--- title: "Jigs and fixtures: purpose and 3-2-1 principle" subject: "Manufacturing and Production Processes" chapter: 10 key: "semester_4/manufacturing_and_production_processes:jigs-fixtures-purpose-locating-principles" source: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/jigs-fixtures-purpose-locating-principles site: BCT Engineering --- # Jigs and fixtures: purpose and 3-2-1 principle Manufacturing and Production Processes · Chapter 10: Jigs and Fixtures for Machine Shops - Asked in: year not given, 6 marks - Video: [Jig & Fixtures-Mechanical engineering Interview Questions,Dimu's Tutorials](https://www.youtube.com/watch?v=p8HnhxknWrM) — shrikant kusnurkar - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_4/manufacturing_and_production_processes/jigs-fixtures-purpose-locating-principles - Whole chapter: https://www.bctengineer.com/md/semester_4/manufacturing_and_production_processes/chapter-10 ## Question Define jig and fixture and differentiate between them. State the purposes and advantages of using them. Explain the 3-2-1 principle of location with a neat sketch. ## Short answer A jig holds, locates, and guides the tool (e.g., drill jig); a fixture only holds and locates (e.g., milling fixture). The 3-2-1 principle uses 3 points on the base to restrict 5 motions, 2 points on a side to restrict 3 motions, and 1 point on an end to restrict 1 motion, leaving only 3 clamping motions. ## Answer A **jig** is a work-holding device that holds and locates the work and also guides the cutting tool (by drill bushes), e.g. a drilling jig. A **fixture** is a work-holding device that holds and locates the work but does not guide the tool; the tool is set relative to it by set blocks or feeler gauges, e.g. a milling fixture. #### Differences | Point | Jig | Fixture | |---|---|---| | Tool guiding | Yes, by bushes | No | | Used on | Drilling, reaming, tapping | Milling, turning, grinding, welding, inspection | | Fixing to machine | Usually not clamped to the table | Bolted to the table, tenon in T-slot | | Weight | Light | Heavy and rigid | | Cutting forces | Smaller | Larger | #### Purposes and advantages 1. Eliminates marking out, trial cuts and individual setting; gives interchangeability. 2. Higher production and shorter cycle time; semi-skilled labour is enough. 3. Consistent accuracy, repeatability and lower scrap. 4. Safe operation and reduced operator fatigue. 5. Allows several operations in the same setup, and permits the use of automatic machines. They are economical only if the batch size is large enough to repay the tooling cost. #### 3-2-1 principle of location When each direction is counted separately, a free body in space has **12 degrees of freedom**: translation in +/- x, y and z (6), and rotation clockwise and anticlockwise about x, y and z (6). The 3-2-1 principle uses only six locating points to restrict nine of them; the clamps restrict the remaining three. ```text z | Plane 1: 3 points on base (XY) | . . . (restrain -z translation and | +/- rotation about x and y: 5) +----- y Plane 2: 2 points on side / (restrain -y translation and x +/- rotation about z: 3) Plane 3: 1 point on end face (restrain -x translation: 1) ``` 1. **Three points** on the base (primary plane) fix the height and the two tilts: 5 freedoms. 2. **Two points** on a side (secondary plane) fix the side position and the rotation about the vertical axis: 3 freedoms. 3. **One point** on an end face (tertiary plane) fixes the last translation: 1 freedom. The 9 freedoms are restricted by the locators, and the remaining 3 (translation in +x, +y and +z, away from the locators) are removed by the clamps. For round work, a V-block with a flat or a locating pin is used instead. A locating surface should be a machined face (not a rough casting surface), and the points should be spread as far apart as possible. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What distinguishes a jig from a fixture? A. A jig guides the tool by bushes; a fixture does not B. A fixture guides the tool; a jig does not C. A jig is bolted to the table; a fixture is not D. They are identical Answer: A. Drilling jigs use bushes; milling fixtures use set blocks. (Key point: Jig vs fixture) 2. Which is an advantage of using jigs and fixtures? A. Reduces the need for large batches B. Eliminates marking out and gives interchangeability C. Needs skilled labour D. Always cheap for one part Answer: B. They are economical only when batch size repays tooling. (Key point: Advantages) 3. What does the 3-2-1 principle do? A. Locates with 1, 2, 3 clamps B. Restrains all 12 freedoms with 6 points C. Locates with 3 points on base, 2 on side, 1 on end face to restrain six freedoms D. Uses 3 clamps on three planes Answer: C. Six points restrain six freedoms; clamps remove the rest. (Key point: 3-2-1 principle) 4. What do the three base points restrain? A. Side position and rotation about vertical B. The last translation C. All rotations D. Height and two tilts Answer: D. Base fixes one translation and two rotations. (Key point: Primary plane) ## Related in Manufacturing and Production Processes - [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 ## Related in other subjects - [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 - [Locating principle and design of a drill jig](https://www.bctengineer.com/question.md?key=semester_2%2Fworkshop_technology%3Ajig-locating-principle-design) — Workshop Technology, chapter 12