Work Hardening in 304 Stainless Drilling and Bandsaw Blade Failure
In a job shop, 40 through-holes Ø12.0 mm are required in 16 mm thick 304 stainless plate. The machinist uses a standard HSS jobber drill with a 118° point in a drill press at 12 m/min and 0.10 mm/rev, with a dab of tapping fluid applied by brush. The plate is held by hand against a stop block because the drill press vise is in use. By hole 12, the drill squeals, the chips turn from curled to fine powder, the hole measures 12.18 mm, and the drill breaks at breakthrough. Nearby, a horizontal bandsaw is cutting 75 mm diameter 304 bar with a 4/6 TPI bimetal blade at 45 m/min and light feed; the blade is lasting only 20 cuts. The shop has a cobalt split-point drill, water-soluble flood coolant, a 3/4 TPI variable-pitch blade, and a clamp kit available.
1. Calculate the drill spindle speed and feed rate. Explain why the drill is squealing, producing powder and breaking, and recommend changes.
Read the worked answer
Spindle speed: n = 1000Vc/(πD) = 1000 × 12/(π × 12) = 318 rpm. Feed rate = 318 × 0.10 = 31.8 mm/min.
Type 304 work-hardens rapidly. A standard 118° HSS drill at low feed with only brush-applied fluid rubs instead of cutting. The surface hardens, the drill squeals and wears, the hole goes oversize, and the drill can break at breakthrough. Fine powder chips confirm rubbing rather than cutting.
Use a 135° split-point cobalt drill, keep the feed constant at 0.08-0.12 mm/rev, and apply flood water-soluble coolant. Reduce speed to about 8-10 m/min if the cobalt drill shows edge breakdown. Do not peck-dwell, because dwelling work-hardens the bottom of the hole. Support the underside with a sacrificial plate and maintain feed through breakthrough to avoid a hardened breakout burr.
2. Evaluate the bandsaw parameters and blade selection for the 75 mm 304 bar. What changes would you make?
Read the worked answer
For 75 mm 304 bar, 45 m/min is too fast for a bimetal blade, and 4/6 TPI is too fine for good chip clearance. The light feed makes the teeth rub and work-harden the stainless, so the blade wears quickly or strips teeth.
Use a 3/4 TPI variable-pitch bimetal blade, 20-25 m/min, flood coolant, and a heavier feed so each tooth cuts. Set the guides close to the work and use a break-in period at reduced feed for the first cut. A 4/6 TPI blade fills its gullets with stainless chips, which causes strip failure rather than normal wear.
3. List the safety corrections required for both operations.
Read the worked answer
Drill press: clamp the plate in a drill vise or bolt it to the table; never hold it by hand or against a stop block alone. Remove the chuck key before starting, wear safety glasses and a face shield, do not wear gloves near the rotating spindle, and clear chips with a brush or hook, not by hand. Make sure the drill is securely tightened and the spindle is stopped before changing tools.
Bandsaw: clamp the bar in the vise, keep the blade guard adjusted, stop the machine before clearing chips, and check coolant flow and blade tension. Keep the floor clean and dry. Both operations require guarding, correct PPE and a workpiece that cannot spin, lift or be pulled into the cutter.