△ Casting draft angle
Casting draft angle · Fundición (ISO 10135) · ISO 1101
Example in plan
How to interpret it
On the drawing, “1.5° draft on all vertical faces unless otherwise specified” states a casting draft angle requirement.
What controls
Minimum taper that the part walls must have in the direction the pattern is withdrawn from the mold (or the part is ejected from the die), so that sand is not torn out and the surface is not scored during stripping. It is dimensioned as an angle or a ratio, and it is always measured relative to the direction of draw, not to the part axis: two walls that look identical on the drawing may need different draft if they are oriented differently relative to the mold. It is not a tolerance: it is material deliberately added or removed, which is why wall thickness is not constant from top to bottom.
How to verify
Checked with a precision bevel protractor on the as-cast wall, or by measuring the thickness at the top and bottom with a caliper and calculating the taper from the difference. On medium and large parts, a portable measuring arm or a 3D scanner compared against the CAD model is the most practical option, because it shows the draft over the whole wall instead of at two points. On a rough-textured cast surface, average several readings: a single measurement on a high grain skews the angle.
Workshop guideline values
- General
- 1°–3° in sand casting of iron and steel, depending on wall height and pattern finish.
- Accuracy
- 0.5°–1.5° in molding with well-finished metal patterns and shorter parts.
- High precision
- Below 0.5° only with permanent-mold or investment casting processes, and almost always negotiated with the foundry.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Measuring draft relative to the part axis instead of the mold's direction of draw.
- Treating it as a manufacturing tolerance that can be tightened at no cost: it is pattern geometry and affects weight and wall thickness.
- Using a drafted face as a machining locator and being surprised that the part goes out of square depending on where it rests.
How to get it in the workshop / if it comes out of tolerance
When you receive a casting drawing, first locate the direction of draw and the parting line; from there you understand why the walls are tapered. In machining, draft is the reason an as-cast face is not parallel to the machined one: avoid locating on a drafted face if you can, because the contact point changes depending on the height where it touches. If the part has machining allowance, check it at the unfavorable end, which is where the draft eats into the stock.
If parts come out with torn surfaces or sand stuck to them after stripping, draft is almost always insufficient or the pattern is worn; it is not a sand problem. If you run out of stock when machining the lower part of a tapered wall, check whether the patternmaker applied the draft inward (removing material) instead of outward. Increasing draft is cheap on the pattern and very expensive once production is running: it is decided before the tooling is made.
Alternative symbol:α
Related symbols
Other symbols in Foundry (ISO 10135)
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