Question 1 of 3: Orthographic views, section, CAD sequences and manufacture of a slotted step block

Question 1: Orthographic views, section, CAD sequences and manufacture of a slotted step block (40 marks)

Question text not reproduced: the examination questions are © Engineers and Geoscientists BC. Open the official past paper (linked at the top of this page) to read the question, then follow the worked solution below.

Reading the pictorial. The part is one solid block with three features.

• Base. A rectangular block about 100 wide × 150 deep × 45 high.
• Upper step. A step about 30 high covers only the back of the base. Its front face is a vertical plane that runs diagonally across the block in plan, from about one-third of the way back on the left side to near the front on the right side. In front of that diagonal face, the top of the base is exposed as a four-sided lower step.
• Slot. A rectangular slot (not a V) about 32 wide and 15 deep runs from front to back through the upper step. It leaves two islands of equal height: a 36-wide island on the left and a 32-wide island on the right. Each island has one vertical hole of about Ø20, near the back.

The holes are taken as through-holes. A plain drilled hole of this kind is normally made through, and the pictorial shows nothing that would limit its depth.

a) Third-angle orthographic views

View selection. The front view looks at the lit face in the lower right of the pictorial, because that face shows the slot notch and both step heights in true shape. The top view sits above it and is the only view that shows the diagonal step face as an edge and the two holes as true circles. The side view is the left side view, placed to the left of the front view as third-angle projection requires. That side is chosen because the diagonal face and the slot walls are visible from the left. From the right they would all be hidden behind the right-hand island, so a right-side view would be mostly dashed lines. Choosing the view with the fewest hidden lines is the standard rule for selecting views.

1. Front. The outline is a 100 × 75 rectangle with the 32 × 15 slot notch cut into its top edge. A visible horizontal line at height 45 marks the edge where the exposed lower-step top meets the diagonal face. Each hole shows as a pair of hidden parallel lines running the full 75 height, with a centreline between them. A vertical hole is seen from the side in this view, so it can never appear as a circle here.
2. Top. The outline is a 100 × 150 rectangle. The diagonal step face is a single visible line, and the two slot walls are visible lines running from the back edge to the diagonal. The holes are true circles with centre marks. There are no hidden lines, because the holes go right through.
3. Left side. This view shows the stepped silhouette: the full 75 height at the back, and the base alone (45 high) at the front. Vertical visible lines mark where the diagonal face meets the left side, the left island's slot corner and the right island's slot corner. A short visible line at height 60 is the front edge of the slot floor. The rest of the slot floor, behind the left island, is a hidden line. The two holes coincide in this view and show as one pair of hidden lines.

b) Section view

Pass a full-section cutting plane A–A through both hole centres, parallel to the front view, and look toward the back. Section A–A replaces the front view. It shows the two bores cut open along their full length, the thin walls between each bore and the slot or the outer side, and the slot profile, none of which the unsectioned front view shows clearly. All material the plane cuts gets 45° section lining. The bores stay white. Behind the plane the cross-section does not change, so no visible lines are added beyond the cut. The cutting-plane line in the top view has its arrows pointing in the viewing direction.

c) Two feature-based modelling sequences

1. Sequence A — build up, then cut. (1) Sketch a 100 × 150 rectangle on the base plane and extrude it 45 to make the base. (2) On the top face of the base, sketch the four-sided footprint behind the diagonal and extrude it 30 to make the upper step. (3) On the back face, sketch the 32 × 15 slot rectangle and cut-extrude it “through all” toward the front. (4) Add two Ø20 holes on the island tops, cut through all.
2. Sequence B — constant profile, then cut. (1) On the front plane, sketch the notched front-view outline (100 × 75 with the slot notch) and extrude it 150 deep. This gives a notched bar with the slot already along its full length. (2) On the top face, sketch the region in front of the diagonal and cut-extrude it down 30. This removes both the step and the front part of the slot in one operation. (3) Add two Ø20 through-holes.

Both feature trees produce the same solid. Sequence A follows how a designer thinks about the part, one feature at a time. Sequence B follows how the part could be made from a bar of constant cross-section, which is the basis of the extrusion route in (d).

d) Manufacturing methods and issues

Methods.

• Machining (one-off or low volume). Start from a 100 × 75 rectangular bar sawn to 150 long and machine it on a CNC vertical mill. The slot runs right through, so it can be cut with an end mill, or in one pass with a side-and-face cutter on a horizontal mill. The diagonal step face is a vertical wall, which is simple 2½-axis contouring with the side of an end mill. On a manual mill, swivel the vise to the diagonal's angle, about 39° from the front edge (tan−1(80/100)), and mill the face straight. Finish by spot-drilling, drilling and, if a fit is needed, reaming the two holes.
• Medium or high volume. The profile in Sequence B is constant along the part's length, so an aluminium extrusion of the notched profile can be cut to length, with only the diagonal cut and the holes machined. This removes far less material than machining from solid.
• Casting. Sand or investment casting followed by finish machining also works. With the parting line at the top face, the slot and all vertical walls draw straight out of the mould, so no cores are needed.

Issues.

• Thin walls. The walls between each bore and the slot or the outer face are only about 6–8 mm thick. Drill wander or clamping force can distort them or break through. Drill undersize and ream, and do not clamp across the islands.
• Burrs and an acute edge. Burrs form where each drill breaks out at the bottom face and where the slot meets the diagonal face. The diagonal meets the right-hand side at an acute angle of about 51°, which leaves a sharp edge that must be broken.
• Removed volume and distortion. Machining from solid removes about a quarter of the bar (about 24 % by volume). Rough-machine first, then finish-machine, so that residual stress released by roughing does not warp the final dimensions.
• Setups. The part needs at least two setups (top and back). The hole positions must be held to the same datum as the slot, either by locating the part from one machined face and one edge or by doing everything in one fixture.
• Casting route. Vertical walls need draft, which conflicts with the square walls drawn, so functional faces need machining allowance. Cored holes this small are not practical and should be drilled.