Polycarb Tube — R24 Compliance Audit + CAD + 3D Print Prototype

Skimmer 2822A · Override 2026-27 · Sheet 0/5 (engineering summary)

EN4 reminder: This is an analyst-produced thinking aid. The team rewrites all rule interpretations, geometry, and procedural steps in their own words for engineering notebook submission. R24 rule text is quoted verbatim from the manual; everything else is the team's design work.

This document supersedes the earlier /polycarb-tube-cad/ package. That review's tube ID recommendations (3.5″, 4.0″) and single-piece funnel cap (4.5″) all violate R24. The corrected approach is documented here.

Critical finding (read first)

The team's documented polycarb tube (ID 2.55″) violates R24 as written in the 2026-27 Override manual. The violation is in the developed-length math: a 2.55″ ID tube made from a single sheet requires a developed length of 8.20″ (neutral axis) or 8.39″ (outer surface) — both over R24's 8.00″ maximum.

This was caught BEFORE the team built the tube. Inspection at competition would flag this immediately.

The math (Sheet 1)

R24 rule text (page 58 of the manual, verbatim):

"Each individual piece of non-shattering plastic cannot be larger than 4″ x 8″ x 0.070″."

A tube made from a flat sheet has developed (unrolled) length equal to its circumference. For ID 2.55″ and wall 0.060″: - Neutral axis circumference = π × (2.55 + 0.060) = 8.20″ — over by 0.20″ - Outer surface circumference = π × (2.55 + 0.120) = 8.39″ — over by 0.39″

Both measurement methods exceed 8.00″. To be safe under any inspector's method, the tube must satisfy the outer-surface limit.

The corrected dimensions

Likely root cause of the team's documentation error: the original 2.55″ value referred to OD, not ID. Somewhere in the doc revision history, the OD value got relabeled as ID. Update the docs to make clear which is which.

Element fit with corrected ID (2.43″)

The tube still works as a pin pickup (pin enters neck-first, cinch grips at mid section). Cup cannot fully enter the tube (rim > ID). This was already true at 2.55″ ID; correction doesn't change cup handling.

Path B reliability — revised conclusion

The earlier CAD review proposed widening the tube to 4.0″ ID or adding a 4.5″ funnel cap to address Path B (loader catch) reliability. Both proposals violate R24 as single-piece designs: - 4.0″ ID tube: developed length = π × 4.12 = 12.94″ (over by 4.94″) - Single-piece 4.5″ funnel cap: developed length = π × 4.5 = 14.14″ (over by 6.14″)

The clamshell two-piece funnel cap (Sheets 4-5) IS R24-legal and achieves the same 4.5″ mouth. Each clamshell half is a 5.74″ × 2.39″ × 0.060″ piece (well within 4×8), and the halves bolt together mechanically (no adhesive). See Sheets 4-5 for the flat pattern and assembly.

Important scope note: the funnel improves Path B catch for pins only. Cups can't enter the tube longitudinally at any R24-legal ID (cup rim 3.16″ > 2.43″ max legal ID). Skimmer remains a pin pickup mechanism; cups are the job of the V5 claw bots (Pelican, Spoonbill, Osprey).

R24 fabrication compliance (Sheet 2)

R24d (verbatim from manual):

"Plastic may be mechanically altered by cutting, drilling, bending, etc. It cannot be chemically treated, melted, cast, or bonded to another part. Heating non-shattering plastic to aid in bending is acceptable."

What's legal vs illegal

The seam closure problem

A tube made from a single bent sheet has a seam where the two edges meet. Closing the seam is where teams most often violate R24. The correct approach is to leave the seam open — held in shape by end brackets/clamps that mechanically fasten the tube to the robot. The seam doesn't need to be closed for the tube to function structurally; the brackets at each end hold the tube's circular cross-section.

This is the standard VEX approach. The tube is essentially a "C" cross-section (with a small open gap at the seam) rather than a fully-closed "O". The cinch mechanism still works because the cinch closes around the outside, applying inward pressure through the polycarb wall onto the element.

