1. Definition of Datum, Tolerance, and Alignment

Datum, Tolerance, and Alignment is the assembly-design discipline that establishes the positional relationships of individual parts and assemblies using a consistent reference system, manages manufacturing and assembly variation within allowable limits, and aligns actual machine axes, surfaces, guides, and drive components to the required positions and orientations.

Each term has the following role:

• Datum defines the reference used to determine position and orientation.

• Tolerance defines the allowable range of dimensional and geometric variation.

• Alignment is the process of positioning and adjusting actual parts and assemblies to match the defined references.

These three elements are not independent.

If the reference system is unclear, tolerances cannot be assigned correctly. If tolerances are inappropriate, assembly alignment may become impossible or excessively dependent on manual adjustment.

The essence of this discipline is therefore:

Establish measurable references, control tolerance accumulation, and reproduce the required geometric relationships in the actual assembled machine.

2. Role in Assembly Accuracy

The final accuracy of a machine is not determined only by the machining accuracy of individual parts.

The following factors act together:

• Part dimensional error

• Datum-surface machining error

• Form error

• Assembly-position error

• Fastening deformation

• Welding deformation

• Load-induced deformation

• Temperature variation

• Bearing and guide clearance

• Variation in adjustment work

For example, even if a linear guide itself has high precision, the linear-motion accuracy of the axis cannot be ensured under the following conditions:

• The frame datum surface is not flat.

• The two rails are not parallel.

• The rail-tightening sequence is incorrect.

• The moving table is twisted.

• The ball screw is not parallel to the guides.

3. Definition of Datum

A Datum is a theoretically exact reference used to measure or assemble dimensions, position, orientation, and geometry.

In an actual part, the following physical features may be used as Datum Features:

• Plane

• Cylindrical surface

• Hole

• Shaft

• Centerline

• Slot

• Locating pin

• Reference block

A datum is not merely a surface marked on a drawing. It must be a repeatable reference that can be used in actual measurement and assembly.

4. Datum Feature and Actual Reference

A datum on a drawing is defined as a perfect theoretical plane or axis, but the surface of an actual part is not perfect.

The following relationships should therefore be distinguished:

• Datum: ideal theoretical reference

• Datum Feature: actual physical feature on the part

• Datum Simulator: measurement or assembly equipment used to establish the datum, such as a surface plate, pin, chuck, or V-block

Example:

• Datum A on the drawing

• The actual machined base surface

• The surface plate used to contact that surface during assembly

These three elements must be connected consistently.

5. Primary, Secondary, and Tertiary Datums

Three levels of datums are generally used to locate a part stably.

5.1 Primary Datum

The primary datum constrains the largest number of degrees of freedom.

A broad and stable plane is generally used.

The primary datum can constrain:

• Translation along one axis

• Rotation about two axes

5.2 Secondary Datum

The secondary datum constrains additional degrees of freedom not constrained by the primary datum.

A surface or axis perpendicular to the primary datum is generally used.

5.3 Tertiary Datum

The tertiary datum constrains the final remaining degree of freedom and fully defines the position of the part.

This structure is generally associated with the 3-2-1 locating principle.

6. The 3-2-1 Locating Principle

The 3-2-1 principle is a basic method for constraining the six degrees of freedom of a part without excessive constraint.

Three Points on the Primary Datum

Three contact points establish one plane.

Constrained degrees of freedom:

• Translation in the Z direction

• Rotation about the X-axis

• Rotation about the Y-axis

Two Points on the Secondary Datum

Two contact points establish the second plane.

Constrained degrees of freedom:

• Translation in the Y direction

• Rotation about the Z-axis

One Point on the Tertiary Datum

The final contact point constrains the remaining degree of freedom.

Constrained degree of freedom:

• Translation in the X direction

If too many contact points are used, part-form error may cause instability or overconstraint.

7. Criteria for Selecting Assembly Datums

A good assembly datum should satisfy the following conditions:

• It has sufficient rigidity.

• Its machining quality is stable.

