally, it increases the energy loss around the crook and decreases the projection and efficiency of the horn.
Another issue with out of parallel slides is that the tension in the horn is different depending on how far the slide is pulled. This could explain why some players report a slide setting that plays the best being different than the one that plays in tune. With a parallel slide, the horn responds the same no matter where the slide is.
Paralleling slide legs is one of my "secret" elements that adds the clarity and clean articulations to my horns!