We are reviewing Constant Surface Speed (CSS) on a LeBlond RKL Series manual lathe in a series of posts. See this introductory post on what CSS is and its benefits.
We are primarily reviewing the Newall E70 CSS Digital Readout (DRO). There are three primary modes on the E70, applicable to most CSS-equipped DROs. There is standard CSS mode, RPM/Diameter mode and Surface Speed mode.
Constant Surface Speed Mode
You would use CSS mode for single point turning tools like a RKL Series manual lathe where the diameter of your workpiece changes significantly. In Surface Speed mode, an operator enters the surface speed in inch mode (surface feet/minute or SFM) or millimeter mode (surface meters/minute or SMM). As the operator moves around the workpiece, the DRO will adjust the RPM of the spindle to maintain a constant surface speed.
In RPM/Diameter mode, the operator knows the RPM they want to use at a certain diameter of the part. The DRO calculates the surface speed, using this formula:
SFM = d x RPM/3.82
The DRO adjusts the RPM of the spindle to maintain the same surface speed as the workpiece is machined.
Maximum RPM is a safety feature on Newall’s E70 Constant Surface Speed DRO and most other units. When machining at a high RPM on an unevenly balanced workpiece, there is a vibration on manual lathes that can become dangerous. On the E70, you can set a maximum RPM for each job that is less than the maximum lathe RPM.
In the meantime, check out video footage of CSS in action on a LeBlond RKL Series manual lathe. The position of the cross feed on the x-axis is displayed in the top left window. The carriage position on the z-axis is displayed on the bottom left window, and the message display window is on the top right, showing the complete setup of the DRO including settings and functions:
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