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Chapter 5-Designing for CNC Turret and Laser Fabrication

Turret presses, also known as CNC punch presses are particularly suited for low to medium quantity production runs. CNC presses are the work horse for "soft-tooled" manufacturing, and are widely used by members of PMA's Metal Fabricating Division. Their versatility and speed are constantly being improved, increasing the run quantities for which they are the most economical choice when compared with dedicated stamping press tooling and processes.

Equipment Characteristics

Machines are constructed with either a Cframe or a bridge frame design. See Figure 1. CNC presses vary considerably in size and speed. The smallest, and therefore least versatile of the group are those with 20 or fewer tools in the turret, 20 tons or less of press capacity and table size of 40 in. (1 m) square or less.

Intermediate-sized units may carry up to 60 tools, have up to 30 ton press capacity, and usually use a table of 50 in. (1.3 m) square. Larger machines carry as many as 72 tools, provide up to 50 tons of capacity, and feature table sizes as large as 60 in. (1.5 m) x 70 in. (1.8 m).

Operational speeds range from 80 to about 300 hits per minute (hpm). This rating is based on the one-inch movement of workpiece material between each "hit" or workstroke.

In practice, the highest operating speeds are usually associated with intermediate-sized machines. The larger machines are often run at less than maximum speed to maintain precision while accommodating the inertia of relatively larger and heavier tools and workpieces.

Operation

Regardless of construction, size or speed, all turret presses operate similarly. See Figure 2. A sheet of workpiece material, gripped at the edges by workholders, is moved across the table into position between the upper and lower portions of the turret by action of two precision lead screws (one in the X axis, the other in the Y axis). Meanwhile, the turret rotates until the appropriate punch and die set is in place.

With the turret pinned in position to assure precise alignment, the program activates the ram, pushing the punch through the workpiece. After the punch is withdrawn, the machine is ready to prepare for the next hit.


Figure 1. Typical turret press designs.

Some turret presses are constructed so that the crankshaft is used to both depress and withdraw the tool. In this situation, urethane strippers surrounding the punch are used to hold down the workpiece during movement of the tool.

Other machines are designed to withdraw the tool by spring action rather than with the ram. This requires a separate punch holder and allows for strippers of metal rather than urethane. Metal strippers can hold the workpiece more securely, particularly during forming operations.

Advantages and Limitations

The CNC press couples the unique advantage of highly flexible production of prototypes with relatively inexpensive production quantities. This is due to the extreme variety of standard punch configurations available which makes the cost and lead time associated with special tooling unnecessary.

Figure 2. (Top) In this closeup a sheet of workpiece material is gripped across the table into position between the upper and lower portions of the turret. The turret rotates to select and position the proper punch and die set. (Bottom) A view of the relationship of the turret table and workholders.

Because of the nature of the process, it is possible to make rapid changes in part configuration, and to make them "on paper" before committing them to metal. In effect, the designer has the unique opportunity of doing design development during initial phases of production.

A related advantage is the fact that lead time from the completion of design to the production of parts can be extremely short using CNC turret press technology.

Larger production quantities are also economically run in the CNC turret press in many cases. When automatic loading and unloading equipment is employed, long periods of economical, unattended operation are possible, but sweeping generalizations regarding length of run are often misleading.

At the beginning of a project many knowledgeable designers take advantage of the quick, low-cost, flexible design opportunities inherent in the CNC turret press, then evolve into more sophisticated and costly tooling as the increasing quantities warrant.

The exact quantity level at which special tooling becomes more cost effective depends on many variables, but in most cases involves several thousands of parts. In general, parts with highly irregular outer contours or large central holes--which require long machine operating time for the turret press-- reach the cross-over point for dedicated or "hard" tools at relatively lower quantities.

Certain design features make the CNC press a candidate for higher quantity production runs. For example, tightly spaced hole patterns and louvers as commonly used for ventilation purposes, could require the use of two or even three progressive stations in a hard tool die to space the openings--at considerable added cost. This added tooling cost gives the CNC press its economical quantity advantage in this example.

Another advantage of CNC press production is the extraordinary design flexibility in configuration and size of features within the part. See Figure 3 for sample parts. Newer machines, especially models with indexable tool stations, make the nibbling of very large and complex features a practical reality.

