Deep Drawing Large Parts in Four Cylinder Presses

by Dennis J. Taylor, President
Taylor Made Solutions, Monroe, NC

The deep draw technology is one of the most challenging in manufacturing. The sizes, shapes, volumes and materials used to produce deep drawn parts cover a diverse range of variables. Individual variables should be evaluated carefully to determine the optimum manufacturing method.

To understand the dynamics of this pressworking operation, a review of pertinent definitions and parameters germane to this use of hydraulic technology is relevant. They are:

• Drawing
• Deep Drawing
• Large Parts
• Four-Cylinder Presses

What is Drawing?
Drawing is forming a shell from a flat sheet of metal into a shape that has side walls. The successful operation results in a balance between forming the metal over a punch without breaking the material, and keeping the material from wrinkling as it is being formed.

How do you do this? A blankholder (or cushion) is required to keep the material from wrinkling. The blankholder supports the blank and also pushes against the blank and draw ring to accomplish the wrinkle-free condition.

What is Deep Drawing?
There are many definitions, which, for limited areas, are correct. Almost everyone has their own working definition. For our purpose let us define a deep drawn part of one which has a draw ratio of greater than 1 to 2.0 (1:2.0). Draw ration is the ratio between the blank diameter and the diameter of the drawn part.

For example, if we start out with a round flat blank having a diameter of 10 in., it is generally accepted that it is not a good idea to try to make a "cup" smaller than 5 in. diameter in one step. Therefore, by definition, in one reduction, a 10 in. diameter less than 5 in. (This is also referred to as a 50 percent solution.) However, if it is reduced to a smaller diameter it is considered deep drawing.

Some extended draw rations (greater than 1:2.0) for a 10 in. blank might be:

• A 10 in. blank drawn into a 3.7 in. cup, 10/3.7 = 2/7 draw ration.
• A 10 in. blank drawn into a 2.94 in. cup = 3.4 draw ration.

These parameters will vary with the type and thickness of material.

Multiple Draws
For example, consider a part with an initial blank size equal to 14.5 in. and a required final cup diameter of 4.26 in.:

• First draw diameter = 14.5/2.0 = a7.25 in.
• Second draw diameter = 14.5/2.7 = 5.37 in.
• Third draw diameter = 14.5/3.4 = 4.26 in.

Notice the blank diameter is divided by 2.0, 2.7 and 3.4, respectively. These are basic formulas for figuring first, second and third diameters.

What are Large Parts?
We have drawn parts in four-cylinder presses as small as 2 in. dia. x 3 in. deep and as large as 18 in. dia. x 32 in. deep. parts such as inkpen housing are referred to as small parts and normally made using transfer or progressive-type manufacturing. Larger parts such as bathtubs are certainly considered deep drawn and considered large. However, these parts are normally made by drawing the metal in one direction using a main cylinder to drive the metal and a blankholder to keep it from wrinkling.

Four-Cylinder Presses
Four-cylinder presses, for the purpose of this article, are presses that have four cylinder functions in a vertical plan; two from the top and two from the bottom. See Drawing. The four cylinders are identified as main (1), blankholder, (2), third (3) and fourth (4). The main and third cylinders are at the top; the blankholder and fourth are the two cylinders from the bottom.

Why Does This Technology Exist?
The parts that our customers make in one operation could be made by other methods. These include spinning, two-piece fabrication, multiple hits on conventional equipment, or other operations.

At the risk of offering an alternative to the way some of you may be making parts, we believe, and it actually has been proven, there is a viable market in the U.S. for parts made from one piece of metal without seams (versus welded or crimped). The absence of foreign material holding the part together allows for more consistent expansion and contraction properties. Obviously, each industry has its own reasons for requiring a one-piece unit.

The fabricator should be aware of this technology, how it works, what the advantages/disadvantages are and, in general, have a good understanding of what is available to make his company more profitable and efficient. (It also could be important to know what his competition is doing.)

You should understand that our approach to deep drawing technology contains no magic. It is based on sound principles. We combine our knowledge of these principles, the technology available in today's controls and the knowledge we have gained in developing multiple-cylinder presses to offer what we believe is an advantage in deep drawing.

What Advantages?
This technology reduces the number of operation required to complete a part. Examples of parts produced accompanying this article -- Fig. 3 through Fig. 6 -- were furnished by Anchor Tool 7 Die Co., Cleveland, OH, a major stamping and fabrication shop with roots in die design and build technology.

