In Photo Chemical Machining there are certain pointers and techniques that a designer should understand regarding the tolerances and dimensions of the etching process. The following are basic guidelines for designing PCM parts.

The key factors in PCM design include metal thickness, metal type and size of part being etched.

Relationship of hole size to metal thickness. As a general rule, the diameter of a hole cannot be less than 110% of the metal thickness.

Remaining surface area in a large field of slots or holes also has certain limitations as to how small the metal width between holes can be. Again it depends on material thickness. If the material is less than .005" thick the space between holes must be at least the metal thickness. Whereas if the material is over .005" thick, the spaces between holes must be at least 1.25 times the material thickness.

Relationship of inside corners to metal thickness is directly proportional to the thickness of the metal. The smallest corner radius would be .002" for piece of metal .002" thick.

Relationship of outside corners to metal thickness is considered to be at least .66 metal thickness. The outside corners tend to etch more sharply than the inside; therefore radii of less than metal thickness are obtainable.

Relationship of bevel to metal thickness. When etching one side of a sheet of metal the etchant attacks the material laterally as well as vertically. The result is the condition of etch configuration for a hole which as known as "bevel." When the material is being etched from both sides (equally) the bevel is reduced.

Tolerances of PCM products depend on the material type, size of panel, equipment used and yield required. As a general rule of thumb a tolerance of +/- 10% of metal thickness can be achieved with PCM.

The above specifications have been kept very general - for more specific dimension and tolerance information as well as design advice, please contact your PCM job shop. Depending on the alloy and the components you wish to produce they can specify the tolerances that are possible. Most have their own in-house CAD staff that can generate tooling from CAD data files as well as blueprints. Tooling is kept very reasonably priced because the same film photo tool is used until revisions are needed.