Turn-Mill Centre tolerances: what is realistic and what it costs
Turn-Mill Centre is a lathe with driven tooling and a Y axis, so milling and turning are completed in one machine. It is used for complex parts that would otherwise move between a lathe and a machining centre. 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, food processing is used as the example: surfaces that clean down between production runs.
What is realistic
The tolerance that can be held on Turn-Mill Centre depends on the feature, the material and the batch size. A value held once on a first article is not the same as a value held across a thousand pieces.
| Aspect | Typical value |
|---|---|
| Process tolerance | ±0.01 mm |
| Measurement | CMM for the milled features, micrometer for the diameters |
| Surface finish | Ra 0.8 µm achievable on both turned and milled faces |
| Batch effect | capability, not a single measurement |
| Documentation | ISO 13485 or AS9100 depending on the part |
| Cost effect | one setup replaces two machines, which is where the saving is |
What it costs to hold
Tolerance and cost are not proportional; they are closer to exponential once a process has to be controlled rather than simply performed. One setup replaces two machines, which is where the saving is, which is why the drawing should say which features carry the function and which do not.
How it is verified
CMM for the milled features, micrometer for the diameters. Calibration dates and measurement uncertainty are recorded with the result, so a later order can be compared with the first instead of being measured from scratch.
Applications
Turn-Mill Centre is used for complex parts that would otherwise move between a lathe and a machining centre, and in practice it appears most often in medical device bodies, aerospace fittings, hydraulic components, instrument housings.
How it fits into a machining route
Turn-Mill Centre 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: one setup replaces two machines, which is where the saving is. 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 ISO 13485 or AS9100 depending on the part. 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.
CMM for the milled features, micrometer for the diameters 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 Turn-Mill Centre?
±0.01 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
17-4PH stainless steel is the usual choice, supplied against ISO 13485 or AS9100 depending on the part.
How is the result verified?
CMM for the milled features, micrometer for the diameters, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
C axis positioning used as if it were a rotary table, without checking the brake.
How long does it take and what does it cost?
A realistic cycle is 15 to 45 minutes for a complete part, and the cost driver is one setup replaces two machines, which is where the saving is.
Related capability
If you are specifying Turn-Mill Centre 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 Turn-Mill Centre in the encyclopedia: Swiss-Type Lathe, Wire EDM Machine, CNC Turning. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
