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OpenFixture Z-Axis Component Alignment Analysis

Component Stack Configuration

Default Parameters

screw_thr_len = 16mm
mat_th = 3mm
pcb_th = 1.6mm
nut_th = 2.25mm
pogo_uncompressed_length = 8mm
pogo_compression = 1mm
pivot_support_r = 4.55mm (calculated from pivot_d + 6)/2

Calculated Dimensions

head_z = screw_thr_len - nut_th = 16 - 2.25 = 13.75mm
base_z = screw_thr_len + 3*mat_th = 16 + 9 = 25mm
base_pivot_offset = pivot_support_r + (pogo_uncompressed_length - pogo_compression) - (mat_th - pcb_th)
                  = 4.55 + (8 - 1) - (3 - 1.6)
                  = 4.55 + 7 - 1.4
                  = 10.15mm

Z-Axis Position Breakdown

BASE ASSEMBLY (Fixed, Z origin at base bottom)

Z = 25mm (base_z)                    ═══════════════════════════ Top of base structure
                                     
Z = 22mm (base_z - mat_th)           ┌─────────────────────────┐ ← TOP CARRIER (tight fit)
                                     │  PCB cutout (exact)     │    Clamps PCB from above
                                     └─────────────────────────┘    mat_th = 3mm thick
                                     
Z = 19mm (base_z - 2*mat_th)         ┌─────────────────────────┐ ← BOTTOM CARRIER (inset)
                                     │  PCB cutout (inset 1mm) │    Supports PCB from below
                                     └─────────────────────────┘    mat_th = 3mm thick
                                     
Z = 0mm                              ═══════════════════════════ Base bottom

PCB Position in Carriers:

  • PCB sits between the two carriers
  • Top carrier bottom surface: Z = 22mm
  • Bottom carrier top surface: Z = 19mm + mat_th = 22mm
  • PCB rests at approximately Z = 22mm

HEAD ASSEMBLY (When Closed)

The head pivots at:

Pivot Z-position = base_z + base_pivot_offset
                 = 25 + 10.15
                 = 35.15mm

When head rotates down (closed position, approximate -8° rotation):

Z = 35.15mm                          ●●●●●●●●●●●●●●●●●●●●●●●●●●● ← Pivot hinge point
                                     
Z ≈ 25mm (head_top)                  ┌─────────────────────────┐ ← HEAD TOP PLATE
                                     │  Clearance holes        │    Z = head_z - mat_th
                                     │  for pogo pins          │    = 13.75 - 3 = 10.75mm
                                     └─────────────────────────┘    (relative to head base)
                                     
                                         ↓↓↓ Pogo pins ↓↓↓
                                     
Z ≈ 22mm (contact with PCB)          ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ─ ← Pogo pin tips contact PCB
                                         (compressed 1mm)
                                     
Z ≈ 22mm (head_base)                 ┌─────────────────────────┐ ← HEAD BASE PLATE
                                     │  ● ● ● (pogo holes)     │    Contains ALL pogo pins
                                     │  Both TOP & BOTTOM      │    (top side AND bottom side
                                     │  test points here       │    test points)
                                     └─────────────────────────┘    Z = 0 (relative to head)

Critical Alignment Issues and Solutions

✅ SOLVED: Carrier Board Horizontal Alignment

Problem: Bottom carrier (1mm inset) was not concentric with top carrier (exact fit). Visible offset in all 4 directions.

Root cause: OpenSCAD's scale() affects ALL coordinates, including the board origin position from KiCAD's absolute coordinate system.

Solution: Compensate for scaled board origin when centering the cutout:

sx_offset = (board_origin_x + pcb_x / 2) * (1 - scale_x);
sy_offset = (board_origin_y + pcb_y / 2) * (1 - scale_y);

Real-world example:

  • Board origin: (155, 113) mm
  • Board center: (155 + 72/2, 113 + 45/2) = (191, 135.5) mm
  • Scale factor: 0.972x (for 1mm inset on 72mm board)
  • Center after scaling: (185.65, 131.71) mm
  • Required offset: (5.35, 3.79) mm to re-center

Verification: Use mode = "check_aligned" in OpenSCAD - cyan and magenta cutouts are now perfectly concentric.

⚠️ OPEN: Head and Carriers Vertical Alignment

Expected PCB position: Z = 22mm (between carriers)

Head base position when closed:

head_base_z_global = (base_z + base_pivot_offset) - head assembly height
                   = 35.15 - (head_z + adjustments for rotation)
                   ≈ 22-25mm range (depends on rotation angle)

⚠️ Architecture Clarification Needed

Current Design (in code):

  • 1 head base plate with pogo holes for BOTH top and bottom test points
  • 2 carrier plates sandwich the PCB
  • All pogo pins come from the same direction (head closes down)

This means:

  • Pogo pins for "top" test points press down directly
  • Pogo pins for "bottom" test points must pass THROUGH the PCB somehow
    • OR there's a design assumption that "bottom" means different layer of same-sided access

🔧 What "Top" and "Bottom" Actually Mean

Looking at the code in head_base_common():

// Loop over test points - TOP
for ( i = [0 : 1 : len (test_points_top) - 1] ) {
    circle (r = pogo_r);  // Pogo pin hole
}

// Loop over test points - BOTTOM  
for ( i = [0 : 1 : len (test_points_bottom) - 1] ) {
    circle (r = pogo_r);  // Same pogo pin hole, same plate!
}

Both arrays drill holes in the SAME plate!

This is a single-sided fixture where:

  • test_points_top = test points on the top copper layer
  • test_points_bottom = test points on the bottom copper layer
  • All pogo pins access from the TOP (head closes down)
  • PCB is held fixed in carriers
  • For bottom-side test points, the pogo pins contact pads on the bottom copper layer that are exposed (no solder mask)

Answer to User's Question

Are the 2 carrier boards inline with the 2 head plates?

There are NOT 2 separate head plates for top/bottom testing.

There is:

  1. 1 HEAD BASE plate (Z ≈ 0 relative to head) - contains ALL pogo pin holes
  2. 1 HEAD TOP plate (Z ≈ 10.75mm relative to head) - structural + pogo clearance
  3. 1 BOTTOM CARRIER (Z = 19mm absolute) - PCB support with 1mm inset
  4. 1 TOP CARRIER (Z = 22mm absolute) - PCB clamp with exact fit

Alignment when fixture closes:

  • Head base (with pogo pins) presses down on PCB at Z ≈ 22mm ✓
  • PCB sits in carriers at Z = 22mm ✓
  • These ARE aligned - the head closes to bring pogo pins into contact with PCB

The "2 head plates" misconception:

  • There's only 1 pogo pin plate (head_base)
  • Head_top is structural, not a second pogo plate
  • Both top and bottom test point arrays use the same head_base plate