Workholding, Fixtures and Machine Maintenance

Workholding, Jigs and Fixtures, and Machine Maintenance

Chapter Introduction

This chapter covers three essential areas of the machinist trade: workholding principles, the design and use of machining jigs and fixtures, and preventive maintenance procedures for machine tools. For the Red Seal exam, you must master not only the practical techniques, but also the applicable Canadian standards, clamping force calculations, and geometric tolerances related to jigs and fixtures. This chapter is designed to prepare you directly for typical interprovincial exam questions.

1. Fundamental Principles of Workholding

1.1 Objectives of Workholding

Securing a raw or semi-finished workpiece on a machine tool must guarantee:

The machinist's golden rule: six points of contact are sufficient to completely position a rigid body in space. Three points define a plane (primary locating surface), two points define a line (secondary locating surface), and one point defines a point (tertiary locating surface). Any additional point of contact creates over-constraint, which can cause deformation or inaccuracies.

1.2 Common Types of Workholding

1.3 Clamping Force Calculation

The required clamping force \( F_s \) depends on the cutting force \( F_c \), the coefficient of friction \( mu \) between the workpiece and the jaws, and the safety factor \( k \) (typically 2.5 to 3).

Basic formula:

\[ F_s = (k × F_c)/(mu) \]

Example: For a cutting force of 1500 N, a coefficient of friction of 0.2 (dry steel on steel), and a safety factor of 3:

\[ F_s = (3 × 1500)/(0.2) = 22 500 N \]

Common trap: forgetting that the coefficient of friction decreases in the presence of lubricant (0.1 to 0.15). Always recalculate using the most unfavorable coefficient.

1.4 Elastic and Plastic Deformation

Excessive clamping can deform the workpiece elastically (returns to its original shape after unclamping) or plastically (permanent deformation). For thin or long workpieces, use soft jaws (machined to the shape of the workpiece) or intermediate supports.

2. Machining Jigs and Fixtures

2.1 Definitions and Differences

2.2 Components of a Jig or Fixture

A complete jig or fixture includes:

2.3 Six-Point Rule (Practical Application)

Frequent error: adding a 7th point of contact. This creates over-constraint. If the workpiece has flatness defects, the jig or fixture will not be repeatable.

2.4 Geometric Tolerances and Jigs/Fixtures

Jigs and fixtures must respect the geometric tolerances defined by ASME Y14.5 (adopted in Canada as a CSA standard). The most relevant for jigs and fixtures:

2.5 Calculating Guide Bushing Position

For a drilling jig, the distance between the bushing center and the reference edge is calculated using the formula:

\[ D = d_{bushing} + frac{d_{drill}}{2} + clearance \]

Where the clearance is typically 0.01 to 0.03 mm for standard drilling.

3. Specific Clamping Equipment

3.1 Hydraulic and Pneumatic Actuators

Actuators (cylinders) provide rapid and uniform clamping, ideal for series production. Key points:

Actuator force calculation:

\[ F = P × A \]

Where \( P \) is the pressure (Pa) and \( A \) is the effective piston area (m²).

Example: Hydraulic actuator with a 50 mm diameter piston, pressure of 150 bar (15 × 10⁶ Pa):

\[ A = pi × (0.025)^2 = 0.00196 m^2 \]

\[ F = 15 × 10^6 × 0.00196 = 29 400 N \]

3.2 Quick-Change Clamping Systems

Quick-change clamping systems (such as System 3R, Erowa, etc.) use reference pallets with a repeatability of ±0.002 mm. They are essential for CNC machining centers where setup changeover time must be minimized.

3.3 Lathe Chucks with Hard and Soft Jaws

Procedure: to machine soft jaws, clamp a reference ring (master) in the jaws, machine the jaws to the desired dimension, then replace the ring with the actual workpiece. This method guarantees perfect centering.

4. Preventive Maintenance of Machine Tools

4.1 Importance of Preventive Maintenance

Preventive maintenance aims to:

The CSA Z432 standard (Safeguarding of Machinery) defines safety and maintenance requirements for machine tools in Canada.

4.2 Typical Preventive Maintenance Schedule

4.3 Verifying Geometric Accuracy

The following accuracy tests are standard (according to ISO 230-1 for numerically controlled machines):

Runout calculation formula: total runout \( V_t \) is the sum of spindle runout \( V_b \) and chuck runout \( V_m \):

\[ V_t = V_b + V_m \]

If the measured runout exceeds the tolerance, you must identify the source (spindle or chuck) by rotating the chuck 180° relative to the spindle.

