Definition
The SW200-1 case design guide defines the geometric, structural, and tolerance constraints required to integrate the Sellita SW200-1 movement into a functional watch case.
It translates movement specifications into case design requirements, forming a direct application of movement-led watch case engineering.
The movement defines the internal constraint system.
The case must resolve it.
A valid SW200-1 case design must not begin with external case shape. It must begin with the movement, then build the case architecture around the movement cavity, stem height, crown tube position, dial interface, rotor clearance, caseback position, sealing system, and movement retention method.
SW200-1 Movement Overview: Design-Relevant Parameters
Only parameters that directly affect case design are considered here:
- Movement diameter: 25.60 mm
- Movement height: approximately 4.60 mm
- Stem height: approximately 1.80 mm
- Automatic rotor system requiring vertical clearance
These parameters define:
- Internal case diameter
- Movement cavity size
- Case thickness
- Crown tube position
- Axial stack structure
- Rotor and caseback clearance
- Movement retention strategy
All internal case design decisions originate from these constraints.
The SW200-1 should not be treated as a generic movement shape placed inside a case. It should be treated as the fixed reference geometry from which the internal case system is derived.
Core Constraint Structure
The SW200-1 defines four primary constraint domains:
- Radial constraint
- Axial constraint
- Stem height constraint
- Rotor clearance constraint
Each domain affects the others. A case can fail even when one dimension appears correct if the full constraint system has not been resolved.
1. Radial Constraint: Movement Diameter
Movement diameter defines the minimum internal case cavity size.
Radial behaviour is governed by Radial Clearance Between Movement and Case.
The radial system must provide:
- Controlled clearance for assembly
- Stable lateral positioning
- Compatibility with movement holders or retaining systems
- Allowance for machining and finishing variation
- A defined insertion path during casing
Failure can result in:
- Interference during installation
- Movement instability under load
- Poor alignment between dial, crown, and case
- Uncontrolled movement shift during crown operation or shock
Radial control defines positional stability.
The case cavity should not simply copy the 25.60 mm movement diameter. It must provide a controlled fit strategy around the movement and the chosen retention method.
2. Axial Constraint: Movement Height
Movement height defines the minimum internal vertical space required by the movement.
The axial system includes:
- Movement height
- Dial thickness
- Hand stack height
- Crystal underside clearance
- Caseback depth
- Rotor clearance
- Gasket and sealing geometry
- Retention method
Vertical spacing is governed by Axial Clearance and Movement Height vs Case Thickness.
Failure can result in:
- Hand-to-crystal contact
- Rotor-to-caseback contact
- Excessive vertical movement
- Poor axial retention
- Unnecessary case thickness
- Functional instability after assembly
Axial control defines vertical integrity.
The movement height is only the starting point. The final case thickness must be derived from the full vertical stack, not from the movement height alone.
3. Stem Height Constraint: Crown Alignment
Stem height defines the vertical position of the crown interface.
Alignment must be resolved through Stem Height to Crown Tube Position Relationship and Crown and Stem Alignment.
The crown system must maintain:
- Coaxial alignment between stem and crown tube
- No angular deviation through the case wall
- Stable engagement during winding, setting, and date correction
- No lateral stem loading
- No forced correction during assembly
Failure can result in:
- Increased friction
- Poor crown feel
- Accelerated keyless works wear
- Stem bending or binding
- Crown tube sealing problems
- Long-term functional failure
Stem height is fixed.
The case must adapt to it.
The crown tube should never be positioned from exterior styling alone. External crown appearance must be resolved after the internal stem axis and tube bore location are correct.
4. Rotor Clearance Constraint
The SW200-1 automatic rotor introduces a dynamic clearance requirement behind the movement.
The case must provide:
- Sufficient vertical clearance above the rotor path
- An unobstructed rotational envelope
- Caseback geometry that avoids contact under tolerance variation
- Clearance that remains valid after gasket compression and assembly
Rotor behaviour is governed by Rotor Clearance Requirements for Automatic Movements.
Failure can result in:
- Rotor contact with the caseback
- Reduced winding efficiency
- Scraping or mechanical wear
- Noise during movement
- Loss of automatic winding performance
Rotor clearance must remain valid under worst-case tolerance conditions.
A caseback that clears the rotor nominally may still fail if gasket compression, movement seating, caseback tolerance, or rotor end-shake are not considered.
Internal Case Geometry
Internal case geometry must accommodate all SW200-1 movement interfaces.
Geometry is defined by:
- Cylindrical movement cavity sized around the movement and clearance strategy
- Seating surfaces for controlled positioning
- Clearance for dial feet, clamps, spacers, or holders
- Caseback depth and rotor space
- Crown tube bore alignment
- Unobstructed insertion path
This behaviour is governed by Internal Case Geometry & Movement Cavity Sizing.
Internal geometry must support both function and assembly.