Figure 3. Sample parts produced on a turret press.

Nibbling, compared to processing with a single punch or special tool, has limitations regarding precision which the designer should keep in mind. Since a dimension of a nibbled feature is affected twice by the punch (once on each side of the feature), twice the normal punch tolerance is generally appropriate. Figure 4 shows nibble marks under high magnification.

Extreme flexibility of over-all part size is another advantage of CNC press production. In practical terms, the throat depth of the machine limits the over-all part dimension in the Y direction of the machine, although it is possible to reposition the part in the X direction during processing to produce a greater length. (Since each repositioning requires additional tolerance, only one repositioning per part is generally recommended.)

Standard and Special Tooling

The opportunity to improve part appearance and precision with inexpensive special tools in the turret press is sometimes overlooked. Particularly where nonstandard shapes and features are concerned the manufacturer is likely to recommend the selective use of specially shaped tools. Special tools, properly designed and used, can significantly improve dimensional control while reducing burrs and enhancing feature appearance. See example of special tooling in Figure 5.

Figure 4. Close up of nibble marks and micro ties.

Forming Operations

In addition to selective perforation and louvering, CNC presses are capable of forming a variety of features in an otherwise flat blank. Circuit card guide slots and recesses, embosses, coined reliefs, countersinks, lanced and formed tabs and small features, nibble-formed stiffening ribs and pierced, formed and hemmed cable path openings are all economically produced using CNC press technology. Certain guidelines must be followed when specifying these features:

  • Height. The feature height may not exceed the clearance between the top and bottom turret--generally 0.350 in. (9 mm).
  • Sequences. Formed features must generally be completed last. Thus pierced areas which are within or immediately adjacent to the formed feature may be deformed as a result of metal stretching during the forming operation.
  • Flatness. Large formed areas and nibble-formed stiffening ribs may create flatness problems because, unlike dedicated tooling, there is usually not sufficient hold down pressure to keep the metal from creeping around the form.
  • Coining. Coined areas should generally not exceed 20% of metal thickness.
  • Burrs. Formed features on a CNC part sometimes preclude machine deburring of the part. In addition, the formed features are produced "up" (formed toward the top of the part as it is clamped in the press) while the burr side is "down.'' This can be a key consideration when designing for the use of inserts and in other situations where burr direction is important.

The size, type and availability of formed features is usually limited only by the designer's imagination. Your precision fabricator can assist you in design, tooling and specification to fully utilize this unique machine capability.

Design Considerations

As in any manufacturing situation, there are process characteristics associated with turret press production which the experienced designer keeps in mind and takes advantage of during product design. Following are some of the potentially significant design considerations.

Figure 5. Example of special tooling (punch and die) for CNC turret press production.

  • Time and Material Utilization. To optimize processing time and material utilization, it is common for parts to be ganged or nested during turret press production. Several parts within the workpiece sheet may be held together during punching by tiny webs or bridges known as "micro ties" which are allowed to remain after punching is completed. See Figure 4.

After punching, the sheet is agitated and the individual parts break apart and are stacked, ready for further processing, without having to be cut apart in a separate operation. The parts are therefore known as "shaker parts" or "shake aparts." See Figure 6.

The designer should be aware of the tiny burr which may remain on the perimeter of the part at the point where the micro tie is broken. Advance consultation on the location and disposition of these burrs can avoid potential subsequent problems.

  • Burr Direction. A burr, no matter how small it may be, inevitably is formed when a punch pierces sheet metal. In pierced features the burr occurs on the side of the part opposite where the punch enters. See Figure 7.

Burr direction is important to the knowledgeable designer because it may be possible to plan the product so that the burr side of the part can be completely concealed--safely hidden from the user--thus saving the cost of deburring in a belt sanding operation.

Figure 6. Example of "shaker parts" or "shake aparts."

The experienced designer also takes into account the fact that clinch hardware is more reliably staked from the burr side of the part, rather than the punch side, and plans the development of the part accordingly.

  • Flatness. The flatness of a workpiece is unavoidably affected by stresses induced and released during punching operations. Generally speaking, the more punching performed on the workpiece, the more bow or "oil canning" distortion is generated. Designs involving closely spaced hole patterns. and where more than 25% of the material is removed by punching, are subject to flatness distortions beyond acceptable levels.