• It is broad and easy to identify.

• It is accessible for measurement.

• It is unlikely to be damaged during assembly.

• It can be shared by multiple parts and modules.

• It can reproduce the same position during reassembly.

• It is related to functionally important positions.

The datum should be selected based on product function and measurability, not only on ease of machining.

8. Functional, Manufacturing, Assembly, and Inspection Datums

Datums may differ according to their purpose.

Functional Datum

A Functional Datum determines the actual function and performance of the machine.

Examples:

• Product-support table surface

• Rotational-axis center

• Machining-tool centerline

• Camera inspection reference surface

Manufacturing Datum

A Manufacturing Datum is used to machine the part.

Assembly Datum

An Assembly Datum is used to physically locate and assemble the part.

Inspection Datum

An Inspection Datum is used for measurement and quality verification.

Where possible, these datums should be aligned.

If they differ unnecessarily, datum-conversion errors are introduced.

9. Common Datum System

When multiple parts form one function, they should share a common datum system.

Examples:

• Use the top surface of the base as the height reference for all modules.

• Use the centerline of the main guide as the axis-position reference.

• Define the product-travel direction as the machine X-axis.

• Use the product centerline as the left-right reference for all fixtures.

A common datum system improves:

• Dimensional relationships

• Tolerance management

• Measurement

• Assembly

• Design changes

• Field reassembly

10. Datum Hierarchy

In a complex machine, datums are also organized hierarchically.

Example:

Factory Installation Reference
→ Machine Base Datum
→ Machine Module Datum
→ Functional Module Datum
→ Subassembly Datum
→ Part Datum

Each lower-level datum should be connected to the upper-level datum.

If the datum hierarchy is disconnected, individual modules may be accurate by themselves but incorrectly positioned in the complete machine.

11. Master Datum

The highest-level reference for the complete machine may be defined as the Master Datum.

A Master Datum is generally selected from one of the following:

• Top surface of the machine base

• Precision table surface

• Main frame reference surface

• Product-transfer centerline

• Main machining-axis center

The Master Datum should:

• Remain stable over a long period.

• Resist deformation under machine load.

• Be accessible during assembly and inspection.

• Be verifiable again after field installation.

12. Coordinate System and Reference Directions

It is important to use the same coordinate system in assembly and control.

Example definition:

• X-axis: product-travel direction

• Y-axis: left-right direction

• Z-axis: vertical direction

• Positive rotation: right-hand coordinate system

The same coordinate system should be used in:

• Mechanical drawings

• Assembly drawings

• Measurement records

• Motion control

• HMI

• Robot coordinates

• Vision systems

Different coordinate-system definitions may cause confusion in motor direction, sensor position, offsets, and correction values.

13. Definition of Tolerance

Tolerance is the allowable range within which the dimensions, form, orientation, and position of a part or assembly may vary.

Tolerance defines both the required accuracy and the practical capability to manufacture and assemble the product.

If tolerances are excessively tight, the following may occur:

• Increased machining cost

• Increased inspection cost

• Increased assembly defects

• Need for selective assembly

• Longer lead time

If tolerances are excessively loose, the following may occur:

• Increased position error

• Interference or excessive clearance

• Reduced accuracy

• Vibration and noise

• Functional failure

14. Main Types of Tolerance

Dimensional Tolerance

Controls allowable variation in length, diameter, thickness, and distance.

Form Tolerance

Controls the accuracy of an individual feature itself.

Examples:

• Flatness

• Straightness

• Circularity

• Cylindricity

Orientation Tolerance

Controls the orientation of a feature relative to a datum.

Examples:

• Parallelism

• Perpendicularity

• Angularity

Location Tolerance

Controls the position of a feature relative to a datum.

Examples:

• Position

• Concentricity

• Symmetry

Runout Tolerance

Controls variation during rotation.

Examples:

• Circular runout

• Total runout

15. Assembly Tolerance and Part Tolerance

Part tolerances and final assembly tolerances should be distinguished.