The case must allow the movement to enter, locate, align, secure, and operate without relying on force or deformation to correct poor geometry.
Tolerance Stack Impact
All relevant dimensions vary under real manufacturing and assembly conditions.
Critical relationships include:
- Movement height vs case depth
- Movement diameter vs case cavity
- Stem height vs crown tube bore
- Hand stack vs crystal clearance
- Rotor envelope vs caseback clearance
- Caseback position vs gasket compression
- Dial seat position vs visual and functional alignment
Tolerance behaviour must be validated under worst-case conditions.
Failure occurs when:
- Clearance collapses
- Compression exceeds limits
- Interfaces lose alignment
- Movement position changes after assembly
- A nominally correct case becomes inconsistent in production
Nominal values are not enough for SW200-1 case design. The case must remain functional across realistic manufacturing, finishing, and assembly variation.
Assembly Constraints
Assembly must be physically executable.
The case design must allow:
- Movement insertion through the case opening
- Correct dial and hand installation sequence
- Crown and stem engagement without force
- Access to securing components
- Caseback closure without disturbing movement position
- Service access where required
Failure can result in:
- Blocked assembly sequence
- Forced installation
- Damaged hands, dial, stem, or movement
- Inconsistent movement seating
- Unreliable final fit
Assembly feasibility must be resolved during design, not discovered during prototype build.
A case that is theoretically correct but difficult to assemble is not a complete engineering solution.
Sealing System Interaction
Sealing performance depends on stable geometry.
SW200-1 case design must account for:
- Caseback position
- Gasket compression
- Crown tube alignment
- Crown seal engagement
- Movement stack height
- Retention method
- Caseback and movement clearance
Failure can result in:
- Under-compression and leakage
- Over-compression and gasket deformation
- Caseback interference with the rotor
- Crown seal misalignment
- Variation between assembled units
Sealing must remain consistent across tolerance conditions.
The sealing system cannot be treated separately from movement fit. Caseback position, rotor clearance, axial stack height, and gasket compression are part of the same vertical design problem.
Structural Requirements
The case must maintain alignment under load.
Structural requirements include:
- Adequate wall thickness around the movement cavity
- Sufficient material around the crown tube bore
- Stable caseback thread or retention geometry
- Controlled deformation under tightening or pressure
- Rigid support for the movement-retention system
Structural instability can result in:
- Alignment loss
- Sealing variation
- Crown tube movement
- Caseback distortion
- Performance degradation over time
The SW200-1 does not only require enough space. It requires a case structure stable enough to preserve alignment, clearance, and sealing behaviour in use.
Common SW200-1 Case Design Failures
Typical SW200-1 case failures include:
- Crown misalignment from incorrect stem positioning
- Crown tube bore positioned from external appearance instead of stem height
- Rotor contact under tolerance variation
- Hand interference with the crystal
- Movement instability due to poor radial control
- Excessive case thickness from unmanaged axial stack-up
- Sealing inconsistency due to uncontrolled gasket compression
- Movement holder or spacer geometry that does not control position repeatably
- Caseback used to force axial retention without proper stack control
These failures usually originate from unresolved constraints rather than one isolated wrong dimension.
The movement must be treated as a connected system.
Movement-Specific Case Architecture
A proper SW200-1 case design is not just a case that can physically contain the movement.
It is a movement-specific internal architecture that resolves:
- Radial fit
- Axial stack
- Stem alignment
- Crown tube position
- Dial seating
- Hand clearance
- Rotor clearance
- Caseback position
- Sealing behaviour
- Movement retention
- Assembly sequence
This is the difference between placing a movement inside a case and engineering a case around the movement.
HorologyCAD treats the SW200-1 as one of its primary reference movements for movement-led, lug-agnostic case architecture.
The internal case system must be correct before external styling, lug form, bezel design, or crown guards are developed.
Design Strategy
Effective SW200-1 case design requires:
- Starting from verified movement constraints
- Defining radial and axial clearance systems
- Positioning the crown system from stem height
- Providing a valid rotor clearance envelope
- Designing the movement retention method early
- Validating tolerance behaviour under worst-case conditions
- Ensuring assembly feasibility
- Aligning geometry with manufacturing capability
All constraints must be resolved before external geometry is finalised.
The external case design can vary. The internal movement-fit architecture cannot be guessed.
Final Statement
The SW200-1 defines the fixed internal constraint system for the case.
A valid case design must:
- Maintain controlled clearance in all directions
- Preserve alignment across all interfaces
- Accommodate tolerance variation
- Support reliable movement retention
- Protect hand and rotor clearance
- Maintain sealing geometry
- Assemble without force
- Perform under real operating conditions
The movement defines the system.
The case must be engineered to match it.
Return to HorologyCAD
HorologyCAD is a movement-led watch case design system for building case architecture around real mechanical movements, manufacturable constraints, and functional assembly requirements.
Return to the main HorologyCAD homepage: