Grade 10 power mechanics Related Drawing – Blending of Lines and Curves Notes

1. Identify and outline sub-sub-strands:
   • Description of blending of lines and arcs
   • Construction: arcs blending two lines
   • Construction: arcs blending two circles
   • Use of blending to construct shapes
   • Application of blending in power mechanics
2. Describe blending of lines and arcs as used in drawing
3. Construct arcs that blend lines (practical steps)
4. Construct arcs that blend two circles
5. Use blending to construct given shapes (fillets, rounds)
6. Appreciate application of blending in power mechanics (stress relief, smooth motion)

1. What is blending?

Blending means joining two straight lines or curves with a smooth arc so that the change of direction is gradual (tangent continuity). In technical drawing this is often called a fillet (internal corner) or a round (external corner). Blending removes sharp corners, giving a neat, safe and mechanically better shape.

2. Why blending matters in power mechanics

• Reduces stress concentration at corners (less chance of cracks).
• Improves flow of belts, cams, shafts and rotating parts.
• Makes parts easier to manufacture (tool paths are smoother).
• Improves appearance and safety (no sharp edges).

3. Tools and materials

Drawing instruments: T‑square, triangle/set-squares, ruler, compass, divider, pencil HB/2H, eraser, protractor or angle measure. Also practise on tracing paper for overlays.

4. Simple visual examples (small diagrams)

5. How to construct an arc that blends two straight lines (fillet) — step by step

Goal: Draw an arc of given radius r that is tangent to two intersecting lines.

1. Draw the two lines and mark their intersection point P.
2. Decide the fillet radius r (use scale, e.g. 6 mm).
3. Using a ruler and set-square, draw two lines parallel to each original line at distance r toward the corner (offset lines).
4. The intersection point O of these two offset lines is the centre of the fillet arc.
5. With compass set to radius r, draw an arc with centre O between the two tangency points where the arc meets the original lines.
6. Erase the parts of the original lines inside the arc to show the smooth blend.

Notes: If lines are nearly parallel, the offset lines may not meet — choose a radius small enough or extend offset lines to find O.

6. How to construct an arc that blends two circles

Goal: Join two circles with a smooth arc (circle tangent to both).

1. Draw the two given circles with centres C1 and C2 and radii R1 and R2.
2. Decide the fillet (blending) radius r required.
3. Create two new circles:
   • If a fillet circle must be outside both circles, draw circle A at C1 with radius R1 + r and circle B at C2 with radius R2 + r.
   • If the fillet must be inside one circle, use R ± r accordingly (choose plus/minus depending on external/internal tangent).
4. Find intersection points of circle A and circle B (their centres line will often give possible centres for fillet). Each intersection is a possible centre O for the fillet circle.
5. With compass set to r, draw the fillet circle at the chosen centre O. Use the arc between tangency points only (erase overlap as required).

Tip: When you add r to each radius you are finding the locus of centres where a circle of radius r would be tangent to the original circles.

7. Use of blending to construct shapes — examples

• Round the corners of a rectangular bracket — replace sharp corners with fillets of radius r.
• Design a pulley groove profile — blend straight flanks to circular crowns for smooth belt contact.
• Create smooth cam followers — blend arcs between profile sections so the follower moves without shock.
• Combine lines and circles to create aesthetic machine covers and guards.

Steps are the same: offset, find centres, draw arcs, tidy up.

8. Classroom and workshop learning experiences (Suggested)

1. Teacher demonstration: Live drawing at the board — construct a fillet between two lines (students follow on worksheets).
2. Hands-on practice: Each student draws fillets of varying radii between given lines and circles using compass and set-square.
3. Group task: Given a simple machine part drawing (bracket, pulley), students work in groups to add fillets where needed and explain why those radii were chosen.
4. Workshop visit: Inspect pulleys, shafts and welded joints; identify places where blending was used and discuss function (stress distribution, safety).
5. Practical assignment: Produce a neat drawing of a plate with four filleted corners and dimension the radii; include instrumented steps (photos or scanned work).
6. Short quiz: Define blending, list instruments, draw step-by-step fillet construction on test paper.

9. Short exercises (practice)

1. Exercise 1: On given intersecting lines (angle about 60°) draw a fillet of radius 8 mm.
2. Exercise 2: Given two circles (R1 = 20 mm, R2 = 12 mm) and centres 60 mm apart, construct a blending arc of radius 6 mm that is externally tangent to both.
3. Exercise 3: Draw a rectangular plate 100 × 60 mm and add rounded corners of radius 10 mm. Dimension the drawing.

10. Assessment and reflection

Short practical test: Students must construct one fillet between lines and one fillet between two circles within time limit. Marking points: correct use of instruments, correct radius, tangency visible, neat erasing of overlaps.

Reflection question for learners: Explain in one paragraph how blending reduces failure in rotating machine parts.

11. Summary mapped to the SLOs

• (a) Sub-sub-strands listed at start — identification done.
• (b) Blending described: smooth tangent arcs that join lines/curves.
• (c) Construct arcs blending lines: use parallel offsets, centre at intersection, draw arc at radius r.
• (d) Construct arcs blending circles: use expanded/shrunken circles (R ± r) to locate centre, draw fillet of radius r.
• (e) Use blending to make shapes: rounds on brackets, pulleys, cams — steps and examples provided.
• (f) Application: stress relief, smooth motion, manufacturability and safety in power mechanics.

• Start with large radii for beginners — easier to find offsets and centres visually.
• Use tracing paper overlays to test fillet radii before committing to the final drawing.
• Relate exercises to workshop parts learners know (pulleys, shaft shoulders) to increase interest.