1.  Calculate the blank size and optimize.

One of the most common hidden costs for drawing parts is the hidden cost from engineered scrap necessary to procure a blank.  When the annual quantity for a particular part number is in the thousands or tens of thousands, custom coil sizes can sometimes be procured.  If not, blank material will normally come from master coil sizes, like 36″, 48″, 60″ or 72″ wide that are cut to width and the “drop”, or material removed in the blank shearing process, still maintains more value.  If quantities are very low, it is common to produce blanks from standard sheet sizes of 8, 10, or 12 foot in length from the various coil widths.  Optimizing a blank diameter and square size to eliminate excessive amounts of engineered scrap or planned drops can dramatically impact final part cost.  Blank size also includes adequate amounts of material for trimming and work holding  to make the part shape.

2.  Design to minimize the material needed for trimming & work holding.

The blank size for the finished part is normally different from the actual blank that is used.  This is due to the method necessary to hold and process the part from start to finish.  Aside from the dimensional requirements, the required surface finish and tolerance of dimensions may require additional material to accommodate secondary processes or to eliminate features that could be permitted, yet are not specified on the drawing.

3.  Plan for thin out in the design phase.

The material formed in the drawing or deep drawing processes experience a lot of surface tension and stretching throughout the cycle of the forming process.  This means that typically, a section of the material will thin out in one or more locations, normally towards the closed end of the shape or the first contact surface on the plug.  Other sections will normally thicken towards the open end of the shape, or the flange.  Allowing for this in the part design will help scrap remain low and the design to work well in the application.

4.  Allow for a taper.

Inherent to the process, due to the punch and die needing  adequate clearance, and to accommodate mill tolerance on metal thickness there is a taper in straight walls of parts.  The taper depends on the metal thickness, the total depth of the part, the grade of material, and the clearance between the punch and die.  Typical taper is between 0.005 to 0.010 of an inch per inch of depth.  In order to minimize taper; minimize part depth, use tooling designed specifically for the application, select materials with better formability that thin less, and accommodate material flow with larger draw radius.  If tapers are not acceptable, secondary forming processes can reduce and sometimes eliminate the taper, however it tends to drive cost.

5.  Select material that has better ability to form.

Selecting the most formable material and/or temper for the draw process may minimize or even eliminate scrap generated during the drawing process.  This sometimes may be difficult if the application requires a certain amount of stiffness and or strength.  Some materials like stainless strain harden when formed and others like aluminum can be tempered to improve mechanical properties.  Sometimes selecting a superior material for the design that well exceeds the properties required by the design may be more cost effective in the long run because the processing costs may be significantly lower, such as drawing 304 stainless steel compared to 410 stainless steel.

6.  Look to use existing tooling.

Many press and drawing companies such as TMS, may have a lot of tooling available for customer use.  Like TMS, a company that invests heavily in its own tooling can produce shapes that are very close to the designed dimensions.  Sometimes, the design can be altered slightly to avoid an engineering charge.  Alternatively, a re-cut charge can be offered when the tool is no longer active and can be repurposed.

7.  Hydro-mechanical drawing (hydroforming) requires less tooling costs.

Hydro-mechanical deep drawing, or hydroforming uses hydraulic fluid as either a punch that pushes metal into a cavity or a die to push metal over a punch.  In hydroforming, a rubber bladder acts as a die with 1,000’s of PSI behind it.  Compared to conventional draw forming, the rubber bladder PUSHES the blank around the plug, instead of pinching and STRETCHING the material thus, putting less stress on the metal and allowing for greater reduction ratios and more elaborate part geometries.

8.  Uniform metal thickness is achieved by hydro-mechanical deep drawing (hydroforming).

Due to the lower forces applied onto the metal during the hydro-mechanical drawing process, the material is not stretched as much, allowing for a more uniform metal thickness to be maintained.  This is especially helpful on difficult to draw shapes that are irregular and asymmetrical, where a metal die would not permit simple metal flow or dynamically change to be forgiving of the metal’s needs.

9.  Optimize for a minimum number of draws.

Each time a part is drawn, it is reduced in size with respect to the diameter of the part or the length and width.  Depending on the percent elongation, some metals can be drawn more than others.  To achieve the maximum amount of reduction a metal is able to be reduced, it is often necessary to go through a series of steps or reductions.  14 gauge stainless steel 304 for example, normally can be reduced 45% the first reduction, and then up to 30% the second.  To further reduce it, SS 304 typically requires an annealing operation and then can be reduced another 15%.  So, if the original blank is 10 inches, the final diameter would be (1-.45)(1-.30)(1-.15) x 10 inches = 3.27 inch diameter of the part.

