Rotary cam lock

On this page

1. Parts
2. Print files
3. How it works
4. Mechanical advantage
5. What limits the force
6. Analyst's calculation
7. Adversarial review
8. Preload retention outdoors
9. Materials
10. Hardware notes from the research
11. Design guidelines
12. Failure modes
13. Existing designs and commercial references
14. Evidence gathered in the sweep
15. Sources

Parts

Per latch: 3 parts to print, 4 items to buy, 3 features printed into the box or lid and the TPU 85A seal. Parts marked † are not drawn in the 3D model. In the 3D gallery you can click a part to see where it sits. The last column links the print file, or an example of the part in a shop (see the shopping list).

Print files

Download all print files for this latch (ZIP, 79 kB)

Print version: The tongue is 8 mm thick, as the guidelines ask, and the keeper's ledge tapers 0.8 mm over the last part of the turn.

Print in ASA (TPU 85A for the seal), 0.2 mm layers, at least 4 walls; settings and the clearances used are in print/README.md. The parts are already turned the right way up for printing. The box and lid coupons are short test sections with this latch's box-side and lid-side features, so you can try the latch before building it into your own enclosure.

All printed parts in their assembled positions, for CAD: rotary-cam-lock-simple.step.

How it works

A quarter-turn cam tongue (or tapered tongue) on a shaft turns behind the frame or panel; a plain tongue gives no defined compression, a roller or tapered cam draws the panel down by up to about 2 mm (Southco E5 roller cam). It is easy to print on a stainless shaft, but the force path is a thin cantilevered tongue in bending with a line contact, so it creeps and carries little load. Fine for dust or splash with foam or thin-lip gaskets, or as a secondary lock, not for a TPU 85A rib.

Mechanical advantage

Taper ramp: F = T / (r tan(alpha + phi)). For r = 12 mm and an 8 deg ramp, T/F = 4.9 mm at mu 0.25 and 3.6 mm at mu 0.15 (frictionless 1.7 mm). With an 18 mm thumb-knob radius the force ratio is about 3.7-5:1 (T handle at 40 mm radius about 8-11:1). A plain cam with no taper has effectively no draw-down. (own calc)

What limits the force

The printed tongue in bending (about 0.05-0.2 MPa per newton depending on thickness; 3-6 MPa sustained allowable gives 25-100 N) and the hub/shaft bearing, not the hand: a thumb knob can easily deliver 165-225 N at 0.8 Nm and overload the tongue.

Analyst's calculation

Analyst's sustained estimate: 25–100 N, typical 60 N; peak 250 N.

ASSUMED PART (own): printed cam tongue 14 mm wide x 8 mm thick, contact 8 mm from the hub root and r = 12 mm from the shaft axis, 8 deg taper, on an M5 A4 stainless screw or D-shaft, thumb knob radius 18 mm (hand torque 0.5-1 Nm; up to 1.5 Nm with a T handle). Printed with the shaft axis horizontal so that in the lock position the tongue lies in the layer plane and bends in-plane.

1. Torque to force: F = T / (r tan(alpha + phi)). Friction values 0.25 plastic/plastic and 0.15 plastic/steel (https://elesa.com/en/elesab2bstoreca/Clamping-handles--Cam-levers--GN9272-A--d1). r tan(alpha+phi) = 4.9 mm (mu 0.25), 3.6 mm (mu 0.15); frictionless 1.7 mm. At 0.8 Nm that is 165-225 N, so the hand can overload the tongue.
2. Tongue as a cantilever (own): sigma = 6 F l / (b h^2) = 6 x 8 / (14 x 64) = 0.054 MPa per N. Sustained in-plane allowable 3 / 4 / 6 MPa (project brief rule for ASA at 60-70 C) x 0.85 root-fillet factor gives F = 48 / 64 / 95 N. A thinner 4-6 mm tongue (typical of printed cams) has 0.1-0.2 MPa per N, i.e. only 15-40 N. I therefore report sustained 25 / 60 / 100 N. Peak: in-plane XY tensile about 37-44 MPa for ASA datasheets (Polymaker https://cdn.shopify.com/s/files/1/0603/6609/files/PolyLite_ASA_TDS_V5.1.pdf) / Kt 1.5 gives about 20 MPa, so about 370 N for the 8 mm tongue; hand-limited at 1.5 Nm = 310 N; I report 250 N as a margin-adjusted peak.
3. Context: a metal eccentric clamping lever (Elesa GN 927.2, size 44) gives 1450 N for 75 N hand force (https://elesa.com/en/elesab2bstoreca/Clamping-handles--Cam-levers--GN9272-A--d1), which shows how far a simple quarter-turn printed cam is from a true clamp; Southco E5 roller cam compresses up to 2 mm of light-duty gasket and its Flush Cup meets NEMA 4 / IP65 but no holding force is stated (https://brandspurng.com/2020/09/23/southco-introduces-new-product-extensions-to-its-successful-e5-cam-latch-series/). A stainless cam on the same shaft would carry several hundred N, but then it is not printed.
4. Seal check: at 2-4 N/mm a 60 N cam can serve only about 15-30 mm of gasket, so a 200 x 150 mm lid would need 25-45 cams. Further web searches were blocked (search budget exhausted); the MakerWorld printed quarter-turn latch page returned HTTP 403, so no printed-cam force data was found.

