Challenges And Tips in CNC Machining Stainless Steel

What is Stainless Steel?

Stainless steel is a specific type of metal alloy composed of Iron, Chromium, and sometimes other elements like Carbon, which are added to provide specific properties. The minimum percentage of Chromium must be present in stainless steel is 11%. Iron is known for its strength and durability but is susceptible to corrosion and abrasions. On the other hand, when Chromium is added to Iron, it makes it more resistant to these issues.

Stainless Steel Machining Processes

Stainless steel can undergo various machining processes:

Milling

Milling is the most commonly used primary machining process for stainless steel. It involves using high-speed rotating cutting tools on a stationary workpiece. When milling stainless steel alloys, it is crucial to select the appropriate tool. Incorrect tools or high speeds can lead to rapid tool wear. Milling stainless steel produces a very precise and smooth cutting edge.

Turning

Turning is a machining process that involves a stationary cutting tool and a rotating workpiece. During this process, the tool physically contacts the rotating workpiece to remove material. Turning is commonly used to machine austenitic stainless steel. When machining stainless steel, minimizing tool overhang for optimal results is essential.

Drilling

Drilling is a machining process typically used with other metalworking techniques to create holes in a workpiece. Stainless steel is often drilled for screw holes, secondary assembly, or aesthetic purposes.

Threading

Mounting screws and fasteners are threaded on steel parts. Threading can occur on the inside or outside of the workpiece, with threading on stainless steel usually taking place on pipes and tubes.

Laser Cutting

Laser cutting is a process that uses an amplified beam of light to melt and remove stainless steel. This method is effective for thin sheets of stainless steel and is preferable to using cemented carbide tools as it does not cause any tool wear. However, it is a costly process that requires highly skilled labor.

Grinding

Grinding is a process used to improve the surface finish of stainless steel. It can also be used to remove material in small amounts, resulting in a more accurate workpiece. During grinding, abrasive wheels are rubbed against the stainless steel workpieces, which can also eliminate burrs created during welding. Grinding helps producing even and polished cutting edges on stainless steel components.

EDM (Electrical Discharge Machining)

Electrical Discharge Machining (EDM) uses electrical pulses to melt metal. The process could be improved in terms of cutting thickness, especially for stainless steel.

Waterjet Cutting

Waterjet cutting utilizes high-pressure water to erode and cut materials. It is capable of cutting thick sheets of stainless steel. However, when low corrosion resistance grades of stainless steel are used, waterjet cutting can cause negative effects.

Important Tools And Equipment For Stainless Steel CNC Machining

Achieving optimal results and maximizing productivity in CNC machining of stainless steel requires specialized tools and equipment designed to handle this durable material. Here are some essential tools and equipment needed for CNC machining of stainless steel:

Cutting Tools

Various cutting tools are used for different stainless steel CNC machining operations, including carbide end mills, carbide drills, carbide inserts, solid carbide readers, and taps.

CNC Machines

The CNC machine is the core of CNC machining for stainless steel, and it can be a CNC lathe, CNC milling machine, CNC machining center, or CNC turning center. Since stainless steel machining requires high demands, it is critical to have a robust and durable design to meet those demands and ensure precise results.

CNC Programming Software

Having specialized programming software is crucial in creating CNC programs. CAM software is also vital as it assists in generating tool paths and converting CAD designs into machine-readable code for CNC machines.

Coolant System

 A reliable system is paramount for achieving effective stainless steel CNC machining. This is because the material has low thermal conductivity, which makes efficient cooling necessary to disperse heat during machining. Proper cooling prevents workpiece deformation and extends tool life. Generally, water-based coolants or cutting fluids with high lubricity are used for this purpose.

Safety Equipment

 Safety is essential in CNC machining, and machine operators should wear proper personal protective equipment (PPE) such as safety glasses, gloves, and hearing protection to prevent accidents and injuries.

Tool Holders and Tooling Accessories

To achieve precise machining results, high-quality tool holders must be used to hold the cutting tools in place securely. These tool holders not only provide stability but also help in reducing vibration, thereby improving both the accuracy and surface finish of the machined workpiece.

Measurement Tools

Precise measurement tools, such as calipers, micrometers, depth gauges, and dial indicators, are indispensable in CNC machining to ensure parts meet specified tolerances for quality control purposes.

Workholding Fixtures

During the CNC machining process, it’s important to hold stainless steel workpieces securely to prevent movement and maintain dimensional accuracy. This is typically achieved through the use of workholding fixtures such as vises, clamps, and chucks. These fixtures are commonly utilized in the industry to guarantee the finished product’s quality.

Benefits of CNC Machining Stainless Steel

Stainless steel is a durable material option for CNC machining, offering long-lasting results. Below are some reasons why stainless steel is an excellent choice for CNC machining projects.

Anti-corrosion

Stainless steel is widely recognized for its outstanding ability to resist corrosion, making it a perfect choice for indoor and outdoor applications. It can withstand water stains and rust and can handle extremes in temperature and pressure. This unique characteristic of stainless steel is its chromium content, which forms a thin layer to protect the steel from oxygen when it comes in contact with oxygen.

