Tools, accuracy and project management

A workshop must make 40 identical metal plates, each with two holes positioned to plus or minus 0.1 mm. Select tools and equipment and justify them.

1. 01Marking and measuring

   Use a digital calliper (reading to about 0.02 mm) rather than a steel rule to set out and check the hole positions, because the plus or minus 0.1 mm tolerance is finer than a rule can achieve.

2. 02Ensure repeatability

   Use a drilling jig to position the holes identically on every plate, so all 40 match without marking out each one by hand - the jig guarantees the repeatable accuracy a batch needs.

3. 03Choose the process

   Drill on a pillar drill (or CNC) for accurate, square holes; CNC would give the highest repeatability if the volume or precision justified its set-up.

Result: A digital calliper meets the plus or minus 0.1 mm measuring precision, a drilling jig gives repeatable hole positions across all 40 plates, and a pillar drill or CNC provides accurate holes - tools and equipment matched to the tolerance and the batch.

(a) A dimension is 50 plus or minus 0.1 mm. State the upper and lower limits and the tolerance. (b) Three spacers, each 20 plus or minus 0.1 mm, are stacked. Find the nominal total length, the total tolerance, and the maximum and minimum overall length.

1. 01Limits of the single dimension

   Upper limit = 50 + 0.1 = 50.1 mm; lower limit = 50 - 0.1 = 49.9 mm.

2. 02Tolerance of the single dimension

   Tolerance = upper - lower = 50.1 - 49.9 = 0.2 mm (twice the plus-or-minus value).

3. 03Stacked nominal and tolerance

   Nominal total = 3 x 20 = 60 mm. Total tolerance = sum of individual tolerances = 3 x 0.2 = 0.6 mm, so 60 plus or minus 0.3 mm.

   $$\text{Total tolerance} = 3 \times 0.2 = 0.6\ \text{mm}$$
4. 04Maximum and minimum

   Maximum = 60 + 0.3 = 60.3 mm; minimum = 60 - 0.3 = 59.7 mm. The assembly can vary by 0.6 mm even though each spacer is within 0.2 mm - stack-up magnifies the variation, which the designer must allow for.

Result: The single dimension has limits 50.1 and 49.9 mm and a 0.2 mm tolerance; three stacked spacers give 60 plus or minus 0.3 mm (60.3 to 59.7 mm) - the tolerances add, so the assembly varies more than any single part.

A firm mass-produces plastic connectors that must fit a socket within a tight tolerance. Explain how QA and QC would ensure quality.

1. 01Quality assurance (prevent)

   Design a reliable moulding process with controlled temperatures and a well-made mould, set achievable tolerances, train operators, and work to a certified quality-management system (ISO 9001) so faults are prevented before they happen.

2. 02Quality control (detect)

   Check the connectors against their limits: a go/no-go gauge quickly passes or fails each sampled connector without measuring the exact size, and statistical process control charts the results to catch the process drifting out of tolerance before scrap is made.

3. 03Combine the two

   QA keeps the process producing good parts and QC verifies the output, so the connectors consistently fit - prevention and detection working together rather than relying on inspecting quality in at the end.

Result: QA designs a reliable, standardised, trained process that prevents faults, while QC uses go/no-go gauges and statistical process control on samples to detect any that slip through - together ensuring the connectors consistently meet their tolerance.

A student must plan the making of a bedside clock: designing and detailing, ordering materials, making the case, making the mechanism mount, assembling and testing. Explain how to plan it and find the critical path.

1. 01Identify dependencies and parallel tasks

   Designing must come first; ordering materials and preparing jigs can run in parallel with finishing the detailing; making the case and the mount can proceed once materials arrive; assembly needs both parts; testing follows assembly.

2. 02Schedule on a Gantt chart

   Place each task as a bar against a timeline, overlapping the independent tasks (ordering while detailing) to save time and marking milestones (materials in, parts made, assembled).

3. 03Find the critical path

   The longest chain of dependent tasks - design, then make parts (whichever part takes longest), then assemble, then test - is the critical path; any delay to a task on it delays the whole clock, so effort and contingency focus there, while off-path tasks have slack.

Result: The clock is planned on a Gantt chart with ordering run in parallel with detailing to save time; the critical path (design to longest part to assembly to test) sets the minimum duration, so delays there delay the project - the focus of project management.