Due to the detailed nature of this section on methods of machining die cast alloys, it has been separated into sections.
Please choose the material you’ll be using for the relevant information:
The complex forms, accurate details, and superior surface finishes obtainable in die casting of small zinc components makes extensive machining generally unnecessary. There is usually very little machining required on Dynacast precision zinc die castings, due to the excellent accuracy that can be achieved through our high standards of toolmaking and good process control. Critical surfaces may need to be trued, holes that are not readily cored may require drilling, while undercuts and threads that are too costly to cast may need cutting.
The machining characteristics of zinc and ZA alloys are very good. Tool wear tests on drilling operations with HSS twist drills indicated lower wear with Zamak Alloys than with SAE-40 brass casting alloy.
A wide range of machining variables can generally be used. High-speed steel cutting tools are usually suitable for all zinc alloys. Sharp cutting edges should be maintained to reduce heat generation, galling, burring and machined dimension variations. Finely ground and polished rake and flank faces provide the best results. Fluted tools such as drills, taps, reamers and milling cutters should have large, polished flutes to reduce friction and aid swarf removal. Moderate to high feed rates and cutting speeds can be used on zinc die casting alloys without any deterioration in tool life.
Drilling of zinc die casting alloys can be achieved under a wide range of operating conditions. Better and more economical drilling can be effected contact Dynacast to find out how.
Zinc die casting alloys are readily tapped and form excellent thread and hole quality. Threads can be either cut or formed with and without cutting lubricants.
Ground thread taps with straight flutes and minimum lead are preferred in production applications, although spiral-flute taps may be advantageous for deep or blind holes, and when the surrounding metal is thin.
Zinc alloys can be easily tapped using fluteless taps to produce a rolled thread. As a rule, fluteless tapping is carried out at higher speeds than cutting taps, and lubrication is essential. The main advantages are the absence of swarf, increased tap life and greater thread strength. The disadvantages are poorer thread appearance due to the characteristic crest, higher tap cost, and the intolerance to deviations from correct core size, which would result in variations in the percentage of thread form.
Our precision zinc die casting process is so precise that the holes are cored to the required size for reaming, thus avoiding drilling operations that require expensive jigs to be manufactured.
Standard reamers can be used for reaming holes in zinc die castings. Inserted blade reamers can also be used if they are satisfactory rigid.
For specific suggestions on your application, contact us for a detailed analysis. Or send us your ideas.
Machining characteristics vary somewhat among the eight commercially available die casting alloys, but the entire group is superior to iron, steel and titanium. The rapid solidification rate associated with the die casting process makes die casting alloys superior to wrought and cast alloys of similar chemical composition. Alloy 380, the most widely used die casting alloy, is better than average. The most difficult alloys to machine are those containing more than 10% silicon. The hard particles of free silicon in these alloys can cause rapid tool wear.
High-speed steel tools are generally satisfactory for machining all but the high-silicon alloys, which are quite abrasive and should be machined with carbide or diamond tools. Carbide tools can be used to machine the high silicon content alloys (such as 390), and generally give a better finish due to the increased hardness of the tool material in comparison to the stock. Diamond tools are only used where exceptionally high surface finishes have to be achieved, particularly on high-silicon alloys, in which the particles of free silicon will in time dull the edge of even carbide tools.
Spiral-flute reamers are preferable to straight-flute reamers when working with aluminum. These are less likely to cause chatter. It is usually better to use reamers with negative spirals (spiraled in the direction opposite to rotation), in order to prevent the reamer feeding itself into the hole.
The low elastic modulus of aluminum (approximately 70 000 N/mm2), means that it will distort more than most metals for a given magnitude of clamping force. It is not necessary to use high clamping forces when machining aluminum, due to the relatively low cutting forces involved. Moderate clamping forces will avoid dimensional variation due to distortion.
Residual stresses can be generated by cold working the surface of a component while machining with an improperly designed cutter. Distortion due to residual stress is most noticeable in slender parts.
For specific suggestions on your application, contact us for a detailed analysis, or FTP your file to us.
Magnesium die casting alloys have excellent machinability due to their close-packed hexagonal structure. There is always a risk of fire when machining magnesium alloys, especially when generating fine chips or swarf. Certain general rules should be followed during the machining and handling of magnesium machining chips:
- Use large feed rates to produce thick chips.
- Do not allow tools to rub on the work piece.
- Avoid any source of igniting the chips.
- Keep the machining place clean and free from excessive build-ups of chips.
- Always maintain an adequate supply of fire extinguishing agents nearby (dry flux, dry sand, D-type extinguisher).
- Use specially approved, inhibited water / oil emulsions or synthetic coolants to minimize hydrogen formation.
- Ensure ample flow of cutting lubricant.
- Enclosed machining cabins must be well ventilated to avoid excessive concentrations of hydrogen.
- Store the wet chips in top-ventilated steel drums in a suitable storage area, away from machining and casting operations.
- Transport wet chips in well-ventilated containers in vented vehicles.
Carbide tools are predominately used for the machining of magnesium alloys. Due to sharper cutting edges of uncoated tools, the problem with formation of material build-up is less when they are used. Normally, high-speed steels are only used for tools with more complex forms, e.g., twist drills, taps, broaches, etc. Polycrystalline diamond (PCD) tools should only be considered when machining long series at high production rates, demanding a superior surface quality. Material build-up is not formed when PCD tools are used.
Good results are normally obtained when magnesium alloys are machined with tools designed for machining aluminum. However, because of the low resistance to cutting and the relatively low heat capacity of magnesium, the tools should have smooth faces, sharp cutting edges, large relief angles, small rake angles, few blades (milling tools), and a geometry that ensures good chip flow during machining. Special attention should be given to the relief and clearance angles, which should be as large as possible to prevent rubbing of the machined surface, which may lead to overheating and material build-up on the tool. Because of the low cutting forces required, these angles can be larger than those permitted for other metals. Chips, rather than coils, are produced during the machining of magnesium.
Traditionally, magnesium alloys were machined without using cutting fluids. Although dry machining is less common today, the low cutting forces required and the high thermal conductivity of magnesium result in rapid dissipation of heat that keeps the machined surface cool. The main arguments for using cutting fluids for the machining of magnesium are:
- Reduced fire risk, due to reduced temperature in the cutting zone.
- Elimination of material build-up on the tool.
- Prolonged tool life.
- Ease of chip removal.
Chips from machining operations must be handled with due regard to safety and environmental considerations, and according to national regulations. Magnesium chips must not be mixed with chips from other materials. Magnesium chips from dry machining operations should be placed in closed, labeled, clean, non-combustible containers made out of steel. The containers should be stored in a dry place, in which there are no risks of water contamination. Chips contaminated with mineral oil should be stored in the same manner as dry chips.
Magnesium chips contaminated with water or water-based coolants should be stored in well-ventilated steel containers. The hydrogen concentration in the container can be kept below the lower explosion limit of 4% with three venting holes with a diameter of 25 mm drilled just below the top cover. The storage area and transport vehicle must also be properly ventilated to prevent accumulation of flammable hydrogen-air mixtures. Prior to storage and transportation, it is important to remove as much of the coolant as possible. Water removal methods currently in use are filtration, centrifugal drying and compression.
If reclaim of machining chips is not economical or practical, the chips must be disposed of in a non-hazardous manner and in compliance with local regulations.
Possible methods of disposal include:
- Dissolving in 5% aqueous ferric chloride.
- Dissolving in seawater.
- Land filling.
For specific suggestions on your application, contact us for a detailed analysis, or send us your file.