CNC Turning of Stainless Steel: Chip Control & Increasing Feed Rates
This post focuses on a medium-volume turning operation for a general machine shop in Central New York. We’ll be talking all about cutting tools, machine tools, speeds and feeds. This machine shop fabricates custom specified stainless steel shafts for use in the paper and print machinery industry. The shop typically works in batches of around 250 workpieces, with a total yearly volume of around 5000 pieces spread out over four different variations.
What is Turning?
In a typical turning application, the workpiece is rotated on a spindle and the cutting tool remains stationary, traversing along the axes of motion to produce a precise cutting action. The turning action removes material and creates chips. A unique challenge in many turning applications is the formation of (what can be) long continuous chips. Uninterrupted chips are generally bad. Short smaller chips are generally good. Long, uncontrolled, continuous chips can lead to problems like nesting which will impact cutting tool performance. Or, worse, can jam/clog the entire machine resulting in scrapped workpiece and an unexpected breakdown.
In this application, the machine being used is a Dooson 2100 CNC Lathe. The parts being machined are 304 stainless steel. Stainless steel is a material that is particularly susceptible to long unbroken chips and nesting. The machine shop wanted to control the chips while at the same time they wanted to improve cycle time: getting the same number of parts done in less time.
Increasing Output while Avoiding Failure
For this machine shop, unexpected breakdowns were unacceptable. The machine operators already had methods in place to avoid the nesting of long stainless steel chips. The question was: How to increase feed rate while at the same time avoiding this known failure mode?
The Horizon Solutions Cutting Tool Specialist analyzed the part and the machine configuration. For this example, we will not give exact part dimensions or exact depths of cuts – or other proprietary machining parameters. We can say that a sample part might be 21″ in length and 1″ in diameter. As shown above in Figure 1, each part has a taper on both ends. Let’s assume for our example that the taper distance is 0.050”. The Specialist calculated a certain CNC setting resulting in a 25% increase in feed rate. This setting avoiding nesting due to the intrinsic chipbreaker of the insert. The grade chosen was a T8315, with a FF chipbreaker engineered for a “perfect fit” of chip control and a feed rate “sweet spot”.
There was a testing period, quality control was verified. The parts were produced with a satisfactory tolerance and surface finish, up to the end users’ specification. The machine operators confirmed that the chip control was beyond satisfactory. The 25% increase in feed rate meant a shorter cycle time, resulting in more parts being finished in the same amount of time.
Success Going Forward
As previously mentioned, this machine shop does a lot of general work. They always have opportunities for ad hoc jobs – that is, if they have the CNC machine tool time available. A typical batch of stainless steel shafts is 250 pieces, with four different possible variations. The turning insert selected was a DNMG 431 FF. This Pramet insert was selected and approved by the shop because of (1) the results of the test and (2) because the same insert could easily be used on all four of the different versions of the shaft parts. No additional or redundant tooling was required.
Each part version does have a different length and different diameter, but will follow the general geometry illustrated. This shop uses a value of $100/hour for “Shop Rate” in determining costs of machining. The shop manager and the Cutting Tool Specialist will be reviewing and comparing the calculated theoretical savings of all the batch jobs of all four versions over a rolling 12-month period versus the actual savings. We are very excited about the potential cost savings, and even more excited about the prospect of “creating” more available CNC machine time for the shop to take on new business in 2017!
What turning application successes have you had? What types of grades or chipbreakers have you used for chip control?