Example:

• Base-hole position tolerance

• Motor-bracket hole position tolerance

• Motor-flange tolerance

• Coupling tolerance

These tolerances combine to produce the final shaft-alignment tolerance.

To meet the assembly tolerance, each part tolerance must be allocated appropriately.

Applying equally tight tolerances to every part is generally not cost-effective.

16. Tolerance Allocation

Tolerance Allocation is the process of distributing a final functional tolerance among parts and manufacturing or assembly processes.

For example, if the allowable sensor detection-position error is ±1.0 mm, the tolerance may be distributed as follows:

• Frame machining: ±0.3 mm

• Bracket machining: ±0.2 mm

• Assembly positioning: ±0.3 mm

• Sensor repeatability: ±0.1 mm

• Adjustment allowance: ±0.1 mm

Tolerance allocation should consider:

• Functional importance

• Manufacturing capability

• Measurement capability

• Assembly method

• Adjustability

• Cost

17. Tolerance Stack-Up

Tolerance Stack-Up is the analysis of how dimensional and positional errors from multiple parts combine to affect the final assembly condition.

Example:

Base Height

• Bracket Height

• Sensor Mounting Height
  − Detection-Target Height
  = Actual Sensing Gap

When the tolerances of these dimensions accumulate, the actual sensing gap may vary significantly.

18. Worst-Case Analysis

Worst-Case analysis assumes that all tolerances occur simultaneously in the most unfavorable direction.

Conceptual calculation:

Total Tolerance = Sum of the Absolute Values of All Individual Tolerances

Advantages:

• Simple calculation

• Conservative evaluation of functional failure

• Suitable for safety and interference issues

Limitations:

• May be excessively conservative

• May increase manufacturing cost

19. Statistical Tolerance Analysis

Statistical tolerance analysis reflects the fact that all errors are unlikely to reach their extreme values at the same time.

Typical methods include:

• Root Sum Square

• Distribution-based simulation

• Monte Carlo analysis

Statistical analysis is useful for balancing cost and quality in mass production.

However, the following conditions are required:

• Process distributions are stable.

• Part errors are independent.

• Sufficient measurement data are available.

• Process capability is controlled.

20. Linear Stack-Up and 3D Stack-Up

Linear Stack-Up

Analyzes length and clearance in one direction.

Examples:

• Axial clearance

• Sensor height

• Product position

• Bearing preload

3D Stack-Up

Analyzes dimensions and angular errors in multiple directions.

Examples:

• Relationship between robot and fixture positions

• Camera and product orientation

• Tool position in a multi-axis machine

• Spatial error between modules

In complex machinery, one-dimensional stack-up analysis may not be sufficient.

21. Tolerance Loop

A Tolerance Loop expresses functionally connected dimensional relationships as a closed chain.

Example:

Base Datum Surface
→ Motor Mounting Surface
→ Motor Shaft
→ Coupling
→ Ball-Screw Shaft
→ Nut
→ Moving Table
→ Base Datum Surface

This loop can be used to analyze:

• Shaft alignment

• Position error

• Height difference

• Clearance

• Preload

The tolerance loop should be established around the functional result.

22. Adjustment Structures and Tolerance Absorption

Rather than solving all variation through machining accuracy alone, adjustment structures may be used.

Typical adjustment methods include:

• Slotted holes

• Adjustment screws

• Shims

• Spacers

• Eccentric bushings

• Threaded stoppers

• Adjustable brackets

• Alignment plates

Adjustment structures absorb tolerance variation, but may introduce:

• Increased assembly time

• Operator variation

• Need for readjustment after reassembly

• Recordkeeping requirements

• Possibility of loosening

Measurement and locking methods should therefore be defined together with the adjustment structure.

23. Use of Shims

Shims are used to adjust height, parallelism, clearance, and shaft alignment.

Application Examples

• Motor-height adjustment

• Bearing-housing height

• Linear-rail parallelism

• Frame-module leveling

• Sensor position

Management Items

• Material

• Thickness

• Number of layers

• Contact area

• Position

• Surface condition

• Retention

• Recordkeeping

Excessive shim stacking may reduce rigidity and stability.

