Custom & Other Castings

Frequently Asked Questions (FAQ)

1. What are ASTM standards and where are they mainly applied?
   ASTM standards, developed by the American Society for Testing and Materials, comprise test methods, specifications, practices, and guidelines for materials, products, systems, and services. They are widely used across industry for the testing and quality control of metals (carbon steels, alloy steels, stainless steels, cast irons, aluminum and copper alloys), pressure vessels, piping, fasteners (bolts, nuts), gaskets, castings, forgings, and many other engineering materials and components.

2. What is the BS EN 1559 standard family about, and which materials does it cover?

   The BS EN 1559 series defines general delivery conditions for castings.

   • Part 1 covers general requirements for all castings except copper alloys.
   • Subsequent parts specify additional delivery conditions for casting steel, cast iron, aluminum, magnesium, and zinc alloys.

   This family does not apply to downstream processing of metal ingots or continuous-cast steel billets.

3. What is dimensional tolerance for castings, and how is it determined and indicated?

   Dimensional tolerance is the allowable difference between maximum and minimum permissible sizes on a casting’s nominal dimension.

   • Tolerance grades range from CT7 to CT13 (per GB/T 6414–1999), with tighter grades for smaller, precision castings and looser grades for large, sand-cast parts.
   • Recommended grades vary with production scale (mass production vs. small batch).
   • On drawings, general tolerance is shown as “GB CT6” or similar; individual critical dimensions may bear specific tolerance values.

4. What is weight tolerance for castings, and how is it determined?

   Weight tolerance expresses permissible variation as a percentage of the nominal (theoretical) weight.

   Grades run from WT1 Kg to WT10 Ton, matched to dimensional tolerance grades.
   Lower and upper weight deviations are normally equal; the lower deviation may be tightened by up to two grades if required.
   Reference tables guide tolerance selection for batch-produced and single-piece castings.

5. What is machining allowance, and how does it affect dimensional accuracy?

   Machining allowance is extra metal left on casting surfaces to ensure final dimensions and tolerances can be achieved.

   Alloys and casting methods have recommended allowance grades (MCL5–MCL10).
   Proper allowance ensures adequate material for machining critical features and achieving specified tolerances.
   On drawings, allowance may be noted by grade code or by listing the actual allowance dimension.

6. What is casting shrinkage rate, and which factors influence it?

   Shrinkage rate is the percentage reduction in length from pattern to casting after solidification and cooling.

   • It depends on the metal’s intrinsic shrinkage, its liquidus–solidus range, and mold design factors (casting geometry, sand type, gating, and riser allowances).
   • Accurate pattern scaling and trial pours help determine the correct shrinkage allowance for high-precision castings.

7. In casting design, what roles do draft angle, non-machined wall allowance, minimum castable hole, and process ribs play?
   Draft Angle: A taper on vertical faces to facilitate mold removal without damaging the mold or the casting.
   Non-Machined Wall Allowance: A negative allowance on pattern walls for areas not to be machined, compensating for parting-line mismatch.
   Minimum Castable Hole: The smallest hole or slot dimension that can be produced as-cast without machining, depending on alloy, casting method, and wall thickness.
   Process Ribs (Shrink and Draw Ribs): Temporary ribs added to control solidification stresses (shrink ribs) or to resist distortion (draw ribs). Shrink ribs are removed during cleaning; draw ribs are removed after heat treatment.