Fabricating and Repairing Jigs, Fixtures, and Gauges
Chapter Introduction
This chapter covers the full range of theoretical and practical knowledge required for the Red Seal exam in the Tool and Die Maker trade concerning the fabrication and repair of jigs, fixtures, and gauges. These devices are at the heart of mass production: they ensure dimensional accuracy, repeatability, and interchangeability of machined parts. You must master not only their design and machining but also the Canadian standards that govern their use and verification.
Definitions and Distinct Roles
Jig
A jig is a holding device that guides the cutting tool (drill, reamer, tap) toward the workpiece. It is typically used on drilling machines or boring machines. The jig features drill bushings that determine the exact position of the drill relative to the workpiece. It is often mounted on a base with positioning elements (rest buttons, stops) and clamping elements (clamps, screws).
Fixture
A fixture is a holding device that positions and clamps the workpiece but does not guide the tool. The tool is guided by the machine tool itself (milling machine, lathe, grinder). The fixture must be rigid and stable to withstand cutting forces without deformation. Distinctions are made between machining fixtures, inspection fixtures, assembly fixtures, and welding fixtures.
Gauge
A gauge is a verification instrument used to check the conformity of a part to a specified tolerance. It does not measure a numerical value but rather indicates whether the part is good (within tolerance) or bad (out of tolerance). Gauges are classified into two categories:
Working gauges: used in production for rapid inspection.
Receiving gauges: used by quality control or the customer for final verification. They are generally more precise than working gauges.
Design Principles for Jigs and Fixtures
The Six Points of Location (Kelvin Principle)
Any rigid body possesses six degrees of freedom: three translations (X, Y, Z) and three rotations (around X, Y, Z). To position a workpiece reproducibly, you must constrain these six degrees of freedom using fixed contact points. The six-point locating principle states:
3 points in the base plane (Z): define the primary support (plane).
2 points in a vertical plane (Y): define the secondary support (line).
1 point in the orthogonal vertical plane (X): defines the tertiary support (point).
Golden rule: Never place more contact points than necessary in the same plane, otherwise the workpiece becomes unstable (over-constrained). A well-designed fixture uses rest buttons or V-blocks for these supports.
Locators and Supports
Rest buttons: Point supports with flat or spherical heads. They are used to avoid interference with machined surfaces.
V-blocks: For locating cylindrical workpieces. A 90° V-block is standard. Contact is made along two generatrices of the cylinder.
Locating pins: For workpieces with bores. A cylindrical pin (precise diameter) and a diamond pin are used to compensate for variations in the distance between bores.
Stops: For positioning the workpiece in translation. They must be placed facing the cutting forces to absorb the thrust.
Clamping
Clamping must be effective (holding the workpiece) but non-deforming. The basic principles:
The clamping force must be directed toward the fixed supports, never toward movable elements.
The point of force application must be at the center of the support area to prevent tipping.
Use swing clamps or cylinders to distribute pressure.
Clamping force calculation: For a drilling operation, the axial cutting force F (in N) is approximated by:
F = 1.2 × D × f × K
where D = drill diameter (mm), f = feed rate (mm/rev), K = material constant (e.g., 2.5 for mild steel, 1.5 for aluminum). The clamping force must be at least 2.5 times the cutting force to ensure safety.
Fabrication of Jigs and Fixtures
Materials Used
| Material | Application | Characteristics |
|---|
| AISI 1018/1020 Steel | Jig bodies, base plates | Good machinability, low cost |
| AISI 4140 Steel (pre-treated) | Fixtures subject to high stresses | High strength, hardenable |
| Grey Cast Iron (class 40) | Milling fixture bodies | Dampens vibrations, stable |
| Tool Steel (O1, A2) | Drill bushings, gauges | Hardenable, wear-resistant |
| Aluminum 6061-T6 | Lightweight jigs, prototypes | Lightweight, good machinability |
| Tungsten Carbide | High-production drill bushings | Extreme wear resistance |
Precision Machining
Fabricating a jig or fixture requires tight tolerances:
Base surfaces: ground to ±0.005 mm over 100 mm.
Bushing bores: bored to ±0.005 mm (H7 tolerance).
Center distances: ±0.01 mm, verified on a coordinate measuring machine (CMM).
Typical procedure:
42.Roughing: Cut the base plate, machine the main contours.
43.Heat treatment (if required): Hardening and tempering for tool steels.
44.Grinding: Grind the reference surfaces to achieve flatness and parallelism.
45.Drilling and boring: Drill locating holes, bore the holes for bushings.
46.Bushing installation: Press in the drill bushings (interference fit H7/p6).
47.Final verification: Complete dimensional inspection on a CMM.
