Stainless steel : custom CNC machining and prototyping

AC Manufacturing is a complete cnc machine shop capable of machining custom parts to customer specification (conventional and 5 axis machining). We can machine to print and accept the most common file formats including: .dwg .dxf .iges .step .sat as well as .pdf files. If you don't see your file format here let us know. Please send us files to: sales@acmanufacturing.com We can accept both small and large run jobs of custom machined parts. We provide Turning and Milling, 3 axis , 3+2 axis & continuous 5 axis machining).

We accept both small and large run jobs, Minimum quantity for order is 1.
Short lead times are often available 2-3 days.
Standard lead time is 2 weeks.

The most common and hence most frequently machined stainless steels in our cnc machine shop are the austenitic types, such as grades 304 and 316 . These are characterised by their high work hardening rates and poor chip breaking properties during machining.

Stainless steel is defined as a steel alloy with a minimum of 10.5 % chromium content by mass. As the name suggests, stainless steel is steel that should stain 'less' and not corrode. It should stain less, but it is not stain-proof. It is normally thought of as being corrosion-resistant, but given the wide range of grades of stainless steel, that is actually not a very useful defining characteristic, because it is the quality and grade of the stainless steel which determine its strength and corrosion resistance por custom cnc machined parts

The base element in stainless steel is iron (Fe), and to this chromium (Cr) is added, which makes it into a corrosion-resistant alloy, an alloy being a mixture of metals. A small amount of carbon (C) is often included to increase hardness and strength. Nickel (Ni) may be added to stabilize the crystalline structure of the iron, and thus increase the integrity at all temperature extremes and to assist with corrosion-resistance. Other alloying elements such as molybdenum (Mo) or titanium (Ti) may be added to improve heat- and corrosion-resistance properties.

One of the qualities of stainless steel is that it is self-repairing after cnc machining processes are done. The chromium, through a process called passivation, forms an invisible layer over the iron to protect it from air and water. If the surface is scratched, the layer quickly rebuilds itself in the presence of oxygen. This is why high quality stainless steel will not rust, even if it becomes scratched during machining. Most quality stock shape materials are stress relieved to ensure the highest degree of machinability and dimensional stability.

Stainless steels are finding increasing use in custom material processing, power generation, transportation, and other applications. The challenges of cutting austenitic stainless can be overcome with both good machining technique and advanced material and insert technology.

Machining stainless steels in our cnc machine shop:
Stainless steels are generally more challenging to cnc machine than standard carbon steel and a general rule is that the higher a steel’s alloying content, the more difficult it is to machine. When machining stainless steels parts it is important to ensure that there is no dwell or rubbing caused by machine vibration or tool chatter. In all machining that involves cutting austenitic and duplex stainless steels, work hardening is more pronounced than in carbon steel or ferritic and martensitic stainless steels. The machined surface becomes harder at the same time as tool-life decreases.

Small training or 'hobbies' lathes and milling machines intended for custom machining mild steel, brasses etc. are unlikely to be substantial enough for the successful machining of stainless steels. Arbours for supporting barrel milling cutters should be stout as short as possible. The arbour supports should be as close as possible to the ends of the cutter to provide maximum support. Tool materials either high speed steel (HSS) or cemented carbide tools can be used for machining stainless steels. We use 5 axis precision cnc machines such as Haas UMC 750 and Haas VF-5SS as well as conventional milling equipment (Matsuura RA-1G, MC 500V PC 2S and MC-510 VG)

Alloy 303 is the most readily machineable austenitic stainless steel; however, the sulfur addition does lower Alloy 303’s corrosion resistance to below that of Alloy 304. Like other austenitic grades, it demonstrates excellent toughness, although the sulfur does reduce this a little as well. CNC machining can be enhanced by adhering to the following rules:

• Cutting edges must be sharp. Dull edges cause excess work hardening.
• Cuts should be light but deep enough to prevent work hardening by riding on the surface of the material.
• Chip breakers should be employed to assist in ensuring swarf remains clear of the work
• Low thermal conductivity of austenitic alloys results in heat concentrating at the cutting edges. This means coolants and lubricants are necessary and must be used in large quantities during machining.

Alloy 304 series stainless steel austenitic, which has a minimum of 18% chromium and 8% nickel. Type 304 has a maximum carbon of 0.07% . It is the standard “18/8 stainless” that is commonly found in pans and cooking tools. Alloy 304 is the most versatile and widely used alloy in the stainless steel family. Ideal for a wide variety of home and commercial applications, Alloy 304 exhibits excellent corrosion resistance and has a high ease of fabrication, outstanding formability after cnc machining. Type 304/304L has good forming and excellent welding characteristics. It is non-hardenable by heat treatment. The austenitic stainless steels are also considered to be the most weldable of the high-alloy steels and can be welded by all fusion and resistance welding processes. Stainless steel work harden rapidly, heavy positive feeds, sharp tooling, and a rigid set-up should be used of cut below the work-hardened layer resulting from the previous passes.

The 400 series group (eg 414) of stainless steels has an 11 percent chromium and 1-percent manganese increase, above the 300 series group. The 400 series is susceptible to rust and corrosion under some conditions. Heat-treating will harden the 400 series. The 400 series of stainless steels have higher carbon content, giving it a martensitic crystalline structure. This provides high strength and high wear resistance after cnc machining. Martensitic stainless steels aren’t as corrosion resistant as the austenitic types.

17-4 PH stainless steel is a precipitation hardenable, martensitic stainless steel that delivers an excellent combination of oxidation and corrosion resistance, and is comparable to 304 in most conditions or environments. Mechanical properties, such as strength and ductility, can be optimized with heat treatment. Because this alloy is hard, forming normally should be limited to mild operations. However, formability can be greatly improved by heat treating before cold working or by use of hot-forming methods.