July 24, 2003
Coil-fed stamping presses are nothing new, but coil feeding processes have changed a lot since the days when press feeders were driven mechanically by crank motion. Influencing these processes are differences between transfer and progressive tooling.
Stamping processes revolve around two basic styles of tooling, transfer and progressive dies. They both require feed-to-length systems but differ in many key areas. Blanking processes could be added but are progressive in nature.
Transfer tooling requires material to be fed into a press and through the tooling, severing the amount of material required to make a blank that is then transferred to the next operation. This transfer occurs within the same press or into another press to work the blank into a finished part.
Progressive tooling requires material to be fed at a preset coil length into a die that works it with each stroke into a finished part by the end of the die. The finished part typically is not severed from the material strip until the final operation, as compared to the transfer tooling, which severs on the first operation.
Each die has a carrier that elevates the material out of the working portion of the die to allow the feed system to move the material progressively from operation to operation within the die. The carrier strip defines the passline of material within the die.
For safe operation of a press, electrical signals, or interlocks, monitor certain events, such as coil feeding. While each process requires more than electrical interlocks between the press and the coil feed line, this article focuses on variables that affect a line's timing to maximize production.
Most feed systems for press applications are feed-to-length. The feed system receives an initiate signal from the press, feeds at a preset length of coil, and waits for the next request to feed. In the most basic interlocking sequence between two pieces of equipment, all that is needed are an initiate signal from the press and a feed complete signal from the feeder.
The resulting operating speed of the feeder, either an air feeder or electronic roll feed, often is through trial and error to ensure sufficient time to feed to length to prevent interference with the tool action in the die. A feed complete interlock control initiates a stop signal if the feed is not positioned by a predetermined point in the press stroke.
Progressive tooling also uses a mechanical interlocking sequence to allow the tooling to position the strip within the die before closure. To accomplish this, the feed system must release the material from the air grippers or feed rolls. However, if the material is released without being held in place by something else, it can fall out of the tool. Most feed systems will not release the strip until something in the tool engages the material and holds it securely and continues to position it within the die.
Because tooling uses pilot pins to secure this function, the interlock is called the pilot pin release. This is the signal between the press and feeder that allows the feed system to release its hold on the material while the pilot pins are entering the material. When the pilot pins have entered the material fully, the feeder must again grip it to ensure that when the pilot pins leave the material (on the press upstroke), the material will not lose its position within the tooling.
A press is at top dead center at 0 degrees, downstroking to bottom between 0 and 180 degrees, at bottom dead center at 180 degrees, and returning to top between 180 and 359 degrees.
Stamping processes parameters are defined in degrees. For example, a press stroke is defined as 360 degrees of rotation of the press crank. Feed signals can be defined as initiating at 270 degrees, or lubrication may be turned on at 300 degrees. A press is at top dead center at 0 degrees, downstroking to bottom between 0 and 180 degrees, at bottom dead center at 180 degrees, and returning to top between 180 and 359 degrees (see Figure 1).
Tooling parameters are defined in inches because a die's working distance is measured in inches from fully open to fully closed and bottomed out. Pilot pins may begin to engage or disengage at 4 inches off bottom, and the die could be fully closed at 2 in. off bottom. For electrical interlocks, tooling parameters typically are converted into degrees to implement proper timing.
The feed window is the total amount of press time in degrees of stroke that the press allows for feeding material safely into the die.
Transfer and progressive tooling differ in the amount of feed window each has. The feed window is the total amount of press time in degrees of stroke that the press allows for feeding material safely into the die. The process defines these parameters as the feed initiate on angle and feed initiate off angle. The total degrees in between these two angles is the window (see Figure 2).
Progressive dies allow feeding to begin as soon as the pilot pins clear the material and the carrier strip has returned to passline. Transfer dies cannot be fed until the transferring mechanism and the part are both safely out of the way of the feeding material, which often reduces the feed window to 40 to 100 degrees.
Press mechanics also influence the feed window. Many processes engineers overlook this area during new tool setup evaluation. In the section on pilot pins, an example was given of where in the press stroke pilot pins enter the material. For the material to be in place before the pilots enter the coil, the material feed motion must be complete. This parameter (the degree position in the press stroke at which the pilots enter the material) often is incorrectly used as the feed initiate off angle. This does not take into account that the interlock looking for feed complete initiates a press stop request that disengages the clutch and applies the brake. Stopping the press in the stroke takes more degrees of press rotation, which could damage tooling if the press closes the die.
Stopping the press in the stroke takes more degrees of press rotation, which could damage tooling if the press closes the die. This press stopping time (the difference between the time the press is told to stop and the actual sensing of the press being stopped) must be checked at the maximum press stroking speed for each die.
This action triggers the press stopping window, which is the distance (in degrees or time measured) the press slide travels after the brake is applied until all motion is stopped. This press stopping time (the difference between the time the press is told to stop and the actual sensing of the press being stopped) must be checked at the maximum press stroking speed for each die (see Figure 3).
Today stampers use controls that synchronize the press and material feeding to maximize press speed and the die feed window. Understanding this synchronization is important for proper press and coil feed setup, even if the system doesn't have extensive controls.
Optimizing a press production system is based on the relationship among three angular settings in the press stroke (360 degrees):
Maximum stroking speed is achieved with the largest feed window possible while still allowing the feed complete check angle to stop the press safely in case of a feed fault. This feed window is the difference in degrees between the feed initiate on and off angles. The feed initiate on angle is the earliest press angle at which material can be fed safely into a press. This signal takes into account all variables, from pilot pins to part exiting. The feed initiate off angle and the feed window are related to the feed complete check angles as a function of the press to coil feed interlock signals.
The feed complete check angle must be set larger than the feed initiate off angle to allow sufficient time for the feed complete interlock signal to be generated and sensed by the press controls. It cannot be set so late in the press downstroke that the clutch stop command will not stop the press before closing on the material.
The feed complete check angle must be set larger than the feed initiate off angle to allow sufficient time for the feed complete interlock signal to be generated and sensed by the press controls. It cannot be set so late in the press downstroke that the clutch stop command will not stop the press before closing on the material (see Figure 4).
Synchronizing control systems maximize feeder performance in a given feed window by utilizing all the feed window available. They do this by calculating the proper feeding speed based on feed length and the window. These systems still need to be interlocked with a proper feed complete check to ensure that die safety is not compromised by pure calculated speed.
Typical Setup Procedure. The minimum safe angle for the die being tested should be selected and then that angle used as a benchmark. The final setting of this tested feed complete check angle is used as the basis for:
Following these procedures may cause operators to calculate a much smaller feed window than previously, which may affect the overall top speeds the system can achieve, because feed windows and required feed lengths determine the feeding speed required. This may result in undersizing the feed system for the application if the equipment supplier is given information that does not truly fit the process parameters. In some cases, the allowable press speed calculated (based on available feeding performance) exceeds the available maximum press stroking speed, which might actually slow down the feeder, saving equipment wear and tear in the process.
Ted Sberna is a stamping equipment and safety consultant with Applied Engineering Concepts, P.O. Box 2135, Georgetown, KY 40324, 859-333-9579, fax 502-867-4892, firstname.lastname@example.org, www.appliedengineeringconcepts.com.
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