Common problems with CNC Lathe and how to avoid them
CNC Lathe is a machine that turns the workpiece against a single point tool to produce cylindrical forms. It is used for shafts, bushings, flanges and any part whose main features are diameters. This entry explains what it is, what the numbers on a data sheet mean, and what to put on a drawing.
It is written from the point of view of the buyer and the drawing office: the values that are realistic in production, the ones that cost money, and the way the result is verified. Where an industry sets the requirement, oil and gas is used as the example: materials that work harden and still have to be certified.
The problems that actually occur
These are the failures that show up on a real job, listed in the order in which they are usually discovered.
- The tailstock not aligned, which puts a taper on a long shaft. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
- Chip control ignored, so a stringy swarf wraps around the part. The cause is usually upstream: a datum that is not the functional one, a fixture that does not repeat, or a specification that cannot be measured the way it is written.
What causes them
In most cases the cause is upstream of the operation, not in it. The tailstock not aligned, which puts a taper on a long shaft is usually a symptom of one of the three causes above rather than a problem in its own right.
How they are prevented
Prevention is duller than diagnosis. Specify diameters that can be finished in one pass wherever the tolerance allows, and the result is confirmed by micrometer, bore gauge and surface roughness gauge before the batch is released.
| Item | Note |
|---|---|
| Failure | the tailstock not aligned, which puts a taper on a long shaft |
| Prevention | specify diameters that can be finished in one pass wherever the tolerance allows |
| Tolerance to hold | ±0.01 mm |
| Finish to check | Ra 1.6 µm turned, Ra 0.4 µm with a wiper insert |
Applications
CNC Lathe is used for shafts, bushings, flanges and any part whose main features are diameters, and in practice it appears most often in shafts and pins, pulleys and rollers, hydraulic fittings, seal carriers.
How it fits into a machining route
CNC Lathe is one station in a route rather than the whole of it. The part arrives with the previous operation finished and leaves with the features it is responsible for completed, so the datum and the clamping method are agreed before the first part rather than discovered on it.The route is fixed before the first part is cut, and it is the route rather than the individual setting that holds the result: cycle time and bar feed efficiency. Where a drawing calls for something the route cannot hold economically, that is raised at quotation rather than after the parts are made.
Standards, documentation and what is issued
The documentation requirement is in-house approval with dimensional reports. Material is supplied against a certificate where the application calls for it, inspection results are recorded with the job, and the records are kept so that a repeat order can be compared with the first instead of being measured from scratch.
Micrometer, bore gauge and surface roughness gauge is the verification step. Where a customer format or a standard report is required, it is agreed before the batch starts, because a report written after the event is a report nobody can use.
Frequently asked questions
What tolerance should be specified for CNC Lathe?
±0.01 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
303 stainless steel is the usual choice, supplied against in-house approval with dimensional reports.
How is the result verified?
Micrometer, bore gauge and surface roughness gauge, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
Chip control ignored, so a stringy swarf wraps around the part.
How long does it take and what does it cost?
A realistic cycle is 3 to 12 minutes for a turned part, and the cost driver is cycle time and bar feed efficiency.
Related capability
If you are specifying CNC Lathe on a drawing and want a second opinion on the route, send the model and the quantity. The first response is a manufacturability review with the likely risk points and a quotation, so the decision can be made on facts rather than on a price alone.
Machining, inspection and documentation are handled in one place, which is why the dimensional report and the material certificate arrive with the parts rather than after them.
Related entries
These entries sit next to CNC Lathe in the encyclopedia: Turn-Mill Centre, Wire EDM Machine, Wire EDM. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
