| There are three types of bends used to form | | | | even the difference of a few thousands of an |
| metal that any sheet metal engineer or press | | | | inch can result in bad parts. |
| brake operator must be familiar with; air bending, | | | | Coining |
| bottom bending and coining. | | | | Coining is a very basic type of bending in which |
| Air Bending | | | | the workpiece is stamped between the punch and |
| Air bending is the most common type of bending | | | | die. The material is put under enough pressure |
| process used in sheet metal shops today. In this | | | | that the punch tip penetrates the material and it |
| process the workpiece comes in contact with the | | | | begins to flow into the die. This method produces |
| outside edges of the die, as well as the punch tip. | | | | excellent accuracy and repeatability, and does not |
| The punch is them forced past the top of the die | | | | require sophisticated machines to execute. It does |
| into the v-opening without coming into contact | | | | however very large tonnages compared to air |
| with the bottom of the v. The v opening is | | | | bending. Often times in excess of 50 tons per |
| typically deeper than the angle which is sought in | | | | inch of material, compared to 1 or 2 tons per inch |
| the work piece. This allows for over bending to | | | | for air bending. Because of these tonnage |
| compensate for the spring-back of the work | | | | requirements wear and tear on the machines will |
| piece. Typically 30 degree tooling can be used to | | | | be much greater than air or bottom bending. |
| fully air bend and 90 or 88 degree tooling can be | | | | Tooling required for coining must be robust and |
| used to partially air bend. There has recently been | | | | this can limit your tooling and geometry options. |
| the introduction of 75 degree tooling to allow for | | | | Because of the tooling restrictions and the large |
| full air bending, without the tooling restrictions of | | | | tonnages required to coin this process is rare in |
| acute punches. (Acute punches are almost always | | | | the press brake world. |
| knife dies with no goose-neck.) Because the punch | | | | Bottom Bending |
| tip does not penetrate the workpiece the inside | | | | Bottom bending has similarities to both air bending |
| radius of the bend is controlled almost entirely by | | | | and coining. In this process the die angle should |
| the size of the v-opening of the bottom die. | | | | match the intended angle of the work piece, |
| The larger the v-opening the larger the radius. | | | | adjusting a few degrees for spring back, hence |
| This has both pros and cons, the pros being that | | | | the existence of 88 degree tooling to achieve 90 |
| the operator can control the radius of a bend | | | | degree angles. The workpiece is first bottomed |
| even when working with the same material and | | | | against the die, then the radius of the punch is |
| thickness just by changing the bottom die. This | | | | forced into the work piece which achieves the |
| can be used to compensate for errors in the | | | | angle of the punch, it is then released and the |
| layout or achieve a wider variety of design | | | | workpiece springs back to meet the die again. |
| options. This same versatility can work against | | | | Unlike coining however the material is not under |
| you as well as it can lead to bad parts if the | | | | so much tonnage that the metal flows. Because |
| wrong dies are used. It is my recommendation | | | | of this there is still spring back which must be |
| that the dies you are using be clearly marked for | | | | compensated for. In order to do compensate the |
| their intended gauges and that using the larger | | | | angle of the punch can be smaller than the angle |
| dies to achieve larger radii be done only under | | | | of the die by a few degrees allowing an |
| engineering approval. Be aware that you should | | | | over-bend when the punch tip is forced into the |
| never use smaller dies to bend heavier gauges | | | | workpiece, it should not be larger or else you will |
| due to the risk of damaging the die. Use the rule | | | | damage the tooling. This method can give an |
| of thumb that the v-opening should always be at | | | | operator excellent repeatability if properly set up. |
| least (Rp. + 2* Mt.) For a list of radii compared to | | | | Setting up a bottom bending operation accurately |
| v-openings refer to your air bend force chart. A | | | | requires knowledge in the tooling, material, |
| further note on air bending is that it should almost | | | | tonnages and even the timing of the different |
| never be used on older, mechanical, brake | | | | steps so it lends itself to a more skilled operator. |
| presses because of their inherent margin of error | | | | |