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Steel Rule Die Cut Parts
Introduction
Steel rule dies are one of, if not the most widely used means of stamping parts from soft to semi-rigid materials in use today. The process is, of course, central to operations specializing in die cutting such materials all around the world, but is also a normal part of many other industries like printing, clothing manufacturing, shoe and other leather products makers, and many others. While technologies like laser and water-jet cutting impose somewhat on the fringes of steel rule die cutting applications, there is no reason to think general use of steel rule dies on the chopping block of technological advancement: Quite the contrary is true, in fact, as advancements in computer-aided design (CAD), industrial laser cutting, and automated rule forming have done nothing but establish the place steel rule dies have in component and consumer part manufacturing as the lowest-cost option for fabricating custom, two-dimensional shapes from the wide array of materials suited to the process.
A bit of history is in order, and I can think of no better source than my own experiences as the General Manager of a company centered on steel rule die cutting for two decades, a considerable portion of which
on behalf of some of the most quality-demanding corporations in the world. Prior to the late 1980's, our dies were made by a highly-skilled craftsmen whose tools were rulers, compasses, protractors, pencils, scroll saws, and manual rule-bending tools. Computer CAD programs and precision plotters then superseded the manual drafting on wood, and by the mid to late 90's, laser cutting had become the industry standard. Soon, automated rule-bending equipment was also added.
The result of this was an almost head-spinning industry-wide improvement in steel rule die quality that directly translated into radically better dimensional tolerances on the parts and products produced by them. Being heavily vested in high-tech industries as far as clientele goes, this perhaps affected us more directly than most, but the timing couldn't have been better: It coincided almost perfectly with the rapid ascent of higher-level, more structured quality control requirements by them. From our vantage point, that meant that sample-approval based on fit and function alone (regardless of drawing conformance) was gone forever, and thousands of our dies were no longer suitable for production. It took nearly a decade to transition from where non-conformance to the new, higher quality standards was the rule rather than the exception.
I've often been befuddled by the lack of a recognized comprehensive standard for steel rule die cut shapes based on material, thickness, and other pertinent variables, and the relatively recent quantum improvements in steel rule die cutting precision would seem to enhance the value of such a standard. Recognition notwithstanding, the following is done primarily to document our manufacturing standards for our own purposes, with the potential value to others in view, also.
(Sorry, all I have at present is a rough outline; more later...)
Tolerance Variables
As opposed to other methods of cutting or stamping two dimensional shapes
from flat sheet stocks (e.g. male-female or milled steel dies, laser or waterjet
cutting). While this remains generally true, technological advancements
have enabled quantum improvements in the controllability of the variables
affecting dimensional precision. The availability of +/-.010" (1/100 inch) tolerances on semi-rigid, thin gauge materials like plastic films and paper
products, for instance, coupled with the very low tooling costs of steel rule
dies make this a viable process for numerous applications without compromising
even the most rigid quality standards.
Nevertheless, steel rule die cutting does involve several variables that are
either unique or uniquely significant to the process, including the following
factors which are divided into three (3) groups according to their respective
place in production and quality control related processes:
1. Tooling Factors
Steel rule dies are generally made to a default +/- .010" tolerances, with
factors affecting this described below. These only pertain to new, unused
tooling, though, so only constitute part of the equation regarding finished
products made from such tooling.
- Rule and Punch Substrate or "Die Board" - Generally
made of laser-cut hardwood laminates (e.g. maple, birch) and Can be
made of aluminum, steel, or plastics for additional stability and/or
longevity. Estimated potential effect +/-.001" per 12 inch length,
which must be doubled to factor in the distance between two edges.
- Steel Cutting Rule - Generally manufactured to +/-.002" edge
location plus is given to minor variations from bending to shape and
inserting in die board, especially in relation to radius edges.
Estimated potential effect +/-.003" max.
