The proper fit issues that effect the Novena Heirloom fabrication process lead to a concern with making minor tweaks for each individual part that is prepared for assembly. Sometimes this kind of thing can be handled by essentially telling a CNC controller that the object is not in plane with the table and just let it do the math to figure out how to compensate. That is typically done for rotational displacements along the Z axis – that’s like spinning a part flat on the table. Each instance could conceivably have a modified parameter in the g-code. But was I was confronted with is a rotational displacement along the Y axis or perhaps the X axis. There are apparently some CNC controllers that can handle this, but mine does not.
My Aciera manual mill can rotate along Y with the table itself. If it were a CNC machine, that would probably suffice to resolve the issue. But CNC mills don’t often have such a feature and mine is no exception. Since I wanted to hold the parts in a vacuum fixture, I resolved to find a solution that would hold the part and simultaneously allow for the subtle X or Y axis tweaks required of the task.
I had earlier made a vacuum table base that uses the CNC machine table T-slots as a plenum. The vacuum pump attaches directly to the mill table as shown above. The sub assembly then bolts down to the table and provides an interface for various insert materials to be used as the working surface. Since my first attempts with it were based upon drawing air through mdf panels, I used one of the previously precut panels for this setup.
I could get slightly better vacuum by sealing off the areas outside of the vac seal. But so far, the system works just fine. I could remake it out of some plastic or aluminum in the future.
As configured above, the setup assumes a panel will be placed on top of this and drawn down to the hard surface by the vacuum while compressing the foam seal around the perimeter. That works quite well but is not applicable to the challenge at hand. So I spent quite a bit of time experimenting with trial and (mainly) error looking for a way to adjust the part hold down assembly on that Y axis. I finally had a minor flash of inspiration and went off the hardware store looking for some relatively thick, closed cell foam tape. Having that in hand, I reconfigured the surface of the insert by removing the 1/8″ round foam cord and replacing it with the newly acquired tape.
Oddly, I have been experimenting with tapes for holding small parts down on the CNC mill and had a particular candidate that was not up to that job but which turned out to be perfect for sealing the intersections of the foam tape. It has no backer, is very tacky and maintains the flexibility of the foam tape at the corners – crucial for what I wanted to achieve.
A pair of index pins can be seen at either side of the tape layout.The table that will hold the part is then fitted over those index pins at three pairs of holes. This is due to the working envelope limitations of my CNC machine. This table version actually came before the final working solution and was intended to be used with the standard foam cord in a flat position on the table in case I found a way to work this out with g-code. That was not in the cards, so it was from here that the thicker foam tape idea sprung.
A few modification later this version of the table materialized. Several features are noteworthy.
- The outriggers with thumb screws are used for tweaking the plane of the table.
- There are wavy foam seals at either end made by cannibalizing some bottom panel composite material to support the closed cell foam tape. They work nicely to seal the vacuum against the wavy bottoms of the panels.
- The closed-cell foam cord along the other two edges are cut in with a horizontally wavy pattern because the point at which they make contact with the bottom panel is at the bottom crest of a wave. The wavy placement of the cord insures that some of the panel contacts the flat surface of the fixture rather than ambigously falling into the cord groove.
- Visible but not clear are the red markings indicating the six positions the panel can take on the vacuum insert. (OK, that’s more than several features . . . )
Most things like this are first sourced from within the shop scrap pile and these screws have been lurking for a long time receiving occasional use for projects such as this. Because this is a very short run application, I just used Appleply with threads cut into it for the screws.
The complete arrangement begins with a lower vacuum table assembly that seals at the bottom against the CNC table/plenum chamber and presents a 1/4 inch closed-cell foam surface at the top. On top of that goes this vacuum fixture with seals designed specifically for the panels that have a number of operations needing to be done on them. The key to its operation is the thickness and flexibility of the 1/4″ foam seals. As shown in the video, the foam seal interface provides enough flexibility to allow adjustment of the vacuum fixture across both X and Y planes.