Their expertise in harnessing the forces of four timed hydraulic press movements combined with controlled heat to the die gives material flow characteristics not normally associated with stamping technology. This is invaluable on the redrawn part shown in Fig. 1 and Fig. 2. Fig. 1 shows the part after the operation, Fig. 2 after the second. This tandem press-production results in a superior part for the customer over any other manufacturing method.

Fig. 1 -- Anchor Tool operator performs visual inspection of redrawn part.

The control system used with this technology provides additional advantages. It allows for manipulating the speed and force of the four cylinders during their strokes as many as eight times. We find this to be an advantage in several different applications.

The control system also is used in areas such as drawing dome or spherical-shaped parts. Obviously, as the blank is drawn, the contact area between the final draw ring and the blankholder ring becomes force (the force necessary to keep the material from wrinkling) is reduced. Having the ability to reduce this blankholder force using the drawing operation reduces the chances of breaking the part and, in effect, increases the chances of successfully drawing the part.

Fig. 2 -- After secondary sizing and blanking operation, the completed part is ready for fabrication. Anchor Tool 7 Die credits four-cylinder technology with reducing manufacturing steps and improving part quality.

Reverse Drawing and Redrawing
The schematic at the beginning of this article illustrates an operation that takes full advantage of all four cylinders and , combined with the control system, offers significant advantages in deep draw press operations. Parts often can be completed with no annealing, because moving the metal through two and three reduction stages without letting it cool provides that effect.

Fig 3. Material: Thickness: Blank Size: Diameter: Draw Depth: Draw Ratio: Remarks: 1008/1012 CRS AKQD 0.117/0.123 18.750 6.960 10.580 height First draw is 47% reduction second draw is 30% reduction One operation: draw and reverse draw.

Today control technology for this type of press has advanced to allow the user to linearly control the force and speed on all four cylinders in the up or down direction. This allows the user to smooth our or eliminate draw lines as well as control the forming process completely through the press.

Fig 4. Material: Thickness: Blank Size: Diameter: Draw Depth: Draw Ratio: Remarks: 1008/1010 CRS AKQD RB 55 max. 0.045/0.051 17.312 4.250 5.850 height First draw is 47% reduction second draw is 30% reduction third draw is 34% reduction One operation: draw, reverse draw and reverse draw.

For example, we know that as a part is being drawn material thickens as it approaches the open end of a shell. New control technology allows us to either increase the blankholder force linearly to reduce material thickening or decrease pressure to minimize the possibility of breaking the part or damaging the tooling.

The speed can be adjusted up and down to maximize production while maintaining high part integrity.

Fig 5. Material: Thickness: Blank Size: Diameter: Draw Depth: Draw Ratio: Remarks: 1008/1012 CRS AKQD 0.088/0.094 14.187 4.575 8.780 height First draw is 40% reduction second draw is 26 1/2% reduction third draw is 28% reduction One operation: draw, reverse draw and reverse draw.

The principle illustrated is reverse drawing and re-drawing, a process that uses a hollow punch in the bottom half of the tooling. These also is hollow punch driven by the main cylinder from the top. Here, the main and third cylinders come down together. Blankholding in the first draw is done by the third cylinder.

The blank is drawn around the outside of the hollow punch on the top to complete the conventional first draw. Then the part is reverse drawn from the outside of the hollow punch to the inside of the hollow punch on the bottom. When the reverse draw is completed, the fourth cylinder comes up and re-draws the part for the third and final reduction.

Fig 6. Material: Thickness: Blank Size: Diameter: Draw Depth: Draw Ratio: Remarks: 1008/1010 H*PO AKDQ 0.115/0.123 15.218 7.090 7.905 height First draw is 50% reduction second draw is 24% reduction One operation: Draw and reverse draw.

This method may sound complicated. However, in practice, the tooling is rather simple and many parts made from a wide range of materials are made using this principle today. The tooling basically consists of rings, pins and punches.

The data presented in this article is based on sound deep drawing principles. These principles, when combined with knowledge gained from our customers and advances in control technology, enable us to reduce the number of operations required to complete a part, in some cases, eliminating the need for annealing as well.

Technology has embraced the deep drawing of large parts. Four-cylinder hydraulic presses may provide a manufacturing advantage for your particular parts. MF