Back in 1971 Robert Plant reminded us that, “sometimes words have two meanings.” He had his Stairway to Heaven in mind, but it also applies in the not-so-rock-and-roll world of metal fabrication.
The word we’re thinking of today is drawing. Drawing, in metalwork, is how wire is made, but it’s also a sheet metal forming process. Wire drawing entails pulling metal through a die, reducing it’s diameter and making it longer. Drawing in sheet metal means deforming a blank over a curved surface. As metal fabricators it’s this latter process we’re most interested in, and today our topic is the variant called, “deep drawing.”
Drawing is a sheet metal forming process where flat material is pushed into a cavity. The starting point is typically a disk or blank that gets deformed to take on a cup or bowl shape. If the depth of that bowl is greater than the radius of the initial blank – meaning the metal has been deformed a long way – the process is called deep drawing.
Drawing and deep drawing processes look rather like stamping. The big difference is that stamping tools are designed to fracture the sheet into the shape needed but drawing tools create deformation. To do this, drawing tools are radiused while stamping tools have sharp edges.
Two other differences are:
The easiest example to visualize is a can, formed by having a circular punch push the blank down into a circular die. Punching through the bottom of the can will turn it into a short tube.
Deep Drawn Parts don’t have to be circular though. A stainless steel sink could be deep drawn, and it’s possible to form more complex shapes than that. Generally speaking though, deep drawn parts should be axisymmetric as this balances out the deformation. It’s also essential that the material be suitably ductile.
There are usually many different ways of making any particular metal fabrication. You can do a lot with stamping or shearing, bending on a press brake and welding, but deep drawing has at least six advantages. These are:
1. Speed.
No other process can match the speed of a punch press moving up and down. It’s usually the most efficient method if you need a large quantity of parts making.
2. Eliminates assembly steps.
Deep drawing produces shapes with closed ends. That avoids the need to cut and weld multiple pieces.
3. Seamless.
A deep drawn can or tube shape has no joins. That makes deep drawing an ideal process for anything that needs to be water or gas-tight.
4. High accuracy.
Parts coming off a forming press are extremely repeatable. Assuming the tooling was made correctly, they’ll also conform very closely to the drawing.
5. Produces complex geometries.
We’ve talked here about simple shapes like cans and sinks, but deep drawing can create more complex forms. How about the oil pan for an engine or complex filter housings?
6. Produces very strong parts
Many metals work-harden as they deform. Essentially, their crystal structure allows a certain amount of movement but beyond that it becomes locked. Deep drawing subjects metal to a lot of deformation, so can result in very hard finished parts.
Deep drawing isn’t a universal solution for complex sheet metal parts of course. The tooling is costly and setup can be time-consuming so it’s more suited to high volumes and long runs. In some cases though, these are more than offset by the resulting part simplification and strength and performance benefits.
At Wiley we know metal forming even better than we know our classic rock. If you’re interested in learning whether deep drawing might be appropriate for your parts, give us a call. We’re always happy to talk about fabrication.