Chapter 12 · 1 hour
Jigs and Fixtures for Machine Shops
Practice questions
Practice questions and answers
2 exam-style questions on this chapter, written for this site from the official syllabus. We haven’t found past IOE papers for this subject yet; if you have some, share them in the community.
- Practice · 5 marks
What are jigs and fixtures? State their purposes and advantages. Differentiate between a jig and a fixture with examples.
Answer
A jig is a work-holding device that holds, locates and also guides the cutting tool (by bushes). A fixture holds and locates the work and sets the tool position, but does not guide the tool.
Purposes and advantages
- Locate and hold the work in the same position every time, so parts are interchangeable.
- Remove the need for marking out, measuring and setting-up, saving time.
- Increase production rate and reduce cost per piece; allow semi-skilled operators.
- Improve accuracy and repeatability; reduce scrap.
- Provide safe and rigid clamping; allow several tools or parts to be handled.
- Make operations on difficult shapes possible.
Differences
| Jig | Fixture |
|---|---|
| Guides the tool by bushes | Does not guide the tool; uses setting blocks or feelers |
| Usually used for drilling, reaming, tapping | Used for milling, turning, grinding, welding, inspection |
| Light, not clamped to the table (often) | Heavier, firmly bolted to the table |
| Tool contact is above the work | Tool position is set by gauge blocks |
| Examples: plate jig, box jig, template jig | Examples: milling vice, turning fixture, welding fixture |
- Practice · 5 marks
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.
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.
top clamp
|
+--------+
2 | work |
pts->| | <- 1 pt (end stop)
+--------+
o o o <- 3 pts (base)
Locators are pins or buttons that touch the work (hardened, ground). Clamps then hold it against the locators; the clamping force acts on the locators, not away from them.
Design points for a drill jig
- Locate on machined surfaces, and keep to the 3-2-1 principle; no over-constraint.
- Clamping must be quick, firm and not distort or damage the work; the clamping force should oppose the cutting force.
- Hardened drill bushes (fixed, renewable, slip) guide the drill, placed close to the work.
- Provide chip clearance, ease of loading and unloading, and poka-yoke so that the work can only go in one way.
- Use cheaper, light, rigid construction (mild steel, case hardened parts); provide feet to rest on the table and gain stability.
- Make it safe, with a mistake-proof design 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.
Written from the official syllabus. Questions and answers are written for this site; check them against your class notes.
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