Benjamin Jacobs - Guitar Fretting Machine

This project was completed for a Precision Machine Design & Implementation course at BU. The semester-long project required the full design of an automated machine for one of several applications. In addition to the design of custom tooling, architecture, and subsystems, all motors and sensors had to be appropriately selected. Torque and timing calculations were performed for each motor to assure the necessary forces and throughput were achieved. The objective was to design a machine that automated the fretting of electric guitar necks, a complex, multi-step process which luthiers conventionally perform by hand. The full presentation goes into greater depth than this overview, and can be downloaded at the foot of this page.

System Overview and Customer Requirements

Isometric system view of the GFM-57 (guitar fretting machine).

System view with housing, including user display, access windows and waste bin.

The specified customer requirements were as follows:

• Accommodate an input rack of 10 necks

• Cut 24 fret slots in the necks

• Press fret wire into the necks and trim to size

• Polish each end of the fret wires to required finish

• Cycle time < 5 seconds per-fret (pressing)

• Batch cycle time < 60 seconds (trimming and finishing)

The machine must also be able to process necks with varying parameters, including:

• Scale sizes from 24-25.5"

• Fretboard radii from 10-16"

• Fretboard width from 1.69-2.25"

System profile view depicting the bottom tool wheel (BTW), which performs fret wire cutting, grinding, and polishing.

System profile view depicting the top tool wheel (TTW), which performs fret slot milling, fret pressing, fret grinding (in tandem with the BTW), and fret wire feeding to the eater-feeder subsystem, detailed below.

Machine Subsystems and Custom End-Effectors

I. Neck Rack & Eater-Feeder Mechanism

Subsystem view of the neck rack and Eater-Feeder mechanism, the latter of which is detailed in the below diagram.

The neck rack uses pneumatic grippers (blue) to secure each neck laterally. These allow for necks of varying radii and fret widths to be secured by one universal mechanism. A retaining bar (grey) is used to fix the necks vertically.

The Eater-Feeder subsystem dispenses, aligns, and presses the fret wire for each of the 24 rows of frets. It consists of three primary components:

1. The Feeder (left) contains the spool of fret wire and a motorized spindle for reversing the spool - once the free end is secured in the eater - in order to properly tension the span of wire.

2. The Gripper (middle) transfers the free end of wire from the feeder, to the eater.

3. The Eater (right) securely fixes the free end of the wire at the opposite end of the neck rack.

II. Top & Bottom Tool Wheels

The Top Tool Wheel (T.T.W.) performs fret slot milling (bottom), fret wire handoff to the Eater (right), fret wire pressing (left), and fret grinding (top).

The Bottom Tool Wheel (B.T.W.) performs fret cutting (top), grinding (left), and polishing (right).

III. Tool Wheel & Custom End-Effector Operation

Both tool wheels cycle rapidly between end-effectors via a bevel gear-driven axle, which is mated to the rear of the wheel housing. This provides the necessary rigidity for machining operations, reducing axial play and backlash, while allowing for efficient tool changes to keep cycle times within required parameters.

A section-view of the Top Tool Wheel (V1) performing fret slot milling.

A section-view of the Top Tool Wheel performing fret wire pressing.

A detail view of the Bottom Tool Wheel's fret cutting end-effector, and the low-profile pneumatic cylinder that drives the cutting mechanism.

Graphic User Interface (GUI)

A complete GUI, tailored to the specific hardware and manufacturing processes of the GFM was designed using MATLAB App Designer.

Page 1 of the GUI prompts the user to input the global neck parameters (scale length, radius, fretboard composition, and nut width), in addition to other inputs relating to fret wire properties, optional neck inlay and nut slot milling, and travel speed.

It also includes indicators that track the state of various locking and safety mechanisms.

Page 2 of the GUI is for monitoring. It features indicators displaying mill spindle speed, and a dedicated panel for tracking supply of expendables, which include fret wire supply, as well as remaining tooling life for abrasive tool heads. Lastly, it displays the current time elapsed for a batch, remaining time, and current and next-up machining stages.

Rendering of GUI displayed on touch screen operator interface.

Process Flow Chart of Machine Operation

A detailed flow chart was created to illustrate the sequential and parallel processes which occur throughout the full manufacturing process. This begins with logging of user-input parameters for the batch via the GUI, and ends with the operator's removal of the fully-fretted necks, before the next batch is begun.

Sensor Selection

Torque Calculations

Motors

Pneumatic Actuators

Timing Calculations, Financial Justification, Bill of Materials

A PDF with detailed process timing calculations, financial justifications, and a full bill of materials is available below.

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