Go-Kart Steering Bracket: Dimensioning Scheme and Line Conventions
A Year 11 student has designed an aluminium bracket that clamps a school go-kart's steering column to the chassis rail. The bracket is made from 40 × 40 × 5 mm equal angle, with two 6 mm bolts through the vertical face and one M8 bolt through the base. The drawing sent to the metalwork technician shows: an overall length of 180 mm, with 40 + 40 + 100 also given along the same edge; a 6 mm hole positioned 25 mm from the left end and also 155 mm from the right end; a 6 mm hole with no centre lines; a bent edge drawn as a solid line on the view where it is hidden; and no projection symbol. The technician has emailed back: 'I can make it, but it might not be the one you want.'
1. Identify the dimensioning faults and describe a corrected dimensioning scheme.
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Three faults, all of which let the technician build a different part than the student intended.
Double dimensioning. 40 + 40 + 100 = 180, so the length is given twice. If the machinist works to the chain and the overall, the errors accumulate and the part will not close. Give the overall once and one chain or, better, dimension from a datum.
Hole position given from both ends. 25 mm from the left and 155 mm from the right on a 180 mm part are the same hole stated twice, and they conflict as soon as either dimension is rounded or tolerance is applied.
Corrected scheme. Choose a functional datum – the face that actually locates against the chassis rail – and dimension every hole from that single edge: hole 1 at 25 mm, hole 2 at 90 mm, M8 at 140 mm from the datum, with the overall length given once. Dimension to visible edges, never to hidden lines, and group dimensions outside the outline.
Also state the hole size properly: a hole for a 6 mm bolt is not 6.0 mm; it needs clearance, about 6.6 mm for a medium fit.
2. Explain the projection symbol and line-type errors, and why they matter to a technician who will not ask a second time.
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Projection symbol. Without it, the technician cannot tell whether the plan belongs above or below the front view. New Zealand practice is third-angle, so the symbol is a truncated cone with the circle on the left and the trapezium on the right. If the drawing is actually drawn in third-angle and the symbol is missing, the technician may mirror the part or drill the holes from the wrong face. Label it and draw it consistently.
Line types. A bent edge that is not visible in a given view must be dashed; a solid line there tells the technician there is a visible edge where there is none. Holes need long-dash-dot centre lines to show they are holes and to give a drilling centre; missing centre lines suggest slots or plain marks.
Weight consistency. If outlines and dimension lines are the same weight the drawing reads as a tangle and small features get missed. Outline about 0.7 mm, dimension and centre lines about 0.25 mm.
Every one of these is a one-way ambiguity: the technician cuts metal once, and scrap aluminium does not go back.
3. The technician must drill the clearance hole for the 6 mm bolt and the M8 bolt. Recommend hole sizes and tolerances, and explain the tolerance-stack risk in the current drawing.
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Nominal is not actual. A '6 mm' bolt measures slightly under 6 mm, but a 6.00 mm hole gives no clearance, so any positional error or burr turns assembly into a hammer job. Use a standard clearance hole: for M6, a medium series clearance of about 6.6 mm (close fit about 6.4 mm, free fit about 7.0 mm). For M8, about 9.0 mm for a medium fit.
Positional tolerance. The hole position should carry a tolerance such as ±0.2 mm from the datum, and the bracket faces that mate with the chassis should be flat within a stated figure. Without positional tolerance, the drawing implies 'perfect', which no drill delivers.
Tolerance stack. With the current drawing the hole is dimensioned 25 mm from one end and 155 mm from the other on a 180 mm part. If the overall length runs +0.3 mm and each end dimension runs −0.2 mm, the two stated positions no longer agree. The technician has to choose one, so the steering column may sit a millimetre off centre and the bracket may not line up with the chassis holes. Baseline dimensioning from one datum removes the stack entirely.