Question 1 of 5: Orthographic views, dimensioning, CAD sequence and manufacturing for a ramped-slot base bracket

Question 1: Orthographic views, dimensioning, CAD sequence and manufacturing for a ramped-slot base bracket (50 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.

Given. The pictorial (reproduced below) shows a base bracket. An obround (stadium-shaped) base plate has a vertical through-hole concentric with each rounded end. A rectangular block stands on the middle of the base, flush with the base’s front and back faces. A slot runs front-to-back through the block between two thin side walls. Its floor is an inclined plane (a ramp): the ramp starts at the front face a little below the base top and rises at about 45° to meet the top of the block roughly four-fifths of the way back, leaving a solid rear wall. The diagonal line in the pictorial is the edge where the ramp meets the near side wall of the slot. Proportions were read along the pictorial’s isometric axes (block about as deep as it is long; base thickness about 0.28, block height above the base about 0.7, each wall about 0.2 and the slot about 0.6 of the block length; hole diameter about a third of the end width). They are used only to draw the views to a consistent scale; every size is "xx" in part b).

a) Orthographic views (third-angle)

Approach. Choose as FRONT the face that shows the slot opening, because it shows the walls, the slot width and the ramp’s front edge in true size. Project TOP above FRONT and RIGHT SIDE to the right of FRONT (third-angle glass box). TOP and FRONT share widths, FRONT and RIGHT SIDE share heights, and TOP and RIGHT SIDE share depths. The RIGHT SIDE view is needed because it is the only view in which the ramp appears as an edge, so its slope can be seen.

Front view. The base is a long rectangle: the rounded ends appear as straight vertical ends, and the tangent seams are not drawn. The block rises in the middle. The slot shows as an open rectangle between the two walls down to the ramp’s front edge, a visible horizontal line that sits below the base-top line because the ramp is cut into the base. Each bolt hole appears as a pair of hidden lines through the base thickness, with a centre line; it is never drawn as a hidden circle. Top view. This shows the true obround outline and both holes as true circles with centre marks. The block outline runs across the full depth of the base. The ramp faces upward, so it is visible: the tops of the two slot walls run back from the front edge, and the ramp’s top edge closes the slot opening about four-fifths of the way back. Right-side view. The base and the block appear as two stacked rectangles. The ramp is seen edge-on as a single 45° line from the ramp’s front edge up to the block top. It is hidden (dashed) because the right-hand wall is in front of it. Both holes project onto the same pair of hidden lines.

b) Fully dimensioned sketch

Dimensioning follows ASME Y14.5 / CSA B78.1 practice. Each size or location is given once, in the view where the feature shows its true shape, and dimensions are placed outside the views wherever practical. No dimension goes to a hidden line. Every value is "xx", as the question instructs. TOP: overall length; hole centre-to-centre spacing; block length, placed symmetrically about the part’s centre line; overall depth; depth from the front face to the ramp’s top edge; "2X R xx" for the rounded ends; and "2X Øxx THRU" for the holes. Because the holes are concentric with the end arcs, the radius and the hole spacing locate both features. FRONT: base thickness; overall height; the wall thicknesses and slot width, chained across the top; and the height of the ramp’s front edge above the bottom. The front-edge height and the top-edge depth together fix the ramp plane, so no angle is needed. That avoids giving a redundant 45° that could conflict with the two sizes.

c) Feature-based solid-modelling sequence

A parametric CAD sequence that arrives at this solid, each step a named feature in the model tree:

1. Base (Extrude). On the TOP datum plane, sketch the obround: two semicircular arcs joined by two tangent lines, with both arc centres on a horizontal construction line and the part centred on the origin. Extrude it to the base thickness.
2. Block (Extrude, merge). On the base’s top face, sketch a rectangle that is symmetric about the vertical mid-plane and coincident with the base’s front and back edges. Extrude it to the block height so it merges with the base.
3. Ramped slot (Cut-Extrude, Mid-Plane). On the side (Right) mid-plane, sketch the closed triangle that lies above the ramp line. Its lower vertex is on the front face at the ramp-foot height, its upper vertex is on the block top at the ramp depth, and its third side runs along the top and front. Cut-extrude it Mid-Plane to a depth equal to the slot width. One feature therefore removes the slot, cuts the ramp into the base, and leaves the two walls and the rear wall. Dimensioning the sketch with the ramp-foot height and the ramp depth keeps the same design intent as part b).
4. Holes (Hole feature). Add a Hole or Through-All cut at each end, constrained concentric to the end arcs so the holes follow any later change to the base length. A Hole Wizard feature records the fastener clearance size.
5. Finishing features (only if the manufacturing route needs them). Add fillets on the wall-to-base and ramp-to-wall internal edges and draft on the vertical faces for a casting. Put them last in the tree so they can be suppressed for a machined version.

d) Manufacturing methods and issues

Machining from solid (one-off or small batch). The part is prismatic, so it is naturally CNC-milled from aluminium or steel plate or bar. The sequence is: face the bottom as the primary datum; contour-mill the obround profile; drill (and ream, if the holes locate a mating part) the two holes; then mill the slot. Issues: (i) The ramp is inclined, so a 3-axis mill needs the part tilted 45° on an angle plate, sine plate or tilting vise. Otherwise the ramp must be surfaced with a ball-nose cutter, which is slow and leaves scallops, or cut on a 4/5-axis machine. (ii) An end mill leaves its own corner radius where the ramp meets the side walls, and where the slot floor would meet a sharp internal corner. The drawing should allow a radius there, or a square-shoulder cutter is needed. (iii) The thin walls either side of a wide slot can deflect and chatter as the slot is cleared, so light finishing passes are needed. (iv) Several set-ups (top, ramp, holes) risk tolerance stack-up. Locate every set-up from the same datums, the base bottom and a side face, or pin through the finished holes. Casting (larger volumes). A sand or investment casting, or an aluminium die-casting, can be machined afterwards only on the base bottom and the holes. The slot and ramp open upward, so with the parting plane at the base bottom the pattern draws straight up with no side core, provided the walls are given 1–3° of draft. Issues: the thick base-to-block junction cools more slowly than the thin walls and risks shrinkage porosity; internal corners need fillets; and machining allowance must be left on the functional faces, since cast holes cannot hold a bolt-fit tolerance. Other routes: a welded fabrication (base plate, wall plates and an inclined plate) is possible but distorts and needs post-weld machining. Metal additive manufacturing suits a prototype but not production.