Question 3 of 3: Functions, datum features and GD&T for a shoulder bracket

Question 3: Functions, datum features and GD&T for a shoulder bracket (30 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 figure. The bracket is a single-piece, L-shaped part that fits over the edge of the baseplate:

• A horizontal plate rests on the top of the baseplate.
• A vertical leg hangs down against the baseplate's side face.
• An upright lug with a rounded top stands on the plate. Its round bore carries the shaft, with the shaft axis parallel to the baseplate top and parallel to the edge the leg sits against.
• Four bolts fasten it. The top view shows two vertical bolt holes near the rounded outer corners of the plate, which clamp the plate down onto the top face. The right-side view shows two horizontal bolt holes near the rounded bottom corners of the leg, which clamp the leg against the side face.

The shaft's position relative to the baseplate is therefore set by two contact faces at right angles: the underside of the plate fixes the height, and the inside of the leg fixes the sideways offset.

a) Functions of the features, in priority order

1. Lug bore. This is the stated function: it holds the shaft at the required height and sideways offset, parallel to the baseplate. Its size and form set the fit with the shaft.
2. Underside of the plate (in contact with the baseplate top). It sets the shaft's height and its tilt about both horizontal axes. It is the largest and most stable contact face.
3. Inside face of the leg (against the baseplate side). It sets the shaft's sideways position and its alignment in plan, keeping the shaft parallel to the baseplate edge. It must be square to the plate underside, or the bracket cannot seat on both faces.
4. Lug body. It carries the bore at the required height, and its length along the bore gives the bearing length that keeps the shaft aligned. It must also be stiff enough to resist shaft loads.
5. Four bolt holes. The two vertical holes pull the plate down onto the top face; the two horizontal holes pull the leg onto the side face. They clamp rather than locate. They are clearance holes, so they must line up with the baseplate's tapped holes. The only thing they fix precisely is the bracket's position along the shaft axis, which is not critical.
6. Inside corner between the plate and the leg. It must clear the baseplate edge, for example with a relief groove or a corner smaller than the baseplate's edge break, so that both contact faces can seat.
7. Remaining outline (rounded corners, plate and leg thickness, lug top radius). These provide strength and safe handling and are not critical to the function; general tolerances are enough.

b) Datum features

1. Datum A (primary): the underside of the plate. It is the largest functional contact face and the first surface to seat when the bracket is mounted. As a plane it constrains three degrees of freedom: vertical position and the two tilts.
2. Datum B (secondary): the inside face of the leg. It is the second contact face, perpendicular to A. It constrains two more degrees of freedom: sideways position and rotation in plan.
3. Datum C (tertiary): the front face of the bracket (the lug and plate face at right angles to both A and B). It constrains the last degree of freedom, position along the shaft axis. It is used only for features whose position along the shaft axis matters, such as the bolt holes.

This A|B|C frame matches how the bracket is actually located in service. It seats on the top face, is pushed against the side face, and is then clamped. A fixture or gauge built to these datums therefore reproduces the real assembly.

c) GD&T controls

1. Flatness on A. Datum A must itself be a good plane before anything can be measured from it.
2. Perpendicularity of B to A. This keeps the two contact faces square, so the bracket seats on both at once without rocking.
3. Position of the bore axis, referenced to A|B (cylindrical tolerance zone). This is the key control. It fixes the shaft's height above A and its offset from B, which is exactly what “positions a shaft accurately relative to the baseplate” requires. Because the zone is a cylinder the full length of the bore, it also keeps the axis parallel to A and B. A refinement such as parallelism of the bore axis to A, and to B, can be added with a tighter value if alignment matters more than location. Datum C is not needed in this frame: moving the bore along its own axis does not move the shaft.
4. Size tolerance on the bore (a fit class suited to the shaft, e.g. H7) and, if the shaft runs in the bore, cylindricity.
5. Position of the four bolt holes at MMC. The two vertical holes are referenced to A|B|C and the two horizontal holes to B|A|C, each hole to the datum it is perpendicular to first. MMC applies because these are clearance holes: the tolerance only has to guarantee assembly (the fixed-fastener case), so bonus tolerance is appropriate.
6. Profile of a surface (or general tolerances) for the lug outline, the rounded corners and the thicknesses. These are not critical to the function.