Machining the ‘unmachinable’

A trick I gleamed from a friend reveals an interesting use of relatively crash-proof soft metal or plastic bed plates on a milling machine. In the shop where my friend works, they prepare the part blanks and secure them to the bed plate with Permacel double-sided tape. Plastic locating pins are used to index the part for the second-side operations. This technique, used along with hot-melt glue filling (see the August 2013 “Shop Operations” column), allows the “unmachinable” to be machined with ease, such as delicate parts that cannot tolerate any clamping force without distortion.

For example, hot-melt glue supports a difficult-to-hold part for two-sided machining, and the part is held with double-sided tape to a freshly surfaced subplate. The first side of the part is faced and then surface contoured to the part’s halfway point. Be sure to slightly overlap the parting line if applying a ballnose endmill.

After machining the first side, spray the machined cavity and part with a little mold release and fill with hot-melt glue. For a large volume of glue, fill an inexpensive Teflon-coated pan on a hot plate to make quick work of melting the glue.

Courtesy of All images: T. Lipton

Double-sided tape holds this workpiece to a freshly surfaced subplate.

Facing and contouring to the halfway point of the first side of the part.

Filling the cavity around the part with hot-melt glue.