Graphic Communication

1. Graphic Communication Tutorial Booklet

2. Dimensions Introduction The system employed is one that is followed by all draughtsman and CAD operators. Using this system allows no misunderstanding of important dimensional details. The same unit of measurement should be used throughout a series of drawings. Stage one — straight line dimensions a) Horizontal sizes are measured from left to right and the distance found is written on top of the dimension line. b) Vertical sizes are measured from bottom to top and the distance found written on the left of the line vertically. c) Angled sizes are shown in a similar manner to horizontal ones, d) Small sizes are shown with the arrows externally situated and the text centred. e) The biggest sizes should always be on the outside of the dimensional grid.

3. Dimensions Stage two — special dimensions a) Sizes from a datum line are set out logically - smallest at the top. b) Angles are measured internally with the in the appropriate place. Notes: Always make your dimensional text neat and easy to read from the bottom right hand corner of the page. If the text looks upside down or is under the line - it is wrong. Do not add the unit measurement except in the drawing details box.

4. Dimensions Introduction This can either use a radius or a diameter. Try and think which one would be more benefit to the next user of your drawing. As an example holes should always be diameters because an engineer will use a drill of the same diameter to machine them. Stage one — diameters a) Diameter dimensions are always written with the measured size after the ‘Ø’ symbol b) Internal dimensioning lines should not conflict with centre lines. c) Pairs of internal dimensioning lines should not cross. d) External dimensions are drawn, whenever possible, outside the main component with thin leader lines. e) Text for external dimensions should be horizontal or parallel to the leader line.

5. Dimensions Stage two — radii a) Dimensions are shown with the size after the ‘R’ symbol b) Internal dimensioning lines should not conflict with centre lines. c) Pairs of internal dimensioning lines should not use the same centre unless it is obvious which is which, d) External dimensions are drawn, if possible, outside the main component with clear leader lines. e) Text for external dimensions should be horizontal when ever possible or if not parallel to the leader line.

6. The Ellipse - Concentric circle method Introduction This system of drawing an ellipse is the more accurate of the two you will see. You willneed to know the sizes of: a) The major axis - the maximum diameter of the ellipse, b) The minor axis - the minimum diameter of the ellipse. Stage one — circles a) Mark the centre lines of the ellipse. b) Draw a circle with the same diameter as the major axis size. c) Draw a circle with the same diameter as the minor axis.

7. The Ellipse • Stage two — construction • Divide the two circles, using a 30/60° set-square, into the same 12 parts. (Additional points be used to build in accuracy.) • From each point on the outer circle project a construction line inwards horizontally. • c) From each point on the inner circle project a construction line outwards vertically. • Where each pair of lines intersect gives one point on the ellipse. • Join the newly constructed points with a smooth curve. • Note: • If the ellipse is to be drawn with the major axis horizontal then the outer construction lines are drawn vertically and the inner ones horizontally.

8. The Ellipse - Trammel method • Introduction • This is a quicker but less accurate method of drawing an ellipse but the advantage it has is that you can use it in real life more easily. • You will need to know the sizes of: • The major axis - the maximum diameter of the ellipse. • The minor axis - the minimum diameter of the ellipse. • Stage one — setting out a trammel • Draw the two centre lines marking 'O’ as the centre. • Measure half the major axis onto each side of the vertical centre line marking each point 'A’. • Measure half the minor axis onto each side of the horizontal centre line marking each point ‘B’ • Take your paper trammel and at one end mark a point ‘O’ • Position point ‘O’on top of the centre spot, line it up with one of the centre lines and mark the position of 'A'. • Repeat, rotating the trammel and mark from 'O‘ to 'B'. • Your trammel is now complete.

9. The Ellipse - Trammel method • Stagetwo — using the trammel • Rotate your trammel so that 'A' on it is on the line 'B - B'. • Rotate further until 4B'is also on the line ‘A - A’ • Mark the position of point 'O' giving one point on the ellipse. • Line up the trammel in a different position and repeat until you have completed your ellipse. • Join all the points with a smooth free-hand curve.

10. Interpenetration - Cylinder intersecting a surface • Introduction • Constructing the curves of interpenetration accurately is important as these are required to complete the junction between two different forms - in this case a cylinder and a flat sheet. • Stage one — preparation • Draw an auxiliary view of the cylinder (circle) directly opposite the plan view. Divide and number the auxiliary view. • Repeat the auxiliary view directly opposite the front view. • Project generator lines onto the front and plan views. • Number both front and plan views accordingly.