24. Clearance and Interference

Clearance and interference should be determined according to the function of the joint.

Clearance Fit

A clearance always exists between the parts.

Applications:

• Sliding parts

• Frequently disassembled parts

• Adjustable parts

Transition Fit

A small clearance or small interference may occur.

Applications:

• Precision positioning

• Shafts and hubs requiring repeatable disassembly

Interference Fit

An interference always exists.

Applications:

• Bearing retention

• Bushings

• Gears and shafts

• Fixed pins

Fits should be selected considering load, rotation, serviceability, and temperature variation.

25. Thermal Expansion and Assembly Tolerance

Machine dimensions change with temperature.

Thermal expansion is approximately affected by:

• Material coefficient of thermal expansion

• Part length

• Temperature change

When different materials are assembled, differential expansion may occur.

Examples:

• Aluminum frame and steel rail

• Steel shaft and aluminum housing

• Plastic cover and metal frame

If thermal expansion is not considered, the following may occur:

• Guide binding

• Increased bearing preload

• Position error

• Loss of clearance

• Sensor-position change

26. Definition of Alignment

Alignment is the process of positioning parts, axes, surfaces, or modules so that they match the defined reference position and orientation.

Main assembly-alignment targets include:

• Frames

• Linear guides

• Ball screws

• Shafts and bearings

• Motors and couplings

• Gears and pulleys

• Tooling

• Sensors

• Cameras

• Machine modules

27. Main Types of Alignment

Level Alignment

Aligning a component to a horizontal plane or reference plane.

Parallel Alignment

Aligning two axes or surfaces parallel to each other.

Perpendicular Alignment

Aligning two axes or surfaces at right angles.

Coaxial Alignment

Aligning the centerlines of two rotating shafts.

Positional Alignment

Adjusting the actual position of a part to its target position.

Angular Alignment

Correcting angular error between axes or surfaces.

28. Frame and Base Alignment

The frame and base are the starting point for overall machine alignment.

Main Items to Verify

• Level of the installation surface

• Flatness of the base

• Squareness of the frame

• Height difference between modules

• Deformation after anchor tightening

• Deformation after load application

General Procedure

1. Inspect the installation surface.

2. Position the machine temporarily.

3. Adjust the leveling bolts.

4. Measure heights at defined reference points.

5. Check frame deformation.

6. Tighten the anchors.

7. Measure again.

8. Apply grouting if required.

29. Linear-Guide Alignment

Linear-guide alignment directly affects linear-motion accuracy and drive load.

Main Control Items

• Straightness of the reference rail

• Parallelism between the two rails

• Height difference between rails

• Block position

• Tightening torque

• Travel resistance

Common Alignment Methods

• Seating against a reference shoulder

• Use of alignment blocks

• Dial-indicator measurement

• Self-alignment using the moving table

• Laser measurement

The rail should not be forced into alignment by deforming it with bolts.

30. Ball-Screw Alignment

The ball screw should be aligned parallel to the linear guides.

Main Items to Verify

• Center of the fixed-side bearing

• Center of the support-side bearing

• Position of the nut housing

• Center of the motor shaft

• Coupling offset

• Axial play

• Rotational torque

Results of Misalignment

• Increased travel resistance

• Increased motor load

• Heat generation

• Vibration

• Reduced ball-screw life

• Position error

31. Rotary-Shaft Alignment

In a rotary-shaft system, the following components should be aligned:

• Bearing housings

• Shaft centerline

• Coupling

• Motor shaft

• Reducer

• Pulley or gear

Main Errors

• Parallel offset

• Angular error

• Axial-position error

• Runout

Alignment has a major effect on vibration and bearing life during rotation.

32. Coupling Alignment

A coupling can absorb a certain amount of misalignment, but this does not eliminate the need for alignment.

