Clarifying Elements to the Deep Draw Design Process – The Draw Ratio
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.
Another note: there is often various terminology that is technically correct when discussing Deep Drawing. For example, at Toledo Metal Spinning, when we deep draw a part in the press before sending it to our metal spinners, this is called a ‘preform,’ or a ‘breakdown.’ Any drawn or stamped cup used for spin would be considered a breakdown/preform, regardless of size, or being a deep drawn form or not.
Deep drawn preforms are particularly useful in the spinning process, as it cuts down on – sometimes over half – spinning time and work compared to laying down an entire blank from flat in the spinning lathe.