Nominal square-tube inside width is not the usable passage. Theoretical width is B−2t, but inside corner radii, longitudinal weld bead, wall tolerance, concavity/convexity, bow and twist reduce it; powder adds thickness on all faces. “35 tube inside 40 tube” does not guarantee sliding.
Values in this article support engineering screening during design review, RFQ and prototyping. They do not replace formal calculations, material data sheets, qualified procedures or contractual standards.
BUILD A CLEARANCE BUDGET
Across two opposed coated faces, film h per face reduces total diametral clearance by about 2h. If an inner tube is 35.0 mm and actual minimum passage 35.6 mm, 0.10 mm powder on each opposed face leaves only 0.4 mm theoretical clearance; bow and particles can then bind it. Free sliding and zero rattle conflict, so use guide pads, rollers, polymer strips or adjustable gibs rather than collapsing metal clearance.
VALIDATION METHOD
1. Obtain actual OD, wall, corner, bead, straightness and twist tolerances from the tube supplier. 2. Control seam orientation; remove the internal bead or use purpose-made telescoping sections where needed. 3. Analyze minimum outer passage against maximum inner section, including coating and debris. 4. Accept by insertion/withdrawal force and lateral play across full travel—not mouth dimension only. 5. Test cross-lot mixed pairs using production material and final finish.