Intro: Strata Bench / Scale Model / Part 2 of 3
After the design idea process happens and you've come to a form you feel good about taking to the next step, it's best to make a scale model before going into fabrication. This is Step 2 of 3 in the making of the Strata Bench in which I will describe the workflow to take a mesh CAD model to a quarter scale plywood model. The hope is that what you learn from making the scale model will help you refine the design before going all in on the final sculpture.
WORKFLOW for making a Scale Model:
- Bring the mesh model into 123D Make to slice layers.
- Export from Make as .eps layout for lasercutting.
- Lasercut the plywood parts on an Epilog machine.
- Assembling and gluing the plywood parts into a solid stacked form.
- Using grinders and hand tools to shape the plywood into a smooth from.
- Evaluating, modifying, and refining the design before fabrication.
Step 1: Slicing in 123D Make
123D Make is a free app that lets you turn 3D models int 2D build plans. You can Download 123D Make at this link, or through the App Store. I used the stacked slice option and added an angle to the slicing rather than just stacking vertically. Its a super "user-friendly" app, so play with it to figure it out. That being said, the app can be buggy, and slows down when you are working with a large scale model. Restart if it crashes or is doing strange things, make sure you have the most recent update and SAVE OFTEN!
- From 123D Catch, you will export the mesh model as a .stl file.
- Open 123D Make and import the .stl.
- All the app functions run top-down on the left side of the window, the order in which they should be used.
Manufacturing Settings - Set this to the size and thickness of the sheet material you will be cutting the parts out of. For the model, I was cutting 3/16" plywood on an Epilogue laser cutter with a bed size of 24" x 36". There are preset material dimensions or you can m
Object Size - Set this to the scale that you want the model to be. I've noticed the program chooses the imported scale, you can reset it to whatever you like. My bench is 96" long, so I set the length to 24" for a 1:4 scale.
Construction Technique - Stacked slices, interlocked, slices, curves, radial slices, and folded panels. I used Stacked slices for the Strata Bench.
Dowels - are optional only with the Stacked Slice option, to help with registration for assembly. I didn't use them.
Slice Direction - allows you to change the angle of slice. I played with the angle until I saw a pattern I liked.
Modify Form - Hollow, Thicken, and Shrinkwrap. I did not use this function for the scale model. See Part 3 of 3 to see how the hollow function works for the full scale parts.
Assembly Steps - Use this to help assemble once you have the parts cut.
Get Plans - See the next step!
Step 2: Export the 2D Layout
Once you've got all the dimensions and construction technique settings in order, you'll export the 2D layout. The last function at the bottom left of the window is "Get Plans", and there are two options for layout - simple and nested. The app automatically generates layouts based on the material dimensions you've entered in the Manufacturing Settings, so make sure those settings are correct before exporting.
- Set the Layout Arrangement to "Simple" or "Nested". (Nesting is still a bit buggy, so it might crash the app).
- Click Get Plans to export the 2D sheet layouts as .eps, .pdf, or .dxf format. Each type deals with layers differently. I tried them all and decided that the .eps format worked best for the next step using Adobe Illustrator to laser cut the parts. Each sheet will export as an individual file to your desktop.
Step 3: Lasercut the Plywood Parts
Here at the Pier 9 Workshop, we have a number of Epilog Laser cutters that can cut a lot of different materials - plywood, acrylic, paper, cardboard, to name a few. Typically when making a scale model, you want to make the model out of the same material as the full scale piece, so in this case, I was cutting 1/4" plywood. The laser cutter bed is 24" x 36", and I was able to get the parts nested into 3 sheets. For more info, here is a pretty good Instructable with detailed instructions about Operating the Epilogue Laser Cutter.
- Open your .eps layout in Adobe Illustrator.
- Make all line weights .001, and set the Artboard to the same size as your sheet material.
- Separate SCORE (red lines) and CUT (blue lines) into individual layers by sheet.
- Hide all other layers you don't want to cut. For each sheet layout, send SCORE before CUT layers for each sheet to keep the parts and their labels registered properly.
