My 2 cents

By Ed Odom

Device for sharpening edged tools – Walter Purdy – 1899 – Patent: US630106A

Device for Sharpening Edged Tools (Walter Purdy, No. 630,106)

The patent by Walter Purdy of Somerset, Pennsylvania, describes the third generation of his sharpening apparatus, a Device for Sharpening Edged Tools (Patent No. 630,106, 1899). This invention focuses on two major refinements to his previous designs:

  1. Varying the elliptical movement to trace a precise curve over the hone.
  2. Providing an improved, positive, non-spring-actuated mechanism for automatically reversing (flipping) the tool during the stroke.

Inventor Background: Walter Purdy

Walter Purdy was an inventor specializing in precision mechanical tools. His repeated patents on this single device demonstrate a commitment to perfecting the mechanical automation of complex, skilled hand motions (the razor honer’s figure-eight stroke), aiming for maximum quality and consistency in sharpening.

Invention and Mechanism

This design retains the core concept of converting rotary motion into an oval path but replaces the spring-driven flip with a gear-and-rack system controlled by external stops.

1. Compound Reciprocating Motion

  • Base Plate (2) and Angular Slot (3): The base plate has a unique V-shaped (angular) slot (3). A stud (11) on the tool-carrier support (12) fits into this slot.
  • Eccentric Drive: A crank-arm (10) on a drive wheel (9) is connected to the tool-carrier support (12).
  • Movement: The eccentric rotation of the drive wheel (9), combined with the constraining guidance of the angular slot (3) and a shifting pivotal connection (27), compels the tool-carrier support (12) to move in a precise, non-elliptical, concave arc (the desired curve) over the hone (30).
    • Function: The tool’s blade remains in contact during the first half of the stroke (heel to toe).

2. Positive, Non-Spring Reversal (Key Innovation)

  • Tool-Carrier (14): Rotatably mounted within the support (12) and provided with a gear-wheel (15).
  • Rack-Carrying Yoke (17): A yoke is pivoted (19) to the tool-carrier support (12) and carries a rack (16) that meshes with the tool-carrier’s gear-wheel (15).
  • External Stop-Arms (21): These are adjustable stop-arms fixed to the base plate (2), located in the path of studs (20) on the yoke (17).
  • Operation (The Positive Flip):
    1. Stroke Right: The tool-carrier support moves right. As it passes the midpoint of the angular slot (3), the right-hand stud (20) on the yoke contacts its fixed stop-arm (21).
    2. Yoke Stops: Since the support (12) continues moving, the yoke (17) is forced to pivot independently on its screw (19).
    3. Rotation: The pivoting of the yoke (17) moves the rack (16) against the tool-carrier gear (15), compelling the tool-carrier (14) to rotate $180^\circ$ (reverse).
    4. Return Stroke: The tool completes the stroke reversed. On the return stroke, the left-hand stud (20) hits its stop-arm (21), forcing the yoke to pivot again, flipping the tool back to its initial position.
  • Result: The flip is accomplished positively and reliably by mechanical contact, eliminating the potential failure and variability associated with springs (used in his previous patent).

Concepts Influenced by This Invention

Purdy’s third-generation sharpener influenced subsequent mechanical designs by establishing principles for high-precision, cam-and-gear based actuation in complex systems.

  • Positive Mechanical Reversal (Non-Spring): The core concept of using fixed external stops (21) to interrupt the travel of a component’s intermediate linkage (yoke 17), thereby forcing a rotational change (gear 15), influenced the design of:
    • Machine Tool Indexing: Mechanisms in automated lathes or transfer lines where a component must be positively rotated or indexed by external stops rather than relying on spring force.
    • Limit Switches and Controls: Systems that use physical contact at the end of a stroke to trigger a necessary reversal or sequential action.
  • Kinematic Synthesis for Contoured Motion: The use of the angular slot (3) to constrain and guide the motion of the drive linkage into a precise, non-circular concave arc is a classic example of kinematic synthesis. This technique is crucial in industrial polishing, grinding, and forming machinery where the tool must follow a complex, specific path over the workpiece.
  • Adjustable Pitch: The provision for adjusting the stop-arms (21) to give a greater or lesser pitch to the tool edge influenced the design of universal sharpening jigs and grinding equipment that require precise, repeatable bevel regulation.