11. Interpenetration - Cylinder intersecting a surface • Stage two — completion • In a logical sequence, project individual points from their intersecting position on the front view onto their corresponding generator lines on the plan view. • Join these points with smooth curves in the correct order -remember that some parts of this line of interpenetration are hidden. • Notes: • It makes no difference which one of the auxiliary views is numbered as long as the front and plan views are both numbered to suit. • The line of interpenetration should always be curved when cylinders are being used no matter what shape the intersecting surface is. The drawing is usually completed with a surface development - the majority of the construction work already being complete.

12. Interpenetration - Prism intersecting a surface • Introduction • This system will demonstrate how to join two straight edges forms together. It will give you similar results no matter how many sides the prism has or how complex the surface is. • Stage one — preparation • Draw an auxiliary view of the prism (in this example a hexagon) opposite the plan view and number it. • Repeat the process directly opposite the front view. • Number both the front and plan views accordingly.

13. Interpenetration - Prism intersecting a surface • Stage two — completion • In a logical sequence, project individual points from their intersecting position on the front view onto their corresponding generator lines on the plan view. • Join these points in the correct order with straight lines -remember that parts of this line of interpenetration are hidden. • Notes: • The lines of interpenetration should be straight unless either the surface or the prism surfaces, or both, are curved. • The system demonstrated here works equally well for other prism cross-sections as well as the hexagonal example. • Extra points may need to be constructed to cater for folds in the surface occurring midway across the flats edges of the prism.

14. Interpenetration - Two cylinders intersecting • Introduction • This system is used to find the curves of interpenetration between two pipes so that a waterproof joint, for example, can be constructed. • An off-set example has been used to make the construction clearer. • Stage one — preparation • Study the two views to determine which pipe is going into the other. This is the one you are going to work with and will be classed as the 'minor' one. • Draw an auxiliary view of the minor cylinder (circle) directly opposite the front view. • Repeat the process directly opposite the plan view. • Project generator lines onto the front and plan views. • Number both front and plan views accordingly.

15. Interpenetration - Two cylinders intersecting • Stage two — completion • In a logical sequence, project the individual minor cylinder points, from their intersecting position on the front view onto their corresponding generator lines on the plan view. • Join these points with smooth curves in the correct order -remember that parts of this line of interpenetration are hidden. • Notes: • The surface development of the minor cylinder is straightforward. • The second development needs a new set of generator lines set to a suitable size for the 'major' cylinder.

16. Isometric - Initial steps Introduction For this example you will be shown how to construct an isometric crate ready to hold a more complex form. Always draw the basic crate lightly first as this will save you time later. • Stage one — front and base edges • Make sure you have only the 30/60° set square available. • Mark the bottom position of the drawing. • Project a vertical line and mark the height of the crate onto it. • Project a line, at an angle of 30° to the left, and measure the depth of the crate, • Project a line, at an angle of 30° to the right, and measure the length of the crate. • Stage two — front and side surfaces • a) Project a vertical line from the length marker and a line at 30°, parallel to the base, from the height marker. This provides you the front surface. • b) Project a vertical line from the depth marker and a 30° line, parallel to the base, from the height marker. This provides you the side surface.

17. Isometric - Initial steps • Stage three — top surface • From the top left hand comer project a 30° line up to right, • From the top right hand comer project a 30° line up to left until it crosses the one above completing the isometric crate. • Note: • Always look to see if you can project the heights, instead of transferring them, making the final construction more efficient.

18. Isometric - Circles Introduction This section will show you how to draw a curved surface onto an isometric drawing. The example following will use a circle going onto a isometric cube but the rules are the same for any shape. When dividing the circle there are different methods - using the one shown here will help you with a more complex shape than a circle. • Stage one — marking centre/datum lines • Draw the circle, with centre lines, as an extra view. • Draw the centre lines across the surface of the cube using vertical and 30° lines.

19. Isometric - Circles • Stage two — dividing circle • Divide the horizontal centre line of the circle into even parts. • Mark these onto a piece of scrap paper (trammel) taking care to specifically indicate the centre point, • Transfer these markers onto the 30° centre line in the isometric view using the indicated centre point on the trammel to help you line up the points. • If the shape being transferred is more complex you may wish to number both sets of markers. • Project a vertical line from each marker until the circle is cut. • Project a similar vertical line in the isometric view stopping and starting at the edge of the cube.