Cryogenic Resistance

Specific stainless steel grades can maintain their stability across a range of temperatures. For example, austenitic steels have exceptional toughness and improved tensile strength when exposed to sub-zero temperatures. However, it is worth noting that not all stainless steel grades possess this characteristic.

High Tensile Strength

Stainless steel has greater tensile strength than mild steel, brass, and aluminum alloys. Tensile strength refers to the force required to stretch a material to its breaking point. This quality is crucial for many products throughout their lifespan, particularly for components that frequently bend and roll.

Outlook

Stainless steel is a type of metal that has a silver-white color which does not corrode or tarnish over time. The metal gets its appearance from the high levels of Chromium present in it. Besides its strength, stainless steel has several other desirable characteristics. Its attractive aesthetics have made it a sought-after option for various items. CNC-machined stainless steel parts have a modern, sleek look and are functional.

Challenges in Machining Custom Stainless Steel Parts

Stainless steel alloys are harder than aluminum and more machinable than titanium. They also possess good strength and temperature resistance. However, machining custom stainless steel parts can be challenging. Here are some of the challenges involved.

Overheating

Stainless steel differs from other metals in that it is not a good conductor of heat. Nonetheless, when working with stainless steel, it tends to heat up quickly, which can be detrimental to both the steel and the tools and CNC machines used in the process.

Difficult Surface Treatment

Selecting the appropriate surface finish for CNC-machined parts made of stainless steel can be difficult. This is because, unlike other metals, the surface finish of stainless steel affects its appearance and other characteristics, such as corrosion resistance, electrical properties, impact resistance, and weldability.

Technical Expertise

Working with stainless steel requires technical skill and careful consideration. This is because machining stainless steel is difficult, and using the wrong cutting tool or speed can damage the material. As a result, machining stainless steel can be a slow and laborious process.

Poor Machinability

Stainless steel is harder to machine compared to other metals due to its quick hardening properties, requiring specialized tools.

Quick Tips For CNC Machining Stainless Steel

When CNC machining stainless steel, several challenges can be prevented by following some quick tips for smooth machining.

Prefer High-Quality Materials

For smoother operations, use high-quality materials for workpieces. Stainless steel comes in many grades, each with multiple quality options. Investing in better-quality materials can save costs and prevent tool breakages.

Work Hardening

Work hardening is a process where stainless steel becomes harder during machining. This happens because of the plastic deformation of stainless steel. Work hardening makes it challenging to machine stainless steel. However, feeding coolant to the cutting tool can decrease the hardening effect.

Rigid Tooling

The machine setup and tool connections should be tight to avoid tool chatter and poor machining. Excessive vibration in the machine bed should also be avoided after workpiece fittings.

Choose Special End Mills and Drills

When machining stainless steel, it is crucial to use tooling made from high-quality materials like molybdenum or tungsten, as well as HSS end mills and drills. These tools are durable and can withstand the stress of the machining process, resulting in minimal wear. Moreover, they improve the surface finish of the material. It is important to avoid using tools that wear easily, as this can slow down machining and increase production costs.

Lubricants

When machining stainless steel, it is crucial to use a lubricant. Lubrication serves multiple purposes for machining:

1. It reduces the friction that occurs between the cutting tool and the metal. This increases the tool’s lifespan by reducing its wear.
2. Lubrication helps to lower the temperature during the machining process, which reduces the likelihood of work hardening and overheating.
3. Lubrication fluids effectively wash away any stainless steel residue that may be left on the workpiece or tools.

Manage Generated Heat

Heat is a common reason for failures in stainless steel CNC machining. The best way to manage the heat is to lower the cutting speed and use coolants. Moreover, it is recommended to stop machining when the material starts overheating.

Use Pecking and Chip-breaking Cycles

One way to control chip formation during machining is to use pecking and chip-breaking cycles. This helps to prevent the formation of long and stringy chips. Another effective method is frequently checking the cutting tools, as worn tools produce more chips.

Protect Your Workpiece from Deformation

When machining a workpiece, various factors can cause its deformation, such as the speeds and feed rate used and the cutting tools. If a worn-out cutting tool is used, it can cause an increase in tool deflection, producing parts that are out of tolerance and deformed. Therefore, using the recommended speed and feed rate when machining stainless steel is crucial to avoid such issues.

Applications Of Stainless Steel CNC Machining

Stainless steel CNC machining plays essential roles in various key industries:

Aerospace

Stainless steel components are used in crucial aviation engine and landing gear systems due to their strength, corrosion resistance, and heat resistance properties.

Marine

Stainless steel’s excellent corrosion resistance in seawater makes it a popular choice for marine applications.

Automotive

Stainless steel is used in automotive parts like exhaust systems and fuel lines due to its corrosion resistance and durability.

Medical

Stainless steel CNC machining offers the necessary precision and accuracy for medical implants, surgical instruments, and other devices.

