program to improve and control the quality of a metal product should
start at the desk of the designer. The metal finisher is restricted
he can do by certain basic principles of mechanical finishing and
electroplating. The engineer should understand the limitations imposed
by shape and size of components to facilitate quality finishing at
acceptable cost. The designer can exert as much influence on the
quality attainable in finishing a part as can the electroplater himself.
ASTM Standard B-5O7 can provide the designer with helpful information.
An important term used in specifying metal finishes is "significant
surfaces". In most products the same standard of quality is not
over every square inch of surface. Instead, the quality specifications
and compliance is expected only for the so-called "significant
defined by mutual agreement between the producer and purchaser
Significant surfaces are defined as those normally visible (directly
or by reflection) which are essential to the appearance or serviceability
of the article when assembled in normal position, or which can be
the source of corrosion products that deface visible surfaces on the
assembled article. When necessary, the significant surfaces shall
the subject of agreement between purchaser and manufacturer and shall
indicated on the drawings of the parts, or by the provision of suitably
Design for Mechanical Finishing
Metal products which are to be coated with copper/nickel/decorative
precious metal or substrates utilizing non-nickel plating processes
by the decorative precious metal finish are generally subjected to
polishing with wheels or mass finishing techniques in preparation
plating operations. This is done to aid in securing an attractive,
mirror-like or satin appearance on the finished part. Mechanical finishing
is an expensive operation. To reduce costs and assist the metal finisher
improving the appearance and quality of the product, the designer
consider certain rules applicable for parts requiring mechanical finishing.
In small parts, which are to be barrel processed, the above rules
- Avoid blind holes, recesses and joint crevices which can retain
polishing compounds and metal debris.
- Avoid intricate surface patterns which will be blurred in polishing.
- Significant surfaces should be exterior, reachable by ordinary
polishing wheels or mass finishing media.
- Avoid sharp edges and protrusions which cause excessive consumption
This includes the requirement that the parts must be sturdy enough
to withstand the multiple impacts of barrel rotation and will not
causing damage or incomplete finishing. Small flat parts, which tend
to nest together, should be provided with ridges or dimples to prevent
Design for Racking, Draining and Air Entrapment
Most metal parts weighing more than a few ounces or that require a
degree of surface finish or a jewelry finish, are not plated in bulk
but are mounted on racks for processing in cleaning and electroplating
tanks. Design considerations relating to racked parts are described
- Consult the plating department to make certain that parts can
be held securely on a plating rack with good electrical contact
without masking a significant surface. Many difficult racking
problems can be solved by design modification.
- Provide for good drainage of processing solutions from racked
parts. Certain shapes tend to trap solution which then causes
contamination by carry over, possible corrosion of the part and
waste of materials. Carry over aggravates the problem of waste
disposal and adds excessive cost due to chemical losses. In design,
avoid rolled edges, blind holes, and spot- welded joints. Drain
holes are especially important in irregular shapes and tubular
- void shapes which can trap air on entry into processing tanks
if this air could block access of solution to areas requiring
treatment. Wherever air can be trapped, hydrogen or oxygen gas
may also accumulate during a cleaning or plating step.