20. Isometric – Circles • Stage three — completing the circle • Find the height from the first marker on the extra view from the centre line to the circle. • Transfer this size, measuring vertically from the centre line, to the isometric view giving you one point on the circle. • Repeat for the rest of the markers in the extra view and the circle will be outlined in the isometric view. • Complete the circular surface by lining in the curve. Turning the paper so that your wrist is over the centre of the curve will help you to draw a smooth curve.

21. Isometric – Circle • Stage four — adding depth • From each constructed point on the isometric circle draw a line, to the left, at an angle of 30°. • Set a pair of compasses to the depth of the shape. • Mark, on each of the lines drawn above, the set depth, from the circle measuring to the left. • Always add an extra point at both ends of the second curve to help complete it smoothly. • You will now have a second outlined curve ready for completion. • The curve tops can be connected with a straight line tangent to both curves. • The curve bottoms can be connected in a similar fashion. Note: More complex forms can be drawn in isometric by using the same system but the corresponding heights will be different.

22. Introduction For this example you will be shown how to construct an oblique crate ready to hold a more complex form. Always draw the basic crate lightly first as this will save you time later. Stage one — front surface a) Draw a construction rectangle with the same length and height to the crate required. This will be the front surface of your crate. • Stage two — side and top surfaces • From the bottom right and both the top corners of the rectangle project lines at an angle of 45° to the right. • Find the depth of the required crate and half this size. You should always halve sizes being projected along the 45° lines. • Mark this size along one of the 45° lines. • Construct the rest of the crate using lines parallel to those already on the page. Notes: Circles and complex shapes are always drawn on the front surface when ever possible, or a surface parallel to it, allowing you to draw all the curves using a pair of compasses. Curved shapes drawn on the 45° surfaces are made more complex because you have to half all the construction work and are rarely attempted.

23. Perspective - Measured — two point Introduction Firstly contain the form within a crate. Then construct this crate to hold the more complex form. Always draw the basic crate lightly first as this will save you time later. Be as accurate as you can - any errors will be multiplied greatly. All lines are construction. • Stage one — vanishing points • Project two lines, parallel to the front edges of the plan view and from the spectator point, ‘SP’ onto the picture plain, ‘PP’ • Now project the two from the picture plain vertically until above the ground line, ‘GL’ • Measure the eye level, 'EL', height, from the ground line, onto both vertical lines giving the left and right vanishing points. • Mark each clearly as vanishing points, ‘VP’

24. Perspective - Measured — two point • Stage two — height line • Project one line, parallel to the front edge of the plan view, onto the picture plain. Either edge will do but the one with the lesser angle will give a more accurate answer. Treat the plan view as rectangular even if it is not, • Project a line from the picture plain, vertically until well above the ground line. • Mark this clearly as the height line, ‘HL’ • All heights, which are measured from the given elevations, are transferred onto this line and use the ground line as the base or zero height.

25. Perspective - Measured — two point • Stage three — front surface • Project from the first vanishing point, lines through the top and base heights on the height line, sufficiently far across the page. • Project, from the spectator point through the front edge of the plan view, onto the picture plain. • Project vertically from the picture plain to connect with the two lines from the vanishing point. • This give you the important front edge of the crate. You could mark this line to remind you later. • Project, from the spectator point, the rear comer of the samesurface in the plan view, onto the picture plain. • Project vertically, in a similar fashion as before, to complete thefront surface of the crate.

26. Perspective - Measured — two point • Stage four — second side and top • Project, from the front edge of the constructed view and towards the second vanishing point, lines from the top and base corners of the crate. • Project a line through the third corner of the plan view and onto the picture plain. • Project this line vertically to give the third corner. This gives you the second side of the crate. • Take the two top corners of the constructed view and project them to opposite vanishing points. • This provides you with the top surface.

27. Perspective - Sketched — one point Introduction This style of drawing is usually used to show room interiors. There is no straight forward way of finding some of the sizes so a good eye is required to make your final drawing look correct. • Stage one — the empty room • Draw the wall that you are looking at and divide the horizontal size into three approximately. • Decide on the height of the viewer and mark this height onto one of the two middle lines giving you the single vanishing point. • Generate four lines from the vanishing point - each one going through one of the corners of the room. • Stage two — far away wall • Draw the height of the furniture in proportion to the back wall. • Generate, from the vanishing point, lines from each corner of the furniture, in a similar fashion as before. • Estimate the depth of the furniture and complete the basic crate -always err on the small side.

28. Perspective - Sketched — one point • Stage three — nearer objects • Draw the back of the new part as if it was touching the rear wall. • Project, from the vanishing point, lines extending through each corner of the new part. • Estimate the distance from (he back wall and construct the back of the piece of furniture. • Estimate the depth once more and complete as before.