10. Add features that leverage the benefit of in-house secondary operations.

 Whenever there is forming, usually there is cutting, finishing, secondary forming, and sometimes joining processes that are needed to complete the part so it is useful.  A deep drawing house usually offers a variety of processes that can improve the usability of your part and with minimal cost depending on part design.  Once an order is placed during the prototyping phase of a project or product life cycle, a reputable manufacturing firm will normally provide a design for manufacturing and assembly (DFMA) review to help identify cost saving opportunities and features that can assist in overcoming design, and or cost barriers.

When it comes to tool design, the most important formula is The Draw Ratio, also known as Limiting Draw Ratio (LDR).

The formula is: LDR = D / d

D = blank diameter

d = cup ID or punch diameter

This ratio shows how large of a blank can be drawn with that particular punch size (which would be the Cup ID size). This is why it is considered a limiting ratio.

Usually in this process of determining how many draws, and what size on each draw is required, we start with the blank size (calculated from the final product volume) and work backwards using this LDR formula for each draw/set of tooling. This generally indicates how many draws are required and roughly what diameter each step should be. At this point, this will be refined by using % reduction formulas. In summary, the LDR will yield a ROUGH drawing process, which is refined even further during the next step of the design process, with the % reduction calculations.   

The Draw Ratio is the ratio the width of the part vs its height. If the width of a part you are Deep Drawing is equal to its height, the ratio will be 1:1. Once you start drawing a part that is deeper than it is wide, you will exceed the 1:1 ratio. This is when you encounter problems.

When the part you are drawing exceeds this ratio, for example, 1.5:1 or 2:1, then the part will need to be drawn in multiple progressive steps. Each step will require its own dedicated draw tooling. If you can keep your parts below the 1:1 ratio, then a single set of tooling will typically be enough. This reduces development and production time. Understanding the Draw Ratio is of great benefit to a customer, when we are in the design process.

This ratio helps to classify a drawn cup as a deep draw or not. Any shape exceeding a 1:1 ratio will be considered a deep drawn cup/form. 

Blank Size, Thickness, Shape, and Part Geometry

Determining the blank size is a very crucial step for a successful draw. The amount of material needed for the final product must be included within the blank. When there are multiple draws for a single product, it can be tricky to determine how much material is needed. Since the draw process causes thinning and thickening of the metals, this is an important first step to a successful draw.

Draw Radii

It is important to note the size, accuracy, and finish of the die entry radius. If the die radius is too small, the material will not easily flow. This results in stretching and fracturing of the drawn product. If the die radius is too large, the material will wrinkle after leaving the pinch point between the draw ring surface and binder. If the wrinkling is extreme, the material flow may be restricted when pulled through the die entry.

Draw Ratio

 The draw ratio is one of the most important elements of maintaining successful deep draws. The draw ratio is the relationship between the size of the draw post and size of the blank. During the forming process, the blank is pressed into a circumferential compression which creates a resistance of metal flow. If there is too much resistance, the metal will fracture. If the draw post is not big enough, the metal will stretch, becoming thinner until it cannot be formed. If the draw post is the appropriate distance from the edge of the blank, the metal will be able to flow, while becoming thicker as it enters into the die cavity.

 The formula for the draw ratio is: D/d ≤ 2 for a successful draw.

D = the blank diameter

d = plug (or post) diameter

If this ratio is greater than 2, re-draws (or break downs) are required. In our industry, this is a general rule of thumb. Certain materials may have more accurate, material-specific rule of thumb ratios. For example, Aluminum is 1.8.

Lubricants and Die Surface Finish

Adding lubricants and a polish to the die surface helps with friction and reduces the chance of galling. Galling is when two metallic surfaces slide against each other, creating friction. This can harm the product and the tooling. Applying lubricant to the blank is a very important step in the deep drawing process, to create the highest quality product, while protecting the draw post tooling. Avoiding galling enables the blank to slide easier, allowing for free flowing of the metal. 

Die Temperature

The die temperature can cause the lubricant to thicken or thin, depending on how hot the die is. When lubricants heat up, their viscosity drops and they thin out. As they get cold, their viscosity increases. Understanding this relationship is key to creating the best quality drawn part, while maintaining the quality of the die. 

