As we’ve discussed previously in this blog, machining is an integral part of the aluminium sand casting process – allowing us to introduce specific features onto the part and giving the exact dimensions and finish required. This is not possible during the actual casting process due to the very small tolerances of the design dimensions.
Post-machining inspection is an important aspect of this work to ensure that the part meets the exact specifications stipulated by the customer. This inspection is carried out using a mixture of co-ordinate measuring machines (CMM) and mechanical equipment, such as micrometers, verniers and plug/thread gauges.
CMMs provide an extremely accurate way to check the measurements of the machining – down to a few microns. At Haworth Castings, we use two Zenith Too CNC CMMs. Specialist CMM software ensures that the part is checked in the same way each time and also verified against the pre-entered drawing specifications. This eliminates any chance of human error.
The machine compensates for any environmental temperature changes, which could potentially alter the dimensions and lead to the part being outside the required specifications. The temperature of the part is measured and entered into the programme. The CMM then calculates each measurement as if the part was at a stable 20 degrees Celsius.
Mechanical gauges, such as thread and plug, are also used to check certain features on the casting, such as tapped holes and bore diameters. These features are quicker and easier to check manually rather than on the CMMs.
The inspection process is used to identify a number of different potential defects. This could include issues with positioning (of the holes or other features), sizing and distortions within the casting. In some cases, the part can be modified to address these issues but in other cases, where there are sizing problems, it may need to be re-cast. All of our machined parts have to be certified before leaving the machine shop to verify that they conform to the highest standards.