Without exact correlation of common datum target points for fixturing at the foundry and machine shop, the chances of scrapping good castings at the machine shop increases dramatically. Below are tips to help ensure every qualified casting produces a finish machined part that meets all requirements of the technical data package.
In order to create a high quality casting, an initial dimensional layout inspection should be performed at the foundry on the first casting to make sure it meets all drawing/model requirements to ensure adequate machine stock has been added on all subsequently machined surfaces. At the foundry, the casting inspection fixture should be produced from known datum target points. At the machine shop, if the machining fixture points are different from the initial casting inspection fixture points, there is a high risk a casting could be rejected or require costly repair in the first or subsequent machining operations. Ideally, the customer, foundry, and machine shop should agree ahead of time on the use and location of the datum target points to ensure all parties are inspecting and setting up from the same target points. Gating design, as well as casting and pattern production techniques, have an impact on location and control of datum targets. Datums should therefore be established prior to the start of pattern construction to ensure the best results.”
Datum features are actual features or locations of a part that are used to restrict the transitional and rotational movement of the part when it is placed in the fixture. Datum targets are specified regions of the datum features used for locating the part in a fixture. Devices used to contact the datum features in a fixture are items such as flat bottom pins, square or rectangular blocks, three jaw chucks, V blocks, etc. The assembly orientation in the X, Y, and Z planes of these devices creates the correlation between drawing/model, inspection, and machining fixtures. The marriage of inspection and machining fixtures assures not only that dimensional inspection layout is not needed on every part, but could also prevent unnecessary and expensive machining of features/surfaces that do not need to be machined or could be left as cast. The main difference between the casting inspection fixture and the machining fixture is the additional amount of clamp forms required to hold the part during the machining operation.
Most modern computer numerical control (CNC) machine shops that machine castings use coordinate measuring via touch probe inspection as the first operation. With the casting resting on the known tooling points of the machining fixture, the touch probe will contact certain programmed features to ensure the casting is oriented properly and can then inspect surfaces to ensure adequate machine stock exists. This inspection can also determine if the setup needs to be shifted slightly to ensure machine clean up on all required surfaces due to unforeseen issues such as overgrinding during foundry processing. The operation of the touch probe can be programmed directly into the CNC to run automatically as part of the machining program.
If the above practices are followed at both the foundry and the machine shop, the chances of machining a good part every time are dramatically increased.
Below is an example of a casting check fixture showing datum tooling point locations in red.
Below are pictures of typical casting check fixtures where the casting is held in place with light pressure from clamps while resting on datum tooling points. Go/no-go gauges then check the critical features of the casting as related to the datum target points.1
In some cases, castings are actually targeted in the casting strightening fixture establishing the exact machining set up points. Generally, these are holes for the X – Y orientation and spot faces to set Z height orientation.2
This typical automotive tombstone production machining fixture shows the importance that every casting datum point must be exact and undamaged to ensure every casting sits in the machining fixture properly.3
1. (Second image) Jancox Metal Products [Online image]. Retrieved March 23, 2017 from http://sholehgisymar08.blogspot.com/2014/05/fixtures-and-gauges.html.
2. Targeting Fixtures [Online image]. Retreived on March 23, 2017 from http://fansteel.com/folder/Wellman/html/foundry_capabilities.html
3. Machining Fixtures and Production Machining [Online image]. Retrieved on March 23, 2017 from http://www.select-tool.com/machining.html