Tolerances
Tolerances should only be specified on critical and functional dimensions. Unnecessary or over-close tolerances may make the castings less economic. Consistent process repeatability will be achieved, even on non-toleranced dimensions.
SIZE RANGE
P.I. Castings Ltd produce investment castings from less than 5mm to 600mm
LINEAR TOLERANCES
| DIMENSIONS |
ALUMINIUM ALLOYS |
STEELS, BRASS & BRONZE |
| Up to 25mm (1”) |
± 0.125mm (± 0.005”) |
± 0.135mm (± 0.005”) |
| 25-505mm (1”-2”) |
± 0.255mm (± 0.010”) |
± 0.265mm (± 0.010”) |
| 50-755mm (2”-3”) |
± 0.325mm (± 0.013”) |
± 0.395mm (± 0.015”) |
| 75-1005mm (3”-4”) |
± 0.405mm (± 0.016”) |
± 0.505mm (± 0.020”) |
| 100-1255mm (4”-5”) |
± 0.455mm (± 0.018”) |
± 0.605mm (± 0.024”) |
| 125-1505mm (5”-6”) |
± 0.505mm (± 0.020”) |
± 0.705mm (± 0.028”) |
| 150-1755mm (6”-7”) |
± 0.555mm (± 0.022”) |
± 0.805mm (± 0.031”) |
| 175-2005mm (7”-8”) |
± 0.605mm (± 0.024”) |
± 0.905mm (± 0.035”) |
| 200-2505mm (8”-10”) |
± 0.655mm (± 0.026”) |
± 1.05mm (± 0.040”) |
| 250-3005mm (10”-12”) |
± 0.705mm (± 0.028”) |
± 1.15mm (± 0.045”) |
| 300-4005mm (12”-16”) |
± 0.805mm (± 0.031”) |
± 1.35mm (± 0.050”) |
| 400-5005mm (16”-20”) |
± 0.905mm (± 0.035”) |
- |
| 500-6005mm (20”-24”) |
± 1.05mm (± 0.040”) |
- |
GEOMETRIC TOLERANCES
| DIMENSIONS |
ALUMINIUM ALLOYS |
STEELS, BRASS & BRONZE |
| Up to 50 mm (2”) |
0.25mm envelope |
(.010”) |
0.50mm envelope |
(.020”) |
| Up to 100 mm (4”) |
0.40mm envelope |
(.015”) |
0.65mm envelope |
(.030”) |
| Up to 150 mm (6”) |
0.50mm envelope |
(.020”) |
0.75mm envelope |
(.030”) |
| Up to 200 mm (8”) |
0.60mm envelope |
(.025”) |
0.90mm envelope |
(.035”) |
| Up to 300 mm (12”) |
0.70mm envelope |
(.030”) |
1.00mm envelope |
(.040”) |
| Up to 400 mm (18”) |
1.00mm envelope |
(.040”) |
1.25mm envelope |
(.050”) |
| Up to 450 mm (18”) |
1.25mm envelope |
(.050”) |
1.50mm envelope |
(.060”) |
SURFACE FINISH
A finer quality finish is achieved than most other casting processes, in the order of 3.2 microns (N8, 125CLA).
DATUM PLANES & TOOLING POINTS
These are best chosen to satisfy the functional requirements of the component. Datum planes across the centre of the casting, rather than at one end, are usually preferable.
Tooling points, showing where the casting will be picked up for machining will enable the same points to be used for dimensional checking. Please ask for our advice, in conjunction with the machinist.
HOLES & SLOTS
Holes and slots should be kept as short as possible in relation to diameter or width. Square holes, 'D' and other odd-shaped holes are as easy to cast as round holes. Blind holes are more difficult than through holes - we will advise you on what can be cast at the enquiry stage.
WALL SECTIONS
Unlike die-castings and plastic mouldings, it is not necessary to have a uniformly thin section all over. In fact, it is often helpful to have some thicker sections to enable metal to be fed into the casting at those points. Good design will enable sound castings to be produced and may result in significant cost savings. Our design advice is at no obligation – we welcome the chance to assist you at the earliest stage.
The section thickness that can be cast depends on the alloy, the surface area and the complexity of the casting. For aluminium and zinc, the typical wall thickness is 2 to 3 mm, but 1.5 mm or less can be achieved on some features. Thicker walls are needed for steel or copper based alloys, usually 3 mm or more, but a thickness of 2 mm or less can be achieved over small areas.
INTERNAL CORNERS
These can be stress raisers - use the largest fillet radii possible. External corners will not have burrs or sharp edges like machined corners.
UNDERCUT FEATURES
Investment Castings are ideal for producing undercut detail. Where complex internal shapes are required, removable inserts in the tool are used. If this is not possible, then soluble wax cores or ceramic preformed cores can be used, but these will inevitably add to the casting and tooling costs.
FINISHED MACHINED AREAS
These can be kept to a minimum thanks to the casting accuracy and flexibility of design. The amount of machining allowance will depend on customers' requirements, but we are ready to advise on this at the design stage. We can also supply fully machined castings, including surface treatments and threaded inserts.
RAPID PROTOTYPING
We can produce Rapid Prototypes in a full range of cast metals using our in-house facilities. (Click for a comprehensive guide.)
TOOLING
Tooling costs are relatively low as aluminium dies are used to produce thewax patterns. Die life is excellent and modifications are relatively easy when design changes are required.
