Diameter drift and taper on a batch of EN8 pins
A batch of 60 pins, 120 mm long, is being turned from EN8 bright bar on a CNC lathe to Ø25.00/24.95 mm with Ra 1.6 µm on the diameter. The shed has no air conditioning; shop temperature climbs from 26 °C in the morning to 36 °C by mid-afternoon, and the machine gets only a 15-minute warm-up at 1000 rpm before the first cut. Tooling: CNMG 120408 carbide insert, cutting speed 180 m/min, feed 0.15 mm/rev, depth of cut 1.5 mm, flood coolant. Parts are held in a three-jaw chuck with 65 mm overhang and no tailstock support. In-process checks with a 0–25 mm micrometer (0.01 mm graduations, calibrated six months ago) gave 24.960 mm on parts 1–10, 24.975 mm on part 20 and 24.990 mm on part 25. On part 25 the free end measures 0.015 mm larger than the chuck end, and the finished surface shows a dull, polished band along the insert flank. Parts 26 onward are still to be run.
1. Part 25 is close to the upper limit at the free end and the trend is upward. Separate the three candidate causes - flank wear, workpiece deflection and thermal growth - and state the checks you would run, in order, to confirm which is acting.
Read the worked answer
Read each symptom separately before touching the machine.
| Symptom | Points to | Reasoning |
|---|---|---|
| OD grows 0.030 mm from part 10 to part 25 | flank wear | a worn flank rubs and springs back, so effective depth of cut falls and the diameter creeps up |
| Dull polished band on the flank | flank wear | rubbing contact rather than a clean shear |
| Free end 0.015 mm larger than chuck end | deflection | radial force pushes the bar away, the free end cuts less and stays bigger |
| 10 °C ambient rise over the shift | thermal drift | shifts the whole diameter, but does not bend the profile into a taper |
So wear and deflection are both acting; thermal drift is a secondary term that shifts the curve rather than tilting it.
Checks in this order (cheapest and non-destructive first):
- Measure flank wear VB on the insert corner with a toolmaker's microscope or wear gauge. For carbide turning, VB ≈ 0.3 mm is the usual rejection limit; a polished band usually means it is already past that.
- Re-measure part 25 at 20, 65 and 110 mm from the chuck with a 0.001 mm micrometer to quantify the taper, then repeat on a freshly indexed insert. Taper that disappears with a sharp tool was force-induced; taper that stays is machine geometry - tailstock/quill alignment, bed twist, chuck jaw bell-mouth.
- Cut one test bar at the start of the shift and another after two hours of running at the same offset; the difference is thermal growth of spindle and slide, and it should track the coolant temperature.
- Check tool overhang and centre height, since excess overhang multiplies deflection and also worsens finish.
2. What do you do with parts 1–25, and what must change before parts 26–60 are run? Comment specifically on the measuring instrument used for the decision.
Read the worked answer
The gauge is not good enough for the decision. A 0.05 mm tolerance band needs discrimination of about a tenth of it, i.e. 0.002 mm. A 0.01 mm micrometer gives only one significant digit across this band, so 24.960 / 24.975 / 24.990 each carry roughly ±0.005 mm of reading uncertainty. Re-measure with a 0.001 mm micrometer or a comparator set on a Ø25 setting ring, after letting the parts and gauge stabilise together on a steel plate for 20–30 minutes.
Disposition. Because the tool wore in one direction, the earliest parts are the safest. Check both ends of every pin, not a sample: parts 1–24 are acceptable if their chuck-end diameter is ≥ 24.950 mm. Part 25 reads 24.975 at the chuck end and 24.990 at the free end - inside limits, but the trend, not the reading, is the real warning: the next few pins would cross 25.000 at the free end.
Before running parts 26–60:
- Index the insert and log VB against tool life.
- Kill the 65 mm overhang - support with a revolving centre, or take a lighter finishing pass (0.5 mm depth at 0.10 mm/rev) to cut radial force.
- Add a 30-minute warm-up cycle and re-zero the tool after it, plus a re-check once the shed reaches its 36 °C afternoon peak.
- First-off inspection on every restart, then every 5th piece plotted on an x̄–R chart with limits at ±0.015 mm, so drift is caught before the limit rather than after.