Gap Frame presses

Gap Frame Presses C05.doc 1993 2005 Rev September 26, 2005 Smith & Associates, 530 Hollywood Drive, Monroe, Michigan 48162-2943

GAP FRAME PRESSES Gap-frame or C-frame presses derive their name from the C-shaped throat opening. In addition to the familiar open back design, this style of machine has a long history of use for portable rivet and bolt hole punching. The press-brake is a type of gap-frame press. Features of Gap-Frame Presses Gap or C-frame presses have many desirable features. These include excellent accessibility from the front and sides for diesetting and operation. The machines also cost much less than straightside presses. The open back is available for feeding stock as well as ejection of parts and scrap.

Example of Old Style Gap Frame Press

Figure 1. An older style open back inclinable (OBI) gap frame press with all guarding removed. Note that the use of the foot actuated mechanical full revolution clutch shown is no longer permissible for most applications. A further advantage of a gap frame press is that the machine is easier to set-up than a straightside press. The diesetter has much greater freedom of access to locate and bolt the die in place.

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Gap-frame presses generally have less overall height than a straightside press of comparable tonnage. This is a valuable consideration when overhead clearance is limited. Example of Gap Frame Press Figure 1 shows an older style of gap frame press with all guarding removed. This style of machine is known as an open back inclinable or OBI press. The press frame secured in a cradle permits the machine to be inclined backward. The machine must be securely anchored to the floor to prevent falling over backwards if inclined. This facilitates gravity loading as well as part and scrap discharge out the open back of the press. 1 The frame of older OBI presses is of cast construction. The most commonly used material is gray cast iron. Figure 1 illustrates two pre-tensioned tie-rods across the open front of the machine. Lugs are cast into the frame of the machine to accept the tie-rods, which are installed as an option to limit machine deflection under load.

A Modern Open Back Stationary (OBS) Press

Figure 2. A modern open back stationary (OBS) press featuring heavy welded plate construction to limit deflection and two slide connections. Verson Corporation The development of timed air blow-off devices and a variety of small conveyors has lessened the demand for the OBI style of presses. Today, many OBI presses are operated in the vertical position. While the OBI style is not obsolete, many press builders press builders supply OBI presses only on special order.

1 D. Reid, Fundamentals of Tool Design, Third Edition, Chapter seven, Design of Pressworking Tools, The Society of Manufacturing Engineers, Dearborn, Michigan, 1991. The source art was taken from this publication.

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Open Back Stationary (OBS) Gap Frame Presses The OBS gap frame press shown in Figure 2 is more compact and often a more robust machine than the older OBI style that it has largely replaced. The OBS press has a box-like structure. Nearly all OBS presses are fabricated of heavy steel plate and assembled by welding. Figure 2 illustrates a machine having two slide connection points. Advantages of Steel Construction An advantage of cast steel or fabricated steel plate construction steel plate over cast iron is that steel has a substantially higher modulus of elasticity and hence, proportionally greater stiffness. The modulus of elasticity for steel is approximately 30,000 ksi (206 gPa) compared to as low as 9,600 ksi (66 gPa) for ASTM class 20 gray cast iron and approximately 24,500 ksi (169 gPa) for ductile (nodular) irons. However, Gray or ductile cast frame construction may be preferred because these materials have superior vibration damping capacity compared to steel. In order to achieve low deflection, the cast frame can be made with a heavy cross sectional design to achieve the desired rigidity while retaining the vibration damping benefits of cast irons.

Limitations of Gap Frame Presses The chief limiting factor of this type of machine is that it has more deflection than a straightside press for a given load. The deflection has both a vertical and angular component. The angular deflection or misalignment occurs due to the spreading of the throat opening as tonnage is developed. In many applications, this angular misalignment under load may not be objectionable. This style of press is popular for short run work where high accuracy of die alignment or close part tolerances is not necessarily controlling factors. However, straightside presses are generally recommended for any application where angular machine deflection would cause unacceptable part quality and accelerated die wear. The lower cost of gap frame construction machines may be poor economy if accelerated tooling wear and quality problems result. Many manufacturers have sourced out gap press jobs to contract stampers because of intolerable tool wear and quality problems. In some cases, workforce skills are blamed for the difficulty. In the writers experience, the contract shop will simply repair the die and run the job in a straightside press. Simply stated, there are a number of jobs that are not suited for operation in a gap frame machine. Success Factors in Precision Applications Precision progressive dies can be successfully operated in gap-frame presses in spite of the angular deflection limitation. Success factors include:

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1. Maintaining excellent machine fit and alignment. 2. Derating the press tonnage, this reduces angular deflection. 3. Operating at high speeds so that much of the punching work is done by the kinetic

energy of the slide, which does not produce machine deflection. 4. Installing properly pre-stressed tie-rods with tubular spacers across the front

opening to reduce deflection. Measurement of Gap-Frame Press Stiffness An older accepted American standard that has been used by builders of gap-frame presses is 0.0015-inch per inch (mm per mm) of throat depth measured from the centerline of the connection to the back of the throat opening. This measurement includes both vertical and angular deflection. Angular deflection is by far the greatest concern because it results in misalignment between the punch and die. ANSI standard B5.52 M specifies both the allowable vertical and angular deflection in machines built to metric standards. ANSI Metric Total Deflection Test Method A practical procedure is to place a jack capable of full rated tonnage at the centerline of the slide and bed. The vertical deflection is then measured between the centerline of the slide and bed. It should not exceed 0.002-inch per inch (mm/mm) of distance from the centerline to the back of the throat opening. Some press builders design for a lower value of deflection. For example, Danly Machine allows only 0.0015-inch per inch (mm per mm) for standard frames. 2