Recommended legal fabrication sequence

1. Cut a 4.00″ × 8.00″ sheet of 0.060″ polycarbonate. Mark the centerline (4.00″ from one short edge) — this becomes the apex of the bend.
2. Mark drill locations for cinch cord holes (two opposed 1/16″ holes at the centerline, at the tube's midpoint when rolled). Mark mounting holes per design.
3. Heat in a kitchen oven at 300°F (148°C) for approximately 5-7 minutes until pliable but not melted. The sheet should bend with light hand pressure; if it sags under its own weight, it's too hot. Test on a scrap piece first.
4. Wrap the heated sheet around a 2.43″ OD mandrel (cut a length of 2.5″ PVC pipe, or use a hardwood dowel). Wear heat-resistant gloves.
5. Hold the shape with hose clamps or large rubber bands while cooling to room temperature (5-10 minutes).
6. Remove the cooled tube from the mandrel. Polycarbonate has 1-2% spring-back; the actual ID may end up slightly larger than the mandrel OD. If the resulting ID exceeds 2.43″, use a slightly smaller mandrel on the next attempt (e.g., 2.40″ OD).
7. Drill the marked holes using a small drill bit. Back the polycarb with a wood block to prevent crack-out.
8. Mount to the robot via mechanical fastening only. End brackets (custom-cut polycarb or aluminum) clamp around the tube and bolt to the swing arm tip.

Clamshell funnel cap (Sheets 4-5)

For Path B catch reliability with pins, add a two-piece clamshell funnel to the tube top.

Geometry

R24 piece count: tube (1) + clamshell halves (2) = 3 pieces total against the 12-piece limit.

Fabrication

1. Print Sheet 4 at 1:1 scale. Use as cut template and inspector tracing.
2. Cut two identical pieces from 0.060″ polycarb. Each piece needs its own 4×8 sheet allotment (or one larger stock cut to two pieces).
3. Drill the hole pattern per piece:
4. 4 seam holes (1/8″ for 4-40 clearance): two near outer edge + two near inner edge, along each straight cut edge
5. 3 rim holes (1/8″): evenly along the inner curved arc
6. Heat-bend each piece into a half-cone shape using a half-cone wooden mandrel. Heat at 300°F until pliable, press against mandrel, cool while held.
7. Drill 6 matching rim holes in the tube's top edge (evenly spaced around the circumference).
8. Assemble:
9. Place clamshell half A around one side of the tube top; insert 3 rim screws (4-40 × 1/4″) from inside the tube, secure with nylock nuts outside the funnel
10. Place clamshell half B on the opposite side; align both seams and rim holes
11. Insert remaining 3 rim screws + 4 seam screws (2 per vertical seam)
12. Tighten all screws snugly — don't over-torque or the polycarb cracks

Hardware list (clamshell funnel only)

OnShape modeling instructions

OnShape is the team's primary CAD tool. The tube can be modeled two ways.

Method A: Simple revolve (fastest)

For most documentation purposes, a simple revolved cylinder is sufficient. This doesn't preserve the "made from a flat sheet" origin but is quick.

1. Open a new Part Studio in your team's Onshape document.
2. Click the Top plane in the feature tree, then Sketch.
3. Draw a rectangle. Dimension it 0.060″ (height) × 4.000″ (width). Position the rectangle so its inner edge is 1.215″ from the Y-axis (this is the inner radius for ID 2.43″).
4. Click ✓ to exit the sketch.
5. Click Revolve in the toolbar. Select the rectangle as the region. Set axis = Y-axis. Set revolution type = Full (360°).
6. Click ✓ — you now have a tube part.
7. Add part properties: right-click the part → Properties. Set material = "Polycarbonate", thickness = 0.060″.
8. Save the part as polycarb-tube-skimmer-rev2.prt (rev2 marks the R24 correction).

Method B: Sheet-metal feature (for R24 verification)

OnShape has a sheet-metal toolkit. This method explicitly shows the flat pattern, which the team includes in the engineering notebook as proof of R24 compliance.

1. Open a new Part Studio. Make sure Sheet Metal tools are visible in the toolbar (toggle in toolbar customization if not).
2. Sketch on the Top plane: draw an 8.000″ × 4.000″ rectangle.
3. Click ✓ to exit.
4. Click Sheet Metal model. Settings:
5. Thickness: 0.060″
6. Bend radius: 1.215″ (inner radius of finished tube)
7. Select the rectangle. The tool extrudes it into a flat sheet metal part.
8. Click Bend in the sheet metal toolbar.
9. Add a bend along the centerline of the long axis (4″ in from one short edge). Bend angle: this gets the half-tube. Repeat for the second bend to complete the cylinder.
10. Flat pattern view: right-click the part in the tree → Flat Pattern. This shows the 4″ × 8″ sheet — print 1:1 for the inspector tracing.
11. Export the flat pattern as DXF for laser cutting (if available): File → Export → DXF.

