Engineering Drawing Practice

The work carried out by the draughtsmen in the AMC Drawing Office varied widely, depending on their abilities, levels of experience and knowledge of the firm's products.

All but the most junior, though, would be expected to take on a particular area of development or the investigation of a problem.

In the normal course of events, this would involve the production of an initial scheme, or layout drawing, from which detail drawings would later be produced. Initially, this would be for prototype manufacture but eventually the drawings would be finalised for full production.

Pencil, eraser and shield pic
Clutch Pencils, eraser and shield

All drawings were initially done on tracing paper, with pencil as the usual drafting medium. For schemes, the paper would be cut off from a roll to whatever size was required at the time. For detail drawings, though, each part, or assembly, would be drawn on one of a range of different sized detail sheets.

These were pre-printed tracing sheets ranging from the largest 40" x 30" size to the smallest 15" x 10" size. They all incorporated borders, information blocks and various standard notes. A grid, or zoning system (similar to that on geographical maps), was incorporated into the margins of the largest sheets to help locate particular features of the drawing.

Sample drawing sheet layout pic
Typical example of engineering drawing
(drawing views superimposed on original linen tracing sheet)

A title block was located at the bottom right-hand corner of each drawing sheet, where the firm's name and address were printed. Included in this block was other standard information such as: copyright notice, system of projection used, units of measurement, general tolerances for dimensions and keys to machining and other symbols used on the drawing.

The title block also had spaces provided for the following information to be inserted by the draughtsman: Drawing number (also repeated in the top left-hand corner), title of drawing, scale, dates and signatures of draughtsman, checker and approver.

Other information to be filled in would include: material specification, finish or treatment required.

Another important pre-printed area of each sheet was the Revisions table, located in various places, which was used to record the history of any changes made to the part.

When carrying out a revision on a drawing, the table would be filled in with a reference letter, the date and a short description of the change(s) made.

When possible, drawings of parts and assemblies would be made full size (scale of 1:1) but obviously, with parts ranging in size from a complete bike to a 6BA screw, this was not always possible. In the case of complex castings, like a cylinder head, several drawing sheets would be required in order to record all the various details in a variety of scales.

Drawing projection symbols diagram
Drawing projection symbols diagram

All drawings at AMC sensibly followed the American (or Third Angle) projection standard, meaning that a view on one end of an item would be positioned close to that end on the drawing.

In the English (or First Angle) system, this same view would be positioned across the other side of the part, which on long items like gearbox shafts could be very confusing.

To carry out the actual drawing work, the draughtsman would select the appropriate size drawing sheet and secure it to his drawing board using either spring board clips for larger sizes, or drafting tape for the smaller ones.

The next stage would be the most important of all, and one that would take several years to fully master; the planning stage.

Before a single pencil line was drawn, the draughtsman would spend considerable time calculating the number and positioning of all the views necessary to fully convey the form and size of every detail of the component to be drawn.

On a complicated component, apart from several outside views, there could be any number of cross-sections (full, half, part, revolved), as well as enlarged details and tables for cam profiles.

Not only would the positioning of the views need careful planning, but the expected positioning of all the dimensions that would later be added to the views was also to be allowed for.

Modern CAD drawing pic
Modern CAD generated drawing
(note extra 'freebie' pictorial views)

Nowadays, with most drawing work being done on computers, this important forethought is largely unnecessary as, with a quick flick of a mouse, everything on the screen can be adjusted as many times as needed in order to end up all the views evenly spaced on the final drawing.

pantograph protractor head and rules pic
Protractor head and rules

Each drawing board was equipped with a pantograph drafting machine that incorporated an adjustable protractor head to which two scale rules were attached at right angles to each other.

The horizontal (x-axis) rule was 18" long and the vertical (y-axis) rule measured 12", and the pantograph mechanism allowed the rules to be positioned anywhere over the whole board area, whilst maintaining perfect alignment.

Normally, the rules would be left in the horizontal/vertical position but when, for instance, an auxiliary view was needed to be drawn at an angle from the main views, the protractor head could be adjusted to allow the scales to rotate relative to the pantograph arms.

draughtsman's tools pic
Selection of draughtsman's tools

Each draughtsman was expected to provide himself with the majority of drawing instruments to carry out his job. These would have included spring bow compasses of at least two sizes, for drawing circles and arcs; fixed and adjustable angled set squares; circle, radius and ellipse templates; various rules and other measuring tools.

Limits and Fits chart pic
Limits and Fits chart

All drawing work was carried out following the recommendations laid down in BS 308 'Engineering Drawing Practice', which covered everything from line styles and lettering to geometrical tolerancing. This adherence to standards was necessary to ensure that the design intent was conveyed without ambiguity to anyone reading the drawing, whether in-house or a sub-contract supplier.

Another standard that the draughtsmen had to work to was concerned with dimensional tolerances. In order that mating parts would have the required degree of clearance or interference, it was necessary to specify the range of sizes that certain features had to lie between, to be acceptable.

If this were done on an ad hoc basis, the result would be the need for an ever-increasing number of checking gauges for use in production.

This problem was prevented by the use of a restricted range of 'Limits and Fits', laid down by British Standards, that the draughtsmen could select from as required.

Even up to the time of the firm's closure, metrication had not yet been taken very seriously by the engineering industry in general (the building industry had made a tentative start, but only in terms of saying that a 2" x 4" section of wood was now called 50 x 100 mm, without actually changing anything!)

This avoidance of metrication might be thought of now as quite strange, especially in the automotive fields, as the cylinder capacities of engines had been measured in cubic centimetres since the beginning, and not in cubic inches as used in America.

Drawing numbers were allocated to parts on a simple 'next one from the book' basis, the book being referred to as the part number bible. This meant that part number 200045 might belong to a complete engine, or frame, whilst the next one, 200046, something as different as a toolbox cover screw.

When each set of drawings for a particular new design, or change, were completed, checked and signed off by the office manager, they would be printed off onto normal paper for issuing to the various departments of the factory involved in their production.

The tracing paper originals would then be filed away in numerical order in various plan chests, fitted out with special compartmented drawers to accommodate the different sheet sizes.

This would have been the responsibility of the drawing office clerk, who would also keep an eye on the condition of the most frequently handled drawings, so that either plastic film copies or linen tracings could be made before the originals became too damaged.