Drilling 150 Plates: HSS Versus Carbide on a Worn Radial Drill
A batch of 150 mild-steel plates, each 40 mm thick, must have a 20 mm diameter hole drilled through. The radial drilling machine has seven spindle speeds - 80, 125, 200, 315, 500, 800 and 1250 r/min - and four feeds - 0.10, 0.15, 0.20 and 0.25 mm/rev. The shop standard for HSS drills in mild steel is 25 m/min with 0.20 mm/rev, and the approach allowance is taken as 0.3 × drill diameter. Loading, clamping and unloading take 0.5 min per plate. A supplier offers a carbide-tipped drill rated at 80 m/min and 0.25 mm/rev, claiming it will finish the batch in under half an hour. The machine has no power feed above 0.20 mm/rev and the plates are clamped on a worn table with 0.15 mm run-out.
1. Select the HSS spindle speed from the machine steps using 25 m/min. Calculate the feed rate, the time per hole including approach, and the total time for 150 plates including 0.5 min handling each.
Read the worked answer
n = 1000v/(πD) = 1000 × 25/(π × 20) = 25,000/62.832 = 397.9 r/min. Machine steps available are 315 and 500; for HSS the calculated speed must not be exceeded, so select 315 r/min. Actual v = π × 20 × 315/1000 = 19.79 m/min, about 21% below the ideal - acceptable and safer on a worn table. Feed rate = f × n = 0.20 × 315 = 63.0 mm/min. Drill travel = thickness + approach = 40 + 0.3 × 20 = 46 mm. Time per hole = 46/63.0 = 0.730 min = 43.8 s. Drilling time for 150 = 150 × 0.730 = 109.5 min. Handling = 150 × 0.5 = 75 min. Total = 184.5 min = 3 h 4.5 min. Judgement: selecting 500 r/min would give 31.4 m/min, 26% above the HSS recommendation, and would cut theoretical drilling time to 69 min, but with 0.15 mm run-out the drill would dull rapidly and the saving would be lost to resharpening and scrap. The 315 r/min choice is the defensible one.
2. Evaluate the supplier's carbide claim. Select the spindle speed actually available, account for the 0.20 mm/rev feed limit, calculate drilling time and total batch time, and state whether the claim is met.
Read the worked answer
Required n for 80 m/min = 80,000/62.832 = 1273 r/min; the nearest step not exceeding it is 1250 r/min. Actual v = π × 20 × 1250/1000 = 78.5 m/min, close to the rating. Feed is limited by the machine to 0.20 mm/rev, not 0.25: feed rate = 0.20 × 1250 = 250 mm/min. Time per hole = 46/250 = 0.184 min = 11.0 s; 150 holes = 27.6 min. Total including handling = 27.6 + 75 = 102.6 min = 1 h 43 min. The claim of under half an hour is false for the batch. It is true only for the drilling portion alone, and only if every hole follows without stoppage. Handling alone is 75 min. Judgement: carbide at 78.5 m/min in mild steel is feasible only with rigid clamping, a true-running spindle, adequate coolant and no interrupted cut. A 0.15 mm run-out on a worn table is enough to chip the corners, so require a demonstration on 10 plates before committing the batch. Otherwise HSS at 315 r/min is the lower-risk route.
3. A trainee sets the HSS drill to 500 r/min 'to finish faster'. Calculate the actual cutting speed, the percentage above the 25 m/min standard, and the drilling time. Explain why the time saving is not worth it.
Read the worked answer
v = πDn/1000 = π × 20 × 500/1000 = 31.42 m/min. Excess = (31.42 − 25)/25 × 100 = 25.7% above the HSS standard. Feed rate = 0.20 × 500 = 100 mm/min; time per hole = 46/100 = 0.46 min = 27.6 s; 150 holes = 69 min, plus 75 min handling = 144 min. Saving against the correct setting = 184.5 − 144 = 40.5 min, about 22%. Judgement: a 25.7% overspeed raises edge temperature roughly in proportion; HSS softens above about 600 °C, so the drill will lose hardness, rub and dull. Over 150 holes the likely outcome is one or two resharpenings and possibly a scrapped hole from a dull drill, which costs far more than 40 min. The correct response is to keep the recommended speed, apply coolant and, if time matters, improve the handling sequence or use the carbide drill under proper conditions.