Metric thread designation and representation on an adjusting spindle
A subcontractor submits a drawing of a stainless adjusting spindle that screws into an aluminium housing. The spindle's external thread is designated only "M24", right-hand, and the designer's note says "fine pitch 1.5". The screwed length is 30 mm, with a 3 mm wide undercut at the shoulder and a chamfer at the free end; the shoulder face seats on the housing. The mating housing drawing specifies the internal thread as M24 × 1.5 – 6H. On the spindle drawing the thread is shown with two continuous thick lines along its length, the end view shows two complete thick circles, and the runout is drawn with a thick line. The surface-texture symbol for the seating face has been placed with its tail resting inside the hatching, and a general note asks for "Ra 1.6 on the thread". Plating after machining adds about 8 µm per flank.
1. Write the correct designation for the spindle thread and account for every field. Explain why the 6g/6H pair is used and whether the plating threatens the fit.
Read the worked answer
Correct designation: M24 × 1.5 – 6g, right-hand (no LH mark needed).
- M - ISO metric thread profile.
- 24 - nominal major diameter in mm. The coarse pitch for M24 is 3 mm, so the pitch cannot be omitted here.
- 1.5 - pitch in mm, fine series. It belongs in the designation, never in a separate note; a note is the first thing lost when the drawing is revised.
- 6 - tolerance grade, applied to the thread diameters (grades 3 to 9 exist; 6 is normal).
- g - fundamental deviation for an external thread, negative: es ≈ −(15 + 11P) µm = −(15 + 16.5) ≈ −32 µm at P = 1.5.
The 6H internal thread has EI = 0, so the −32 µm allowance on the screw guarantees clearance even at maximum material. Plating is the trap: 8 µm per flank adds about 16 µm on diameter to the screw and removes clearance from the nut, eating half the allowance. The fit should be checked as plated, and if a plated screw is required the drawing should say so (plate after thread, or state that dimensions apply after plating), or a larger deviation such as 6e should be used.
2. Correct the thread representation on the spindle drawing, in the longitudinal view and in the end view, and say what changes when the spindle is shown assembled in section with the housing.
Read the worked answer
Longitudinal view, external thread. The crests (major diameter = 24 mm) are drawn as continuous thick lines; the roots (minor diameter, about d − 1.08P ≈ 22.4 mm) as continuous thin lines parallel to them, stopping at the runout. The runout itself, and the limit of the full thread, are continuous thin lines - the drawing uses thick, which makes the runout read as a real shoulder. The chamfer at the free end and the 3 mm undercut at the shoulder are shown and dimensioned as real geometry, not implied.
End view. The major circle is continuous thick and complete; the minor circle is continuous thin and drawn as about three-quarters of a circle, open in one quadrant. Two thick complete circles are wrong: they read as a plain turned groove.
Assembly section. The screw's visible thread takes precedence over the nut's. For an external thread in section, hatching runs out to the major diameter, with the minor as a thin line; for an internal thread the reverse holds - hatching out to the minor (crest), major drawn thin. At the engagement the external thread is drawn and the internal is masked. Hatching never crosses a thread as if it were solid material.
3. Deal with the two texture requirements: the symbol on the seating face and the general note "Ra 1.6 on the thread". What should the drawing actually say?
Read the worked answer
Seating face. The symbol is misplaced: its apex must touch the line representing the surface, on the outside of the material, and it must be readable from the bottom or the right. Sitting inside the hatching with its tail buried, it names no surface at all. Move it onto the shoulder line or its extension, and pair it with the requirement that actually matters: surface roughness alone does not make a face seat squarely, so add a perpendicularity or runout tolerance on that face referenced to the thread axis as datum. Ra 1.6 plus a geometric control is the functional specification; Ra 1.6 alone is not.
"Ra 1.6 on the thread". A texture symbol on a thread flank is normally meaningless: there is no single flat surface to point at, and thread flank quality is controlled by the thread tolerance grade and the manufacturing process. If a roughness really must be imposed, the symbol is placed beside the thread designation or stated in a note that names the designation, e.g. "M24 × 1.5 – 6g; flanks Ra 1.6", never as a free-floating symbol. The likely intent is a smooth crest and a clean runout, which is better expressed as a general note for the turned surfaces plus the thread callout, leaving the flanks to 6g.