Servo Applications: Prolonged Die Life; Increased Tolerances, Faster Forming Speeds

This is another comparison between the typical press and servo press.  To the left side; the parts are rails of stainless steel with a production run of 100,000 pieces.  With the typical press the speed is only 20 spm but in the servo the average is 50 SPM with 40 times the die life and better tolerance because for pendulum motion profiles and ability to slow the stroke speed at BDC. That's money saved! The right side are handles made of hi-tensile steel.  This is just blanking so the speed is just about the same.  You do not have to worry about the die speed angle.  But there is a difference in die life and tolerances, again because you can slow down the stroke speed at bottom dead center.  And, of course, the tolerances are getting better because when you’re at bottom dead center of the working area you can slow down the speed and that will increase the die life.  Lesson learned; you can run tighter tolerances on the servo because you can run with slower speeds at BDC, whereas with a conventional press you have to open the tolerances because there is more friction, more heat.  You’ll end up breaking punches.

Servo Applications: The Ability to Heat Material In-Die

Like a digital camera, a mac book, where you can heat the material, typically only able to be done in a hydraulic press.  You see the curve a to b you load on the material face down.  B to c you’re holding, without pressure, so to heat the material of magnesium to specific temperature to then down and then go back.  D and E is where the metal is being pushed to create the sides of the chassis.  The dies is actually heating.  This could be magnesium, titanium, other alloys that require heating or warm forming.  8mm and 0 mm is height above bottom dead center.

Servo Press Technology: Breaking Down the Choices

The flexibility and efficiency of servo presses are challenging tooling engineers and part designers to rethink the way they form.  For any company hunting to gain a competitive advantage, having a servo press will be a mainstay.  When it comes to a servo press investment, there are varying differences between manufacturers to consider that will impact performance, reliability and cost.  While you dream of new and creative ways to form, be sure to rethink power, construction and control.

Rethinking Power

All servo presses replace the main motor, flywheel, clutch and brake with a servo motor.  The differences are the types of motors and drive trains.  The first option is a “hybrid” drive configuration.   With a hybrid, a knuckle or link-type drive train is used with a standard high rmp/torque AC servo motor.  This hybrid drive configuration allows for a compact and efficient motor, but has drawbacks.  For one, the need to “down-gear” a high-rpm motor to generate torque requires more parts, belts and mechanical components, increasing maintenance and service.  Because of the knuckle or link-style, you still rely on mechanical linkages to generate thrust which can negatively affect tonnage and torque, especially high in the stroke.
The second option is a “direct drive” servo motor.  This motor has a dramatic hi torque/low rpm ratio. In some cases the max rpm is only 340, with a torque rating of 14000 N-m. At such a ratio, less than 40% of the torque is required to accelerate the drive train, leaving 60% or more to accelerate or decelerate the slide or develop press force.  This configuration eliminates energy-consuming linkages, down-gearing and other mechanical obstacles. The pinion shaft is connected directly from the motor, and in some instances, directly to the motor (part of the motor, actual rotor.) Only the pinion gear and main gear are needed.   This configuration provides better transmission, less maintenance and more consistent energy thru the stroke. Some manufacturers have large capacitors to capture any unused energy, like when the press is not under load, and use it when the time is right.

Rethinking Press Construction
A servo press’s programmable slide speed, position and dwell will open your mind to creative metal drawing and in-die processes.  But this versatility introduces new strains, twists and forces on the press frame.  Most servo press manufacturers offer conventional gap frame, straight side, or tie-rod constructions.  However, other manufacturers argue that conventional structures are not rigid enough to hold tight tolerances under longer dwells and draws without stretching, and elongating the frame. These manufacturers employ a completely different design. Some call it monoblock, others call it honeycomb.  In essence, both design the press with low-impact speed in mind and are intended to “box-in” horizontal and parallel support on the slide and bolster, which are located in the center of the frame.  Thus, all support is targeted to where it’s needed most, reducing the risk of press damage, as well as floor space.
Speaking of press damage, you cannot overlook the importance of an overload protection system.  Most manufacturers offer an upgraded version of the traditional hydraulic overload, understanding that new forces are being entered into the forming process which could alter the way a hydraulic overload works.  Other manufacturers have done away with the hydraulic overload and replaced it with an electronic, programmable overload , which may perform better over longer dwells operating closer to rated tonnage.

Rethinking Control

Every manufacturer gives you a means to select and adjust pre-programmed stroke profiles or create freestyle profiles. Beyond that, a servo press controller should offer more than what you’re used to.   You should have more standard ways to integrate and orchestrate peripherals, a robust back end for collecting and analyzing data, and a user-friendly front end to put that data to use.  The controller is the key to making your servo investment pay off.  If you’re not using the most up-to-date and scalable control technology, you’ll soon be upgrading and paying more.
As you weigh the options in servo press technology, you have more choices than ever.  With these choices come more information and a better understanding of what to expect, which should help you make the right decision.