Pillar Drilling Stainless Steel Bracket Without Coolant
A student is drilling eight 8 mm diameter holes in a 5 mm thick sheet of 304 stainless steel to make a bracket. The pillar drill is set to 1200 rpm. The student uses a standard HSS twist drill, applies firm downward pressure, and holds the sheet by hand because the vice is being used. No cutting fluid is used. The drill screeches, the chips turn blue, the hole comes out about 8.3 mm, and the sheet snatches and spins, bruising the student's wrist. The workshop has a machine vice, clamps, a coolant supply, cobalt HSS drills, a centre punch, and a range of speeds from 200 to 3000 rpm. The bracket has a hole position tolerance of ±0.2 mm.
1. Explain why the drilling operation produced oversized holes, blue chips, and a snatching workpiece.
Read the worked answer
Stainless steel work-hardens and has low thermal conductivity, so heat stays at the cutting edge. At 1200 rpm with an 8 mm drill, the surface speed is about 30 m/min, which is too high for a standard HSS drill in 304 stainless; a typical surface speed is nearer 10 to 15 m/min, giving roughly 400 to 600 rpm. The high speed plus no coolant generates enough heat to turn the chips blue and soften the drill edge. A dull, hot drill rubs and pushes rather than cuts, which enlarges the hole and work-hardens the surface.
The oversized 8.3 mm hole also comes from drill wander and vibration. The drill was not started in a centre-drilled guide hole, and the sheet was held by hand, so the drill could move sideways as it entered. When the drill broke through, the cutting forces changed suddenly and the thin sheet snatched. Hand holding gave no resistance to the snatch, so the sheet spun and injured the student. A machine vice or clamps would have controlled both position and kickback.
2. Recommend corrected drilling parameters and a setup that would hold the ±0.2 mm position tolerance. Include speed, drill type, coolant, and workholding.
Read the worked answer
Use a cobalt HSS drill, or a carbide drill if available, because it retains hardness at the higher temperature generated in stainless. Set the spindle to about 450 to 500 rpm for an 8 mm drill, corresponding to roughly 11 to 12 m/min surface speed, and use a slow, steady feed with cutting fluid or flood coolant. Peck drill to clear chips and avoid work hardening. Do not let the drill rub without cutting.
For position, mark out from a datum, centre punch, and use a centre drill or spot drill to create a rigid start. Then drill the 8 mm hole, preferably through a drill jig or with the workpiece clamped against a fence so each part is located identically. Clamp the sheet in a machine vice or with clamps directly to the table; never hold it by hand. Support the underside to reduce breakout and vibration. Check the first hole with a caliper or pin gauge before drilling the rest. If the hole position is critical, drill slightly undersize and ream or file to final size, but the main control is a rigid setup and a centre-drilled start.
3. The student's wrist was injured. Identify the safety controls that should have prevented this and state what the workshop supervisor should do before permitting further drilling.
Read the worked answer
The main control is never to hold work by hand on a pillar drill. The sheet should have been clamped in a machine vice or held with clamps against a stop, with the table and vice secure. The supervisor should also have checked that the correct speed, drill and coolant were used, because the snatching was made more likely by the poor cutting conditions. A risk assessment for the task should identify rotating-tool entanglement, ejection and workpiece rotation. The student should wear suitable eye protection and avoid loose clothing or gloves near the drill.
After the injury, the supervisor should stop the machine, apply first aid and record the incident. Before further drilling, the supervisor should inspect the machine, confirm the guard and emergency stop work, and demonstrate the correct clamped setup. The student should be re-briefed on the safe procedure and supervised until competent. If the student cannot safely clamp the work, the task should not proceed. The vice being in use is not a reason to hand-hold; the work should wait, or another safe clamping method should be found.