Drawing Instruments

Today, engineering drawings are very seldom constructed by hand.  CAD technology is the universal choice in industry.  However, it is useful to include a discussion of the instruments that were historically used for this purpose.  Manual drafting is an appropriate way teach the fundamentals of engineering graphics.  The most important concepts of engineering graphics are applicable whether they are taught with manual instruments or by CAD. Manual drafting can instill in students a sense of care and professionalism that is required in producing engineering drawings.  Each and every line, letter and number must be precise and accurate.  A small error on a drawing can produce expensive and disastrous results.  Learning to draw with instruments is analogous to learning math without a calculator.  You wouldn’t want to do it as a professional, but you need to understand it is that what is being automated by the computer.

Pencils & Lead

All pencils are not the same.  You cannot produce quality engineering drawings with ordinary pencils and lead.  Pencil lead is made from graphite and is available in various hardness grades.   The hardness of the lead is controlled by changing the clay content in the graphite.   Technical lead hardness is specified using the H-B (hardness/blackness) scale.  9H is the hardest, and 7B is the softest. Engineering drawings can be constructed with two or three hardness grades (4H, F and B).

Pencils and pencil leads are available in a variety of hardness levels.

A hard pencil is used to lay out light lines that are not intended to be seen by the reader.  These are called construction lines.  Construction lines are used to precisely position various features on the print.  When using a hard lead, keep in mind that the goal is an extremely light line.  Use only very light pressure.  A good pencil grade for construction lines is 4H.

Generally, a drawing requires two line weights: thin and thick.  For these two a grade F (thin) and a grade B (thick) are used.  These leads would ideally be held in a type of pencil known as a  lead holder.

Lead holders such as this were used in manual drafting.

Leadholders make mechanical drawing more productive.  By rolling the pencil, they can be used for extended periods of time without sharpening.

Lead holders were the tool of choice for people who had to do a lot of mechanical drawing.  They hold a large diameter (usually 2mm)  piece of lead and can be used for extended periods of time without sharpening.   A skilled draftsperson would roll the pencil as they drew to keep the point sharp.

For lettering, where the pencil rolling technique cannot be employed,  a mechanical pencil is used.   Either hardness grade F or HB lead is adequate  for lettering.  The easily obtained lead diameter of .5  is best for lettering.

Paper

High quality paper used to be essential to producing professional engineering drawings.  Today, engineering drawing are usually just printed on bond paper (if they are printed at all).  Bond paper is the type of paper you are probably most familiar with.  It is commonly used as “printer paper.” Some companies have gone to a “paperless” policy, only allowing access to electronic documents. This prevents accidentally using a print that is out of date.

Historically, drafting paper had to be transparent because reproduction methods worked by shining light through the drawing.  Today this is less important.  Two types of paper that were used were vellum and Mylar®.  Vellum is a vegetable-based thin tracing paper, and Mylar® is brand name of a polyester film.

In the United States, paper is sizes are indicated by a letter designation.  These sizes are standardized by ANSI (American National Standards Institute).   Size “A” or “A size” paper is 8.5 by 11.0 inches.  All of the other sizes can be folded town to this approximate dimension.

ANSI standard paper sizes. Architects’ paper sizes are slightly larger.

Drawings should be folded in such a way that they can easily be read without having to unfold the entire sheet.  Below is an illustration of the customary folding method for D size paper.  Other paper sizes are folded in a similar manner.

Paper is conventionally folded so it can be read a section at a time without completely unfolding it.

Drawing Boards and T-Squares

While some drawing can be done on an ordinary desk, a drawing board is really a better choice.  A simple piece of hardwood plywood can be used for student work, and the addition of a quality board cover will make the board perform extremely well.

A simple piece of plywood with a clean edge and a quality board cover can make an excellent drawing board.

If the board is to be used with a T-square it will need to have a clean, smooth edge on the left and right sides.   A T-square provides a horizontal reference that all lines will be constructed from.  The head of the T-square slides on the edge of the board and holds the blade perpendicular to the edge.  Be sure to choose a T-square with transparent edges as this makes it much easier to use than the solid kind.

A T-square provides a horizontal reference for all drawing geometry.

A more expensive but easier to use alternative to a T-square is a parallel straight edge.  This tool is permanently installed on a drawing board but serves the same function as the T-square.  In the days when professionals drafted by hand a drafting machine was the best choice.  Since CAD has replaced manual drafting, these machines are too expensive to justify for student work.

Compasses

Compasses are used to draw medium to large arcs and circles.  Small circles are better drawn with circle templates (discussed later).  Mathematically, compasses can be thought of as instruments to lay out a set of points that are a particular distance (a radius) away from a given point.  This means that compasses useful for laying out distances from known points to find unknown points.  Examples of this use are described in the chapter on geometric construction.

The most accurate way to set a compass is to mark the dimension of the radius with a light construction line and set the compass to the mark.

A quality compass is required to do precision work.  When performing geometric constructions, it is essential that the compass remain set to an exact distance, so a compass with a positive locking feature is often helpful.  As a compass pierces the paper when used, it is important to use them on a drawing surface that is designed for this purpose such as linoleum or hardwood.  Be careful not to use them on surfaces that could be damaged by the point.

For a compass to make a crisp line it should be sharpened as shown in the illustration below.  A piece of sandpaper is used to grind the lead to a taper.   Sandpaper pads  are made for this purpose.  To avoid smudges,  the lead and soiled sandpaper must be kept away from drawing board and other instruments.  Wipe the excess graphite from the compass with a tissue before drawing with it.