Drill Bushings
Bushings are standardized (ANSI B94.33 or ISO 4247). Their length should be 2 to 3 times the drill diameter for proper guidance. The clearance between the bushing and the drill is critical:
Standard clearance: 0.01 to 0.03 mm (for standard drilling).
Reduced clearance: 0.005 mm (for precision drilling).
Increased clearance: 0.05 mm (for deep drilling, to evacuate chips).
Types of bushings:
Fixed bushings: pressed directly into the jig.
Renewable bushings: mounted in a master bushing, replaceable without disassembling the jig.
Head bushings: with a shoulder for high-thrust applications.
Gauges: Design and Fabrication
Limit Gauges (Go / No-Go)
The fundamental principle is Taylor's rule: the GO gauge must check the maximum material condition (MMC) of the part, meaning the largest dimension for a shaft and the smallest dimension for a bore. The NO-GO gauge checks the least material condition (LMC).
Example: For a bore with a diameter of 20.000 mm (tolerance +0.021 / 0):
GO gauge (plug): diameter 20.000 mm (must enter).
NO-GO gauge: diameter 20.021 mm (must not enter).
Gauge Tolerance Calculation
According to ANSI/ASME B89.1.5 (adopted in Canada), gauges have a manufacturing tolerance and a wear allowance:
Manufacturing tolerance (T): ±10% of the part tolerance (minimum 0.002 mm).
Wear allowance (W): ±5% of the part tolerance (minimum 0.001 mm).
Formula for a plug gauge:
GO diameter = Minimum bore diameter (MMC) + T/2 (positive side).
NO-GO diameter = Maximum bore diameter (LMC) − T/2 (negative side).
Worked example: Bore 20.000 to 20.021 mm. Part tolerance = 0.021 mm.
T = 0.10 × 0.021 = 0.0021 mm → rounded to 0.002 mm.
W = 0.05 × 0.021 = 0.001 mm.
GO gauge: 20.000 + 0.001 = 20.001 mm (tolerance ±0.001).
NO-GO gauge: 20.021 − 0.001 = 20.020 mm (tolerance ±0.001).
Special Gauges
Snap gauge: For shafts. GO = maximum diameter, NO-GO = minimum diameter.
Thread gauge: GO = full thread plug, NO-GO = truncated thread plug (fewer threads). The NO-GO must not thread more than 2 turns.
Form gauge: For checking radius, angle, depth (e.g., dovetail gauge).
Combined gauge: Checks multiple dimensions simultaneously (e.g., position, perpendicularity).
Repair of Jigs, Fixtures, and Gauges
Damage Diagnosis
Common failures include:
Wear of drill bushings: ovalization, increased diameter.
Deformation of base plates: warping (exceeding 0.02 mm over 300 mm).
Chips on gauges: burrs on measuring edges.
Loss of hardness: due to overheating during grinding.
Repair Procedures
88.Cleaning and inspection: Degrease, check critical dimensions with a micrometer or CMM.
89.Re-grinding: If deformation is less than 0.1 mm, grind the base surface to restore flatness. Caution: this reduces the jig thickness; you must verify the remaining rigidity.
90.Bushing replacement: Extract the worn bushing (with a puller or by drilling it out), bore the housing to the next size, press in a new bushing.
91.Build-up: For worn gauges, a layer of hard chrome (0.01 to 0.02 mm) can be deposited and then ground back to nominal size. Hard chrome has a hardness of 68-72 HRC.
92.Localized heat treatment: To restore the hardness of a gauge, torch hardening followed by tempering can be performed, but this risks deforming the part. Prefer replacement if the part is small.
Repair Tolerances
A rule of thumb: a worn gauge can be repaired if the wear does not exceed 20% of the manufacturing tolerance. Beyond that, it must be replaced. For a jig, re-grinding must not reduce the base plate thickness by more than 10% of its nominal value.
Canadian Standards and Applicable Codes
Measurement and Calibration Standards
In Canada, metrological traceability requirements are defined by:
CAN/CSA Z540.3: Requirements for calibration programs for test and measuring equipment. This standard requires that all instruments be calibrated with an uncertainty less than 25% of the tolerance of the part being inspected.
ANSI/ASME B89.1.5: Specifications for limit gauges (plugs, rings, snaps). This is the reference for gauge dimensions and tolerances.
Electrical Safety (if applicable)
If the jig or fixture incorporates electric actuators (electric cylinders, sensors), the installation must comply with the Canadian Electrical Code, Part I (CE Code) (C22.1). Rule 8-200 (Section 8) applies to control circuits: it requires overcurrent protection and compliant grounding. Connectors and wiring must be protected against mechanical damage (Rule 12-100).
