For any roll former operator, two fundamental questions are key to achieving optimal performance: "At what speed can I run the line and still ensure proper cutoffs?" and "How accurately can I achieve the desired cut length?" These questions are inherently connected, as faster line speeds make it increasingly challenging to maintain precise length tolerances for any given shape. In this post, we’ll explore some of the most common and effective methods for measuring accurate cutoff lengths in relation to line speed, helping you balance efficiency and precision in your roll forming operations.
Positive Type Measuring Method for Cutoff Tooling
If a roll-formed shape requires a length accuracy of ±1/64 inch or better, using a positive stop has traditionally been the only option—and it remains the most cost-effective and reliable method. However, with some of today's high speed presses, especially the air presses available that can run at very slow speeds (possibly below 75 F.P.M.), can hold this tolerance using a photoelectric eye measuring system or a rotary encoder system.
A positive stop is made of a traveling plate on the measuring table that is attached directly to the flying cutoff die by means of a rod. When the part hits the stop plate, the stop plate pulls the die along, and the die hits a switch on the bolster which trips the press.
The only advantage of this type of measuring is the ability to hold close-length tolerances. But that is a big advantage.
A key disadvantage is the high likelihood that light-duty parts will buckle when they hit the flag at higher speeds. Some light-duty aluminum parts cannot use the positive stop at any economical speed without buckling. Also at increased speeds, if the part is too strong to buckle, it might still flex and bounce on the flag causing variances in the length. There might also be damage to the end of the part at times, depending on the part cross section. These problems all contribute to keeping line speed down when running a positive stop.
The positive stop is an accurate, simple measuring system with setup being easy. You will just measure between the blade and the stop plate to set length without much further adjustment. It's up to the experience of the designer and operator to determine ahead of time if the cross section and/or gauge of the part is strong enough to be run economically using a positive stop.
Flag Trip Cutoff Measuring Method
Another method of measuring is the flag trip method which hits a plate attached to a pivot pin. The whole unit resembles a flag. This method is probably the most widely used, with many variations. The switch, attached to a housing, is adjustable for the full length of the runout table. When the lead edge of the part hits the flag, the switch trips the cutoff press. The flag then pivots out of the way, letting the part pass.
The advantage of the flag trip method is the ability to run a given shape at significantly higher speeds than with the positive stop method. This is due to the absence of resistance when the part contacts the flag, preventing any buckling of the shape on the runout table.
The flag trip method would always be used if it weren’t for the fact that it results in a greater error of the cutoff length. The accuracy depends on many factors, such as the stability of the line speed, the consistency of the flag switch, keeping the stock at 90 degrees to the flag when it hits the flag, etc. However, most important to accuracy is the consistency of the press to repeat, keeping the identical elapsed time from signal impulse to hitting the part with the blade.
All measuring and press controls associated with all measuring systems but positive stop systems should be D.C. filtered. The sine wave variation in AC 60-cycle current can result in a prohibitive error by itself. When using a flag trip, the associated cutoff equipment is much more critical to the accuracy of the line. So, many times, if no more money can be spent on the line, the choice must be to use a positive stop.
Using a Gripper with the Flag Trip Method
The "positive stop" and the "flag trip" are the two primary measuring methods in use. However, when the line needs to run faster than the positive stop allows, yet requires its level of accuracy, or when the flag trip introduces too many variables with a particular press, a solution that combines both speed and accuracy becomes essential. A "gripper" can fulfill this role by offering the speed benefits of the flag while remaining independent of the press for accuracy.
The gripper is made of an air cylinder, a gripper pad of serrated steel or polyurethane rubber which pivots down onto the stock, a fixed bottom block of serrated steel or polyurethane rubber, and a framework which supports the air cylinder and pivot block.
The flag switch now triggers a valve, which fires the gripper cylinder and pushes the pivoting gripper pad down onto the stock. When the gripper grips the stock, the stock moves the die forward. The forward motions of the die trip a switch on the bolster to fire the press. The air cylinder is much quicker and much more repetitive than a crank type punch press. Since the stock is already gripped and moving at line speed when the press is actuated, it does not matter if the press is late or early. The length is already determined. To shave off time for high-speed operation, it's possible to trip the press at the same time as the gripper or by a pressure switch which reads pressure buildup from the gripper cylinder.
