Taper and concavity on a newly installed centre lathe
A 2.2 m between-centres centre lathe was installed on a new 150 mm concrete slab, grouted after one week and commissioned without formal levelling. Six weeks later the operator reports that a 300 mm test bar turned between centres measures 0.060 mm larger at the headstock end than at the tailstock end, and facing a 200 mm diameter disc gives 0.030 mm concavity. A 0.02 mm/m precision level on the cross slide shows the bubble 3 divisions toward the tailstock when the carriage is at the headstock end and 1 division at the tailstock end; along the bed the level reads +0.04 mm/m at the headstock and -0.01 mm/m at the tailstock. Foundation bolts were tightened by feel. The machine stands beside a doorway where afternoon sunlight falls on the headstock, the ambient is 34 °C, swarf has collected under the tailstock end, and the machine has never been re-levelled since commissioning.
1. List the candidate causes of the taper and concavity, and state the order in which you would check them. Explain why the headstock must not be touched first.
Read the worked answer
The symptoms are consistent with bed twist, not a faulty headstock.
- Twist from the level data: the longitudinal readings differ by 0.05 mm/m between the two ends of the bed. Over the 2.2 m bed that is an order of magnitude more than the 0.02 mm/m normally accepted, and a twisted bed guides the carriage in a helix, producing a taper that is larger at the headstock end and a concave face when the cross slide is fed.
- Thermal and settlement effects: sunlight on the headstock and a warm ambient distort the bed; swarf packing under the tailstock end lifts one corner; bolts tightened by feel give uneven pad loading and a rocking bed.
Order of checks:
- Shade the machine, clean out the swarf, let it soak at workshop temperature for at least 12 h, then repeat the level survey.
- Check for rocking with a 0.02 mm feeler under each foot and confirm all pads carry load.
- Level the bed in both longitudinal and cross planes.
- Only then check spindle-axis parallelism with a test mandrel, cross-slide squareness, and tailstock offset.
The headstock is dowelled and scraped to the bed; shimming it to hide a twist locks in a fault that will move again as the foundation settles, and it destroys the reference geometry. A tailstock error would affect only work turned between centres, but it would not produce concavity when facing.
2. Describe the levelling correction and the acceptance re-test you would carry out, with the figures you would work to.
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Procedure:
- Clean the foundation pads and levelling wedges; release the holding-down bolts just enough to unload the bed.
- Level longitudinally at both ends and in the cross plane at the headstock and tailstock, adjusting wedges until the level reads within 0.02 mm/m in both planes, with no more than 0.02 mm rocking at any foot.
- Torque the bolts in stages, diagonally, re-checking the level after each stage; re-check after 24 h because grout and concrete creep.
Acceptance tests after levelling:
- Turning test: light cut, 0.2 mm depth, 300 mm between centres, measure at three positions with a micrometer reading to 0.001 mm. Taper must be within 0.02 mm over 300 mm (0.067 mm/m); the present 0.060 mm is 0.2 mm/m, three times over.
- Facing test: face a 200 mm disc and check with a straight edge and feeler or a dial indicator on the cross slide; concavity must be within 0.02 mm over the diameter, against 0.030 mm now.
- Spindle axis: test mandrel in the taper, indicator on the carriage, 0.02 mm in 300 mm in both vertical and horizontal planes.
Record all readings against the machine's history card; the direction of any remaining error tells you whether twist is gone and whether the cross slide or headstock is genuinely out.
3. After correct levelling, the 300 mm taper is still 0.035 mm but facing is now within 0.010 mm. What does this pattern tell you, and what would you do next?
Read the worked answer
Levelling has removed most of the fault: facing came from bed twist and is now acceptable, but a residual taper that is still larger at the headstock end points to the spindle axis not being parallel to the bed in the horizontal plane, or to tailstock offset.
Isolate it:
- Turn a test piece held only in the chuck, unsupported by the tailstock, over the longest length the chuck will safely carry. A taper here means the headstock axis is out; no taper here but a taper between centres means the tailstock is offset.
- Put a test mandrel in the spindle taper and sweep it with a dial indicator mounted on the carriage, at the headstock end and 300 mm out, in both planes. A horizontal-plane error of 0.035 mm in 300 mm is the value to correct.
Actions depend on the result:
- Tailstock offset: adjust the tailstock set-over screws and re-cut a test bar; this is a normal setting, not a repair.
- Headstock out of alignment: check that the locating faces and dowels are clean and undamaged, check spindle bearing preload and that the headstock clamp bolts are correctly torqued, then scrape or shim the headstock to the bed and re-dowel - a job for a planned stoppage with the geometric test sheet, not a quick adjustment.