High pressure strippers to clamp the material during processing can under normal conditions reduce the bow to generally acceptable limits. Greater flatness can be achieved, at additional expense, through subsequent leveling operations.

In many cases the designer will have a definite preference for which side of the workpiece should contain any bow distortion (positive or negative), and can specify accordingly--allowing the manufacturer to process the workpiece from the appropriate side.

  • Edge Conditions. Certain edges of parts processed in a CNC press may exhibit characteristics of interest to the designer, particularly where nibbling is involved.

The overlapping of the punch during nibbling operations inevitably leaves characteristic markings on the edge of the nibbled feature. These marks are primarily of cosmetic interest, and are often not measurable.

Figure 7. The normal metal deformation created by a piercing operation.

If the presence of nibble marks are an important cosmetic concern, the designer may wish to consider special tooling to eliminate the condition.

"Scalloping" is a condition where the nibble marks become exaggerated and protrude to the point that they are measurable. See Figure 8. Scalloping sometimes occurs in a location where it can be accepted by the customer. If it is objectionable, special tooling may avoid the need for nibbling, and should be considered.

Obviously, it is especially important that questions regarding these edge conditions be discussed early in the design process, and that agreement between customer and supplier be reached in advance of production.

Figure 8. Example of scalloping.

Likewise, "breakout," which occurs normally on all punched edges, can become a significant factor in thicker materials and should be treated accordingly. See Figure 7. For material thickness of 0.075 in. (1.9 mm) and greater, the effects of breakout should be discussed wherever hole diameters are critical or clinch hardware is to be inserted.

  • Clamp Marks. Small indentations along the outer edge of one side of the workpiece may result from the gripping action of the workholders. These clamp marks are seldom a problem, and may be eliminated entirely by positioning the part in the workpiece sheet so that the perimeter, containing the marks, is cut away and discarded after processing.
  • Feature Location. While specific requirements must dictate the details of part design, there are two general guidelines regarding the positioning of features which experienced designers often find useful:
  1. Avoid placing holes and other features unnecessarily close to one another. Narrow webs can produce flatness problems and twisting of the material. (For more information see Stamping Production Chapter).
  2. Avoid requirements for inserting clinch hardware in holes too near the edge of the part. And, for economy, design for all clinch hardware to be inserted from one side of the part. (See the Inserted Fasteners Chapter).

Dimensioning Practices

If there is a single area where the designer can accomplish the greatest benefit in producibility and economy of manufacture, it is perhaps in communicating effectively with the supplier, using appropriate detailing practices on drawings. Following are a few basic guidelines which can make an enormous difference:

First, select a meaningful datum in the body of the part--passing through hole centers, if possible--rather than using an edge or corner of the part. (See Dimensioning Practices in the Press Brake Chapter). There are several reasons for this suggestion.

It avoids problems of possible misalignment of the part, distortion from clamping, etc. It allows for more precise measurement by avoiding measurements from edges which may be tapered and therefore dimensionally uncertain. It facilitates accurate inspection. And, it avoids unnecessary accumulation of tolerances.

Second, on related hole patterns, dimensioning and tolerances should be within this pattern with only one dimension linking to the general datum. Better quality control and function of the product can be expected.

Third, highlight the truly significant dimensions. Critical dimensional relationships can be protected, if they are known.

Dimensional Precision Capabilities

All machine tools are subject to finite limitations of dimensional accuracy, and the turret press is no exception. Published machine accuracy figures may not always reflect the true tolerance capability of machines in actual hard use. The electronic and mechanical inaccuracies combine for the total dimensional variation experienced in practice.

Depending on machine make, type and condition, the plus-minus feature tolerance may vary from 0.005 in. (0.13 mm) to 0.015 in. (0.38 mm). Program corrections can often be used to improve the inherent machine inaccuracies.

Machine repeatability, however, is 0.002 in. (0.05 mm) T.I.R. as long as lead screw progression is in one direction, since then the mechanical tolerances are not compounded.

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Excerpt taken from Design Guidelines for Metal Stampings and Fabrications -- 2nd Edition copyright © 1995 Precision Metalforming Association

Purchase the new Third Edition of Design Guidelines for Metal Stampings and Fabrications copyright © 2004 Precision Metalforming Association at Marketplace today!

 



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