Coolant Failure on the Manual Lathe
A Year 12 student is turning a 40 mm diameter mild steel shaft to 38.00 mm on a manual lathe. The coolant pump has been intermittent for two shifts; the workshop supervisor has logged it but not repaired it. The student notices the workpiece is hot to touch, the surface finish has become smeared and the swarf is turning blue near the tool. The depth of cut is 1.0 mm radial, spindle speed 640 rpm, feed 0.15 mm/rev. The student's teacher is absent and the only other person present is a first-year student who has just finished the lathe induction. The job is due for the client at 3 pm and it is 1:30 pm.
1. Identify the hazards in this situation and explain what the student should do before continuing the cut.
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The main hazards are hot workpiece and hot swarf (blue swarf indicates cutting temperatures well above what bare hands should contact), inadequate coolant leading to poor chip evacuation and possible tool breakage, and unsupervised operation with an inexperienced bystander present.
The student should not simply continue. The safe sequence is:
- Stop the machine and let the spindle come to rest before reaching near the chuck or workpiece.
- Isolate or lock out the lathe if the fault cannot be resolved, so no one restarts it.
- Report the coolant fault to the workshop supervisor or another qualified staff member, referencing the existing log entry.
- Do not use the first-year student as a supervisor - an induction-completed peer is not a competent person for supervising another operator.
Continuing without coolant risks burns, tool shattering and a scrapped workpiece, so the correct judgement is to escalate, not to push on to meet the 3 pm deadline.
2. Explain how the cutting parameters and the coolant fault together explain the symptoms observed, and what parameter change would be a sensible interim measure if the supervisor authorises a short run.
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At 640 rpm the surface speed on a 40 mm bar is roughly:
v = π × D × N / 1000 ≈ 3.14 × 40 × 640 / 1000 ≈ 80 m/min
That is a reasonable speed for mild steel with flood coolant, but with intermittent coolant the heat stays in the tool and workpiece. Combined with a 1.0 mm radial depth of cut and 0.15 mm/rev feed, the heat generated per pass is high, which explains the blue swarf (high interface temperature), the hot workpiece and the smeared finish (built-up edge and rubbing rather than clean shearing).
If the supervisor authorises a short run, sensible interim measures are to reduce spindle speed (lower surface speed reduces heat), reduce depth of cut and take more passes, and apply manual coolant or cutting fluid by brush with the machine stopped between passes. These reduce heat but do not fix the pump, so the run should be short and monitored.
3. The client rings at 2 pm asking whether the job will be ready. Draft the key points the student should communicate, and justify why honesty about the delay is the correct professional choice.
Read the worked answer
Key points to communicate:
- The job is in progress and the shaft has been roughed to near size.
- A coolant fault on the lathe has affected surface finish and finishing cuts cannot be completed safely to 38.00 mm tolerance until it is resolved.
- The fault has been reported and logged and a supervisor is aware.
- A revised completion time will be confirmed once the repair is scheduled - do not promise 3 pm if it cannot be met.
Honesty is correct because: (a) a shaft finished without adequate coolant may not hold the 38.00 mm tolerance or the required finish, so promising delivery risks a rejected part; (b) safety faults must be reported, not hidden, and misleading the client about why work stopped undermines the workshop's safety culture; (c) early warning lets the client plan around the delay, which is more professional than a late surprise. The student should not blame the supervisor personally - describe the fault, not the person.