Generative Sheet Metal Design - General Information

Generative Sheet Metal Design - General Information

Overview: The most important part of a design is to effectively communicate your design intent to manufacturing and quality control personnel. The best sheet metal part design is only effective if the part can be manufactured efficiently. It is important to adhere to specific sheet metal terminology and notation. The manner in which tolerances are called out on the drawing and the areas of the part that are dimensioned can cause a perfectly good part to be scrapped, or worse, a bad part to be accepted.

Bend Terminology

There are several terms unique to sheet metal: Inner Mold Line (IML), Outer Mold Line (OML), Center Line of Bend, and Neutral Axis:
- The IML is the intersection of the two inner surfaces of the bend.
- The OML is the intersection of the two outer surfaces of the bend.
- The Center Line of Bend refers to the axis of the bend radius where the machine or die is set that creates the bend.
- The neutral axis is where the tensile forces (at the outside of the bend) and the compressive forces (on the inside of the bend) are zero.

Sheet Metal Dimensioning

The basic rules of design apply when dimensioning a sheet metal part for manufacturing or inspection. There are several rules that are unique to sheet metal. This figure illustrates the following rules:

- Dimensions should be given between surfaces having a functional relation to each other.
- Dimensions should be given between surfaces controlling the relationship of mating parts.
- Radial dimensions should always be to the inside bend radius.
- Dimensions should be consistent to the inside or outside surfaces.
- Do not dimension from the inside of the part to the outside of the part.
- Use the OML intersections to dimension the length of angled flanges.

Standard Notes

When dimensioning sheet metal parts, you can account for many features and conditions by using standard notes in the title block of the drawing or record them as common shop procedures. You can group many sheet metal designs as families of parts because they use similar features or are made of similar materials. By including a standard note in the title block that states "Unless otherwise specified...," you can focus on exceptions to the normal. Typical techniques used to handle standard conditions are listed below.

Bend Radii

When designing parts made from the same material, it is typical to design to a common inside bend radius. A typical title block note may state:
Unless otherwise specified.
- All inside bend radii to be .062.

Corner Radii

It is more economical to produce a sheet metal part with an allowance on the outside corners rather than forcing them to remain sharp. A typical title block note states:
Unless otherwise specified.
- 1/16 corner radii allowed.

Feature Tolerance

When feature locations and sizes are designed with very tight tolerances, additional operations are usually required to finish the part. This adds time and cost to an otherwise simple process. Most sheet metal feature size and location tolerances can be specified by a note stating:
Unless otherwise specified.
• Hole location + .010 on centers.
• Hole size + .005 diameter.

Shape Deviation

Because the sheet metal process deforms material, it is not practical to assume features maintain their true shape. Holes tend to stretch oblong if they are near a bent edge. This condition can be captured in a note stating:
Unless otherwise specified.
- Shape deviations acceptable within feature size limits.

Burrs and Sharp Edges

Because the edges of the material can become sharp and dangerous to handle during the sheet metal process, it is important to specify an edge condition to protect manufacturing personnel and the end-user of the product. You can define edge conditions in a general note in a variety of ways. Some methods are listed below.
- Break all sharp edges and corners .015.
- Remove all burrs.
- Burrs not to exceed .005.
- Condition for handling.
- Break all sharp corners except cutting edges.
- Specify burr up or down.