Sometimes cast parts can be used in the form they leave the mold. But often, they need to be machined to provide sealing surfaces or threaded holes. Machining is the most versatile and accurate of all manufacturing processes in its capability to produce a diversity of part geometries and geometric features.
In the machine shop, cast parts can present challenges. To machine them you can adapt your machining practices accordingly. Therefore, it helps to know about
- the casting process and
- the physical characteristics of castings.
Aluminium casting methods and machinability
In any casting process, molten metal fills a cavity the shape of the part you want. When the metal solidifies, it has taken on the shape of the cavity. The various casting processes create the cavity in different ways and use different methods to introduce the metal into the mold.
- Sand casting uses special casting sand, that is compacted around a three-dimensional pattern, which forms the part cavity and the channels through which the metal will flow. The pattern is removed and the cavity remains. The mold is made in two halves, top half and bottom half. Where they come together is called the parting line. On the cast part, you can usually see where the parting line is. The surface of the part picks up the texture of the mold, which may be fine or coarse, depending on the sand used.
- Die casting is done in a reusable steel mold. It is the fastest of these casting processes. Molten metal is forced at high pressure into the mold. The mold is cooled by air or water until the part is solid and can be removed. Cycle times of a minute or less are common. Die casting can produce thin wall sections and yields parts with a good surface finish. Porosity can be an issue.
- Permanent mold casting also uses a steel mold, but the metal enters the cavity by gravity, or at low pressure and/or under vacuum. The cycle time is slower than for die castings, often three to five minutes. Parts have a good surface finish, and often have better mechanical properties than die cast parts.
- Investment casting is known as the lost-wax process, uses patterns that duplicate the final part, made from wax or a similar meltable material. Investment casting can produce netshape or near-net-shape parts with excellent surface finish.
Dimension tolerances of aluminium castings
Castings bring a degree of difficulty to the machining process. They have quite loose tolerances. Even on investment castings, which tend to hold tight tolerances, they are likely to be in the neighborhood of a few thousandths per inch. Two-part molds will produce a visible parting line on the casting. Often, the part is designed so this parting line is on a surface that will be machined. If it will not be machined, and the parting line is intrusive, it will be removed with a belt sander. Metal may leak out between the mold halves, producing flash on a part. For die casting processes, an automated trim press may be used for cleanup. Sometimes this may cut into the part, or sometimes fold over the flash rather than remove it .
Machined surfaces and finish allowance
Creating a quality machined surface on a cast part isn’t just between you and the metal. It’s a team effort that includes you, and the foundry, and your customer.
The areas you finish machine need to have enough metal for you to remove the entire cast surface, allowing for the tolerances of the casting. The foundry needs to make the part selectively oversized to provide enough material so you can machine over the whole area. This additional material is called the “finish allowance” or the “machine stock.” If there isn’t enough machine stock, after you’ve run the cutter over the surface, some as-cast surface may remain, and you will have to scrap the part .
Porosity inside heavier sections of die cast parts can be an issue, so you won’t want to cut into them too deeply.
Fortunately, die casting holds tight tolerances, in the range of a couple of thousandths, so with a properly designed casting, you don’t need to remove much material, and can stay clear of the porosity.
Other types of castings may also have porosity problems, so when you cut into the piece, pits show in the surface. You can work with the foundry to remedy the situation, or, if necessary, you can convince your customer to find another foundry that can produce better-quality castings .
Features of machining of cast aluminum
Typically, parts made of cast aluminum alloys are easy to machine, including all types of cutting. Low cutting force allows you to remove a large volume of metal in one pass. The quality of the surface after machining depends on the machining parameters such as cutting speed, cutting geometry, lubrication and cooling.
The more speed, the less roughness
When machining aluminum, it is recommended to use high cutting speeds in order to achieve the minimum possible roughness of the machined surface, all other things being equal.
Effect of silicon on cutting tool wear
In addition to the microstructure – defects, pores, inclusions – the silicon content has the greatest influence on the wear of a cutting tool. Thus, cutting tools for processing modified hypoeutectic silumins have the highest service life, while hypereutectic silumins for the production of internal combustion engine cylinders cause very significant wear to the cutting tools.
Chips during machining of silumins
When cutting softer aluminum cast alloys, in particular most hypoeutectic silumins, narrow spiral or short brittle chips are formed. In this case, suitable cooling emulsions and lubricants are widely used.
During mechanical processing of hypereutectic silumins, small chips are formed with the formation of powdered silicon. In combination with a lubricant, this powder acts as an abrasive. In some ways, the machining of these types of cast alloys is similar to the machining of gray iron castings .
When cutting parts made from eutectic silumins, which are characterized by a very soft matrix, long spiral chips are formed. In addition, the relatively high ductility of the material leads to aluminum sticking to the cutting edges of the tool. These phenomena require the use of lubricant, which, in turn, leads to deterioration in the quality of the machined surface.
Cutting tool for cast aluminium
For mechanical processing of aluminum castings, cutting tools made of high-speed steels, hard alloys and ceramics are used. Diamond tools are often used for finishing.
See cutting tools for aluminium
Allowances for castings for machining
The following machining allowances are used for the main methods of casting aluminum alloys :
- sand casting: 1.5-3.0 mm;
- hill casting: 0.7-1.5 mm;
- injection molding: 0.3-0.5 mm.
See also about chips Machining of aluminium
To minimize the loss of aluminum during remelting of chips, they are sorted by type of alloy and pressed into briquettes. In addition, the shavings must be protected from contamination by moisture, lubricants and iron impurities, as all this significantly reduces the value of aluminum scrap. Aluminum shavings are not a fire hazardous material and do not require fire prevention measures. If grinding of aluminum parts is used, the aluminum dust collection system must be explosion-proof.
- Mechanical machining of aluminium castings //Aluminium Casting Alloys – Aleris International, 2011