🔍 Key Concepts

• Base circle vs. pitch circle vs. root circle on a gear • How a radius is shown in a mechanical drawing (from gear center to a circle) • The difference between circular measurements along the tooth and radial measurements from the center

💭 Think About

• On the illustration, which lettered dimension clearly runs from the center of the gear out to one of the concentric dashed circles rather than along the tooth surface? • Among the concentric circles shown, which one represents the base circle (where the involute tooth profile begins), not the outside diameter or root diameter? • Which of the labeled lines along the bottom of the figure is a length along the circumference (a pitch), and which are true radii from the center?

✅ Before You Answer

• Confirm that a radius must start at the gear center and end on a circle, not run along the tooth flank or around the circumference. • Identify which dashed circle is the base circle, distinct from the pitch circle and the root circle. • Eliminate any letter that clearly measures tooth height, thickness, or circular pitch instead of a radial distance from the center.