To strictly follow the ANSI metric angular deflection test method the angular misalignment is measured with a test bar and two dial indicators. The procedure is as follows:

1. A rectangular test bar is attached to the slide with an angle plate or other means to assure that it is at a 90 angle to the slide face.

2. Two dial indicators spaced 3.937-inches (100 mm) apart are placed on a single

supporting rod attached to the press bolster. 3. Both dial indicators are carefully adjusted to zero.

2 The Danly Machine OBI frame deflection standards as of 1994 were the identical to the older Joint Industrial Council (JIC) standards in use before the adoption of ANSI metric standards. The older JIC standard is much more conservative in rating allowable press angular and total deflection than the metric ANSI standard. See Mechanical Power Presses General Purpose Single Point Gap Type (Metric), ANSI Standard B5.52M, American Society of Mechanical Engineers, New York

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4. A large hydraulic jack is placed directly under the connection and full press tonnage applied.

5. The dial indicators should show a difference of no more than 0.0047-inches (0.12

mm). This corresponds to an angular deflection of 0.0012-inches per inch, or 0.0144-inches per foot.

Since this measurement is intended to pick-up an angular value, measuring the difference from front to back across the slide face will give similar results. For many applications, angular misalignment under load is harmful. If the job cannot be run in a straightside press, some reduction in angular deflection can be achieved by installing tie-rods across the open front of the press. Adding Tie-Rods Some older gap-frame presses have existing lugs for tie-rod installation in the front of the machine. A few domestic press builders also are currently tie-rod lugs them on new machines. In other cases, the manufacturer may supply lugs, which can be welded in place. Provided the rods are properly pre-stressed, a significant reduction in deflection results. The best method of installing tie-rods is to use them in conjunction with tubular steel spacers around the rods. The spacers help ensure that the tie-rods are not over-stressed, and serve to further stiffen the machine. In a case study of a 200-ton (1779 kN) gap-frame press installing properly pre-stressed tie-rods with correctly machined spacers, reduced the total deflection from 0.026-inches (0.66 mm) to 0.0134-inch (0,34 mm). The tie-rod diameter is 3.000-inches (76.2 mm). Each spacer cross-sectional area is 10.5 square inches (6774 square mm). 3 Important Considerations When Adding Tie Rods Figure 1 shows how tie-rods that are installed on the front of the press to reduce the angular deflection under load. The tie-rods are pre-stressed a fixed amount. Care must be exercised not to over-stress them, which can cause die misalignment. The use of carefully fitted tubular steel spacers around the tie-rods is advised. No attempt should be made to increase press force capacity by this method. At best, adding pre-stressed tie-rods to a gap frame press will result in reduced total angular misalignment. The physically limiting factor is that the cross sectional area of the tie-rod and spacer is small compared to that of the press frame. In addition, adding tie-rods to the front of the press limits access to the die opening, and makes the point of operation more difficult to guard properly.

3 C. Wick, J. T. Benedict, R. F. Veilleux, Tool and Manufacturing Engineers Handbook, Volume 2 fourth edition - Forming, pages 5-12 to 5-13, The Society of Manufacturing Engineers, Dearborn, Michigan 1984.

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The following design criteria are recommended if tie-rods are to be added. The spacer should have 1.5 times the area of the tie-rod. In addition to adding stiffness, the spacers will reduce the alternating load in the tie-rod thread. The tie-rod area should be sufficient to support half the force capacity of the machine. A conservative nominal pre-stress in the tie-rod is approximately 14,000 psi (96,516 kPa). Bulldozers Figure 3 illustrates a hydraulic bulldozer. Mechanical bulldozers originated in the middle of the nineteenth century.

A Hydraulic Bulldozer

Figure 3. A hydraulically actuated bulldozer style press. Pacific Press Technologies.

The early mechanically driven machines were typically double or triple back-geared and, hence capable of exerting great force. The design found widespread application in all sorts of heavy plate fabrication such as punching rivet holes and bending operations. Metal working bulldozers were in use before the development of the familiar earth-moving bulldozer. Bulldozers are essentially a powerful C-frame press laid on its back. They are still used for heavy fabrication work, and are built in capacities from 25 to over 1,000 tons (222 to 8896 KN). Modern machines are hydraulically powered which provides excellent control of the forming speed and force desired. Hydraulic bulldozers are excellent for straightening applications as well as punching plate and structural steel shapes. The dies required are simple and inexpensive. The ability to handle the workpiece with an overhead crane and the unrestricted work area suits them for forming hoops, rings and large irregular structural shapes. NOTES: _____________________________________________________ _____________________________________________________________

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Gap Frame presses

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