Food and Beverage

Free-machining stainless steel components are commonly found in the food and beverage industry due to their sanitary qualities and resistance to corrosion, where they are used in processing machinery, storage tanks, and other applications.

Available Stainless Steel Alloys For CNC Machining

Several stainless steel alloys are suitable for CNC machining, just like there are different types of titanium alloys. Below are some examples:

SS303 (Free machining Stainless steel)

This type of stainless steel is called austenitic and is very easy to machine. It is also widely available in the market.

Advantages:

– It has good resistance against corrosion.

– It is moderately priced.

– It has excellent machinability.

Disadvantages:

– It is not suitable for marine applications.

– It does not harden when subjected to heat treatment.

Applications:

– Electronic hardware

– Aircraft fittings

– Bushings

– Nuts, bolts, and screws

SS304 (Stainless steel)

This is one of the most common grades of stainless steel that has good weldability and is made up of stainless steel.

Advantages:

– It can be welded easily.

– It can be machined easily.

– It is cost-effective.

– It has good corrosion resistance.

Disadvantages:

– This material is prone to cracking and corrosion.

Applications:

– Heat exchangers

– Architectural applications

– Screws

– Automotive parts

– Food handling

SS416 (Free machining stainless steel)

This stainless steel is the easiest to machine.

Advantages:

– It exhibits moderate corrosion resistance.

– It boasts outstanding strength.

– It has excellent machinability.

Disadvantages: 

– Unsuitable for marine or chlorine-saturated environments.

– Limited availability and poor weldability.

Applications:

– Shafts and gears, nuts and bolts, pumps, and valves.

SS Grade 17-4 (PH stainless steel)

This grade of martensitic stainless steel is precipitation-hardened and contains up to 30% chromium.

Advantages:

– Resistance to corrosion

– High level of toughness

– High strength

Disadvantages:

– It is susceptible to being damaged by heat.

Applications:

– Aircraft turbines

– Oil and gas

– Nuclear reactions

430F stainless steel is popular in machining due to its excellent feed and speed. It is essentially ferritic and has more sulfur added for improved machinability when compared to 430.

Surface Finishes Choices For Stainless Steel Machined Parts

Different surface finishing techniques are available on stainless steel machined parts, including polishing, etching, brushing, and blasting. The chosen finishing option can significantly impact the corrosion resistance of the stainless steel surface.

Polishing

Stainless steel objects can have various types of finishes. Most stainless steel parts typically have a #3 or #4 finish or a more reflective mirror finish of #8. Most manufacturers prefer the mirror finish as it provides a highly reflective appearance. The reflective nature of the mirror finish makes it look very polished and shiny, reflecting light beautifully.

Brushing

Brushed stainless steel has a slightly rough surface that reveals a subtle silver-gray color beneath it. Unlike the highly reflective mirror-polished finish, the brushed finish doesn’t reflect light as much, which makes the steel’s color slightly muted. Many designers prefer this finish for its unique look. Please note that the brushed finish requires a bit of abrasion to achieve.

Blasting and Pickling

The standard finish for stainless steel objects is achieved through blasting and pickling. This involves mechanically cleaning the stainless steel product using bead blasting, followed by chemical cleaning by dipping it in a stainless steel passivation tank. The resulting finish appears matte and has a modern look.

The blasted and pickled finish is perfect for achieving an industrial look, effectively covering imperfections. It is also a popular choice for marine projects and other projects that are exposed to harsh weather conditions.

FAQ

Why is machining stainless steel challenging?

Working with stainless steel can be difficult due to its high strength and exceptional ductility. The material will harden and produce much heat during machining, making it challenging to work with. This can cause cutting tool wear to increase, which further adds to the difficulty of manufacturing stainless steel.

Which one is the most suitable machining grade of stainless steel?

430F stainless steel is a ferritic grade of steel that contains more sulfur, making it more machinable than 430 stainless steel due to its excellent feed and speed.

What is the recommended milling speed for stainless steel?

It is generally advised to use a Surface Feet per Minute (SFM) of 100 to 350 when milling stainless steel. The chip load should range from 0.005 inches for 1/8-inch end mills to 0.006 inches for 1-inch end mills.

Which material, stainless steel 303 or 304, is easier to machine?

Type 303 stainless is an austenitic stainless steel widely known for its excellent machinability. It is a free-cutting material considered the most machinable of all austenitic stainless steel. On the other hand, grade 303 steel is not suitable for welding. 

Meanwhile, type 304 stainless steel cannot be hardened by heat treatment but is more weldable than type 303. It is not a free-cutting material, making it less ideal for machining.

Conclusion

Stainless steel provides numerous benefits to manufacturers due to its exceptional properties. It is highly resistant to corrosion, can withstand harsh environments, and has a longer service life than many other metals, making it an excellent choice for creating solid and durable products. Although it can be more challenging to work with, the benefits are often worth the effort. Therefore, designers should opt for a reliable CNC machine shop such as Enze to create high-quality stainless steel products at an affordable cost. Contact us now for a fast offer.