- Machined Punches - Machined punches (standard round and other shapes) punches are
usually made to +/- .001" tolerances.
- Squeezed Oval Punches - An economical alternative to machined punches, these are actually made by squeezing inexpensive standard round punches into oval shapes. Tolerances on these type of hole punches generally runs around +/- .015" on overall length and width, with shape anomalies like irregular radii on short sides and concaving of the long side edges, which is often called "peanuting," a feature that increases as the ratio between long and short sides increases; i.e. the more the round punch is squeezed or the narrower the oval.
2. Production Process Factors
Beyond actual tooling dimensions, dimensions of steel rule die cut parts can
notably vary from actual die dimensions due to process-related factors like
materials, die wear, ejection materials, and equipment, as follows:
- Cutting Rule Flexing - A minimal factor in most cases since any
significant amount would directly interfere with usability of die; it must
be perpendicular to material to stamp through it cleanly, so any bending
would cause rule to "lay over" and cease cutting altogether (like striking
a bent nail with a hammer). Flexing occurs in response to lateral pressures asserted by the rule and bunch bevels, ejection materials, and material being cut being compressed within a cavity or gap. Off-center bevels can create such issues in themselves or contribute to other elements, as well. Estimated potential effect +/- .002"
max. per rule-edge.
- Cutting Rule and Punch Edge Sharpness - Since materials are
actually stamped "cookie-cutter" style using sharp-edged steel rule, wear
on the cutting edge is a normal consequence of use (particularly relative
to number of setups as much or more than number of "hits"). This not
only affects the actual dimensions, but also and often more significantly,
it degrades the cleanness of cut edges, making them rough and jagged, for
instance, instead of sharp. Theoretically, center bevel rule of typical
.028" thickness can affect dimensions by as much as .014" if worn to
complete bluntness, though dies are normally repaired well before such
wear occurs. Realistic estimated potential effect of +/- .005" max
per edge.
- Ejection Rubber, Springs, Strippers - Although normally
included in the die making process, the placement of ejection materials in
and around cavities is more of a factor in setting up production, so is
included here rather than above. Since parts stamped with
steel rule dies naturally remain in place rather than being sheared
through material like male-female dies do, mechanical ejection from the
die cavity must be facilitated. This is generally done using rubber
or sponge rubber materials placed around the cut edges, but springs may be
employed in various forms (e.g. pins, stripper plates), as well. The
general aim with ejection is to minimalize the stress it places on the material being die cut while still cleanly ejecting cut parts from the tooling.
(See notes on Rule Flexing and Material Distortions.)
- Material Distortions - Since the process involves linear force
from cutting edges and ejection materials as parts are stamped, materials
(especially rubbers and sponges) tend to compress and extrude away from
the pressure points of the tooling. This can create bulges, indentions,
concavities and other cut-edge irregularities, as well as affect overall
dimensions. Such irregularities are directly related to the force
generated between material and die as parts are stamped, so edge sharpness
and ejection material volume and placement are major factors in the degree
of distortion-related variances, as is the stroke velocity of the press.
Mechanical feeding devices can also cause stretching of material passing
through a die cutting press.
3. Post Production Factors
- Material Shrinkage, Swelling - Some materials common to steel
rule die cutting processes like closed cell sponges and semi-rigid plastics
like flexible vinyl (PVC) are given to temperature, humidity and other
environmentally-related post-production changes, especially shrinkage.
In house tests, for instance, have shown that even minimal exposure to
100ºF+ heat (as one would expect during
summertime transit) for a few hours can significantly shrink some
closed-cell sponge materials. Since materials given to such
reactions are generally easily stretched or otherwise conformed to the
requirement, this is usually more of a concern for quality inspectors than
users, but bridging that gap is largely the point here. Another
related factor is the stabilizing effect of laminated adhesive backings
with removable, peel-off liners, again, especially on sponge and foam
materials. This can hide an otherwise evident shrinkage until the
liner is removed, allowing the material to relax.
