There is tons of information available on CNC machining over the internet. Everybody is discussing “What is CNC Machining” but we are here to take you a step further. It is material machining with different alloys. Let’s be more specific; today’s focus is on stainless steel.
According to latest research on nitrogen-added austenitic stainless steels, alloys can significantly increase stainless steel strength for machining. (Research Paper by Yasuhito Kawahara, Shunya Kobatake}
You will read some of the elements used to create stainless steel and then some of the attributes and types of stainless steel used in the industry today. Similarly, why some of those materials can be a real challenge to machines?
Also Read:
Properties of Stainless Steel as a CNC Material
For stainless material, there might have to be a 10.5% chromium and 1.2% carbon content or less. The base material here is iron. Similarly, specific amounts of other materials or elements are added to change the unique physical properties. The purpose is to make stainless steel suitable for each industry or market.
For example, materials used in food and medical applications have particular properties. These properties allow them to be easily cleaned or sterilized. The oil and gas industry or energy industry, on the other hand, uses stainless with additives. It is to protect against highly corrosive liquids and high heat and pressure.
Even the automotive and high appliance market uses stainless steel for durability and aesthetics. Depending on what elements are added and how much dictates:
- The tensile strength
- Melting point
- Oxidation
- And corrosion resistance for each type of stainless steel.
Sulfur is added at low levels to improve machinability and formability. It will form sulfide inclusions in the material. But these inclusions act as chip breakers. It aids in lubrication by forming a thin layer at the cut surface. The additional sulfur allowed the creation of some more common 400 stainless steels. Similarly, it is used generally with the “F” designation at the end.
Why choose stainless steel for CNC machining?
Stainless steel is widely used in CNC machining due to a mix of attractive characteristics. These features enable them for various purposes. Here are some significant reasons:
Corrosion Resistance
Stainless steel is a corrosion-resistant material. The chromium inside it makes a passive oxide layer. This element protects the metal from rusting and degradation. It makes it ideal for parts vulnerable to harsh environments, fluids, or weather conditions.
High Strength and Durability
Stainless steel has good tensile strength, which means it can withstand significant pulling forces without breaking. Furthermore, it has high yield strength, which lets it resist permanent deformation under pressure. This feature makes it suitable for parts requiring structural integrity or those facing wear and tear.
Versatile Elements
Different grades of stainless steel exist, each with a unique blend of properties achieved by adding various elements. By employing such variation, you can choose a particular type suited to your project’s requirements. For instance, some grades focus on high strength while others emphasize improved resistance to corrosion or better machinability.
High Machining Capacity
Some grades of stainless steel may be more challenging to machine than materials like aluminum. However, tool advancements and proper machining practices have made it an excellent choice for CNC machining. Some are formulated with machinability in mind, utilizing things like sulfur. It is to improve chip formation as well as reduce cutting forces.
Alao Read:
Machining in Different Types of Stainless Steel
Machinability varies significantly in different types of stainless steel depending on the elements added. A condition called built-up edge is when the material adheres to the cutting edge. It is usually caused by low cutting speed and hard work on the material at the point of cutting.
Increasing the shear or rake angle and the RPM will aid in heat creation. However, each type of stainless steel has many other dynamics.
Thermal Conductivity in CNC Machining Stainless Steel
Thermal conductivity is the material’s capacity to transfer or conduct heat. It is one of the major factors in machining stainless steel. The reason is its meager ability to create the heat necessary to form and break a proper chip.
So, in machining a workpiece with poor conductivity, an increased amount of heat transfers into the tool. It highlights the importance of appropriate coolant flush to carry away heat chips and tool coatings. It acts as a heat barrier between the tool substrate and the material being cut.
Elements Added in Stainless Steel Base Material
Moving now into the elements added to each base material to form the various types, the table below lists all of these alloying elements. These are added in different amounts and at different stages of the material development.
Each one gives the material the desired qualities. As more and more of the elements are added, tool parameters decrease, and tool life can be compromised.
Of course, first up is chromium. As we mentioned a material must include a minimum of 10.5% chromium to be classified as stainless. When chromium is added, the material can create a passive film called chromium oxide. This stops the form of rust and creates an immediate barrier when exposed to oxygen. This is the base of all stainless steel.
Carbon is added in low amounts to certain types of stainless to increase the hardness and strength ability. Low-carbon types like 304L are often used in welding situations.
| Element (Periodic Table Group) | Effect on Stainless Steel | Alloy Range (%) |
| Chromium (Group 4) | Creates passive film (chromium oxide) for corrosion resistance | Min 10% |
| Carbon (Group 14) | Increases hardness and strength | Low amounts |
| Nickel (Group 10) | Increases heat and corrosion resistance | Up to 25% |
| Molybdenum (Group 6) | Improves pitting corrosion resistance | 0.5 – 7.0 |
| Nitrogen (Group 15) | Improves strength and pitting corrosion resistance | Varying |
| Titanium (Group 4) | Improves toughness in ferritic steels | Varying |
| Niobium (Group 5) | Increases hardness in martensitic alloys | Varying |
| Copper (Group 11) | Improves precipitation hardening or corrosion resistance | Varying |
| Sulfur (Group 16) | Added in low amounts to improve machinability by breaking chips | Low amounts |
Alloys Used in Stainless Steel for Machining
The geometry of stainless steel is chosen according to the nature and amount of alloying elements. The hardness and toughness should always be considered. Knowing this, it is possible to select the cutting tool and the kind of tool path we might need accordingly.