29. Perspective - Sketched — two point exterior Introduction This style of drawing is often used to show the exteriors of buildings and other large objects. It is a more natural method of sketching and will make your final drawing look correct to your eye. • Stage one — building the crate • Decide, and draw lightly across the page, a horizon line giving you the two vanishing points - one at each end of the line. • Construct the front edge of the crate one third of the way from a vanishing point and at a suitable vertical position on the page. • Project, to each vanishing point in turn, lines from the top and the bottom of this front edge, • Estimate the length of the form along the longer projection lines and draw a vertical line joining the two, • Estimate the depth of the form along the shorter projection lines and draw a vertical line joining the two, • Complete the crate by joining the top left point with the right hand vanishing point and the top right point with the left hand vanishing point.

30. Perspective - Sketched — two point exterior • Stage two — completing the form • Construct as much as you can of the form which is actually touching the outside of the crate. Horizontal lines are projected towards the vanishing points. Vertical lines are still vertical. Angles have to be constructed. • Project, again towards the vanishing points, through the interior of the crate to build up the internal parts of the form.

31. Perspective - Sketched — two point (interior) Introduction This style of drawing is often used to show the building interiors. It is a very natural method of sketching and will make your final drawing look correct to your eye. Sizes have to be estimated. • Stage one — building the room • Decide, and draw lightly across the page, a horizon line giving you the two vanishing points - one at each end of the line. • Construct the rear corner of the room one third of the way from a vanishing point and at a suitable vertical position on the page. • Project, from each vanishing point in turn, lines from the top and the bottom of this rear edge towards the edge of the paper.

32. Perspective - Sketched — two point (interior) • Stage two — adding the furniture • Estimate the height and position of the furniture compared with one of the two constructed walls. • Draw the back of the piece of furniture where it would touch the wall. You may need to imagine it being moved in the room. • Project, from the correct vanishing point, lines from the top and the bottom of this rear surface towards the edge of the paper. • Estimate the depth of the part. • Complete the constructed crate for the part. • Complete the interior and exterior details of the part. Horizontal lines are projected towards the vanishing points. Vertical lines are still vertical. • Angles have to be constructed.

33. Planometric Introduction Planometric drawing are mainly used to show building interiors quickly. The are easily to both construct and to visualise. Always begin by constructing a suitable crate in light lines. • Stage one — floor area • From the starting position draw a line at an angle of 30° to the left and mark the length of the room along it. • From the starting position once more draw a line at an angle of 60° to the right and mark the width of the room along it. • The angle between the two lines making up the front corner should always add up to a right angle. • Complete the rectangle which will have the same dimensions asthe Plan of the room but will be drawn at an angle of 30°. • Stage two — floor units • Construct the plan view of the room with in the floor area starting with the furniture that is actually touching the floor. • From each corner of the furniture project a vertical line upwards. • Set a pair of compasses to the furniture height. • Mark each line and connect them in turn to give form the outline.

34. Planometric – Interior View • Stage three — wall units • Add the positions of the wall units onto the floor plan as above. • Project lines vertically as before. • Mark the heights onto these line. You will need two sizes - the top and the bottom of each unit. • Connect these points in turn to give you the required outline. • Stage four — walls, windows and doors • The height of the walls is now easily constructed. • The doors are projected from their floor plan position with the door height measured vertically, • The windows are constructed in a similar fashion but require two heights - the top and the bottom. • Finally, hard line the correct lines taking care to only draw the surface edges that can be seen.

35. Supplementary Views - Auxiliary Elevation Introduction These are drawn using the same method as any other elevation — the heights are measured from a previously completed elevation and the widths are projected from the plan. • Stage one — preparation • Project construction lines, parallel to the direction of view, from each of the outside corners of the plan into the area where the auxiliary is to be drawn. • Draw, at right angles to these last lines, an appropriate base line. • Stage two — completion • Working from one surface on the plan view at a time:Project, from the plan, each corner of the surface. • Find the height of the corner measured in the elevation from thebase line. • Link with the previous corners logically. • Continue this sequence until all the surfaces in view are complete. Notes: If the form was symmetrical then in stage 1(b) a centre line would be correct used instead of a base line. Hidden detail is rarely asked for in an auxiliary plan and is only used to help the viewer understand the view.

