Ø40 H7/g6 bush and shaft: limits, duplicated dimensions and misplaced surface texture symbols
A workshop is making a replacement bronze bush and steel shaft for a water pump. The fit is specified as Ø40 H7/g6. From the standard tables the shop foreman reads: IT7 = 25 µm, IT6 = 16 µm, and the fundamental deviation for a g shaft at this size is es = −9 µm. The drafter's sheet gives the bore as 'Ø40 H7' and the shaft as plain 'Ø40' with no tolerance anywhere; a general note in the corner says only 'H7/g6'. The bore is dimensioned twice, once in the section and once in the end view. One dimension to the keyway is taken from a hidden line. Surface texture is shown by a symbol placed directly on the hatching of the section, with '1.6' written below the short arm; a note elsewhere reads 'all other surfaces 6.3'. The shaft journal is required to be Ra 0.8 but no symbol appears on it. The customer rejects the sheet and asks for the limits to be written out as well as coded.
1. Calculate the limits of both members, state the maximum and minimum clearance, and classify the fit. Show the arithmetic.
Read the worked answer
Hole, Ø40 H7 (H = hole basis, lower deviation zero):
- EI = 0, so minimum hole = 40.000 mm
- ES = +25 µm, so maximum hole = 40.025 mm
Shaft, Ø40 g6:
- es = −9 µm, so maximum shaft = 39.991 mm
- ei = es − IT6 = −9 − 16 = −25 µm, so minimum shaft = 39.975 mm
Clearance:
- Maximum clearance = max hole − min shaft = 40.025 − 39.975 = 0.050 mm
- Minimum clearance = min hole − max shaft = 40.000 − 39.991 = 0.009 mm
Classification: the clearance is positive at both extremes, so this is a clearance fit (a close running or sliding fit). It is hole-basis: the hole is fixed as H and the shaft deviation is chosen to produce the running clearance, which is the normal workshop arrangement because holes are made with fixed-size reamers and gauges while shafts are turned to size.
Presentation on the sheet: write both forms, e.g. 'Ø40 H7 (+0.025/0)' for the bore and 'Ø40 g6 (−0.009/−0.025)' for the shaft, using the upper deviation above the lower in the tolerance block. The unambiguous limits 40.000/40.025 and 39.991/39.975 let the inspector work without the tables, which is exactly what the customer has asked for.
2. Identify the dimensioning and surface-texture errors on the sheet and state the correct practice for each.
Read the worked answer
Dimensioning faults
- Shaft with no tolerance. A nominal Ø40 on a fitted shaft is meaningless - the turner could work to 39.5 mm and still claim compliance. Give the code and the limits as calculated.
- Note 'H7/g6' instead of dimensions. A fit code in a corner note does not dimension either feature. Put the code and limits on the features themselves; keep the note only as confirmation.
- Bore dimensioned twice. Each feature is dimensioned once, in the view where it reads most clearly - here the section. A repeat in the end view must be marked '(REF)' or removed; duplicated dimensions invite contradiction when one is revised.
- Dimension taken from a hidden line. Never dimension to hidden detail. Show a removed section or local section through the keyway and dimension the visible edges there, or dimension the keyway in the end view where its edges are visible.
- Dimension and extension lines should not cross, and dimensions should be outside the outline where space is tight.
Surface texture faults
- Symbol placed on hatching. The texture symbol is applied to a surface, so it sits on the visible outline or its extension line, or on a leader with an arrow touching the surface. Hatching is not a surface.
- Value on the wrong arm. The Ra number goes above the long arm of the tick, reading from the bottom or the right of the drawing; the short arm carries the value only when a second requirement such as a machining process is also given.
- Machining symbol missing. Because the surface is machined, the tick must carry the bar across the long arm (material removal required); a plain tick means removal is not required.
- Missing Ra 0.8 on the journal. The most critical surface has been left without a symbol. Add it on the journal outline; it is never inferred.
- General note. 'All other surfaces 6.3' should be written with the basic symbol in brackets next to the title block, so it applies to every surface not individually marked.