Interpenetration with Axes at an Angle

≈ 42 min

Interpenetration with Axes at an Angle

CAPS Grade 11 requires interpenetration at 30°, 45°, 60° and 90° with axes in line. The method does not change — but the symmetry that made 90° easy disappears.

What changes at an angle. At 90° the curve is symmetrical about the centre line, so half the points can be mirrored. At 30°, 45° or 60° the branch enters obliquely: the curve is longer on one side than the other and must be plotted in full. Mirroring at an angle is one of the most common errors in this topic.

Extra care is needed at the extremes. The highest and lowest points of the curve, and the points where it changes from visible to hidden, do not usually fall on one of your twelve generators. Add extra generators there specifically, or the curve will be flattened exactly where its shape matters most.

Worked example. A 45° branch into a header pipe: the curve on the upstream side is short and tightly curved, while the downstream side stretches out along the header. Plotting only six points misses the tight upstream turn entirely and the marked-out pipe is cut short.

Core checkpoint: At an angle, plot the whole curve. Add generators at the extremes rather than assuming twelve evenly spaced ones are enough.

Why may the interpenetration curve not be constructed as a half and mirrored when the branch enters at 45°?

The branch enters obliquely, so the two halves of the curve are genuinely different lengths and shapes.

Because mirroring is never permitted in EGD.

Because the curve is a straight line at 45°.

Because there are too few generators to mirror.

Why should extra generators be added at the highest and lowest points of the curve?

Those extremes rarely fall on an evenly spaced generator, and missing them flattens the curve where its shape matters most.

To make the drawing look more detailed.

Because the standard twelve generators are not allowed.

To check that the twelve generators were spaced correctly.

Which angles of interpenetration does CAPS Grade 11 require?

30°, 45°, 60° and 90°.

Only 90°.

Any angle the learner chooses.

15°, 30° and 45°.

A pipe is marked out from a curve plotted with only six points, missing the tight upstream turn. What happens at the weld?

The cut deviates from the true curve, leaving a gap the welder must fill.

The pipe fits perfectly — six points is sufficient.

The pipe is too long and must be trimmed.

Only the appearance of the joint is affected.