Sand the compass lead flat on one side.
The resulting sharp point should look like this.

It is good practice to align the point and lead ends of the compass so they are even.  This will allow you to use the compass without too much difficulty.  The lead and point are adjusted by loosening the knurled wheels on the side of the compass.  The point and lead should extend about 3/8 of an inch beyond the instrument.

The point and lead should extend about 3/8 of an inch beyond the instrument.

To use the compass, set the compass to the approximate radius and press the point end into your paper at the previously marked center point.  Adjust the screw until the lead is properly placed.  It is difficult, if not impossible to set the compass before placing it in the drawing position.  Lean the compass slightly into the direction of rotation.  If a thicker line is desired, slightly open or close the compass and repeat the circle.

Many compasses come packaged with an alternate tip that can replace the pencil lead.  This allows the compass to function as a set of dividers.

Dividers

Dividers are used to mark off a series of equal distances.  They look similar to compasses except they do not have a drawing point.  Instead they are equipped with two sharp needle points.  Since compasses are available with extra point, they can be used as dividers.  Unless students will be using dividers frequently, there is no need for to purchase a separate set of dividers.

Dividers are similar to compasses but have two sharp points and no drawing lead.

Triangles

Triangles are essential tools for manual work.  Two triangles are required: a 30-60-90° and a 45-45-90°.  A small size (4 to 6 inches) is convenient for students. The two triangles can be used with the T-square to produce vertical lines or lines at 30°, 45°,  or 60° to horizontal.

Triangles are used with the T-square and with other to make vertical and angled lines.

The triangle can be used together to produce angles at 15° increments.

Triangles can be used together to make angles in any 15 degree increment.

Triangles can also be used to draw parallel lines, but this will be discussed later.

Scales

Because engineering drawings are not always drawn full size, ordinary rulers are not usually used for manual drafting.  Instead, instruments called scales are employed. Scales are precision instruments with fine graduations or marks.  They are designed to let the drafter draw scale drawings without having to convert from full scale to a different scale.

If a large part needs to be documented on a small piece of paper it can be scaled down by an appropriate amount.  For example, on a half-scale drawing every inch on the drawing represents two inches on the actual part.  When drawing very large objects, such as houses or other structures, a scale such as 1/4 = 1′-0 (a quarter inch equals one foot).  Scales are available for a variety of drawing scales.  These are dealt with in more detail in a later chapter.

Scales are used to make measurements only. They are never used as a straight edge.

Scales are not to be used as straight-edges.  They are strictly for making measurements.  To use the scale, draw a construction line longer than the length required.  Lay the scale against the line and make a small tick mark from the line directly perpendicular to the scale.  Since it requires such high precision, laying out with scales is done with a very hard, sharp pencil.

Erasers

The best eraser to use when learning engineering graphics is a white vinyl eraser.  These are available in block form or in a convenient pen package.  Some (but not very many) mechanical pencils come with a quality vinyl eraser.  Be sure to check the quality of the eraser.  You will probably need to purchase an eraser separate from your mechanical pencil.

When drafting was done manually, electric erasers were employed to speed the process.  There is little reason to purchase one of these as a student.

Protractor

A drafting tool many students already have is a simple plastic protractor.  Protractors are instruments that measure angles.  Students will need a protractor that can measure at least to the nearest degree.  Triangles should be used to lay out lines in 15° increments; otherwise, a protractor can be used.  A discussion on angle measurement is included in a later chapter.

Even student protractors should be able to read to the nearest degree.

Miscellaneous Drawing Instruments

If you have purchased a drawing instrument kit, it may have some of the following items that will make mechanical drawing easier and more efficient.

Dry Cleaning Pad

A dry cleaning pad is a mesh bag filled with gum eraser powder.  As a drawing is being constructed, smudges from will tend to form on the paper.  A dry cleaning bag can be squeezed over the surface of the drawing and dabbed gently to remove the smudges.

Drafting Brush

Pencil drawing are especially susceptible to smudging.  So when an eraser or dry cleaning pad is used you must never use your hand do wipe the drawing.  Instead, a drafting brush is used.

Eraser Shield

It can sometimes be difficult to erase a line that crosses another line.  This is one of the many uses for an eraser shield.  Eraser shields are made from thin sheet metal and are perforated with various shapes.  The eraser shield is laid on top of the line to erase, shielding the neighboring lines that are to remain on the drawing.

A series of holes across most eraser shields is a convenient way to make hidden (dashed) lines.  The line of  holes is aligned with the line to be dashed and the spaces are erased away.  This is a good way to make consistently spaced hidden lines.

Eraser shields are a convenient tool for making dashed lines.

French Curve

A French curve is designed to create curves that cannot be constructed with a compass.   To use a French curve, plot points that are known to lie on the curve and fit the instrument to the points.

French curves are useful for making curves whose radius changes.

French curves are available in a wide variety of shapes and sizes.  They typically only fit a short section of the curve.

French curves are available in a wide variety of shapes and sizes.

Flexible rules are available for the same purpose, but they do not produce as good results as French curves.

Drafting Tape

Drafting tape is similar to masking tape, except that it has a more easily removable adhesive backing.  Blue painters’ tape can be used in place of drafting tape for student or professional work.  It is less expensive and is easier to find.

Circle Template

Making high quality circles (especially small ones) is difficult with a compass.  A circle template allows circles to be constructed with lead holders and mechanical pencils.  A circle template is a plastic tool that has been perforated with round holes of various diameters.  The holes are oversize by a small amount to accommodate the pencil width.  Even large circles are better drawing with circle template if the correct hole diameter is available.