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the punch is withdrawn, the machine is ready to prepare for the next hit.
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:
- 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).
- 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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