Electronic Cutoff Measuring Systems
Electronic measuring systems use a rotary encoder measuring wheel, a pressure wheel, and a predetermining counter to read the counts put out per revolution by the rotary encoder while riding on the material. These pulses are converted to the pre-set length by the controller, which then outputs a signal.
There are two popular methods of using rotary encoders with flying cutoff presses.
- "Open Loop" measuring uses rotary encoding to produce an output signal at a pre-set length, which then fires the press. When the cutoff blade or punch enters the material, the die is pushed by the material. When the cutoff cycle is complete, the die is returned to its rest position by an air cylinder or springs. “Open Loop” is used extensively with air presses or high speed hydraulic presses because of the press’s ability to be very quick and repetitive. However, with any “Open Loop” system the faster the line speed, the less accurate the lengths will be.
- “Closed Loop” electronic measuring operates much like “Open Loop” systems except that the die is accelerated to match line speed by a servo motor. The correction of any error is controlled by feedback from a second rotary encoder attached to the servo motor which tells the controller where the die is relative to the proper cutoff point. When the die reaches line speed match, and “locks it” electronically at the proper cutoff position, the controller sends an output to fire the press. The “Closed Loop” system is usually very expensive, but can be worth the price when high speed and accuracy are required.
Piloting Cutoff
With the increasing use of prenotching in-line to eliminate costly secondary operations, the use of piloting in the cutoff die has grown as a form of length measuring.
The simplest form can be demonstrated in a blade-type cutoff die for an angle, where a prenotch slot on the edge of the material must be picked up. The pickup finger is spring-loaded and constantly rides the stock edge until a notch comes through. The finger then drops into the slot pulling the die along until the press is tripped.
A 45 degree angle is ground on top of the point of the pickup finger, and the outer area of the blade is also ground on 45 degrees. As the blade cuts, it also pushes the pickup finger out of the stock. A variation of this that works for any formed shape is to have a cam pull the pickup finger out of the stock as the blade is cutting.
A couple of basic methods are used to keep the pickup finger from dropping back into the slot it was just pulled from.
- Spring load the finger forward and leave enough slope in the finger retainer so that when it is pulled out of the slot, it will pop forward under the stock.
- Provide a lock-out of some sort that will hold the finger out at least until the die is returned to its resting position against the die stop.
If the pickup finger must pilot on one of two or more openings in line, the timing must be set up so the finger picks up in the correct opening. An air cylinder is used to hold the pilot out.
If the finger is to pilot on the first opening through, then the release after the cutoff cycle is done; the finger must be delayed to let the succeeding openings pass before dropping the finger back onto the stock. This can be done with a time delay, a crank switch, a flag trip by the lead edge of the next part coming out, or by a rotary encoder signal.
If the finger is to pilot on an opening other than the first one through, or pilot on one of a continuous line of openings, then the release of the finger must be signaled by the lead edge of the part coming out hitting a flag trip on the runout table, or by a rotary encoder signal. This will drop the finger onto the stock between openings.
Piloting and Accelerating
Sometimes, the line must pilot on a prenotched opening, but the gauge of the part is light and the die fairly heavy. As the finger drops into the opening, it tears the stock when trying to get the die moving. In cases like this, the opening for the pilot-in must pass the pickup finger. Then release the finger onto the stock and accelerate to catch the opening at the same time.
In most cases, the press is tripped with a small switch mounted in the die, which is actuated when the finger drops into the opening. This eliminates timing problems and jam-ups from miss-hits.
As we've discussed, there are several essential considerations for roll former operators concerning line speed and cutoff length accuracy. Faster line speeds make it challenging to maintain tight length tolerances, so selecting an effective measuring method is crucial.
The positive stop method provides high accuracy, especially for length tolerances of ±1/64 inch, making it a reliable choice but with speed limitations and potential part damage risks. Alternatively, the flag trip method enables higher speeds but sacrifices some accuracy due to factors like line stability and switch consistency. For those seeking both speed and precision, a “gripper” system combines elements of both methods, while electronic cutoff systems (open and closed loop) offer advanced accuracy for high-speed operations, especially with costly closed-loop systems that provide real-time adjustments.
Additionally, piloting methods assist with precision when prenotching is used, ensuring accuracy even in complex setups. By understanding the strengths and limitations of each method, operators can select the best approach based on production needs and tolerances.