--- title: "Locating principle and design of a drill jig" subject: "Workshop Technology" chapter: 12 key: "semester_2/workshop_technology:jig-locating-principle-design" source: https://www.bctengineer.com/q/semester_2/workshop_technology/jig-locating-principle-design site: BCT Engineering --- # Locating principle and design of a drill jig Workshop Technology · Chapter 12: Jigs and Fixtures for Machine Shops - Asked in: year not given, 5 marks - Video: [3 - 2 -1 Principle of Location | Jigs and Fixtures | Production Engineering and Robotics | #zafarsir](https://www.youtube.com/watch?v=ofx2rbBciyg) — Impact Academy Official - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_2/workshop_technology/jig-locating-principle-design - Whole chapter: https://www.bctengineer.com/md/semester_2/workshop_technology/chapter-12 ## Question Explain the 3-2-1 principle of location with a figure. State the points to be considered in the design of a drill jig, and name any two types of drill jigs. ## Short answer The 3-2-1 principle uses six locators to arrest all six degrees of freedom: three on the base, two on a side, one on an end. Drill jig design points: locate on machined surfaces, clamp firmly without distortion, use hardened bushes close to work, provide chip clearance and foolproofing. Two types: template jig and box jig. ## Answer #### 3-2-1 principle A rigid body in space has six degrees of freedom (three translations and three rotations). It is fully located by six points of contact arranged as follows: - 3 points on the base (primary plane) stop vertical movement and two rotations. - 2 points on a side (secondary plane) stop one horizontal movement and one rotation. - 1 point on an end (tertiary plane) stops the remaining movement. ```text top clamp | +--------+ 2 | work | pts->| | <- 1 pt (end stop) +--------+ o o o <- 3 pts (base) ``` Locators are hardened, ground pins or buttons that touch the work. Clamps then hold the work, and the clamping force must push the work towards the locators, not away from them. #### Design points for a drill jig - Locate on machined surfaces and follow the 3-2-1 principle, with no over-constraint. - Clamping must be quick and firm and must not distort or damage the work; the cutting force should act towards the locators, not against the clamps. - Hardened drill bushes (fixed, renewable or slip) guide the drill and are placed close to the work. - Provide chip clearance, easy loading and unloading, and foolproofing (poka-yoke) so that the work can be loaded only one way. - Use light but rigid construction (mild steel, with case-hardened wearing parts); provide feet so the jig rests steadily on the table. - Make it safe to use and economical for the production quantity. #### Types of drill jigs Template jig (a plate with bushes laid on the work), plate jig, box (tumble) jig, leaf jig, channel jig, indexing jig. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. In the 3-2-1 principle, what do the three points on the base do? A. Stop one horizontal movement only B. Stop vertical movement and two rotations C. Stop rotation about the vertical axis only D. Clamp the work on top Answer: B. The primary plane's 3 points restrain one translation and two rotations; 2 and 1 points restrain the remaining three. (Key point: 3-2-1 principle) 2. Why does the principle use six points in total? A. A body has six degrees of freedom (3 translations, 3 rotations) B. Six clamps are needed C. A jig has six faces D. Six bushes are required Answer: A. 3 + 2 + 1 = 6 contacts remove all six degrees of freedom; more would over-constrain. (Key point: Six degrees of freedom) 3. How should clamping be arranged in a drill jig? A. Force away from the locators B. Clamping force pressing the work against the locators and opposing cutting force C. Clamp as hard as possible regardless of distortion D. Use no clamps Answer: B. Clamps hold work against the locators, firmly but without distorting the work. (Key point: Clamping in jig design) 4. Which is a drill jig design point? A. Soft bushes placed far from the work B. Hardened drill bushes close to the work, with chip clearance and mistake-proof loading C. Allow work to enter in any orientation D. Locate on rough cast surfaces Answer: B. Hardened bushes guide the drill; chip clearance and poka-yoke are needed, and location is on machined surfaces. (Key point: Drill jig features) ## Related in Workshop Technology - [Jigs and fixtures: purpose and differences](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Ajigs-and-fixtures-purpose-differences) — Workshop Technology, chapter 12 ## Related in other subjects - [Drill jig and fixture design considerations](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Adrill-jig-milling-fixture-design-principles) — Manufacturing and Production Processes, chapter 10 - [Jigs and fixtures: purpose and 3-2-1 principle](https://www.bctengineer.com/question.md?key=semester_4%2Fmanufacturing_and_production_processes%3Ajigs-fixtures-purpose-locating-principles) — Manufacturing and Production Processes, chapter 10 - [Principle and complexities of route location](https://www.bctengineer.com/question.md?key=semester_6%2Ftransportation_engineering_i%3Alocation-principle-complexities) — Transportation Engineering I, chapter 2 - [Short note: Location of sounding points](https://www.bctengineer.com/question.md?key=semester_4%2Fsurveying_ii%3Asounding-point-location-hydrographic) — Surveying II, chapter 5, asked 2×