This 9.4-lb. (4.26-kg) aluminum electrical components enclosure was investment cast, sav- ing hours of machining while reducing material costs and CNC production time.
5
Design for Additional Feature Tolerances
As a general rule, linear tolerances
on investment castings are +/- 0.01 in. (0.025 cm) with dimensions up to 1 in. (2.54 cm) and +/-0.003 in. (0.008 cm) for each additional inch of dimension up to 10 in. For dimensions exceed- ing 10 in. (25.4 cm), allow a tolerance of +/-0.005 in. (0.013 cm) for each additional inch. Additional tolerances are applied to each of the features designed into investment castings. Fol- lowing are the guidelines: Flatness—Flatness tolerance is the
total deviation permitted from a plane and consists of the distance between two parallel planes. T e degree of fl at- ness exhibited in an investment casting is determined by the amount of volu- metric shrinkage the pattern wax and
molten metal undergo during cooling. T is shrinkage usually is in the center of the mass and referred to as “dish.” Dish can be controlled by special- ized techniques but never eliminated completely. General fl atness tolerances cannot be quoted, as they vary by confi guration and alloy (Fig. 1). Straightness—T e tolerance cover-
ing the straightness of an axis is the diameter or width within which the axis must lie. To correctly measure axial straightness on a shaft, bar or plate, the tolerance zone must be measured. Straightness may present a problem
with certain casting types. A thin- walled, short component may bend, while a long, tall component may not. Roughly, a constant section thickness will have an axial bow of 0.005 in./in. Ribs and gussets will inhibit warping and hinder mechanical straightening. Parallelism—Parallelism
Fig. 1. Flatness is the allowable dishing in a component, in addition to basic tolerance.
is a relative planar measure with respect to the datum plane. It is aff ected by casting confi guration and straight- ness. A tolerance more than 0.01 in. of the total indicator reading may require second- ary machining. Perpendicularity—When
38 | METAL CASTING DESIGN & PURCHASING | Nov/Dec 2011
This 1-lb. (0.54-kg), 8620 steel invest- ment cast component, converted from a machined-from-billet part, incorporates cam tracks and spline, a series of projec- tions that fi t into slots on a corresponding part.
specifying perpendicularity, use the longest plane for reference, establish- ing the datum plane with three tooling points. Some improvement on toler- ance can be aff ected by straightening. Roundness—“Out of round” is de-
fi ned as the radial diff erence between a true circle and a given circumference. It is the total indicator reading when a component is rotated 360 degrees or it can be calculated by taking 50% of the diff erence between the maximum and minimum condition. Concentricity—Two cylindrical sur-
faces sharing a common point or axis as their center are concentric. Any di- mensional diff erence in the location of one center with respect to the other is the extent of eccentricity. Concentricity tolerance varies according to straight- ness at 0.005 in./linear in. of diff erence between points of measure (Fig. 2). Angularity—Angular tolerance
depends on the confi guration form- ing the angle and the alloy being cast. However, angular tolerance on invest-
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60