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How to CNC machine from two sides

Two-sided machining a thin-walled model of a cellphone front

One of the characteristics of mobile communications equipment is the very short life-cycle of the mobile phones (cellphones). This implies that cellphone manufacturers have to develop new products within a very short time frame. As the design of the phone also plays a major role in the the product being accepted or rejected, it is clear that designing cellphones is a challenging activity.

The use of physical prototypes has proven to be a major factor to achieve successful designs. Not only to be used by designers, but also needed for communication with suppliers, marketeers, managers, and end-users. Also see the fascinating book "Serious play" written by Michel Schrage, for an explanation why prototypes cannot be missed.

Rhino wireframe
Flamingo rendering
CAD design in Rhino as wireframe, and a rendering in Flamingo.

Unfortunately we cannot show real-life cellphone projects on this galley page: this branch is so sensitive that the Deskproto users that manufacture cellphones cannot allow us to show any product. So we were very happy with this cellphone geometry example created in Rhino by John Brock of Robert McNeel & Associates in Seattle (USA). He modelled this thin-walled front cover for a mobile phone, and also create the rendering in McNeel's renderer Flamingo.

This geometry is used in Lesson 4 of the DeskProto Tutorial, to teach you about Two-Sided machining. As you will need the geometry to replay this lesson, the cellphone STL-file has been made available as a free download !

Machining side one
Machining reference planes
Two sided milling : machining side one, and next the reference plane.

A shell-shaped part like this front cover has to be machined from two sides to create a thin-walled prototype. DeskProto makes this very easy, as you can use the Two-Sided Milling Wizard to achieve this. In this wizard DeskProto will add a frame of material around your model, and four bridges (support blocks) to connect the model to the frame. After machining the model can be removed from the frame, and finally the bridge's remains can easily be removed and manually smoothed.

Machining side two
Machining some small details
Two sided milling: machining side two, and a detailing operation.

When machining from two sides the main problem (at least for unexperienced operators) is to again find the starting position after turning the block upside down. DeskProto makes this very easy by using a ruler on the machining table (clearly to be seen on the pictures). The ruler is parallel to X and located on Y=0, with an end-stop on X=0. After machining the first side, DeskProto tells you to machine reference planes on the back side and the left side of the block. The original block does not need to be exact. After that you can turn the block upside down, position the reference planes to the rulers, and continue machining with the same workpiece zero point. Piece of cake !

Cellphone model
The resulting shell model of the cellphone's front cover, immediately after machining. The model stands in front of a mirror. Note that the four support bridges are clearly visible.

A major advantage of DeskProto's subtractive approach of RP is that this model can be machined in almost any material, so for the prototype the same material can be applied as for the final product. Second advantage is that the model is very smooth due to the use of ballnose cutters. Because of this it can easily be finished to a glossy presentation model.