The Guard That Cracked When a Part Flew Out
We built a machine guard from 1 mm aluminum sheet. The guard enclosed a press. On the floor, a part (a small metal blank) ejected from the press and hit the guard. The aluminum sheet cracked. The part flew through. Luckily, no one was hurt. The problem: the guard material was too thin. Aluminum sheet (1 mm) isn’t impact-resistant. For a machine that ejects parts (or has flying chips), the guard must withstand impact. We switched to 4 mm polycarbonate (transparent, impact-resistant). A flying part hits the polycarbonate and bounces off (doesn’t crack). The mistake was using thin aluminum for a guard on a machine with flying parts.
Machine guard material selection balances visibility, impact resistance, and cost. This article covers the choices.
Guard Material Options
Polycarbonate (Lexan)
A transparent plastic (4–6 mm thick). Impact-resistant (won’t crack when a part flies). Clear (you can see the machine through it). Standard for guards that need visibility.
Best for: Machines with flying parts/chips (press, grinder), where visibility is needed. It’s the impact-resistant standard.
Wire Mesh (Perforated Sheet)
A steel mesh (grid of holes). Air flows through (good for ventilation). You can see through it (blurry). Lower impact resistance than polycarbonate (a small part can pass through a large hole). Use small holes (for small parts) or as a secondary guard.
Best for: Fences around robots (article 74), ventilation, general enclosure.
Aluminum Sheet (Solid)
A solid aluminum panel (1–3 mm). No visibility (opaque). Rigid, cheap. But not impact-resistant (thin sheet cracks on impact). For areas where nothing flies (a static cover, a side panel).
Best for: Static covers, non-critical panels, where nothing can eject.
Steel Sheet (Welded)
A steel panel (2–3 mm). Very strong (withstands impact). But opaque (no visibility), heavy, and rust-prone. For high-impact areas (a chip chute, a blast enclosure).
| Material | Impact | Visibility | Best For |
|---|---|---|---|
| Polycarbonate (4–6 mm) | High (won’t crack) | Clear | Flying parts, press, grinder, visibility |
| Wire mesh | Medium (small holes) | Blurry (see through) | Robot fences, ventilation |
| Aluminum sheet (1–3 mm) | Low (cracks) | Opaque | Static covers, non-critical |
| Steel sheet (2–3 mm) | Very high | Opaque | High-impact (blast, chips) |
Step 1: What Can Fly Out?
The guard material depends on what can eject from the machine:
- Nothing (static): The machine doesn’t eject parts (an assembly station). A thin aluminum sheet (or a polycarbonate window) is fine.
- Small parts (a press, a grinder): Parts or chips fly. Use polycarbonate (impact-resistant) or steel.
- Large/fast objects (a robot, a spinning tool): Use a fence (article 74) at a safe distance, plus polycarbonate windows.
The guard material rule: If anything can fly out (press, grinder, chips), use 4–6 mm polycarbonate (transparent, impact-resistant). The guard that cracked had 1 mm aluminum on a press. Polycarbonate won’t crack. For non-ejecting machines, aluminum sheet is fine.
Step 2: Visibility
The operator needs to see the machine (to monitor it). A polycarbonate window (clear) lets them see. An opaque aluminum sheet blocks the view. Use polycarbonate windows (even if the rest is aluminum) where the operator watches the process.
For a fully automatic machine (no operator watching), a solid panel is fine (cheaper). For a machine the operator monitors, add a polycarbonate window.
Step 3: Chemical and Temperature Resistance
Polycarbonate is sensitive to certain chemicals (solvents, some oils). Check the environment. For a washdown (food/pharma), use stainless steel (not polycarbonate). For high temperature (near an oven), polycarbonate softens (use glass or steel).
Step 4: Guard Frame
The guard material mounts on a frame (aluminum extrusion, article 74). The panel slides into the extrusion’s groove. For polycarbonate, drill holes carefully (it cracks if over-tightened). Use rubber gaskets (don’t bolt polycarbonate directly to metal — it needs room to expand/contract with temperature).
A Guard Material Checklist
- What can eject? (Nothing, small parts, chips?)
- If flying: use polycarbonate (4–6 mm)?
- Does the operator need visibility? (Window?)
- Is the environment corrosive? (Washdown? Stainless?)
- Is the temperature high? (Polycarbonate OK?)
- Is the guard framed in extrusion? (Panel slides in?)
- Are the polycarbonate holes drilled? (No cracking?)
- Is there a gasket? (Thermal expansion?)
- For robot fences: wire mesh (article 74)?
- Is the guard impact-tested? (Part eject test?)
The Bottom Line
Machine guard material selection matches the material to what can fly out. The guard that cracked had thin aluminum on a press. Use 4–6 mm polycarbonate for machines that eject parts (transparent, impact-resistant). Use wire mesh for robot fences, aluminum for static covers, steel for high-impact. Add a window where the operator watches. The guard that held the flying part wasn’t the thickest sheet — it was polycarbonate.
