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Why I Wrote The Sheet Mechanic (And Why Calculations Aren’t Enough)

For engineers who already know the math—but still lose projects. For the last few years, I’ve been sharing technical guides here on Mechanical Design Handbook —how to size a motor, how to calculate fits, and (as you recently read) how to choose between timing belts and ball screws. But after 25 years in industrial automation, I realized something uncomfortable: Projects rarely fail because the math was wrong. They fail because: The client changed the scope three times in one week. A critical vendor lied about a shipping date (and no one verified it). The installation technician couldn’t fit a wrench into the gap we designed. University taught us the physics. It didn’t teach us the reality. That gap is why I wrote my new book, The Sheet Mechanic . This is not a textbook. It is a field manual for the messy, political, and chaotic space between the CAD model and the factory floor. It captures the systems I’ve used to survive industrial projec...
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Engineer's Guide to Nylon (PA): Grades, Properties & Uses

Assortment of Polyamide (Nylon) parts: white PA66 gears, black oil-filled bushings, and glass-filled structural brackets

Figure 1: The versatility of Nylon grades. Shown here: White PA66 spur gears for impact strength, black oil-filled bushings for self-lubrication, and grey glass-filled brackets for structural stiffness.

The Toughness King: Polyamide (Nylon)

PA (Polyamide), commercially known as Nylon, is the workhorse of the engineering plastic world. While POM (Acetal) is known for precision, Nylon is known for Toughness.

First developed by DuPont, Nylon bridges the gap between plastics and metals. It is one of the few plastics capable of withstanding high impact loads, high temperatures (under the hood of cars), and aggressive chemicals, making it a staple in heavy industry.

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Decoding the Grades: PA6 vs. PA66

Not all Nylons are the same. The numbers following "PA" refer to the molecular structure, which dictates the performance:

1. PA6 (Cast Nylon)

Often called "Cast Nylon," this grade offers excellent damping properties and impact resistance. It is slightly easier to machine and is often available in massive rod sizes for making large gears and sheaves for cranes.

2. PA66 (Extruded Nylon)

The most common "General Purpose" grade. PA66 is stiffer, stronger, and has a higher melting point than PA6. It is the standard for automotive engine covers, cable ties, and machined bushings.

3. PA12 and PA11

These are high-performance grades with lower moisture absorption. They are flexible and incredibly tough. Today, PA12 is the gold standard for SLS and MJF 3D Printing.

The Achilles Heel: Moisture Absorption

Engineering Warning:
Unlike POM (which ignores water), Nylon is Hygroscopic. It absorbs water from the atmosphere—up to 8% by weight at saturation!

Implications:
  • Dimensional Growth: As Nylon absorbs water, it swells. A precision bushing designed with tight clearance will seize up on a humid day. Always design with extra clearance.
  • Property Shift: Water acts as a plasticizer. "Wet" Nylon is tougher and more impact-resistant, but "Dry" Nylon is stiffer and stronger.

Advanced Materials: Glass-Filled Nylon (PA-GF)

To compete with aluminum, engineers add glass fibers to the Nylon matrix.

Glass-Filled Nylon (e.g., PA66-GF30) contains 30% glass fiber. This creates a material that is incredibly rigid, creep-resistant, and capable of withstanding high heat.
Common Use: Automotive intake manifolds, power tool housings (drills/grinders), and structural brackets that replace die-cast aluminum.

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Machining & Processing Tips

  • Coolant is Mandatory: Nylon is a poor conductor of heat. Without coolant, the heat from machining will cause the plastic to melt and gum up around the cutter.
  • Sharp Tools: Use high-positive rake tools similar to those used for aluminum.
  • Drying: Before injection molding, Nylon pellets MUST be dried (dehumidified) for several hours. Molding wet Nylon results in "splay" marks and structural failure.

Summary of Properties

Feature Benefit Drawback
Strength High tensile strength (80+ MPa) Drops when wet
Wear Excellent abrasion resistance Abrasive to mating parts if Glass-Filled
Heat High Heat Deflection Temp (HDT) Melts quickly if friction builds up
Chemicals Resists oils, fuels, and solvents Attacked by strong acids

Typical Applications

  • Automotive: Oil pans, engine covers, radiator end-tanks.
  • Industrial: Silent running gears, wear pads, conveyor rollers.
  • Electronics: High-voltage switchgear and cable connectors.
  • Consumer: Power tool bodies, zip ties, and ski bindings.

References

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