Adversarial review

Independent recalculation: cam T/F 3.56-4.86 mm; tongue stress 0.0536 MPa per N; sustained allowables 3 / 4 / 6 MPa x 0.85 give 48 / 63 / 95 N. The analyst's typical of 60 N is reproduced within 5 percent. Source checks: the Elesa GN 927.2 page is live and states mu 0.25 plastic/plastic and 0.15 plastic/steel. For size 44 it lists 75 N manual force and 1450 N clamping force, as claimed (the page's extracted table has an unlabeled column with 33, which I ignored). The Southco/brandspurng page is live: roller cam gives up to 2 mm light-duty compression, Flush Cup meets NEMA 4 and IP65 with E5 or E3 latches, and no holding force is stated. The Polymaker TDS was read locally: XY tensile is 43.8 MPa, bending strength XY 73.4 MPa. The analyst's '37-44 MPa for Polymaker' is a misattribution, since 37 MPa is the Bambu figure from the brief. Specimens were 70 C chamber, 100 percent infill, and the sheet says not for design. The MakerWorld quarter-turn latch page is HTTP 403 and unverifiable; no printed-cam force data exists in the sources I could read. My search budget was exhausted, so I could not search for more. A cam latch is the weakest sealing mechanism here, and the force is limited by the tongue, not the hand. Confirmed, with peak raised.

Issues raised:

• Taper arithmetic confirmed: r tan(alpha+phi) = 4.86 mm (mu 0.25) and 3.56 mm (mu 0.15), so 0.8 Nm gives 165-225 N. A thumb knob can therefore overload the tongue, as the analyst said.
• Tongue cantilever confirmed: sigma = 0.0536 MPa per N for 14 x 8 mm at 8 mm reach. With 3 / 4 / 6 MPa x 0.85 that is 48 / 63 / 95 N, which supports 25 / 60 / 100 N. The typical 60 N assumes an 8 mm thick tongue; thin 4-6 mm cams (the common printed design) give only 15-40 N.
• Peak is understated. The analyst's 250 N is a 'margin-adjusted' figure, but peak was meant to be the breaking load. 20-27 MPa in-plane (datasheet XY flexural strength is 73 MPa) divided by the sigma-per-N coefficient gives 370-500 N, and the hub, fillet and shaft bearing reduce that. I use 350 N. Peak is separate from sustained and does not change the ranking.
• Even 100 N serves only 25-50 mm of gasket at 2-4 N/mm, so a 700 mm path would need 15-35 cams. Seal suitability 2 stands. The taper over 90 deg (about 2.65 mm of draw-down) does give fine stroke control, but no hard stop is defined by the cam itself.
• The shaft hole bears on a printed hub: an M5 washer bears on about 55 mm2, so at 100 N the bearing is about 2 MPa, which is fine. Creep of the printed hole around the steel shaft is a real long-term loss but is not quantified.
• Zinc-plated shafts rust within months; specify A4. The Southco E5 cam latch figures (up to 2 mm compression, Flush Cup NEMA 4 / IP65, no holding force) confirmed from the cited page.

