Tips for Mastering Stainless Steel Machining

Reader Question:

Our shop has grown from primarily aluminum machining to now working in a diverse range of materials. Some we’ve figured out quickly, but stainless steel seems to be the most difficult for us to master. Can you offer some advice on managing tool wear in stainless?

Miller’s Answer:

Your struggles are understandable. In my opinion, of all materials we are faced with as machinists, stainless steel is the most difficult to understand. Nonferrous materials are easy to machine, while irons and carbon steels are, if nothing else, predictable. Super alloys are very difficult, but by comparison, the portfolio of materials under this umbrella is much smaller, so there is less to solve, and you know it will be difficult. Stainless has the unique position of a diverse range of alloys, but also being very sensitive to the processing of that alloy. This means stainless steel as a genre can be uniquely described as being soft or super hard, free machining or gummy, as well as cheap or expensive.

Due to this diverse genre, the most important thing when trying to get a grip on stainless is to make sure you are very specific in how you talk about it. I find machinists often oversimplify stainless as a singular type (304, 416 and so on) without understanding the more pertinent information they need, or their tool vendor needs to help them.

To solve the problem of discussion, it can be simplified to classification, special characteristics, forming type and hardness.

Classification is simply the alloys name, and it gives us some general information. This would be 304, 416, 17-4 and so on. You may also see terms such as austenitic, martensitic, ferritic, precipitation hardened and duplex used. These rank from easiest to machine to hardest, respectively, and refer to the family of alloys based on them.

Special characteristics are those letters you see after the alloy number. 304“L” and 316“L”, for example, are low carbon versions of the same base alloy, while “H” would be high carbon. Some alloys have added sulfur to enable more efficient chipbreaking as well. Therefore, if you have a baseline recommendation from a speed and feed chart for a certain material, you can use these special characteristics to tune it.

After establishing the alloy, the forming type is the most important — and most overlooked — aspect when discussing stainless steel. The problem of ignoring the forming type is that an alloy like 316 can be annealed (heat treated to reset the material to its base hardness), or it can be forged (or cold rolled), and work hardened up to more than double its original hardness. Therefore, it’s not enough to simply say “it’s 316L” and expect a magical combination of speeds and feeds. The cutting solution for something at 180 Brinell is entirely different than something at over 400.

Lastly, hardness is as simple as the above point. When in doubt about the material, just find the real hardness from the material certification and base your starting speeds and feeds there. A softer stainless can be treated more like a low-carbon steel, and a harder one has many parallels to titanium. There may be some further tuning required if you witness premature tool wear, especially chipping and built-up edge. However, those are much easier to solve as you refine your process rather than scratching your head with the wrong initial cutting parameters.

Once you are talking about stainless with the right mindset, next you must address process specific issues that could be hindering your performance, or ability to adapt to stainless steel and its many forms.

First is coolant use and delivery. If you do dry machining or minimum-quantity lubricant (MQL), you can continue with these processes, but be aware of new potential for built-up edges compared to standard steels. If coolant is used, it must be applied in large amounts. High pressure and volume are a must with any material that makes long chips, and especially when those chips are considered sticky. If you feel your coolant is inadequate for the task, it may be time to upgrade that system, or into a machine with through-spindle coolant.

Next is to explore your tooling. For the most part, end mills can cover a broader range of materials. There is a lot of overlap for steel and stainless-steel solid end mills and drills. However, for inserted tools, whether it’s turning or milling, you will need to source material specific inserts. Due to the gummier nature, a dedicated insert for stainless will be slightly sharper to combat this and reduce the cutting forces for harder applications, thereby reducing chatter and protecting the machine long term.

While it may not be technical, some headache you may be experiencing is administrative in nature. Being late on an order because of mishaps at the machine is a pain and will make the mastery of stainless feel more cumbersome. Planning for this new venture with some sensible spare tooling will go a long way. A shop should also allow adequate time for jobs at first, until you get your standard processes settled in. Lastly, make sure your people are a part of this as well. Any new business strategy takes time to develop, so while some may see training events as time away from the shop, that small time away from the shop can shorten the learning curve in a big way.