Clamshell funnel in OnShape (sheet-metal)

The same sheet-metal approach works for the clamshell halves but requires non-rectangular flat patterns.

1. New Part Studio with sheet metal enabled.
2. Sketch on Top plane: draw the half-annular sector (use the dimensions from Sheet 4: inner radius 1.826″, outer radius 3.223″, sector angle 125.7°).
3. Convert to sheet metal: thickness 0.060″, bend radius 0.5″ (arbitrary — actual bend is gradual via cone surface).
4. Bend along the centerline to form the half-cone. The "bend" in this case is the curvature along the radial direction — this is more accurately modeled as a Form feature with a conical form tool rather than a true bend.
5. Alternative method: skip sheet metal, use Loft between two circles (bottom 2.55″ OD, top 4.5″ OD) with thickness 0.060″ via Thicken afterward. Simpler but doesn't yield a flat pattern.
6. Add a Split feature on the YZ-plane to divide the frustum into two halves.
7. Export each half as STL for 3D print prototype, or DXF for laser-cut flat patterns.

Saving versions

After each design iteration, save a version: top-right menu → "Save version" with a descriptive name (tube-rev1-initial, tube-rev2-r24-corrected, etc.). Versions are immutable — judges can verify design history in the engineering notebook.

TinkerCAD modeling instructions

TinkerCAD is simpler than OnShape and works well for the 3D print prototype. It runs in a browser, no install needed.

Tube

1. Go to tinkercad.com, sign in with the team's Google account.
2. Click Create new design.
3. From the right panel, drag a Cylinder shape onto the workplane.
4. Click the cylinder to select. In the dimension box (top-left), set:
5. Length: 64.77 mm (= 2.55″ OD)
6. Width: 64.77 mm
7. Height: 101.6 mm (= 4.00″)
8. Drag another Cylinder onto the workplane. Set dimensions to:
9. Length: 61.72 mm (= 2.43″ ID)
10. Width: 61.72 mm
11. Height: 120 mm (taller than outer so it cuts through completely)
12. With the inner cylinder selected, click Hole in the inspector (right panel). It turns transparent.
13. Select both cylinders (Shift+click). Click Align in the toolbar; center on X and Y, bottoms on Z.
14. Click Group (Ctrl+G). The hole subtracts; you now have a tube.
15. Verify the wall thickness: select the tube, switch to side view. Measure should show ~1.52mm (= 0.060″) wall.
16. Export: click Export → STL. Save as polycarb-tube-prototype.stl.

Clamshell funnel halves

1. Create a new TinkerCAD design.
2. Drag a Cone (or "Truncated cone" if available) onto the workplane. Adjust to:
3. Bottom diameter: 64.77 mm (= 2.55″)
4. Top diameter: 114.3 mm (= 4.50″)
5. Height: 25.4 mm (= 1.00″)
6. Drag a second truncated cone with slightly smaller dimensions (so the result has a 1.5mm wall):
7. Bottom diameter: 61.77 mm
8. Top diameter: 111.3 mm
9. Height: 30 mm (taller for clean subtraction)
10. Set the inner cone as a Hole. Align and group to get a hollow frustum.
11. Now split into halves: drag a Box onto the workplane. Make it 60mm × 200mm × 200mm. Position it so it covers exactly half the frustum (one side of the centerline).
12. Set the box as a Hole. Align it with the frustum, group to subtract — you have half the frustum.
13. Export as STL: clamshell-funnel-half.stl. (Print two copies for the assembly.)

TinkerCAD limitations: units default to metric, precision 0.01mm. Fine for prototype. For actual fabrication drawing, use OnShape.

Bambu P2S 3D print prototype

⚠ R24h: 3D printed plastic parts are NOT permitted on the V5RC competition robot. Label all 3D prints clearly: PROTOTYPE - NOT FOR COMPETITION USE.