- Measuring Techniques - Natural edge variations and other
factors occasionally necessitate defined measuring techniques between
suppliers and customers, especially on thicker and more pliable materials
where actual dimensions between cut edges can vary between the two surfaces
of a die cut shape. Also, since many pliable and elastomeric
material and shape combinations should be measured in a somewhat ambiguous
"relaxed" state, this too can be a root cause of discrepancies in
measuring techniques. Other measurement related ambiguities arise on
dimensions centered on radius center points and other hypothetical
references. (See Measuring Techniques below.)
While it may too ambitious to quantify how and to what degree all of these
process variables may affect production tolerances, basic dimensional tolerance
ranges based on materials can be established well enough to serve as defaults
for most steel rule die applications as long as room is allowed for exceptions where merited.
Material Classes
Grouped somewhat according to steel rule die cutting process related factors:
- Plastic Films and Sheets
- Sheet Rubber
- Closed Cell Sponge Rubber
- Open Cell Foams and Sponges
- Paper, Organic Fiber, and Composition-type Gasket Sheets
- Compressed-type Gasket Sheets
- Fiberglass, Ceramic Fiber, Woven Cloth, Insulation Materials
- Cloth, Textiles
- Screen Wire Cloth, Foils, other thin gauge and soft metals.
Measuring Techniques
Because of certain edge features unique to steel rule die cutting, the pliable nature of some materials,
and other factors, some simple measurements can yield ambiguous or false
readings. Therefore, it is important that suppliers and customers
coordinate measuring techniques. The following outlines basic default
procedures for dimensional inspections:
Definitions
For these purposes, two dimensional parts stamped from flat sheet stock
materials are assumed, as is a typical drawing layout where "top" and "bottom"
views display the two dimensional die cut shape, whereas "side" or "edge" views
reveal only the rectangular perimeter of the part as a straight line or
rectangle illustrating the thickness of the part/material and any laminated
features (like adhesive backing).
"Hypothetical reference points" refers to points on finished parts that are
used as dimensional references on drawings, but are not visually identifiable on
finished parts, such as radius and circle center points.
Devices
It is further assumed that measuring equipment includes a properly calibrated
video or magnifying optical (preferred) comparator or similar device capable of
non-contact two-dimensional measurements (X, Y axis).
Obviously, some measuring devices work better in some applications than
others, and these differences can result in misreads on dimensional measurements
and conformance disputes. As needed, suppliers and consumers should
coordinate the devices used to measure parts to assure similar results.
Some common issues include:
- Contact devices like calipers, micrometers, and many 'go/no-go' type
gauges are only as valid as the trueness of the contact points to the actual
surface or edge (see below). It is practically impossible to discern the
actual point of contact with such devices on very soft materials that are made
to yield to external forces on contact within a few thousandths of an inch
accuracy. In effect, though designed as absolute contact devices, these
must be visually oriented like tape measures and rulers, which diminishes
their accuracy considerably.
- Video comparators can give false reads if measuring surface to surface
distances when edge anomalies (see below) require other reference points be
used.
Natural State
Another common issue regarding measuring steel rule die cut parts arises from
the soft and pliable nature of many of the materials processed this way.
Problems arise when material and shape combinations won't naturally return to
their original, relaxed state. For example, a thin-wall, thin-gauge rubber
ring gasket defined by OD and ID dimensions can be nearly impossible to restore
to the original "perfectly" round state for measuring purposes. Like
contact measuring devices, even fixtures that force parts to a shape may
inadvertently cause elongation or other distorts, invalidating any measurement
taken from it as such. It is impossible to discern the point between
relaxed and elongated or otherwise distorted points of some parts.
