Pinch Rods for Squaring Cabinets – Disc-Tip Heads + Zero-Set Diagonal Scale, Use Your Own Sticks

A printed kit that turns two of your own wooden sticks of equal length into pinch rods (squaring sticks) for checking a box, cabinet carcass, drawer or frame for square during glue-up. Each end gets a head with a round disc tip: pressed into an inside corner, the disc touches both walls, so its centre always sits on the corner's bisector - a glue fillet or a slightly rounded corner in the corner line does not move it. The heads rest on the front edges, and a scale collar with a zero-set cursor shows the difference between the two diagonals directly, with its sign.

Validation status (please read): designed and checked in CAD (geometry, fits, calculated readings) and in the slicer (no supports). It has not been physically printed or tested - lock slip, cursor grip on real sticks and reading repeatability are still unverified. All pictures are renders of the CAD model or drawn diagrams; the carcass, sticks and hardware are modelled stand-ins - none is a photograph.

Features

• Disc tips register on both walls of the corner, 8 mm from the corner line - not on the corner line itself. Concave glue fillets and rounded inside corners up to 6 mm do not touch them (calculated in CAD).

• A square opening reads exactly 0. At 0.5° out of square the reading is within 0.27 mm of the true diagonal difference (calculated for 8 opening sizes; it slightly under-states it).

• The heads rest on the front edges (carcass on its back): both diagonals are measured in the same plane and you can let go to read. Only the disc tips enter the opening, so the rods never hit a wall (openings from 1:5.7 to 5.7:1).

• Zero-set cursor and a -20 ... +20 mm scale with 1 mm ticks: zero on the first diagonal, read the second. "+3" means the second diagonal is 3 mm longer - no number to remember.

• Your own sticks, any length, so any carcass size. The printed parts are the same for every length.

• One plate per set, no supports; buy only an M5 x 16 bolt and an M5 nut.

Reach (two equal sticks, tip to tip)

Calculated from the part lengths and the full cursor range; the inside corner-to-corner diagonal is about 17-23 mm longer than the tip-to-tip span.

• 2 x 300 mm sticks: 403-535 mm

• 2 x 450 mm sticks: 553-835 mm

• 2 x 600 mm sticks: 703-1135 mm

• 2 x 900 mm sticks: 1003-1735 mm

• 2 x 1200 mm sticks: 1303-2335 mm

Which files?

Measure your sticks first and pick the preset for them (heads and fixed collar sockets are nominal +0.4 mm, sliding tunnels +0.8 mm):

• Ready-to-print PrusaSlicer projects (one plate each: 2 heads, scale collar, keeper collar, cursor, knob):

  • pinch_rod_heads_for_20mm_square_sticks_complete.3mf - 20 x 20 mm square sticks (about 99 g)

  • pinch_rod_heads_for_3-4in_square_sticks_complete.3mf - 3/4 in (19.05 mm) square sticks (about 95 g)

  • pinch_rod_heads_for_15mm_square_sticks_complete.3mf - 15 x 15 mm square sticks (about 79 g)

  • pinch_rod_heads_for_3-4in_round_dowels_complete.3mf - 3/4 in (19.05 mm) round dowels (about 100 g)

• STL files of every part for each preset (head, scale collar, keeper collar, cursor, knob).

• model.py + stroke_font.py: the CadQuery source with the stick shape and size, socket and sliding fits, disc tip radius and scale range as parameters, for other stock. Run python stroke_font.py once to create font_polys.json (the engraved scale font) before running model.py.

Print settings

• PETG recommended (wipes clean of glue, tougher); PLA is fine indoors.

• 0.4 mm nozzle, 0.2 mm layers, no supports, parts oriented as supplied: heads upside down (flat top on the bed, disc tip up), collars standing on an end, cursor upside down, knob flat.

• Each set is one plate on a 220 mm-class bed; on a 180 mm bed print the heads + cursor + knob first, then the collars.

• About 7-8.5 h per set on a generic 220 mm-class profile (PrusaSlicer estimates).

• PETG on smooth PEI or glass needs a release aid such as glue stick.

Assembly

You need epoxy or CA glue (PVA wood glue bonds poorly to PETG), an M5 x 16 hex bolt (ISO 4017) and an M5 hex nut (ISO 4032). Optional: a 3-3.5 mm wood screw in each head (pilot hole in the head top). Dry-fit everything first; the parts lie flat side down (disc tips down, scale up).

1. Cut two straight sticks to the same length (see Reach).

2. Stick 1: glue a head on one end, stick end against the stop inside the head, disc tip down. The open slot under the socket is intentional (the stick sits on two 45° seat edges).

3. Thread the keeper collar onto stick 1 from its free end, through its open tunnel - its blind tunnel must open toward that free end.

4. Glue the scale collar on the free end of stick 1 (scale up).

5. Stick 2: push it through the scale collar's open tunnel toward the first head and glue its end into the keeper's closed tunnel. Glue the second head on its other end, disc tip down.

6. Push the nut into its slot in the scale collar and thread the bolt through the knob's hex pocket into the nut, its tip short of the inner flap.

7. Snap the cursor onto stick 2 next to the scale collar.

How to use it

Lay the carcass on its back (front edges up) and wipe squeeze-out off the front edges at the corners.

1. Rest both heads on the front edges at two opposite corners, slide the rods apart until each disc touches both walls of its corner, tighten the knob and slide the cursor to 0.

2. Loosen, move to the other two corners, open or close until both discs touch, tighten and read: +3 = this diagonal is 3 mm longer.

3. Set the rods to half the reading (+1.5), tighten, and clamp across the long diagonal until the rods drop into both diagonals.

The heads also transfer inside widths: disc to disc = inside width - 16 mm.

Limitations

• Not physically printed or tested (see Validation status). It does not replace a precision square.

• Readings depend on straight sticks of equal length and on the lock not slipping; the scale has 1 mm ticks and a ±20 mm range (beyond it, re-zero).

• The heads need roughly flush, free front edges at the corners: a proud face frame, a rebate or a clamp jaw on the corner blocks them. It checks the opening at the front edge plane only.

• Concave glue fillets up to 6 mm are cleared; convex squeeze-out beads must be wiped. Glue can run through the open seat slot under a head onto its rest face - wipe it.

• Flat frames need to be at least about 13 mm thick (the tip reaches 12 mm below the rest face) or raised. On thin (~6 mm) drawer sides at very shallow rod angles, hold the head while locking.

• Works with the four preset stick sizes; other sizes need the source. Sand oversize sticks; sticks 0.3 mm or more undersize let the cursor slide - wrap one turn of tape under it.

Pictures

Renders of the CAD model and drawn diagrams, not photographs: the rods across a carcass (cover), the disc tip in a corner from inside the box, why a disc tip beats a point, the scale and knob, how to read it, an eye-level view, the printed kit, the reach chart and the four stick presets.