36. Supplementary View – Auxiliary Plan Introduction Auxiliary plans are used to give a different view point from above an object. They are only used to make the understanding of the total drawing easier to understand. • Stage one — preparation • Project construction lines, parallel to the direction of view, from each of the outside corners of the front elevation into the area where the auxiliary is to be drawn. • Draw, at right angles to these last lines, an appropriate base line. • Stage two — completion • Working from one surface on the front view at a time: • Project, from the front, each corner of the surface. • Find the depth, from the base line in the plan, of the corner. Link with the previous corners logically. • Continue this sequence until all the surfaces in view are complete. Notes: If the form was symmetrical then in stage 1 (b) a centre line would be correct used instead of a base line. Hidden detail is rarely asked for in an auxiliary elevation and is only used to help the viewer understand the view.

37. Supplementary Views – True Shape Introduction The true shape is used to show the exact shape of a complex sloped surface. The sizes are taken from two views in a similar manner to that of auxiliary views. • Stage one — preparation • Project construction lines at right angles to the surface in question from the top and the bottom of it in the front elevation into the area where the true shape is to be drawn. • Draw, at right angles to these last lines, a centre line. • Stage two — completion • Project one point on the edge of the surface, at right angles to the surface, into the true shape area. • Find the depth of the point, from the centre line on the plan. • Mark this size from the centre line in the true shape parallel tothe line in (a). • Link with the previous point constructed in the true shape. • Repeat until all the points have been projected and the true shape is complete. Notes: If the form was not symmetrical then in stage 1 (b) a base line would be correct used instead of a centre line. Only the single surface of the true shape is shown - no hidden details or any other surface are shown.

38. Surface Developments – The cone Introduction A blank version of the cone's development should always be drawn before any additional sections are removed. • Stage one — preparation • Split your base circle into equal parts using the 30/60° set-square and number each sector logically. • Number the apex as a zero. • Set your compasses to the length of one edge of the cone. • Find, and mark with a zero, the apex of the surface development. • Draw a large arc long enough to hold the complete development. • Project a line from the apex to the arc at one end of the arc. This is the starting position and should be numbered to suit.

39. Surface Developments – The cone • Stage two — completion • Set your compasses to the size of one section on the circle. • Mark this size, along the arc from the start and number. • Repeat this until your number is the same as you started with. • Project construction lines to the apex from each mark on the arc. • The outline of the cone surface development is now complete. • You are now ready to subtract any of the extra parts. Notes: Allfinished lines should be curved, because the base is round, even if a straight edged part intersects the cone. All internal, non-edge, lines should be construction weight lines.

40. Surface Developments – The cylinder Introduction The majority of the work should already be complete when you come to the surface development. Using the same numbering system will help speed things up. The blank development is always drawn first. • Stage one — blank development • Project a single horizontal base line. • Mark a starting position at one end and mark it with the first number from the circle - this is the join in the development. • Set a pair of compasses to the size, on the outside of the circle, between two of the equally spaced generator markers. • Mark this size, from the starting point, along the base line and number logically one more than before. • Repeat this stage until you arrive back at the beginning number. • Project vertical lines to the maximum height of the cylinder.

41. Surface Developments – The cylinder • Stage two — removing parts • In a logical sequence project the individual heights of each generator and mark on the corresponding development line. • Join these points with a smooth curve and complete the development outline. Notes: Always look to see if you can project the heights, instead of transferring them, making the final construction more efficient. An Engineer will start the development from the smallest cross-section of material - in this case point *9S - to minimise any seam-

42. Surface Developments – The Right Prism Introduction Prepare the development without any sections removed first as the final stages will become much easier. • Stage one — blank development • Project a horizontal line from the base of the original elevation. • Mark and number the starting point. • Measure and mark the length of the first side and number to suit. • Continue this process until the all sides have been completed and the number inserted is the same as the first one. • Project vertical construction lines to the height of the prism from each of the numbered markers. • Complete the blank development.

43. Surface Developments – The Right Prism • Stage two — removing parts • Using a logical sequence find the heights from the original view and mark onto the corresponding construction lines in the development - project them along if possible. • Join development outline with suitable finished lines. • Complete development by finishing fold lines. Note: Always look to see if you can project the heights, instead of transferring them, making the final construction more efficient.

44. Surface Developments – Right pyramid Introduction The system you are about to follow is the same no matter how many comers the pyramid - three, four, six or eight. • Stage one — preparation • Number each base corner of the plan view, • Number the apex of the pyramid with a zero. • Construct the true length ‘TL’ line. • Set your compasses to the length of the true length line. • Mark the apex point of the surface development.

