Pulley Fit, Induction Heater and Drill Speeds on a Pump Overhaul
A fitter is overhauling a centrifugal pump during a short maintenance window. A replacement pulley must be mounted on a 40 mm shaft, and the drawing calls for an H7/k6 transition fit. Micrometer readings at three places on the shaft are 40.018 mm, 40.020 mm and 40.016 mm; two readings of the pulley bore are 40.000 mm and 40.002 mm. The pulley is alloy steel with a coefficient of linear expansion of 0.0000115 per degree C. The shop has an induction heater, a 20-tonne hydraulic press and a soft-faced mallet, but no oven. Separately, a 10 mm dowel hole must be drilled 9.8 mm and then reamed. The material calls for a cutting speed of 25 m/min for drilling and 6 m/min for reaming. The radial drill offers 160, 200, 315, 400, 500, 630, 800, 1000 and 1250 rpm.
1. Judge whether the measured shaft and bore give an acceptable H7/k6 fit at 40 mm, and recommend how the pulley should be assembled.
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Take means, because a single reading can hide ovality or taper.
- Shaft mean = (40.018 + 40.020 + 40.016) / 3 = 40.018 mm
- Bore mean = (40.000 + 40.002) / 2 = 40.001 mm
- Interference = 40.018 - 40.001 = 0.017 mm
Compare with the specified fit. For a 40 mm nominal size, H7 gives the hole 0.000 to +0.025 mm, and k6 gives the shaft +0.002 to +0.018 mm. The worst permitted interference is therefore 0.018 - 0.000 = 0.018 mm, and the worst clearance is 0.025 - 0.002 = 0.023 mm.
Measured interference of 0.017 mm lies inside the band but at the tight end - the shaft is sitting near its upper k6 limit. The fit is acceptable; the method is not. A 20-tonne press on a 0.017 mm interference risks scoring and galling the bore, and the mallet is out of the question. Use the induction heater: the pulley must grow by the interference plus a working clearance of about 0.005 mm, i.e. 0.022 mm on diameter. Required temperature rise = 0.022 / (0.0000115 x 40) = about 48 degrees C. Heating to roughly 70 to 80 degrees C gives margin, stays far below any tempering range for alloy steel, and lets the pulley slide on by hand with the key located first.
2. Calculate the spindle speeds to set for the 9.8 mm drill and the 10 mm reamer, and choose from the available speeds.
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Use N = 1000V / (pi x D), with D in mm and V in m/min.
Drilling, D = 9.8 mm, V = 25 m/min:
- N = (1000 x 25) / (3.1416 x 9.8) = 25,000 / 30.79 = 812 rpm
- Nearest available speed is 800 rpm. Choosing 800 is correct rather than 1000, because the next step up is nearly 25% above the calculated value and would burn the drill point.
Reaming, D = 10 mm, V = 6 m/min:
- N = (1000 x 6) / (3.1416 x 10) = 6,000 / 31.42 = 191 rpm
- Nearest available is 200 rpm; 160 rpm is also defensible as the conservative side of the nearest step.
Judgement points beyond the arithmetic: reaming speed is deliberately about one quarter of drilling speed because a reamer has many cutting edges and must not rub or chatter; the reamer must be fed in and out at the same feed rate and never reversed while in the hole, otherwise the flutes score the finished bore. The 0.2 mm diametral stock left by the 9.8 mm drill (0.1 mm per side) is a sensible reaming allowance.
3. The first bore, checked after reaming, measures 10.04 mm and shows a spiral mark. Using the same data and setup, what is the most probable cause and what would you change?
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A 10.04 mm bore on a nominal 10 mm hole is 0.04 mm oversize - far outside any normal reamer tolerance - and the spiral mark is the diagnostic clue, not the size alone.
Most probable causes, in order of likelihood given the setup:
- Reamer run too fast or fed too quickly, so the flutes rubbed and cut a helical track instead of shearing cleanly. If the operator set 800 rpm for the reamer instead of 200 rpm, this is the first suspect.
- Reamer not concentric with the drilled hole, because the drill wandered - a 9.8 mm drill at 800 rpm in a hand-fed radial drill can deflect - leaving the reamer to follow a bent axis and cut on one side only.
- Chips packed in the flutes or the reamer reversed in the hole, which scores a spiral and bell-mouths the bore.
What to change: re-drill with a properly ground, correctly aligned drill (or spot and pilot first), hold the reamer speed at about 200 rpm with a steady feed, clear chips, and never rotate backwards in the hole. Check the reamer's diameter and runout before reuse. Since 0.04 mm oversize will not hold a dowel, the practical remedy is to bore or ream oversize to the next dowel size rather than to attempt to shrink the hole, and to scrap the assumption that the machine's 800 rpm setting is acceptable for finishing operations.