Computer controlled machining:
I wanted to create a chair by bringing flexibility to a rigid material using alternated lattice cuts. I considered doing those cuts on plywood, but decent quality plywood doesn´t come cheap locally. The most widespread available plywood is an E grade cheap material used more for crates and construction jigs. It´s possible to use this material in waffle type assemblies. Its true waffle – based designs are relatively simple to assembly without nails, screws and simple tools, but for seats they can take a lot of material and results in frequently heavy objects. I wanted to try an alternative path, using and extrapolating what I´ve learned from lattice cuts to give flexibility to rigid materials.
Because of this I tried to make my design in 1/8″ and 1/4″ extruded polypropylene sheets. Considering I wanted to make a chair it would be good for garden / patio use. Additionally PP has the ability of being hinged by kerfing.
The design idea was made from developable single curvature surfaces. I wanted to scale up some of the lattice patterns I´ve used previously on the push – tab assignment, but on polypropylene.
Detail adjustment on the latch + slot encounter. Since they meet at a 57 deg angle, I had to adjust the width of the opening and the lenght of the bistatic latch.
Basic surface model with a developed flat main surface integrating seat, backrest and bottom, Like a continous surface around the side panels.
Based on the developed flat main surface I´ve created the lattice pattern. For material reasons and to fit the part on a single sheet of 2,44 x 1,22 meters, I resorted to create backrest, seat and their bounding curvatures. The distance between the axis of each external positive curve was measured in order to be translated to the side panels.
The side panels (in red) are made in 1/4″ PP. The outline have an array of bistatic ellastic latches which connect with slots located on the seat + backrest surface. In addition to that, they have a curved midzone that creates the tapering that follows the backrest shape. To add structural rigidity, I´ve added two transversal structural panels (in black.) also in 1/4″ which connect both side panels. Here are shown placed on a 1,22 by 2,44 meter sheet.
From the DXF archive I created a toolpath in Mastercam to upload to our Thermwood CNC router. I´ve made a test sample first with a 400 x 500mm section of the final archive which went well. I´ve put the sample part in my hands and pushed hard with my knee trying to break it. Although it flexed, it never reach a point of fracture.
What went differently and (sadly ) wrong was the cut of the big final part. The router had a tendency to lift the PP sheet up from the bed despit is vacuum assisted. We resorted to nailing it to a sacrificial MDF to avoid lifting, but on the longer lines the 1/4″ diameter flat end mill lifted it. Apparently the mill got tangled with the dense plastic and that made it to pull the material off the bed.
We tried hard, but this pulling – off problem became worse and broke the part twice… We were afraid that so many nails will cause an accident so I´ve decided to remove the failed part away. I´ll try it next with a less intrincate pattern, probably using slotting instead of using a cutting pattern that goes through the material. But that will happen this week. I´ve worked nonstop from thursday until an hour ago. I´ll finish it as soon as I can.