Another typical case involving the natural state of die cut products involves
the presence of removable liners, such as those protecting pressure sensitive
adhesive backings applied to other materials before die cutting. Since
these are essentially a part of the packaging, not the end product, measurements
should be taken from the material itself, not the liner, which can obscure
readings on some devices if not removed prior to measuring. Also,
materials given to elongation should be allowed to relax to their natural state
following removal of the liner, as either the adhesion to the liner or the
removal of the same can affect dimensions.
Where natural state measurements are not practically attainable, there are
any number of ways such issues can be resolved between suppliers and consumers
of steel rule die cut products to assure consistent dimensional quality
conformance. For instance, quality procedures can be established to measure
parts in the web, the webbing itself, or destructive linear circumference
measurements, etc.
Cut-Edges
Steel rule die cut parts will almost always exhibit some variations in the
perpendicularity of cut edges when viewed from a side or edge view, especially
under magnification. The degree of these effects on thin plastic films,
papers, and other non-elastic, stable materials is usually negligible, but
becomes rather pronounced on thicker varieties of plastic sheet stock, rubber,
sponge, and foam materials. As a rule, the thicker, softer, and more
elastic a material, the greater the cut-edge anomalies will be.
Because of this, it is necessary to define cut edges for measuring purposes
as follows:
- Interior Holes and Cutouts - Innermost edge from top or bottom
view.
- Exterior Perimeter - Outermost edge from top or bottom view.
Holes, Cutouts, other ambiguous Center Points
Center points of interior holes and cutouts, radii on interior or exterior,
and other hypothetical reference points should be located at the absolute center
between the extreme edges of a hole or cutout on both the X and Y axis (as
oriented on drawing), or as required by the design.
Round Hole Diameters
Measured on both X and Y axis (as oriented on drawing), and, provided both
are within tolerance, stated as a single average between the two.
Concentricity
Measured as half the difference between wall widths on OD-ID shapes on both X and Y axis
(as oriented on drawing). Provided both are within tolerance, this can be
stated as a single average of the two readings.
Corners, Angles, Radii
There are two ways to form a corner or angle with steel cutting rule in a die
-- bend the rule around it or cut and join two pieces together. Either is
generally viable, but, as a rule, die cutters prefer fewer rather than more rule
joints for the sake of die strength and integrity and to minimize snags between
die cut parts and the material from which they are stamped.
Although seldom an issue, bending rule around a corner or angle yields a
slight radius rather than a zero radius corner. In cases where such
details matter, designers can note drawings to allow radius corners with a
maximum value (e.g. .094") to avoid quality rejections.
Since arc shapes assume a hypothetical center point and consistent arc,
precise measurement can be ambiguous, especially on smaller radius and shorter
angled arcs. Larger arc-shaped edges that require less severe bending of
the rule to form can be made and measured without any particular difficulty,
although portions that are blended into other arcs and angles can be nearly
impossible to absolutely dimension or measure. When significant, design
engineers should note drawings that radii should be "blended" to account for
this common feature of steel rule die cut products.
Measurements of arcs are best done with a computerized comparator or similar
device as an average of a minimum of 3 points spanning the arc center point and
avoiding end points where blended into adjoining edges.
Edge Features
Generally considered undesirable side-effects, the following features are
commonly exhibited on the cut-edges of steel rule die cut parts:
- Beveling - Typically occurs on soft, pliable varieties of plastic
sheets and other non-elastic materials given to conforming to applied
forces. Results from cutting rule bevel and rule flexing outward
from cavity as it penetrates through stamping cycle.
- Concave/Convex Cross-Sectional Edges ("hourglassing") - Typically
occurs on elastic materials like rubber, closed-cell sponges, and (to a
lesser degree) open-cell foams and sponges. Results from compression
forces from rule, punches, and ejection materials that cause outward
extrusion ("flowing") of elastic materials from the same as die penetrates
material through stamping cycle. Generally forms concave exterior
and convex interior edges as materials tend to flow outward from center.
- Nicks, Burrs, Rough Edges -
- Bowed, Wavy Edges -
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