CNC Milling Machine vs Cylindrical Grinding Machine: which one fits your part
CNC Milling Machine is a machine tool that removes material with a rotating cutter while the workpiece is clamped to a moving table. It is used for producing prismatic parts, pockets, slots and contoured faces to a drawing. 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, robotics is used as the example: repeatability across a family of near identical parts.
The short answer
CNC Milling Machine and Cylindrical Grinding Machine overlap, but they are not interchangeable. Choose CNC Milling Machine when the requirement is producing prismatic parts, pockets, slots and contoured faces to a drawing; choose Cylindrical Grinding Machine when the requirement is holding diameter tolerance and roundness on shafts, pins and bores after heat treatment.
Side by side
| Point of comparison | CNC Milling Machine | Cylindrical Grinding Machine |
|---|---|---|
| What it is | a machine tool that removes material with a rotating cutter while the workpiece is clamped to a moving table | a machine that finishes external and internal diameters with a rotating wheel and a rotating workpiece |
| Used for | producing prismatic parts, pockets, slots and contoured faces to a drawing | holding diameter tolerance and roundness on shafts, pins and bores after heat treatment |
| Typical tolerance | ±0.02 mm | ±0.002 mm |
| Surface finish | Ra 1.6 µm as milled, Ra 0.8 µm with a finishing pass | Ra 0.2 µm standard |
| Cost driver | driven by setup count and the number of tool changes | stock allowance and the number of diameter changes |
Which one fits your part
Work from the feature that has to work. If the requirement is producing prismatic parts, pockets, slots and contoured faces to a drawing, CNC Milling Machine is the safer call; if it is holding diameter tolerance and roundness on shafts, pins and bores after heat treatment, Cylindrical Grinding Machine wins on cost or on lead time. Where the drawing does not make it clear, that is the question to settle before quotation rather than after.
A note on specification
Write the drawing so that either route can be considered where the function allows. That keeps the quotation competitive and stops a process being specified where a result should have been.
Applications
CNC Milling Machine is used for producing prismatic parts, pockets, slots and contoured faces to a drawing, and in practice it appears most often in general jobbing work, jig and fixture bodies, machine frames and brackets, prototype housings.
How it fits into a machining route
CNC Milling Machine 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: driven by setup count and the number of tool changes. 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 9001 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.
Measured on a CMM after deburring 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 Milling Machine?
±0.02 mm is realistic in production. Tightening it beyond the functional requirement increases cost without improving the part.
Which material is usually used?
6061-T6 aluminium is the usual choice, supplied against ISO 9001 with dimensional reports.
How is the result verified?
Measured on a CMM after deburring, with the results recorded so a repeat order can be compared with the first.
What is the most common mistake?
Long end mills used beyond a sensible length to diameter ratio, which shows up as chatter on the wall.
How long does it take and what does it cost?
A realistic cycle is 15 to 40 minutes for a typical bracket, and the cost driver is driven by setup count and the number of tool changes.
Related capability
If you are specifying CNC Milling Machine 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 Milling Machine in the encyclopedia: CNC Lathe, Swiss-Type Lathe, High Speed Machining. Reading them together gives a fuller picture of how the process, the material and the measurement affect each other.
