Common problems with Vertical Machining Centre and how to avoid them
Vertical Machining Centre is a milling machine with a vertical spindle and an automatic tool changer, run from a single programme. It is used for batch work on prismatic parts where several operations have to run without an operator. 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, semiconductor is used as the example: cleanliness and surface condition that matter more than the nominal size.
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.
- Tool lengths set by eye, which puts the first part out of tolerance. 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.
- A fixture that swarf collects in, so the second part sits higher than the first. 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. Tool lengths set by eye, which puts the first part out of tolerance 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. Design a datum face that can be indicated in one setup, and the result is confirmed by CMM or height gauge against the drawing before the batch is released.
| Item | Note |
|---|---|
| Failure | tool lengths set by eye, which puts the first part out of tolerance |
| Prevention | design a datum face that can be indicated in one setup |
| Tolerance to hold | ±0.01 mm |
| Finish to check | Ra 1.6 µm as milled |
Applications
Vertical Machining Centre is used for batch work on prismatic parts where several operations have to run without an operator, and in practice it appears most often in enclosures, manifolds, bracket families, repeat production parts.
How it fits into a machining route
Vertical Machining 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: setups and tool changes, not the metal removal. 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.
CMM or height gauge against the drawing 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 Vertical Machining 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?
6082-T6 aluminium is the usual choice, supplied against in-house approval with dimensional reports.
How is the result verified?
CMM or height gauge against the drawing, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
A fixture that swarf collects in, so the second part sits higher than the first.
How long does it take and what does it cost?
A realistic cycle is 10 to 30 minutes depending on the number of faces, and the cost driver is setups and tool changes, not the metal removal.
Related capability
If you are specifying Vertical Machining 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 Vertical Machining Centre in the encyclopedia: 5-Axis Machining Centre, CNC Lathe, Reaming and Boring. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
