Interpretation of Drawings and Work Planning

Interpreting Drawings and Work Planning

Module Introduction

This chapter covers the full range of skills required to read, interpret, and apply technical drawings, as well as to effectively plan machining work. For the Red Seal exam, you must master not only reading orthographic views and tolerances, but also the logical sequence of operations, tool selection, and compliance with Canadian standards. This module typically represents 15 to 20% of the exam questions, making it a high-yield area for your preparation.

Reading Technical Drawings

Types of Drawings and Their Uses

The assembly drawing shows the complete assembly of a part or mechanism. It allows you to visualize the relationships between components, functional clearances, and the assembly order. The detail drawing presents an individual part with all its dimensions, tolerances, and surface specifications. The shop drawing often combines both: it shows the part to be manufactured with all the information necessary for machining.

You must also be able to distinguish schematic drawings (simplified representations of hydraulic, pneumatic, and electrical circuits) from dimensional drawings that are used directly for manufacturing.

Orthographic Views and Projection

The projection system used in Canada for machining drawings is third-angle orthographic projection (American National Standard). In this system, the front view is placed in the center, the right-side view is to the right of the front view, the top view is above it, and so on. This is the reverse of the European first-angle projection used in Europe.

Practical rule: In third-angle projection, the view you see is the one you would get by rotating the part toward you. If you place a part in front of you and tip it to the right, you get the right-side view.

The projection symbol on the drawing title block is a truncated cone: for third-angle, the small circle is at the bottom; for first-angle, the small circle is at the top.

Lines and Their Meanings

Frequent trap: Center lines must never be used as dimension lines. Hidden lines must not cross dimension lines.

Sections and Cuts

A section is a view that shows the interior of the part by cutting it imaginarily. The types of sections you need to know:

Hatching indicates the cut material. The spacing and angle of the hatch lines (generally 45°) must be uniform for the same part. If two adjacent parts are sectioned, their hatch lines must be oriented differently.

Dimensioning and Tolerances

Dimensioning Principles

Dimensioning must be complete (all necessary dimensions are present), clear (no ambiguity), and functional (dimensions originate from important reference surfaces). You must know the following rules:

Dimensional Tolerances

Tolerance is the allowable deviation between the nominal dimension and the actual dimension. It can be expressed in three ways:

The ISO tolerance system uses uppercase letters for bores (H, G, F, etc.) and lowercase letters for shafts (h, g, f, etc.). The letter indicates the position of the tolerance zone relative to the zero line, and the number (IT01 to IT18) indicates the degree of precision.

Reference values for ISO (IT) grades:

Fits

A fit is the relationship between two assembled parts (a shaft and a bore). Three families:

Calculation formula: Maximum clearance = maximum bore − minimum shaft. Minimum clearance = minimum bore − maximum shaft. For an interference fit, we speak of maximum and minimum interference.

Example: Bore 25.00 H7 (25.000 to 25.021 mm), shaft 25.00 g6 (24.980 to 24.993 mm). Maximum clearance = 25.021 − 24.980 = 0.041 mm. Minimum clearance = 25.000 − 24.993 = 0.007 mm. This is a guaranteed clearance fit.

Geometric Tolerances (GD&T)

GD&T (Geometric Dimensioning and Tolerancing) is governed in Canada by CSA B78.2 (equivalent to ASME Y14.5). The main symbols you need to know:

The geometric tolerance is written in a feature control frame: symbol, tolerance value, datum reference(s). The datum is a theoretical reference surface, axis, or point from which measurements are taken.

Maximum Material Condition (MMC) rule: The Ⓜ symbol after the tolerance value means the tolerance applies when the part is at its maximum material condition. This allows the tolerance to increase as the part moves away from maximum material condition, which facilitates manufacturing without compromising function.

Surface Finishes

Roughness Parameters

The most common parameter is Ra (arithmetic average roughness), expressed in micrometers (µm). It represents the average of the profile deviations from the mean line.

Surface symbol on the drawing: The basic symbol is an inclined V (✓) with the Ra value written above it. A circle at the junction of the branches indicates a surface without material removal (as-is). A horizontal line above the V indicates machining with material removal.

Trap: The Ra value is not the maximum depth of surface irregularities. Two surfaces with the same Ra can have very different profiles (sharp peaks vs. wide valleys).

Materials and Treatments

Steel Designations

In Canada, steels are designated according to the SAE/AISI (Society of Automotive Engineers / American Iron and Steel Institute) four-digit system:

The last two digits indicate the carbon content in hundredths of a percent. Example: 1045 = 0.45% carbon.

Heat Treatments

Rockwell hardness: The most common scales are HRC (diamond cone, 150 kg load) for hardened steels and HRB (1/16" ball, 100 kg load) for mild steels. A hardness of 60 HRC corresponds approximately to 650 HB (Brinell) or 700 HV (Vickers).

Work Planning

Drawing Analysis and Operation Sequence

Before any machining operation, you must:

Golden rule: Finishing operations must be done last, after all roughing operations. Reference surfaces must be machined first and used for all subsequent operations.

Cutting Parameter Calculations

Cutting speed (Vc) is expressed in meters per minute (m/min). It depends on the workpiece material and the tool.

Formula: Vc = (π × D × N) / 1000

Where D = diameter in mm, N = rotational speed in rpm.

Recommended cutting speeds (turning, carbide tool):

Feed rate (f): expressed in mm/revolution for turning, in mm/tooth for milling. The feed rate directly affects the surface finish: a lower feed rate gives a better finish.

Depth of cut (ap): the thickness of material removed in one pass. For roughing, depths of 2 to 5 mm are used; for finishing, 0.2 to 0.5 mm.

Cutting power calculation: P (kW) = (Vc × ap × f × Kc) / 60,000

Where Kc is the specific cutting energy (approximately 2000 to 3000 N/mm² for steels).

Operation Order for a Typical Part

Let's take the example of a stepped shaft with a central bore:

Applicable Canadian Standards

CSA B78.2 and Technical Drawings

CSA B78.2 (Technical Drawings — General Practices) is the Canadian reference for dimensioning and tolerancing. It is harmonized with ASME Y14.5. You must know the basic principles: dimensions are in millimeters, angles in degrees, and tolerances apply to the nominal dimension unless otherwise indicated.

General Tolerances

When no tolerance is indicated on a dimension, the general tolerances from the title block apply. Typically:

Canadian Electrical Code

Although less directly related to machining, the Canadian Electrical Code, Part I (CE Code) (C22.1) applies to machine tools and their electrical installation. Rule 8-200 concerns branch circuit conductors and circuit protection. For the exam, you should know that a machine tool must be grounded in accordance with Rule 10-200 and that control circuits must be protected in accordance with Rule 14-100.

Traps to Avoid

Summary

To succeed in this section of the exam, practice reading complete drawings, calculating fits, and planning machining sequences in a logical order. Speed and accuracy come with practice.