During CNC machining, it is necessary not only to ensure the dimensional accuracy of the machined surface itself, but also to guarantee the positional accuracy between different machined surfaces. Correct selection of locating datums plays a significant role in maintaining positional accuracy between surfaces and in arranging the machining result properly.
What is a Datum?
Geometric elements of a part, such as points, lines, and surfaces, have specific dimensional and positional relationships. During part design and machining, certain points, lines, or surfaces are often used to determine the geometric relationships of other features. These reference points, lines, or surfaces are called datums.
Types of Datums
According to their functions, datums can generally be divided into two categories:
- Design datums
- Process datums
1. Design Datum
A design datum is the reference datum specified on the engineering drawing of a part.
During part design, the points, lines, or surfaces used as references to determine the positions of other geometric elements are referred to as design datums. Design datums are defined by designers according to assembly requirements and are used to establish dimensional and positional relationships between geometric features.
On part drawings, dimensions and positional tolerances are specified relative to these design datums.
For example:
- Surface A and surface B may serve as mutual design datums.
- The axis of a Ø30 mm outer cylindrical surface may use the axis of a Ø50 mm outer cylindrical surface as its design datum.
- In another case, line D beneath plane C may serve as the design datum.
In engineering drawings, there is usually only one design datum in a given direction.
2. Process Datum
A process datum is a datum used during manufacturing operations. According to their applications, process datums can be divided into:
- Operation datums
- Locating datums
- Measurement datums
- Assembly datums
Operation Datum
An operation datum is the reference used in an operation drawing to specify the dimensions, geometry, and positional relationships of the surfaces machined during that specific operation. Whenever possible, the operation datum should coincide with the design datum.
Measurement Datum
A measurement datum is the reference used when measuring the dimensions and positional relationships of machined surfaces on a workpiece.
Assembly Datum
An assembly datum is the reference used during assembly to determine the relative position of a component or subassembly within a product. For example, during gear assembly, the bore and end face of the gear may respectively serve as radial and axial assembly datums.
Locating Datum
A locating datum is a datum used for positioning the workpiece during machining.
The point or line used as a locating datum is always realized through an actual surface, known as the datum surface.
- When a workpiece is positioned using a plane, that plane serves as the locating datum surface.
- When positioning is achieved using a cylindrical or conical surface, the axis of that cylindrical or conical surface acts as the locating datum. In contrast, the surface itself acts as the datum surface.
For example, when shaft-type components are positioned using two center holes, the inner conical surfaces of the center holes serve as the locating datum surfaces during machining of the external cylindrical sections.
Locating datums are further divided into:
- Rough datums
- Finish datums
During the initial machining operation, only unmachined blank surfaces can be selected as locating datums. Such datums are called rough datums. In subsequent operations, previously machined surfaces may be used for positioning. These are called finish datums.
Selection of Finish Datums
In process planning, the correct selection of locating datums is critical for ensuring machining accuracy and arranging machining operations properly.
Typically, finish datums are selected first according to machining requirements. The process route is then determined backward to identify which surfaces should be used as rough datums to machine the required finish datums.
Selecting finish datums should facilitate machining accuracy while ensuring convenient and reliable setup. The following principles are commonly followed.
Datum Coincidence
The datum coincidence principle means that the selected locating datum should coincide with the design datum whenever possible, thereby avoiding positioning errors caused by datum mismatch.
For example, suppose the design dimensions are:
- ( A ± Ta/2 )
- ( B ± Tb/2 )
Surfaces P and N have already been machined and inspected.
When milling plane M using the adjustment method, two schemes may be adopted to guarantee the design dimension ( B ±Tb/2 ):
- Using design datum surface N as the primary locating datum
- Using the bottom surface P as the primary locating datum
If the locating datum does not coincide with the design datum, positioning errors caused by datum mismatch will occur.
Unified Datum
The unified datum principle means that the same locating datum should be used as much as possible throughout different machining operations.
This minimizes setup errors caused by changing datums, helps ensure positional accuracy between surfaces, and reduces fixture design complexity.
For example:
- Shaft components are commonly positioned using two center holes throughout most operations.
- Housing components are often positioned using one surface and two holes.
In practice, the datum coincidence principle and the unified datum principle cannot always be satisfied simultaneously, and compromises may be necessary depending on actual manufacturing conditions.
Self-Referencing Datum
In certain finishing operations where machining allowance must be small and uniform, the machined surface itself may be used as the locating datum.
The positional accuracy of this surface relative to other surfaces is ensured by previous operations, while the current operation mainly improves surface finish and dimensional accuracy.
Typical examples include:
- Floating reaming
- Broaching
- Centerless grinding
- Thread tapping using taps or dies
In these processes, the previously machined surface itself serves as the locating datum.
Mutual Datum and Repeated Machining
When a uniform machining allowance or high positional accuracy between two surfaces is required, the two surfaces may alternately serve as datums for each other during repeated machining operations.
For example, in a lathe spindle:
- The bearing journal and taper bore require very high coaxiality and machining accuracy.
- The journal may first be ground using the taper bore as the datum.
- The taper bore may then be ground using the journal as the datum.
Through repeated machining, the required accuracy can be achieved.
Another example is precision hardened gears:
Because the hardened layer is thin, grinding allowance must remain small and uniform.
- The tooth surface may first be used as the datum to grind the bore.
- The bore may then be used as the datum to grind the tooth surface.
This ensures uniform grinding allowance and high positional accuracy relative to assembly datums.
Selection of Rough Datums
Selection of rough datums should:
- Ensure positional relationships between machined and unmachined surfaces
- Distribute machining allowance reasonably
- Provide suitable finish datums for subsequent operations
The following principles are commonly applied.
(1) Ensure positional relationships between machined and unmachined surfaces. In this case, unmachined surfaces should be selected as rough datums. If multiple unmachined surfaces exist, the surface with the highest positional accuracy requirement relative to the machined surface should be selected.
(2) Ensure sufficient machining allowance on all machined surfaces. The surface with the smallest machining allowance should generally be selected as the rough datum.
(3) Ensure uniform machining allowance on important surfaces. Important surfaces may themselves be selected as rough datums. For example, machine tool guideways require:
- High hardness
- Uniform wear resistance
- Uniform material structure
Therefore, the machining allowance removed from the guideway surface should be small and uniform.
By selecting the guideway surface itself as the rough datum, the machine bed legs can first be machined, and the machined bed leg surfaces can then be used to position the workpiece for guideway machining, ensuring uniform material removal.
(4) Ensure stable positioning and reliable clamping. The surface selected as a rough datum should be as flat and smooth as possible while avoiding:
- Forging flash
- Casting riser marks
- Other surface defects
It should also have sufficient surface area to ensure stable positioning.
(5) Avoid repeated use of rough datums. Since rough datums are unmachined blank surfaces, their dimensional accuracy and surface finish are relatively poor.
If the same rough datum is repeatedly used in the same direction, the relative position between the workpiece, machine tool, and cutting tool cannot be guaranteed during each setup, resulting in significant machining errors.
