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Stackable wedge-and-weight jig for an angled acrylic mould

Context

The DIY carbon-fin project needed a simple way to support an acrylic mould sheet at an angle during laminating or gluing. The selected low-cost direction combined wooden wedges with bricks, pavers, or other available weights. The follow-up requirement was to make the wedges composable so the setup could be adjusted without cutting a new fixed wedge for every angle.

No target angle, acrylic dimensions or thickness, process pressure, support span, or permissible mould deflection was recorded. No jig was built or tested in the conversation.

Candidate jig

Use paired opposing wedges as adjustable-height supports:

  • Two matching wedges face opposite directions.
  • Sliding them relative to each other changes their combined height while keeping the upper bearing surface approximately parallel to the base.
  • Identical pairs are placed at several support stations.
  • The acrylic angle is established by the height difference between its two supported edges.
  • Each pair is locked using clamps, a slotted bolt, pins, or another positive restraint.
  • Straight timber spreaders sit between the wedge stations and acrylic to distribute load and maintain straight support lines.
  • Anti-slip material may be placed at suitable timber interfaces, provided it does not create uneven compliance.
  • Lateral and downhill stops prevent the sheet from moving under gravity or handling loads.
  • Weights sit on the support frame or timber spreaders rather than directly on unsupported acrylic.

Flat modular risers can provide coarse height adjustment, with the opposing wedge pairs providing fine adjustment.

Geometry

An opposing-wedge pair is an adjustable-height support rather than an adjustable-angle surface by itself. For two support lines separated horizontally by distance \(d\), with a height difference \(\Delta h\), the acrylic angle is:

\[ \theta = \arctan\left(\frac{\Delta h}{d}\right) \]

Every station along a given support line should have the same height so the acrylic is not twisted. Adjusting the angle requires changing the relative heights of the two support lines in a controlled and repeatable way.

Advice received

The response initially offered fixed wooden wedges, a hinged support, a PVC frame, and a clamp-and-brick arrangement. The chosen hybrid used:

  • evenly spaced wooden wedges;
  • rubber or non-slip material;
  • bricks or weights to prevent movement; and
  • a straight plank under a flexible acrylic sheet to spread support.

Composable variants suggested were:

  • stacked wedges;
  • opposing wedge sliders;
  • a modular riser plus replaceable wedges; and
  • laminated shims for small adjustments.

Opposing wedge sliders were the most useful of these ideas for fine, measurable height adjustment.

Editorial corrections

  • Stacking wedges with the same slope increases height but does not inherently change the supported-surface angle.
  • Sliding a single fixed-slope wedge changes its contact position, not its slope.
  • The acrylic angle comes from the height difference between support lines, not from one opposing-wedge pair alone.
  • Cardboard is not a dimensionally stable shim around resin, load, or changing humidity.
  • Loose wedges and bricks do not provide positive locking.
  • Bricks placed directly on acrylic can create point loads, bowing, scratches, and cracking.
  • A small number of isolated supports may allow a large sheet to sag or twist. Support spacing and spreader stiffness must match the acrylic dimensions, thickness, and applied process load.
  • Soft anti-slip layers can introduce unequal compression and should not substitute for a mechanically restrained frame.
  • Heat-bending a lip into the acrylic would alter the mould surface and introduce residual stress, so it was not retained as part of this jig.
  • Moving a wedge after setup can disturb the mould plane; final adjustment should be measured and then locked before layup.

Verification before layup

  1. Confirm both support lines are straight and coplanar along their length.
  2. Measure wedge-pair heights and calculate the expected angle.
  3. Measure the acrylic angle at several positions.
  4. Check the mould face with a straightedge for sag, bow, and twist.
  5. Apply representative weights or process pressure and repeat the measurements.
  6. Confirm every wedge pair is positively locked.
  7. Confirm the acrylic cannot move laterally or downhill.
  8. Protect the mould face from scratches, grit, timber marks, and brick contamination.