Compressed Gases (if pneumatic cylinders)
Pneumatic fixtures using compressed air must comply with CSA B149.1 (Natural Gas and Propane Installation Code). Although this standard primarily concerns combustible gas, Rule 5.4 addresses pressurized piping and requires leak tests at 1.5 times the service pressure before commissioning.
Practical Calculations for the Exam
Center Distance Tolerance Calculation
For a jig with two drill bushings spaced L (mm) apart, the center distance tolerance ΔL is given by:
ΔL = ±(T_part / 2) × (L / D)
where T_part = part tolerance (mm), D = distance between the bushings and the reference point (mm). This formula is used to verify whether a jig can produce parts within tolerance.
Hydraulic Clamping Force Calculation
For a hydraulic cylinder with diameter d (mm) under pressure P (MPa):
F = P × (π × d² / 4) × 1000
Example: d = 32 mm, P = 7 MPa → F = 7 × (3.1416 × 1024 / 4) × 1000 = 7 × 804.25 × 1000 = 5,629 N.
Allowable Gauge Wear Calculation
Allowable wear = W × (part tolerance / 100)
Example: part tolerance = 0.050 mm, W = 5% → wear = 0.0025 mm. If the GO gauge measures 20.000 mm, it is acceptable up to 20.0025 mm.
Inspection and Verification Procedures
Verifying a New Jig
118.Dimensional inspection: Measure center distances with a CMM (uncertainty ±0.002 mm).
119.Perpendicularity check: Verify that the bushings are perpendicular to the base (tolerance 0.01 mm over 50 mm).
120.Functional test: Machine a test part and inspect it with a gauge. If the part is good, the jig is validated.
Verifying a Gauge in Service
Frequency: Every 6 months for a working gauge, every 12 months for a receiving gauge.
Method: Comparison with master gauges (grade 00 or 0) calibrated at an accredited laboratory.
Criterion: The gauge is compliant if its dimension is within the manufacturing tolerance ± the wear allowance.
Pitfalls to Avoid
126.Confusing jig and fixture: The jig guides the tool; the fixture does not. This is the most frequent trick question.
127.Forgetting Taylor's rule: The GO gauge checks MMC, the NO-GO checks LMC. Reversing these roles makes the gauge unusable.
128.Neglecting the six points of location: Placing 4 rest buttons in the base plane creates instability. Always respect 3-2-1.
129.Using a drill bushing that is too long: A bushing longer than 3× the diameter causes overheating and drill seizure.
130.Ignoring gauge wear: A worn GO gauge (too large) allows out-of-tolerance parts to pass. Check regularly.
131.Grinding a hardened gauge without cooling: Grinding heat exceeds the tempering temperature (200°C) and softens the gauge. Use abundant lubrication.
132.Forgetting Canadian standards: The Canadian Electrical Code applies to electrified fixtures. Do not cite provincial standards (they are not on the interprovincial exam).
133.Calculating insufficient clamping force: Always multiply the cutting force by a safety factor of at least 2.5.
Summary
The jig guides the tool; the fixture holds the workpiece; the gauge verifies conformity.
Six-point location (3-2-1) is the foundation of any holding device.
Drill bushings should have a length of 2 to 3× the drill diameter and clearance suited to the application.
Limit gauges follow Taylor's rule: GO = MMC, NO-GO = LMC. Their manufacturing tolerance is ±10% of the part tolerance.
Gauge repair is limited to 20% of the manufacturing tolerance; beyond that, replacement is required.
The standards CAN/CSA Z540.3 and ANSI/ASME B89.1.5 govern metrology and gauges in Canada.
For electrical fixtures, comply with the Canadian Electrical Code, Part I (CE Code) (Rule 8-200). For pneumatic circuits, CSA B149.1 (Rule 5.4) applies.
Clamping force and allowable wear calculations are essential for the exam.
Final Exam Tips
Memorize the formulas: Clamping force, gauge tolerance, allowable wear. They appear consistently.
Visualize the diagrams: A typical drilling jig has a base, 4 rest buttons, 2 pins, and 2 clamps. A plug gauge has a handle and two ends (GO and NO-GO).
Read questions twice: The traps are often in the words "jig" vs. "fixture" or "GO" vs. "NO-GO".
Use the units: The International System (mm, N, MPa) is the standard in Canada. Convert inches to mm (1 in = 25.4 mm) if necessary.
Use technical terminology: Use the exact terminology (e.g., "drill bushing," "rest button," "snap gauge").
This chapter covers the full scope of competencies from the "Fabricate and Repair Jigs, Fixtures, and Gauges" block of the National Occupational Analysis. A thorough review of these concepts, combined with practice on the calculations, will prepare you effectively for the Red Seal exam.