The prototype lets the team: - Verify dimensions before cutting expensive polycarb stock - Test element fit (pin and cup waist insertion) - Test cinch action with a string/cord cinch - Iterate quickly on the design

Print settings (tube)

Slice polycarb-tube-prototype.stl in Bambu Studio or OrcaSlicer:

Print settings (clamshell halves)

For each half-frustum:

Print orientation note (critical for tube)

The tube prints standing up vertically with the long axis (4″) perpendicular to the build plate: - First layer: a thin ring (bottom edge of tube) - Each subsequent layer: same ring stacked upward - Final layer: top edge of tube

This orientation produces clean walls and accurate circular cross-section. Don't print lying on its side — that creates overhangs on the top half and dimensional distortion.

Post-processing

1. Remove the brim with a deburring tool or hobby knife.
2. Sand the top and bottom edges lightly to remove layer-line burrs.
3. Test-fit a pin: should slide in neck-first with light friction at the mid section.
4. Test-fit a cup: should NOT enter (rim wider than ID, by design).
5. For the clamshell halves: drill the hole pattern using the same template as the polycarb version.

Iterating

If first print's ID is too tight or too loose: - Open the TinkerCAD design, adjust the hole cylinder diameter ±0.5mm - Re-export STL, re-slice, re-print - 3D printing's main value here is fast iteration — expect 2-3 versions before settling

What the prototype CANNOT do

The 3D printed prototype is dimensionally accurate but mechanically very different from polycarbonate: - Plastic is more brittle (PLA) or more flexible (PETG) - Cinch force needed to grip the element will differ - Tube's spring-back/stiffness differs

The prototype is a dimensional check, not a final mechanical test. Once the team confirms ID/length/cinch hole locations, fabricate the actual polycarb tube and bench-test that.

What's in the CAD package

Action items for the team

In priority order:

1. Update tube dimensions in all team docs to reflect ID 2.43″ / OD 2.55″:
2. polycarb-tube-onshape-guide.html — change ID 2.55″ → ID 2.43″
3. polycarb-tube-fabrication-guide.html — same correction
4. polycarb-tube-mentor-handout.docx/.pdf — same correction
5. spartan-hero-polycarb-tube-intake.html — same correction

6. Add a "Rev history" note explaining the R24 correction (Rev 1 → Rev 2 corrected for R24 outer-circumference limit)

7. Add R24 compliance section to the Skimmer page (private site skimmer.html):

8. Explicit dimensional table showing each polycarb piece on the bot, with developed-length math
9. Count against 12-piece R24 limit

10. Inspector tracing strategy (1:1 PDF prints of flat patterns from OnShape)

11. Build the prototype first before cutting polycarb:

12. 3D print the tube + clamshell halves on the Bambu P2S
13. Bench-test pin fit, cup non-fit, cinch action with string
14. Iterate the 3D design if needed

15. Only after the prototype works, fabricate the real polycarb version

16. Add inspector preparation to the team's pre-event checklist:

17. 1:1 PDF tracings of all polycarb pieces
18. Spare polycarb pieces (in case inspector wants to verify dimensions destructively)

19. Photo documentation of the bend process (proves heat-bending, not melting)

20. Notebook narrative for the R24 discovery:

21. "We planned to build a tube at 2.55″ ID. While reviewing our design against R24 we realized the developed (unrolled) length would exceed 8″. We recalculated the maximum legal ID at 2.43″. The tube still meets all our functional requirements for pin pickup; the cup interaction was unchanged since cups can't enter the tube at either dimension. We added a two-piece clamshell funnel to improve Path B pin catch reliability — each piece fits within R24 limits with mechanical (not bonded) assembly."

This is a strong engineering notebook story: discovery, math, correction, fix-in-design rather than fix-after-build, and documented compliance evidence. Judges score this kind of rigor highly.

Cross-references

• Earlier (superseded) review: /polycarb-tube-cad/ — kept for iteration history, but ID recommendations there have been updated by this audit
• Bench-test protocols: /phase-a-bench-tests.html — Protocol 1 (Path B catch) now references the clamshell funnel rather than single-piece funnel
• Skimmer code: skimmer-ez-template-example.cpp — no software changes needed (PathMode coordinator unchanged)
• Skimmer Path A roller: /skimmer-roller-cad/ — covers the front roller intake (Path A), separate from this tube review

End of R24 compliance audit. SVG sheets follow as separate files.