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| Thin walls and uniform wall thickness always produce superior castings with minimum porosity, distortion and tolerance spreading. Constant section thin walls are preferred but large changes of section can be accommodated through critical analysis of the component design and special tooling design features. |
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 Wall thickness and flatness over large surfaces are better controlled if through-holes and ribs are incorporated in the design. |
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Wall section tolerances: |
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Flatness Generally held to ±0.13mm (0.005") per 25 mm (1.00") square. Total Indicator Reading (T.I.R.) will, however, depend on configuration. |
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Straightness Generally held to ±0.1 mm (0.004") max per linear 25 mm (1.00"). Total Indicator Reading (T.I.R.) will, however, depend on configuration. |
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Squareness and Angularity ±0.5 degree Note: Angular tolerances are affected by irregular geometries. |
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When tolerancing, use of a central feature as a datum may allow detail to be cast that would otherwise carry too large a tolerance. Where a mechanical interface feature is required on a surface, it helps to use the centre of the feature as a datum. |
| Improved tolerances can generally be achieved by machining. This however introduces additional cost and lead time. The more machining operations that can be avoided, especially milling operations, the more cost effective the MM casting process becomes. | |
cost effective the MM casting process becomes. Drawings should indicate the essential tolerances and should relax those which are unimportant. |
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 This casting is a maze of holes. Its perfect surface is due to easy mould removal.    |
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| The mould material used by the MM casting process is applied as a slurry and is easily removed by water jets. This enables fine through holes to be cast. Length to diameter ratios can vary upwards from 4:1 dependent on diameter. Blind holes can be cast provided the depth to diameter ratios do not exceed 3:1. Holes of under 0.5 mm (0.020") diameter are often cast. Elongated holes can easily be cast allowing the take up of tolerances within an assembly. Threads are best not cast but provision of a hexagonal cavity around a bolt hole allows the use of a captive nut or bolt, removing the need to tap a hole. Also holes can be cast which are suitable for self tapping screws. |
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To reduce cost and improve accuracy a cast counter bore can be used in many through hole applications where a tight tolerance is rarely needed over the full length. |  (above) Example of support using a counter bore  |
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Drill starts can be incorporated to aid drilling in cases where blind holes may be too long to cast. | |
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Where a component is to be clamped to a shaft, a clamping slot can be cast, rather than machined. To make reaming of a through hole easier and more accurate a removable support bridge should be included. |
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| Special surface effects - such as textures and knurling can be incorporated in the MM casting process. Surfaces which need to be blemish free should be highlighted on a drawing. |
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(left) An example of highlighting a drawing to show important cosmetic surfaces. |
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| Integrated Labelling It is possible to include many types of permanent product identification which can eliminate further need for labelling. Logos and text may be produced with minimum cost if the design ensures that detail is in the line of draw of the wax from its die. Ideally, cast labels should use raised lettering on the casting face (engraved into the wax tool). Where such information must be below a functional surface, it can be sited on a pad sunk into the cast face. Preferred depth of engraving: 0.25 mm - 0.50 mm (0.010" - 0.020") Types of labels (click label in bold for a pop-up example) |
Instructions : Product ID |
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Various heat treatments can be applied to hardened components. This is carried out generally to aid subsequent machining and enhance component stability. Suffixes to the material grade are used to denote the heat treatment carried out: UK and equivalent USA and EN suffixes are shown below: |
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