Preload retention outdoors

Poor. The cam is a rigid fixed-travel element on a thin printed tongue: TPU set (30-58 percent force loss at 70 C, https://www.dream3d.co.uk/wp-content/uploads/2022/12/Ultrafuse_TPU_85A_TDS_EN_v2.5.pdf) combines with tongue bending creep and line-contact wear, so little of the original preload survives hot summers. It cannot be re-tightened.

Materials

ASA for the cam and knob; PC (UV-stabilised) improves tongue creep margin about 1.3-1.5x (own judgement). A stainless or die-cast cam is the realistic way to more than about 100 N. Avoid PETG and PA6-CF outdoors.

Hardware notes from the research

M5 A4 stainless screw or D-shaft, A4 washer, A4 nyloc or lock nut; optional stainless cam or roller; optional spring plunger detent. A threaded stud with lock nut would make the cam height adjustable.

Design guidelines

• Tongue at least 8 mm thick and 14 mm wide with R at least 3 mm fillet at the hub; reach no more than 12 mm from the shaft axis.
• Print with the shaft axis horizontal so that the tongue in the lock position lies in-plane, not across layers.
• Taper ramp 3-8 deg over the last 90 deg of rotation, with a rotation stop and a detent bump; a roller or low-friction face reduces wear.
• Metal shaft or stainless bush in the hub (A4 M5); never let the printed hole bear on the thread.
• Use only with a light gasket (thin lip or foam) or put one latch every 15-30 mm.
• Add a separate printed hard stop beside each latch, since the cam itself defines no squeeze.
• For loads over about 100 N substitute a stainless cam or use another mechanism.

Failure modes

• Tongue creeps or cracks at the hub fillet.
• Printed hole creeps around the steel shaft, so the cam drops and loses preload.
• Cam rotates back with vibration without a detent.
• Line contact wear; zinc-plated shafts rust within months outdoors (12-72 h salt spray).
• Gasket leaks because force is simply too low.
• Cold brittleness at sharp corners of the tongue.

Existing designs and commercial references

• MakerWorld quarter-turn latch (compression capable) — Four printed parts, 90 deg turn, taper compression, default for 2.5 mm acrylic panel; no force data (per lead; HTTP 403 to my fetch)
• Southco E5 cam latch extensions (metal reference) — Roller cam up to 2 mm compression; Flush Cup accessory NEMA 4 / IP65; no holding force stated (fetched)

Evidence gathered in the sweep

SUSTAINED (own est.): printed 20-60 N, steel-shaft cam <100 N; PEAK 100-250 N. EVIDENCE: Southco E5 roller cam up to 2 mm compression, Flush Cup accessory NEMA 4/IP65 https://brandspurng.com/2020/09/23/southco-introduces-new-product-extensions-to-its-successful-e5-cam-latch-series/ ; MakerWorld quarter-turn latch (compression capable): 4 printed parts, 90 degree turn, taper compression, 2.5 mm acrylic default panel, no force data https://makerworld.com/models/3070853 ; E5 316 stainless cam latch 670 N rating is for the metal part (via Gripwell/Southco sheet, search text). CONFIDENCE: low.

Sources

• Southco E5 cam latch extensions — Fetched: up to 2 mm light-duty compression, NEMA 4 / IP65 for the Flush Cup; no load rating given
• Elesa GN 927.2 page — Fetched: friction coefficients (0.25 / 0.15) and metal cam-lever table (75 N hand gives 1450 N clamp for size 44)
• Polymaker PolyLite ASA datasheet — ASA XY and Z tensile figures as given in the project brief, not re-fetched
• MakerWorld quarter-turn latch — Not retrievable (HTTP 403); description via the lead's search summary
• Ultrafuse TPU 85A datasheet — Compression set figures from the project brief, not re-checked

Sources the reviewer flagged (dead, blocked or misdescribed):

• https://makerworld.com/models/3070853 - HTTP 403, unverifiable (compression-capable 90 deg latch, no force data).
• https://cdn.shopify.com/s/files/1/0603/6609/files/PolyLite_ASA_TDS_V5.1.pdf - cited for 'XY tensile about 37-44 MPa'; Polymaker's own XY value is 43.8 MPa (Z 32 MPa); 37 MPa is Bambu's figure. Specimen conditions (70 C chamber, 100 percent infill) flatter real parts.