Graph by Okuma
HSSC prevents chatter by automatically adjusting variables according to pre-set or custom-programmed parameters.

Automated Chatter-Prevention for CNC Lathes

June 25, 2014
No manual control necessary to avoid harmonics, maintain high productivity Harmonic Spindle Speed Control Variable Spindle Speed Threading Reduce vibration, reduce scrap

Machine tool builder Okuma wants its customers to know they have the means to prevent lathe chatter without manual control. Eliminating harmonics that can produce tool chatter or poor surface finish is the real goal, to improve turning and enhance the quality of finished parts — that is, to improve productivity.

According to a white paper recently released by Okuma, operators of its horizontal or vertical CNC lathes with P200 or newer control (also available as an option on the P100 control) have tools to prevent chatter by allowing for RPM changes. Harmonic Spindle Speed Control (HSSC) and Variable Spindle Speed Threading (VSST) — functions available with Okuma’s THINC®- OSP CNC control — are effective at solving harmonics problem, according to the developer. In particular, HSSC and VSST are useful when machining parts with high L/D ratios, thin-walled parts, or thin-walled parts with threads.

Harmonic Spindle Speed Control

The concept of Harmonic Spindle Speed Control (HSSC) is not new, but it is a technique that has had to be executed manually. “Basically this function constantly changes the RPM of a lathe spindle in order to prevent the build up of harmonics or vibration (chatter) during a cutting operation,” according to Okuma.

To implement HSSC, a machinist would carefully adjust the spindle speed control knob to vary the RPM.

Now, machinists can use the HSSC function built into Okuma’s control to alternate the RPM of a CNC lathe spindle on a constant basis in order to prevent harmonics or chatter (see diagram).

With this function, the OSP control (along with some NC code) automatically adjusts the following variables according to pre-set parameters, or parameters that are programmed to suit particular tasks:
•  Amplitude (Q): the percentage amount of variation in the RPM
•  Interval time (R): how long to remain at the peak or valley of RPM change, and
•  Speed variation cycle (P): the pitch of the RPM variation.

The result is reduced or eliminated chatter during turning operations.

Okuma advises using HSSC when absolute accuracy is required during difficult machining tasks.

Variable Spindle Speed Threading

In the past, changing RPM during threading was nearly impossible because most machines cannot maintain thread pitch when changing RPM, thus scrapping the part. However, Okuma reports that it identified some thread cutting applications in which chatter can exist and can be very difficult to control without the ability to vary the RPM.

Now, with the power and speed of the OSP CNC control, it has developed the means to vary RPM while accurately maintaining the lead during threading, thus preventing the introduction of chatter. Okuma noted this novel function is used widely on its LOC lathe product line by machinists supplying oil-and-gas industry parts.

VSST is a standard function with Okuma LOC machines, and is available as an option for others. It allows an operator to use the “spindle speed override switch” during the threading process to adjust the spindle speed manually without losing the thread pitch. By varying the RPM, harmonics build-up is avoided, so chatter is prevented, and providing a quality thread.

Okuma noted that VSST allows users to do two things:
•  First, and most common, it lets users adjust RPM manually during the threading cycle. This function is enabled either by turning it
on by parameter (for continuous use) or by M-code in the program (for occasional use)
•  Second, VSST allows users to reprogram the speed for each threading pass—
a function that’s not often used. Most users set the parameter and vary the speed manually, so no programming is required.

Okuma’s HSSC and VSST are effective in controlling harmonics and dampening vibration. Machining parts for aerospace, oil drilling equipment, automotive, and agriculture equipment industries each have applications that require harmonic control. As various industries increase the need for smaller, more precise parts, often fabricated from alloyed steels or composites, there is no need to fight vibration-induced defects that cause scrap. When machining parts with high L/D ratios, thin-walled parts, or thin-walled parts with threads, HSSC and VSST help to prevent defects and maintain high productivity.

Avoiding scrap caused by harmonic-induced vibration can be a challenge using conventional solutions. HSSC and VSST functions on an Okuma CNC lathe make it easier to achieve consistent surface finishes for your parts. This creates manufacturing efficiencies that increase productivity and establishes a competitive edge in the marketplace.

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