Nickel
Nickel is used in stainless to increase its resistance to heat and corrosion. This can lead to poor chip formation at higher levels and increase the cutting temperature even higher than chromium alone. Adding molybdenum improves resistance to pitting corrosion.
It can be used in more aggressive environments like:
- Pharmaceutical equipment
- Outdoor fixtures and hand railings
- Marine parts and fittings.
The downside is the material has an increased ability to work harden, and sharp geometries are the most effective here.
Nitrogen
Nitrogen is a good alternative to carbon in high-strength austenitic stainless steel. It delivers significantly greater solid solubility than carbon.
Titanium
Titanium is used to improve the toughness of ferritic stainless steel and the hardness of martensitic stainless steel. This combination can be challenging to machine, and high shear geometry is needed to exhibit strength characteristics superior to typical nickel-based austenitic stainless steels, which makes it best suited for parts that really can’t be subsequently annealed after the fact.
Niobium
Niobium increases the hardness of martensitic alloys, which improves machinability. Subsequently, it improves the strength of austenitic materials.
Also Read:
Aluminum CNC Machining
Titanium CNC Machining
How to select a cutting tool for stainless steel applications?
When choosing a high-performance cutting tool to manufacture steel parts, you must consider a few decisive factors to consider:
- Material properties:This includes the type of stainless steel, its hardness, and its work-hardening characteristics.
- Cutting parameters:These parameters include speed, feed, and depth of cut. Cutting parameters impact both chip formation and tool life.
- Coolant selection:Using the proper coolant can significantly improve tool life and chip formation.
- Tool geometry:The geometry of the cutting tool, such as the rake angle and relief angle, will also affect chip formation and tool life.
- Tool coating:The type of coating on the cutting tool can improve its wear resistance and lubricity.
Stainless Steel Machining Process
Compared to the softer material, stainless steel machining is more challenging. The reasons are its high strength and work hardening. However, if professional machinists incorporate adequate techniques, they can effectively make high-precision steel parts.
Stainless Steel Machining Product Examples by Alloy
| Process | Alloy Example | Product Example |
| Turning | 304 (General Purpose) | Shafts, pins, bushings, fasteners |
| Milling | 316 (Marine Grade) | Impeller blades, pump housings, valve bodies |
| Drilling | 430 (Trim & Molding) | Brackets, hinges, appliance trim |
| Tapping | 17-4 PH (High Strength) | Aerospace parts, medical implants, fasteners |
Below are the main steps to machine stainless steel:
Turning
A metal carousel can be imagined. The process seems complicated. However, CNC machines keep rotating the steel workpiece to give it a cone shape. Turning is an excellent process for making parts such as shafts, pins, and bushings. To cleanly break off the cuttings, we use stainless steel-specific tools with more teeth.
Milling
This is comparable to a tiny router that scrubs away material at high speed. Tools go back and forth over the work, making pockets, slots, and every other weird shape you could think of. During milling, we have to keep things cool using lubricants to avoid angering cranky stainless steel that does not like getting too hot.
Drilling
This is an easy one. We drill holes into materials using a spinning bit for screws, pins, and so on. In this case, it’s essential to have sharp drill bits that remove chips well because stainless steel doesn’t appreciate clogged-up holes.
Tapping
Imagine screwing your hole in place instead of just drilling it out. After drilling, there is usually another unique threaded tool called a tap. It creates threads within the drilled hole allowing for screws to fit tightly in them. Tapping stainless steel needs expert professionalism to prevent wear and tear.
Surface Finishes Options for Stainless Steel Machined Parts
Generally, a steel machined part has a natural surface finish. The part has minor marks. Just think about the matte finish. We have mentioned some popular surface finishing techniques for stainless steel parts:
Buffing
Imagine giving your part a good scrub with sandpaper. Buffing uses special tools to smooth tool marks and make the surface shiner. This method provides a brushed look, almost like shiny metal brushed with a wire brush. The shiner look of the part significantly depends on the brushing frequency.
Polishing
This is like buffing on steroids! Superfine polishing compounds are used to give a mirror-like finish. Think of a disco ball – that’s some severe polish! It’s great for parts that need excellent finishing.
Bead Blasting
Bead blasting is like giving your part a sand bath! Tiny beads are blasted at the surface, creating a matte, textured finish. This can hide any little imperfections and make the part easier to grip.
Conclusion
CNC machining for stainless steel is no doubt a challenging task. However, studying its alloys will assist you in making high-quality custom parts. There are numerous advanced studies being held on making more precision parts. But choosing a reliable CNC company can expedite your complex projects.
Consider Yijin Hardware as your most trust-worthy on-demand manufacturing partner. We have all expertise and tools to make custom parts. In addition, with customized surface finishing, we provide rapid turnaround time.
Faqs
What are the methods of stainless steel machining?
Generally, there are six methods for CNC stainless steel machining:
- Turning
- Milling
- Drilling
- Tapping
- Laser-Cutting
- Waterjet Cutting
The first four are considered the most common methods.
What is the machinability of stainless steel?
Stainless steel is a tough material. This feature makes it difficult for machining. However, professional companies with advanced machining tools can work on this material with greater precision.
Is 304 or 316 easier to machine?
304 stainless steel is more simple to machine compared to 316 stainless steel. 304 has a lower nickel content, which makes it prominent compared to 316.
Can I machine stainless steel?
Yes, most definitely. Joking apart, you need to invest a lot of money in equipment and skills. Generally to machine DIY projects, you need to buy machines with different axis such as 3,4, and 5.
