Machining Stainless Steel

Stainless steel is designed to handle harsh conditions—from seawater, to acids, to bio-corrosion from the human body—and not breakdown. It is used regularly for things like automotive and aerospace parts, medical devices (because of its biocompatibility), and in food handling applications (because it is easily sterilized).

The hardness that gives stainless steel its durability can also make it a challenge to machine. If you’re struggling with getting good cuts or want to extend the life of the tools you use on stainless steel, here are some key things to consider.

Handle the Heat

Stainless steel absorbs heat. If too much heat stays in the piece you are cutting, it will experience hardening—which will ruin the part. There are several key ways to help control the heat.

1. Remove chips. When cutting stainless steel, the heat transfers to the chips. This makes it critical to design the tool for efficient chip removal. Tools designed for stainless steel should have enough flutes and enough space between them to easily evacuate chips. This will help to avoid part hardening and keep the cutting area cooler throughout the life of the tool. 
2. Increase strength at the cutting edge. Small changes in tool geometry can make the tool stronger and better suited to handle the material demands of stainless steels. For example, by changing from a normal faceted relief (primary-secondary) to eccentric relief, the cutting edge will be strengthened and will be better able to handle a harder alloy like stainless steel.
3. Reduce chip size. Talk with your custom tool manufacturer about whether a chip breaking feature would work in your application. In hard alloys like stainless, it can be difficult to extend tool life, but making the chips smaller as they are cut can help them to exit the cutting area more easily and reduce the chances of re-cutting the chips. Both can help tools to last longer.
4. Add coating. Coating is a cost-effective way to help reduce heat, and in turn, may help to dramatically extend the life of your tools. 

Whether you’re cutting 304, 316, 410, 17-4, or any other stainless steel, there are ways to ensure that heat is kept as low as possible. Working with a design team can help to develop the geometry best suited for each specific application.  

The Right Speed and Path

Several alloys and material grades fall under the category of stainless steel, so guidance on cutting speed ranges and feed rates will vary. In many applications, it may be possible to achieve between 100-350 SFM for stainless steel. 

Over the past decade, the use of high efficiency machining methods has made stainless steel easier to mill. Machinists used to move more slowly through stainless and bury a tool deep into it. This works against the natural hardness of the metal. Experts now recommend trochoidal or dynamic motion paths combined with optimized geometry to get the best results with stainless steel.

Utilize Application Specific Stainless Steel Designs 

New grades of stainless steel are constantly being developed as manufacturers search for alloys that deliver the properties they need in their finished products. If the grade of stainless steel you are machining has changed, the tool you are machining it with should probably change, too, especially if you want to optimize how long it lasts. 

This could mean reexamining the geometry of the tools, adding a coating, or simply adjusting your feed and speed rates. Here again, expert tool manufacturers will be able to give you insights into matching your tools to your cutting material so you can optimize your operations and your budget.