It is critical to select the correct lubricant for each deep drawing process. Each lubrication brand, type, and formulation performs differently at different temperatures, depending on their intended use. Certain lubrications need to increase to a certain “working temperature” before they will exhibit any friction-fighting properties at all. In contrast, other lubricants only work in a cold or room-temperature environment. When determining the correct lubrication, the tool temperature, (mid-run and at rest) blank material, and draw severity are all taken into account.

Binder Pressure

At Toledo Metal Spinning, we use pinch and pressure to control the material flow. Binder Pressure is a machine setting that controls the upwards force and/or pressure in the press that will be applied through the draw ring/binder, which sits on top of the cushion pins. The draw ring pressure rises, while the die pressure and slide force is in a downward motion, this is how the blank is “pinched.”

N and R Values

The N value is known as the Work Hardening Exponent, or the Strain Hardening Exponent. This describes steel’s ability to stretch. The larger the material’s N Value is, the more the material is able to elongate without necking, or deforming.

The R Value, also known as the Lankford Coefficient or Plastic Strain Ratio, describes the ability of a material to flow or draw. The blank size affects the ability of metal to flow because the press’ speed need to allow for time for the material to flow through. For a more technical explanation, it is a measure of how resistant a metal alloy is to thinning. Mathematically, it is the ratio of the true width strain to the true thickness strain at a specific value of longitudinal strain, up to the point of uniform elongation.

Common materials in deep drawing include Stainless Steel, Copper, Aluminum and Cold Rolled Steel. 

Deep Drawing and Stamping each require a design process, with considerations of how the materials will affect the manufacturing process, production costs, and the ease of manufacture. Material thickness, the type of bending or formability involved are also characteristics that will be different for each process, depending on the shapes being formed, and the shape of the end product. While there are many similarities between these two processes, there are not as many differences.

What is Metal Stamping?

Stamping is a manufacturing process when coils or flat sheets of material are formed into specific shapes. The Stamping process is used to make small changes to parts, such as bends, tabs, or embossments. These features tend to be much shallower in depth than a deep drawn part. Stamped parts start flat and go through a sequence of stamps from a press, where new features such as small tabs, are folded in, or holes are punched out. These features are very sharp, detailed, and precise. Stamping is a broad term that includes many specific forming techniques such as embossing, blanking, punching, bending, flanging, and the list goes on. Each of these methods involve short, quick, and abrupt hits or press movements.

At Toledo Metal Spinning, our stamping capabilities lie with our Komatsu Mechanical Press, where we intertwine our deep drawing abilities with stamping and are able to pierce holes, or form small tabs or flanges. Before the integration of our laser cutting technology, we used to cut blanks with this press.  

How is Stamping different from Deep Drawing?

Deep Drawing is used to make larger features such as as cups, pans, or domes. We draw parts from our two hydraulic AP&T presses. Drawing a cup requires exerting a significant amount of pressure on a flat sheet, and gradually drawing it over a die to sculpt it into the cup shape. Forming these shapes requires much more pressure over a longer period of time than a quick stamp. If the pressure is not controlled properly or is performed too fast, the metal will fracture or break, and will not be usable.  

The shape of the part is the main difference between a stamped or deep drawn product. Drawn parts will have more pronounced curves in the shapes, and will be larger than a stamped part. Below is an example of one of our deep drawn cups. Take note of the defined edges and curves, while its strength and durability is present. 

Hydroforming is a specialized Deep Drawing process also known as Sheet Hydroforming. Sheet hydroforming allows various materials to become complex and structurally sound parts. It allows for asymmetrical or irregular shaped geometries, while conventional Deep Drawn parts are symmetrical and uniform throughout the entire shape. Another big difference is the depth of the parts produced by each. The Hydroform Press cannot produce shapes with sharp edges or angles, and the shapes are not as deep as Deep Drawn parts.

The Hydroforming process begins by placing a laser-cut blank over a die (typically made from rubber). High pressurized water forces the blank down over the mold, forcing the blank into its new shape. Both Hydroforming and Deep Drawing are both excellent, seamless manufacturing processes that do not require any welding.

The Deep Drawing process is better suited for larger production runs, or smaller runs for applications or components that will have long-term usage. The Hydroforming process is a popular choice when we are producing complicated parts, which irregular shapes.

At Toledo Metal Spinning Company, we use a combination of stamping, hydroforming, and conventional deep drawing for a variety of applications. One application is to utilize our press capabilities for preforming a spinning operation. One advantage for using a press preform includes the benefit of flowing the metal towards the flange of the part, giving more material in this area verses a thickness reduction from our spinning process.