Items to Verify

• Horizontal offset

• Vertical offset

• Angular error

• Shaft-end spacing

• Coupling insertion depth

• Set-screw torque

Where necessary, shims may be used beneath the motor to adjust height.

33. Pulley, Sprocket, and Gear Alignment

Pulleys, sprockets, and gears require correct shaft parallelism and position.

Items to Verify

• Pulley-face alignment

• Shaft parallelism

• Belt centerline

• Chain centerline

• Gear mesh

• Backlash

• Contact pattern

Misalignment increases belt edge wear, chain disengagement, gear noise, and bearing load.

34. Tooling Alignment

Tooling directly determines product position and machining or assembly function.

Reference Targets

• Product datum surface

• Clamp position

• Tool center

• Product-transfer centerline

• Inspection reference

Items to Verify

• Position

• Height

• Parallelism

• Perpendicularity

• Repeatability

• Repeatability after replacement

Tooling datums should preferably be connected to the product-design datums.

35. Sensor Alignment

A sensor must have the correct relationship to its detection target.

Main Control Items

• Sensing distance

• Sensing angle

• Detection range

• Reference position

• Adjustment range

• Repeatability

• Bracket rigidity

Photoelectric sensors and vision sensors are particularly sensitive to orientation and angular error.

36. Camera and Vision Alignment

The following should be managed when aligning a vision system:

• Camera position

• Working distance

• Optical axis

• Lens orientation

• Lighting angle

• Product datum

• Calibration pattern

Camera-bracket deformation and vibration may also affect the measurement result.

37. Module-to-Module Alignment

When several machine modules are connected, their positional relationships must be controlled.

Examples:

• Conveyor height

• Product centerline

• Guide spacing

• Position of robot relative to equipment

• Upstream and downstream equipment connection

Required Elements

• Common datum

• Locating pins

• Height adjustment

• Connection plates

• Measurement reference points

• Field-adjustment range

38. Alignment Measurement Tools

Various measuring instruments are used for alignment.

Basic Measuring Tools

• Straightedge

• Feeler gauge

• Level

• Surface plate

• Vernier caliper

• Micrometer

Precision Measuring Tools

• Dial indicator

• Test indicator

• Height gauge

• Precision level

• Autocollimator

• Laser alignment system

• Laser tracker

• Coordinate measuring machine

The selected instrument should have resolution and accuracy sufficiently better than the required tolerance.

39. Measurement Uncertainty

Measurement results themselves contain uncertainty.

Main causes include:

• Instrument accuracy

• Operator variation

• Measuring force

• Datum-surface condition

• Temperature

• Vibration

• Instrument setup

• Repeatability variation

If the required tolerance is similar to the measurement uncertainty, reliable acceptance judgment becomes difficult.

40. Basic Alignment Procedure

A typical alignment procedure is as follows:

1. Confirm the datum surface.

2. Clean the datum surface.

3. Temporarily fasten the part.

4. Establish the reference side first.

5. Install the measuring instrument.

6. Measure position and angle.

7. Adjust using bolts, shims, or adjustment mechanisms.

8. Tighten to the specified torque.

9. Re-measure after tightening.

10. Record the result.

Because the measured value may change after final tightening, reinspection is required.

41. Alignment Sequence

Alignment should generally be performed in the following order:

Base → Frame → Reference Guide → Secondary Guide → Drive System → Tooling → Sensors → Modules

If lower-level parts are aligned before the upper-level references are established, rework may be required.

42. Alignment Hold Point

Critical alignment items should be verified before proceeding to the next operation.

Examples of Hold Points:

• Frame level

• Reference-rail straightness

• Parallelism between rails

• Parallelism between ball screw and guide

• Coaxiality of rotating shafts

• Tooling position

Cover installation or downstream module assembly should not proceed before approval.

43. Alignment Change Caused by Fastening

Bolt tightening can change part position and shape.