4.4 Lubrication

Lubrication is the most critical factor for the longevity of machine tools. Key points:

Rule of thumb: if the machine operates 8 hours per day, the centralized lubrication reservoir should be filled weekly and filters replaced every 3 months.

4.5 Coolant System Maintenance

The coolant (cutting fluid) must be:

Applicable standard: CSA Z94.4 (Selection, Use, and Care of Respirators) is not directly relevant here, but CSA Z1000 (Occupational Health and Safety Management) requires a chemical risk management program for cutting fluids.

5. Safety and Canadian Standards

5.1 Canadian Electrical Code, Part I

The Canadian Electrical Code, Part I (C22.1-21) applies to the electrical installations of machine tools. Key rules:

Exam trap: the Canadian Electrical Code is a national document, but each province may adopt amendments. The Red Seal exam is based on the national code, not provincial amendments.

5.2 CSA B149.1 (Natural Gas and Propane Code)

Although less directly relevant to the machinist, CSA B149.1 applies if the machine uses a gas burner (for example, a heat treatment furnace). Key requirements:

5.3 CSA Z432 (Safeguarding of Machinery)

This standard defines requirements for guards and safety devices on machine tools. Essential points:

6. Setup and Removal Procedures

6.1 Workpiece Setup Procedure on a Jig or Fixture

6.2 Common Setup Errors

6.3 Safe Removal Procedures

7. Advanced Calculations for Jigs and Fixtures

7.1 Clamping Force for Milling

For a milling operation, the tangential cutting force \( F_t \) is:

\[ F_t = (P_c)/(v_c) \]

Where \( P_c \) is the cutting power (W) and \( v_c \) is the cutting speed (m/s).

The required clamping force \( F_s \) to prevent slippage is:

\[ F_s = (k × F_t)/(mu × n) \]

Where \( n \) is the number of clamping points.

7.2 Calculating the Deflection of a Clamped Workpiece

The maximum deflection \( delta \) of a cantilevered workpiece under a force \( F \) is:

\[ delta = (F × L^3)/(3 × E × I) \]

Where \( L \) is the length, \( E \) is the modulus of elasticity (210 GPa for steel), and \( I \) is the moment of inertia of the cross-section.

For a rectangular section of width \( b \) and height \( h \):

\[ I = (b × h^3)/(12) \]

Example: Steel workpiece 100 mm long, 20 × 20 mm cross-section, cutting force of 500 N:

\[ I = (0.02 × (0.02)^3)/(12) = 1.33 × 10^{-8} m^4 \]

\[ delta = frac{500 × (0.1)^3}{3 × 210 × 10^9 × 1.33 × 10^{-8}} = 0.0006 m = 0.6 mm \]

This deflection is unacceptable for a tolerance of ±0.05 mm. You must add an intermediate support or reduce the cantilevered length.

7.3 Positioning Tolerance for Jigs and Fixtures

The positioning tolerance \( T_p \) of a jig or fixture element is calculated using the formula:

\[ T_p = frac{T_{workpiece}}{√(n)} \]

Where \( T_{workpiece} \) is the workpiece tolerance and \( n \) is the number of elements contributing to the error (jig/fixture, machine, tool, thermal expansion).

Rule of thumb: the jig or fixture should not consume more than 30% of the total workpiece tolerance.

8. Specific Maintenance for CNC Machines

8.1 Checking Axes and Encoders

CNC machines use optical or magnetic encoders to measure axis positions. Maintenance includes:

Compensation formula: the positioning error \( E_p \) is the difference between the commanded position \( P_c \) and the actual position \( P_r \):

\[ E_p = P_c - P_r \]

If \( E_p \) exceeds 0.02 mm over 300 mm, compensation must be entered into the controller.

8.2 Tool Changer Maintenance

8.3 Managing Alarms and Error Codes

The most common error codes and their meanings:

9. Documentation and Traceability

9.1 Maintenance Records

Each machine must have a maintenance logbook containing:

9.2 Documentation Standards

CSA Z1000 (Occupational Health and Safety Management) requires that maintenance records be kept for at least 3 years. For machines with regulatory requirements (e.g., overhead cranes), the period is 5 years.

Traps to Avoid

Summary

Review Questions (Red Seal Style)

This chapter covers the full range of knowledge required for the Red Seal exam on workholding, jigs and fixtures, and machine maintenance. Review the formulas, standards, and traps listed above, and practice with the review questions. Good luck with your preparation!