Novena Heirloom – achieving proper fit

I’ve worked with curved and bent wooden forms as a luthier and camera maker for many years. The beauty of a fair curve is one of my favorite things in the design of wooden objects. Often one curved surface will be joined to an adjacent flat surface as is the case when building a guitar. For many of my camera designs, the same holds true as curved surface joins flat with a proper fit. The fitting process is tricky under the best of circumstances but, once the joint is made, there is an opportunity to fair the intersection between members at the joint by careful sanding or scraping.

Curved surface joined to flat in the Pinoramic 120 Series 2
Curved surface joined to flat in the Pinoramic 120 Series 2

I often will actually glue up the joint with one surface rough cut to overlap the other for trimming after the joint is complete. This is pretty standard practice in woodworking and it relies on pre-established datum to insure proper orientation of the completed part. When the glued assembly has cured, the excess material can be trimmed off with a router, a plane, a sanding block or file or any number of other methods that will leave a proper fit.

The aluminum port plate of the Heirloom laptop was trimmed to proper fit by a series of sanding operations one of which is seen here. The port plate has a pair of threaded bosses that align it with the side plate. So the port plate was left slightly long at both ends in anticipation of this fitting operation. Of course the parts are not yet anodized when this procedure occurs. That will happen after final hand finishing and bead blasting for these parts.

Sanding the port plate for proper fit with the side plate
Sanding the port plate for proper fit with the side plate

A prominent feature of the Novena Heirloom is the wavy bottom surface of the enclosure. The intended benefits of this form are threefold.

  • It produces a monocoque structure that contributes greatly to the stiffness of the complete assembly.
  • The gaps resulting when the panel is placed on a desktop are intended to increase airflow and contribute to cooling the computer system.
  • The visual effect when examining the bottom of the computer is a pleasing surprise.

The side plate design incorporates a curved flange whose profile matches that of the bending form for the wood composite that makes up the wavy bottom panel. The reality of producing such a bent form in a wood/glass fiber/cork composite includes the natural tendency for the material to respond more actively to the environment than metal. Then there is the fact that a composite form coming out of such a process requires quite a bit of cleanup. This includes sanding to remove seam tape and minor amounts of epoxy that makes its way through the pores in the wood to the outer surface. This means that when the composite and aluminum forms are presented to each other for gluing, there will be minor discrepancies in proper fit.

The two components are brought together in a press with a resilient-surface form to remove the effects of those discrepancies and produce the required proper fit. But because of the relative complexity of the mating surfaces, the alignment between the top of the aluminum side plates and the solid wood rails glued to the top edges of the composite bottom panel varies.

The solid front rail for the composite bottom panel.
Holding the solid front rail against the composite bottom panel.

Developing fixturing and procedures to achieve proper fit under these conditions took about a week of concentrated thought, trial-and-error and reliance on previous experience. Part of the solution lie in developing a fixture table for the CNC mill that could accommodate tweaking for the side-to-side variations that occur in the bottom-panel-to-side-plate fits for each assembly. (That is dealt with in another post.)

The key difference between this process and the ones described previously is the fact that when the final glue-up occurs, the aluminum side plate will be anodized and the composite panel pre-finished, thus eliminating and prospect of fairing the fit afterwards. And since the fit along the rear edge of the bottom panel assembly also governs the hinge placement, all of the fitting must be done before final assembly of the side plates to the bottom panel.

The image below shows a proper fit being measured to apply to the remainder of the assemblies. A granite surface plate is being used just because it was handy and definitely flat.

Checking for proper fit.
Checking for proper fit.

This measurement is used to determine height for trimming of the remaining composite panels where the solid rails will attach. (That is also covered in another post.) When that rail-to-composite-panel assembly work has been completed, the fitting procedure shown in the video below will be used to set up the assembly with the necessary rotation on the Y axis to achieve a proper fit between the rail/composite assembly and the aluminum side plates.

 

The resulting data is used to set up the custom, tilting vacuum table covered in another post.

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