Test fixture crosshead
CJ Ackley | FEA | Project brief

STATUS
Structural brief; loads and supports to define. Detailed design and analysis pending.

THE PROBLEM
A benchtop fixture applies force to a specimen, but the fixture also deflects. Its movement can become part of the reported displacement. This study compares crosshead profiles within a fixed mounting envelope.

DESIGN QUESTION
Which machinable crosshead keeps fixture deflection within the measurement budget?

SCOPE
- Compare a solid crosshead with alternative machined profiles.
- Evaluate centered and off-center loading using small-deflection elastic analysis.
- Check how the predicted movement changes when the support assumptions change.

CONSTRAINTS
Working load: 500 N, proposed
Deflection allowance: Less than 0.05 mm, proposed
Interfaces: Fixed mounting points and specimen clearance
Model scope: Small elastic deflections under specified static loads

ENGINEERING DECISIONS (PROPOSED)
Support stiffness and profile comparison
Compare candidate profiles under the same loads, mesh policy, and support assumptions. Bracket the mounting stiffness instead of treating an ideal fixed support as the only answer.

ALTERNATIVE
Optimizing one detailed model can produce an attractive low-mass shape, but the ranking may disappear when the actual mount is more flexible.

TRADEOFF
A sensitivity study needs more solves and can leave several candidates tied. The final choice must also account for tool access and drawing tolerances.

PROPOSED TOOLS AND METHODS
Parametric CAD, Linear elastic FEA, Beam calculation

VERIFICATION SCENARIO (NOT YET RUN)
A lighter candidate meets an eccentric load
Compare the solid baseline and machined candidates at the proposed 500 N load, including an off-center load and the defined range of support stiffness.

PASS CONDITION
Report whether each candidate meets the proposed 0.05 mm fixture-displacement allowance across the support cases. Flag any ranking that reverses with the assumptions.

EVIDENCE TO RETAIN
Reproducible solver inputs, a beam reference case, a mesh-refinement table, reaction balance, and displacement measured at the same defined location.

MEASUREMENT AND LIMITS
Use three successive mesh levels and a proposed 2% change limit for the displacement of interest. Evaluate stress away from mathematical support singularities. The mesh limit checks numerical stability, not agreement with a physical fixture.

COMPLETION CRITERIA (PLANNED)
- The reference model agrees with its analytical assumptions within a stated tolerance.
- Candidate comparisons include convergence and support sensitivity.
- The selected machinable design has drawings and a physical validation plan.

VERIFICATION PLAN
Analytical comparison
Compare a simplified version with a beam calculation before analyzing the detailed shape.

Mesh refinement
Refine the mesh until displacement and representative stress values are stable. Treat support singularities separately.

Support sensitivity
Vary realistic support stiffness and load placement to show how much the assumptions affect the result.

PLANNED DELIVERABLES
- Baseline and candidate CAD models
- Reproducible structural model, load cases, and convergence data
- Selected drawing and a report comparing stiffness, stress, and manufacturability

SCOPE NOTE
The load and deflection values are proposed requirements. No analysis results or load rating are being claimed.

NEXT STEP
Choose the material, crosshead envelope, and a defensible representation of its supports.