Main causes:

• Tightening sequence

• Torque variation

• Uneven contact surfaces

• Insufficient part rigidity

• Shim position

• Bolt-hole clearance

Precision parts should therefore be fastened using methods such as:

• Gradual torque increase

• Symmetrical tightening

• Tightening from the center outward

• Repeated measurement during tightening

• Re-measurement after final tightening

44. Alignment Change under Load

Even if a machine is accurately aligned without load, its position may change when actual loads are applied.

Loads to consider include:

• Moving mass

• Product weight

• Machining reaction force

• Cylinder force

• Belt tension

• Cable-carrier load

• Piping reaction force

Where necessary, alignment should be measured under both no-load and rated-load conditions.

45. Alignment Change during Operation

During machine operation, alignment may change because of:

• Thermal expansion

• Vibration

• Joint relaxation

• Structural deformation

• Bearing temperature

• Lubrication condition

• Floor settlement

Realignment or retightening may be required after initial commissioning.

46. Static Alignment and Dynamic Alignment

Static Alignment

Measures position, parallelism, perpendicularity, and related characteristics while the machine is stationary.

Dynamic Alignment

Measures motion error and vibration during operation.

Examples:

• Shaft runout

• Straightness of axis travel

• Position repeatability

• Vibration

• Load variation

• Thermal displacement

Correct static alignment does not always guarantee correct dynamic performance.

47. Accuracy Verification after Assembly

The following may be verified after assembly:

• Straightness of linear motion

• Parallelism between axes

• Perpendicularity between axes

• Position accuracy

• Repeatability

• Rotary-shaft runout

• Backlash

• Travel resistance

• Drive torque

• Vibration

Verification results should be linked to design tolerances and functional requirements.

48. Distinction Between Compensation and Mechanical Alignment

Some position errors can be reduced through control compensation.

Examples:

• Position offset

• Pitch-error compensation

• Vision-coordinate correction

• Robot-coordinate correction

However, the following problems should not be addressed only through software compensation:

• Large shaft misalignment

• Excessive friction

• Guide binding

• Bearing side load

• Structural deformation

• Mechanical interference

Mechanical alignment should be established first, followed by control compensation.

49. Reassembly and Datum Restoration

The original position should be restorable after component replacement or transportation.

Required structures include:

• Locating pins

• Precision pilots

• Reference shoulders

• Datum marks

• Measurement reference points

• Shim records

• Adjustment records

If position is determined only by bolt-hole clearance, reassembly repeatability may be poor.

50. Field Installation and Alignment

Even a machine aligned at the factory should be checked again after field installation.

Main reasons include:

• Transportation shock

• Frame deformation

• Floor condition

• Leveling

• Anchor tightening

• Module separation and reassembly

Field-alignment items include:

• Base level

• Module height

• Product-transfer centerline

• Upstream and downstream equipment connection

• Guide-joint alignment

• Safety-device position

51. Main Symptoms of Misalignment

Misalignment may appear as:

• Increased motor current

• Increased travel resistance

• Binding at specific positions

• Vibration and noise

• Bearing heating

• Uneven belt wear

• Reduced position repeatability

• Poor product alignment

• Unstable sensor detection

• Premature seal wear

These symptoms may appear to be control problems even when the root cause is mechanical misalignment.

52. Causes of Misalignment

• Contaminated datum surface

• Burrs on datum surfaces

• Incorrect shimming

• Incorrect tightening sequence

• Uneven tightening torque

• Welding deformation

• Part-machining error

• Tolerance accumulation

• Insufficient bracket rigidity

• Measurement-method error

• Temperature difference

• Load-induced deformation

53. Methods for Improving Misalignment

• Re-machine the datum surface.

• Clean the datum surface and remove burrs.

• Adjust shims.

• Apply adjustment screws.

• Modify the locating structure.

• Increase bracket rigidity.

• Reallocate tolerances.

• Change the assembly sequence.

• Standardize the tightening sequence.

• Improve the measuring system.

• Measure after thermal stabilization.

Repeated misalignment is more likely to indicate a problem in design or datum strategy than a problem with the operator alone.

