Metal Fabrication: Material Types – Steel, Aluminum, AR SteelBLOG > Selecting die V opening

Selecting die V opening

Selecting die V opening

The V opening we use for our fabrication deeply affects the results we get on our part

The V opening we select is determined by the blueprint of our part, it’s thickness and material type

Let’s see why:

How the blueprint conditions our V selection:

The blue print details might call for a specific operation that will determine or condition our V die selection.

hemming detail will determine that we need a hemming die set.

A back bend present on the part will limit our choices to dies with enough height to allow for such operation.

A large radius bend will require a larger V opening and of course a short flange will call for a V opening that's not wide enough to make the flange fall into it.

Non the less, a certain material type and thickness called in the blueprint for a part, might require a specific V opening. Such is the case of abrasive resistant steels. 

All these aspects limit our choices, condition the options and in some cases simply force us to choose a certain V opening 

There are bending apps or tables that will help us choose the correct V. They all start by asking the material type and thickness and they give back an ideal V opening with some options.

Each option implies a series of consequences to radius, force and minimum flange.

Let's see why this happens. 

How V opening affects the iR of our part

Although some fabricators are used to relating the internal radius of the part with the punch radius, this is not always how bending works. Most of the times the iR is a result of the V opening used.

On most fabricated parts, the ideal iR=1 thickness. This is true because if the radius was smaller than 1x thickness, it would mean that the material taken from the radius is gone somewhere. On plate bending we see how our bend shows "side bumps" if the iR is smaller than 1x thickness. 

Of course an iR larger than 1x thickness is not a problem from this point of view.

​So, how does it work between V opening and iR?

Well, first of all it depends on the material UTSThe stronger the material the larger the iR will result on a given V opening. 

​Empirically we know for example that on mild steel the iR is equal to 1/8 of the V used. While on stainless steel this ratio is 1/6 of the V.

Other materials like Hardox can never be bent to obtain iR=1xT because they would crack due to lack of ductility.

The above ratios do tell us something quite interesting. They are the ground on which is based the most common rule of thumb for every job shop:

 V=Tx8 (or 6 in small gauge)

This rule has some variations. The most common being that on thick plate we use V=10xT. The reason for this is that thicker plate tends to loose some ductility, so we use a larger V opening to distribute the force on a larger area and avoid cracks in the sheet.


On our next post we will see the relation between our V opening and the flat blank we need to cut for our part……