45. Surface Developments – Right pyramid • Stage two — completion • Draw a large arc long enough for the surface development. • Project a line from the apex to one end of the arc. • Set compasses to the length of the first side of the pyramid. • Mark this size from the first point on the arc along the curve. This is the second corner. • Repeat this stage until you have returned to your first number. • Join each point on the arc, in turn, with a straight line. • Join each corner with the apex of the surface development. Note: All additional heights to be transferred onto the surface development must be projected horizontally onto the true length line first.

46. Tangency - Inside curves Introduction This method will demonstrate how to join two circles with an inside arc so that they meet smoothly. The sizes of the three curves must be known. Accuracy is important and two compasses are helpful. A mix of this and the next system will solve complex curves. • Stage one — preparation • Mark the centre lines and draw the two known circles lightly. • Mark one centre 'A' and the second centre ‘B’ • The centre for the arc will be called ‘0’ • Stage two — finding the centre 'O' • Add the radius of the circle 'A' to the radius of the arc. • Set your compasses to this size and draw an arc, from 'A’ in the general position you expect point *0* to be. • Add the radius of the circle 'B' to the radius of the arc. • Set your compasses to this size and draw an arc, from 'B’ until it makes a cross with the first one. • This is the centre ‘O’ • Stage three — completion • Project construction lines from *0* to the other two centres. • Where these lines cross the circles will show you where each arc will start and finish. • Reset your compasses to the radius of the three curves and complete the shape.

47. Tangency - Outside curves Introduction This method will demonstrate how to join two circles with an outside arc so that they meet smoothly. The sizes of the three curves must be known. Accuracy is important and two compasses are helpful. A mixture of this and the previous one will solve complex curves. • Stage one — preparation • Mark the centre lines and draw the two known circles lightly. • Mark one centre 'A’ and the second centre ‘B’. • The centre for the arc will be called ‘0’. • Stage two — finding the centre '0’ • Subtract the radius of the circle 'A' from the radius of the arc. • Set your compasses to this size and draw an arc, from 'A’ in the general position you expect point '0' to be. • Subtract the radius of the circle 'B' from the radius of the arc. • Set your compasses to this size and draw an arc, from 'B’ until it makes a cross with the first one • This is the centre ‘0’. • Stage three — completion • Project construction lines from ‘0' to the other two centres. • Where these lines cross the circles will show you where each arc will start and finish. • Reset your compasses to the radius of the three curves and complete the shape.

48. Tolerancing - Basic terms • Introduction • Tolerances give you the minimum and maximum sizes of any machined object. • This information allows you to make absolutely sure that: • if two parts must move when connected together then they will move and not be too tight. • if one part has to be pressed into another that it will not be too loose and fall out, or too tight to fit in the first place. • The nominal size is the one shown as standard. • If there is only one tolerance number present then it is used in both calculations. • Stage one — external tolerances • To find the minimum size of a shaft find the nominal diameter • of the part and subtract the lower of the tolerance numbers. • (Example: 100 – 0.1 = 99.9) • To find the maximum size of a machined shaft find the • nominal size and add the higher of the two tolerance • numbers. • (Example: 100 + 0.2 = 100.2) Minimum diameter = 100 – 0.1 = 99.9 Maximum diameter =100 + 0.2 = 100.2 • Stage two — internal tolerances • To find the minimum diameter of a drilled hole find the nominal diameter and subtract the lower of the tolerance numbers. • (Example: 75 – 0.1 = 74.9) • To find the maximum diameter of a drilled hole find the nominal diameter and add the higher of the two tolerance numbers. • (Example: 75 + 0.05 = 75.05 ) Minimum diameter = 75 – 0.1 = 74.9 Maximum diameter =75 + 0.05 = 75.05

49. True Length – Basic Construction Introduction A true length is required when constructing the surface development of a right pyramid- The system demonstrated is the same no matter how many corners the base of the pyramid has. • Stage one — projecting from the plan • Set a pair of compasses - point on the centre apex of the pyramid, pencil on the base corner. • Rotate until the compass pencil cuts thehorizontal centre line. • Mark this point onto the centre line. • Stage two — finding the true length • Project this point vertically downwards to the ground line of the front elevation. • Mark this point carefully. • Join this point to the apex on the elevation using a line style which makes it obvious. • Mark this line clearly as the true length line, ‘TL’ Notes: The true length is an important datum line so be extra careful with your accuracy. More complex forms will need more than one true length line so label each one carefully and logically.

50. Glossary of terms