54. Relationship Between Datum and Design for Assembly

To improve assembly performance, datum structures should be easy for the operator to understand and use.

A good assembly-datum design:

• Makes the datum direction clear.

• Makes reversed assembly difficult.

• Allows the part to locate itself.

• Simplifies measurement.

• Reduces the number of adjustments.

• Provides repeatable positioning after reassembly.

55. Relationship Between Datum and Serviceability

After maintenance, the original position of a component should be restorable.

Datum structures for serviceability include:

• Locating pins on replacement parts

• Datum marks on sensor brackets

• Motor-shim records

• Tooling reference blocks

• Post-replacement measurement procedures

• Stored calibration values

56. Drawings and Datum Indication

Assembly drawings should clearly indicate:

• Datum identification

• Datum priority

• Critical dimensions

• Geometric tolerances

• Reference axes

• Measurement positions

• Adjustment directions

• Tightening sequence

• Final allowable values

If only datum symbols are shown without explaining the actual assembly method, different interpretations may occur in the field.

57. Alignment Procedure

An alignment procedure should include:

• Target assembly

• Datum surfaces and datum axes

• Measuring instruments

• Instrument setup method

• Measurement positions

• Target values

• Allowable tolerances

• Adjustment method

• Tightening torque

• Re-measurement method

• Record format

58. Measurement and Alignment Records

Critical assembly-accuracy data should be recorded.

Record items include:

• Machine serial number

• Module number

• Measurement item

• Measurement location

• Measured value

• Allowable value

• Measuring instrument used

• Instrument calibration number

• Operator

• Measurement date

• Adjustment details

59. Major Design Deliverables

60. Datum Review Questions

• Is the machine Master Datum clearly defined?

• Are functional datums connected to assembly datums?

• Are primary, secondary, and tertiary datums defined?

• Is overconstraint avoided?

• Do multiple modules share common datums?

• Can datum surfaces actually be measured?

• Are datum surfaces protected during assembly?

• Can the same datums be used during reassembly?

61. Tolerance Review Questions

• Is the final functional tolerance defined?

• Are part tolerances and assembly tolerances distinguished?

• Has tolerance stack-up been analyzed?

• Have items requiring Worst-Case analysis been identified?

• Is statistical tolerance analysis applicable?

• Are tolerance and manufacturing cost balanced?

• Are locations requiring adjustment structures clearly identified?

• Have temperature- and load-induced changes been considered?

62. Alignment Review Questions

• Are alignment targets and alignment sequence clearly defined?

• Are the reference side and adjustment side distinguished?

• Can suitable measuring instruments be used?

• Is there enough space for measurement access?

• Are temporary fastening and final fastening distinguished?

• Is remeasurement performed after final tightening?

• Have both no-load and loaded conditions been reviewed?

• Is alignment change checked after operation?

• Is an alignment procedure defined for field reassembly?

63. Conclusion

Datum, Tolerance, and Alignment are core disciplines that determine the assembly accuracy of general machinery.

Datum defines where a part or module should be located and how it should be oriented.

Tolerance limits manufacturing and assembly variation to an acceptable range.

Alignment positions and adjusts actual parts and assemblies to reproduce the designed geometric relationships.

The key is to manage the following relationships consistently:

• Functional datums and assembly datums

• Upper-level and lower-level datums

• Part tolerances and final assembly tolerances

• Tolerance accumulation and adjustment range

• Alignment references and measurement methods

• Temporary fastening and final fastening

• No-load alignment and operating-condition alignment

• Factory assembly and field reassembly

• Measurement results and product performance

An appropriate datum system prevents different operators from positioning parts in different ways.

Appropriate tolerance design guarantees final function without imposing unnecessary machining cost.

An appropriate alignment procedure ensures that guides, shafts, bearings, motors, tooling, and sensors operate in the required positions and orientations without excessive load or interference.

Through this approach, assembly accuracy becomes not an operator-dependent result, but a repeatable product characteristic maintained across design, manufacturing, assembly, measurement, and commissioning.