- Print each layer individually. In the Epilog dialogue box, set the material dimensions, set Job Type to "Vector", and set the laser cutter settings (speed, power, frequency). Material dimensions vary, so make sure you test the laser cutter settings for your material. (Scoring is typically 100% speed with a lower power setting.)
- Calibrate the laser cutter focus to your material thickness, turn on the dust collector open the gate, and start the job. Repeat for each CUT and SCORE layer for all sheets.
Step 4: Assembly and Glue Up
Once all the parts are laser cut, start to organize them into sections that can be glued up into stacks. For this step you'll need wood glue, a damp rag to clean up glue squeeze out, and lots of small clamps! I like Quick-Grip and Spring clamps for a smaller glue ups like this. You'll glue the parts into small stacks that will eventually be glued into one solid form of stacked plywood. This will require some strategic planning to make sure you can clamp subsequent parts together.
- Get your stack ready. I did stacks of 6 parts at a time.
- Spread glue onto each layer, both sides. I use my fingers to spread glue.
- Work quickly as the glue will start to set within 10 minutes.
- Clamp the stack so that you get good, even pressure. Some glue should squeeze out the sides - clean it up with the damp rag. Leave in the clamps for at least 30 minutes. Keep combining stacks until it is one solid piece.
Step 5: Grinding Tools and Safety Equipment
Now for the fun step. The model is assembled and the glue is fully set. It is ready for grinding. I like to use a few different kinds of grinder to shape plywood. Here's a list of the tools I used:
TOOLS FOR SHAPING:
- 4 1/2" Angle Grinder with variable speed.
- "Holey Galahad" abrasive discs for the angle grinder (made by King's Arthur's Tools).
- Pneumatic Die Grinder.
- Double cut carbide bits with 1/4" shank for the die grinder.
- Sand paper.
- Rubber mat and clamps for work-holding.
- Face Shield.
- Ear protection.
- Respirator, snug fit with particulate cartridges (see my Instructable about How to Clean your Respirator Mask)
- Gloves are optional - just be real careful! The can easily get caught in the spinning tools.
Step 6: Carve It Smooth
I've used these tools a lot. If this is your first time shaping something, use caution and be careful! Always pay attention to where your hands are when the tool is on, always wear safety equipment, and make sure the piece is securely clamped to the table.
I start by rough grinding the layers smooth with the coarse Holey Galahad disk on the 4 1/2" angle grinder, at full speed. I recommend using the secondary handle, having both your hands on the tool and the piece clamped down. Move the piece around as you go, and make sure you don't hit the clam or anything else with the disk, as the metal tongs will break off if they hit metal.
For more detailed shaping or for spots with tighter curves, switch to using the die grinder with a carbide burr, like in the photo. For this tool, I will hold the model in one hand and use the die grinder in the other hand. This tool is much less aggressive and will leave a smoother surface.
Once the shape is defined, smoothed, and the deeper scratches from the grinder are gone, I'll switch to sand paper for a final smoothing. Start with 150 grit and work your way to 220. Once you are satisfied with the surface, you are done sanding.
Step 7: Evaluate - Modify - Refine
The first model looked awesome, but I quickly realized the design had some flaws that needed to be fixed. The footprint was too small for the size of the bench - when I pushed on one of the overhanging seats, it started to tip over and become unstable. The legs needed to be altered to make the bench more stable.
There are two ways to do this. I could have added to the physical clay model to make the feet bigger, and repeated Part 1 of 1 to make a new 123D Catch scan. The other option was to scan the plywood scale model and modify the mesh digitally - which is what I did. I used another program called Mesh Mixer to modify the mesh model, using their free-form mesh sculpting tools. (A Mesh Mixer Hot Key cheat sheet is attached as a PDF).
One other modification was to mirror the bench - I thought it would look better. This added one step (and yet another program) - I imported the mesh into Rhino and mirrored the surface. Since I was in Rhino, I was able to pull dimensioned drawings as well, by using the "Make2D" command. I exported the 2D dimensioned orthographic drawings into Illustrator at 1:1 scale, added some seated people silhouettes, and made these renderings to show people in scale to the bench.
Step 8: Repeat
To ensure the modifications were to my liking, I repeated Part 2 of 3 and made another quarter scale model before making the full scale bench. After model #2, I was ready for full scale fabrication.