Replacement bush: a fit calculation, a broken dimension chain and a filing instruction
A multi-skill assistant is given a hand-drawn detail print, at scale 1:2, for a replacement bearing bush and its mating conveyor-gearbox shaft. The bush bore is dimensioned Ø40 +0.039/0 and the shaft journal Ø40 −0.025/−0.050, with the note "Ra 1.6 bore, do not scale drawing". The print gives an overall length of 120 mm made up of a 45 mm hub, a 45 mm body and a 32 mm flange. The store has one sawn blank 130 mm long, and the only boring bar available leaves about Ra 3.2. The supervisor says: "Just file the bore to fit the shaft on the bench; the last one worked." The shaft measures Ø39.965 mm and the old bush bore measures Ø40.050 mm. Production wants two bushes by the next shift, and the drawing is signed only by a trainee draughtsman.
1. Is this a clearance or interference fit? Compute the limits, and say whether the measured shaft and the old bush conform to the drawing.
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Fit type
- Hole limits: 40.000 to 40.039 mm.
- Shaft limits: 39.950 to 39.975 mm. The smallest hole (40.000) is larger than the largest shaft (39.975), so there is always a gap: this is a clearance (running) fit.
Limits of clearance
- Maximum = 40.039 - 39.950 = 0.089 mm
- Minimum = 40.000 - 39.975 = 0.025 mm
Do the parts conform?
- Shaft 39.965 mm lies inside 39.950-39.975: conforms.
- Old bush 40.050 mm is above the hole maximum of 40.039 by 0.011 mm: does not conform, even though it 'worked'. Fitted to the shaft it gave 0.085 mm clearance, only just inside the 0.089 maximum and near the loose end of the band, which is exactly where a bush rumbles, leaks and wears fast. A hand-finished oversize bore also loses roundness and taper that a two-point measurement does not reveal. The old bush is not evidence that filing is acceptable; it is evidence of why the gearbox is noisy.
2. The length dimensions are 120 overall, with 45, 45 and 32 making up the chain. How do you resolve this, and why can you not simply scale the drawing?
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The chain does not close 45 + 45 + 32 = 122 mm, but the overall length is stated as 120 mm. One dimension is wrong by 2 mm, and the drawing carries an explicit 'do not scale' note.
Why scaling is not an option A 1:2 print shrinks every error by half and adds paper stretch, printer scaling and measurement error. Scaling can never recover a tolerance of a few hundredths, and the note is an instruction from the drawing office, not a suggestion.
What to do
- Identify the functional dimensions: the 120 overall length and the two 45 mm spacings locate the bush in the housing and on the shaft, so they are almost certainly correct.
- The flange is therefore the dependent dimension: 120 - 45 - 45 = 30 mm, not 32 mm.
- Raise a formal query to the design/drawing office with the arithmetic, and get written confirmation (revised drawing or signed email) before the flange is machined. The drawing is signed only by a trainee draughtsman, which is an added reason to confirm rather than guess.
- Record the query and the answer on the job card.
Material is not the constraint: a 130 mm blank still covers either 30 or 32 mm, so the flange can be left until the answer arrives.
3. How do you respond to the instruction to file the bore to fit, and what is a workable plan that also meets Ra 1.6?
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Why the instruction must be refused The tolerance band is 0.039 mm. Hand filing cannot control size, roundness, cylindricity or taper to that band, cannot be measured reliably without a bore gauge, and cannot produce Ra 1.6 - a file leaves a rough, torn surface. The boring bar available leaves Ra 3.2, so finishing is still one operation short. Filing also removes the possibility of correction: once the bore is oversize, the bush is scrap.
Workable plan
- Rough bore leaving 0.2-0.3 mm on diameter.
- Finish by fine boring with a sharp nose radius and reduced feed, internal grinding, honing or reaming to 40.00-40.02 mm.
- Verify with a calibrated bore gauge or inside micrometer at two planes 90° apart, and check the finish with a surface comparator (or profilometer if available).
- Match to the shaft in hand (39.965 mm) so the assembled clearance lands around 0.035-0.055 mm - comfortably inside the 0.025-0.089 band.
Communication Explain the arithmetic to the supervisor, offer the plan above, and ask for the finishing resource to be booked. If he still insists, ask for the instruction in writing and copy it to the maintenance in-charge; a written instruction protects both of you, and the register entry shows the technical objection was raised before the part was scrapped.