Scott Zitek - Fab Academy 2014

Computer-controlled machining

Week 7 Assignment

The assignment for this week is to "make something big". Big is a relative term and I wanted to make something that I can use. I also didn't have the time and money to make a house using the Shopbot. So I decided to build a side table that could be used beside a couch or a bed. I was interested in making either a press-fit design or a laminated design (several layers of material glued together).

Design

I started out in FreeCad and decided that 3D was overkill since I planned make something pretty simple. I switched to CorelDraw and started sketching 2D concepts of what the side table should look like. I began with a straight forward construction but wanted to make something that would hold itself together and could be unassembled to save space if desired. My first design relied on the ability to stretch open the structure enough to attach it to the top and then use puzzle piece interlock to hold it in place. I didn't think wood would flexible enough to do this.

My first design needed to stretch open to hold the top board in place.

1. I then considered a design where the four sides would surround the top like an open bottomed box. The sides were easy to design but attaching the top as part of the press fit construction was more complicated. I didn't want the top to just press onto the top and I didn't want parts going through the top surface.



Evolution of my design from straight slots with slanted leg edges to slanted slots and legs.



2. The design that evolved was something like a puzzle that I wasn't absolutely positive could be assembled. All four sides had to be pushed together diagonally simultaneously to squeeze in to hold the top on.



Concept of how table would assemble.



3. Now that I had an idea how my table would assemble, I used CorelDraw to make an accurate scale version of the design. I plan to make the table out of left over 4' x 4' piece of 1/2" thick cabinet grade plywood that I found. I used a caliper to measure the plywood in a few places and decided that it was actually only 0.47" thick. So I create the design with 0.47" wide slots.

Prototype

Since I wasn't absolutely positive the design could be assembled; I scaled the design down and cut a prototype out of cardboard using the laser. As part of the previous press fit assignment I determined that a 0.14" slot width was good for the cardboard I use. I divided the desired cardboard slot width (0.14") by the current plywood slot width (0.47") to determine that I needed to scale the design to 29.8% of the original size.

I was able to assemble the laser cut cardboard prototype.

Test cuts on the Shopbot

1. With confidence that the design would work, I used CorelDraw to export the full-scale design in DXF format.


2. For several years we have been using the version of Partworks that came with our Shopbot. It was really getting outdated. Luckily, we were recently able to upgrade to Vetric Aspire. I have never used Aspire before, so decided to use it to generate the Shopbot program for this project. Since Aspire had not been used before, I had to create the tool configuration for my tool.



Tool configuration for the 1/4" single-flute straight-cut Onsrud 48-005 tool I used.



3. Not wanting to waste time and materials, I first used the Shopbot to cut a few test pieces out of a scrap piece of the material I planned to use. This allowed me to test my workflow, tool settings, feeds and speeds, slot widths, and other things.



I was amazed how perfectly the parts fit together. The Shopbot was more precise than I expected.

Using Aspire to program the Shopbot

1. I used Aspire to make sure that I didn't have any duplicate vectors or open vectors.



Aspire did not find any duplicate or open vectors.



2. It is very important that your material is firmly attached to the Shopbot bed. Clamps stick up and can easily be hit by the moving spindle. I find it is best to use a cordless screw gun to screw the material down to the Shopbot spoil board. However, you need to be very careful not to put screws where the Shopbot will hit them. The easiest way to avoid this is to add the hold down screw locations right into your design. I drew small circles everywhere I wanted a hold down screw. I then created a Shopbot program to barely mark the surface where the screws go. Since I am using hard plywood, I pre-drilled holes and then put a hold-down screw in each of these places.



Setting up the Shopbot to mark the locations for the hold down screws. The small magenta colored dots are hard to see so I added some arrows to show where some of the drill marks are located.



3. I then used the Aspire profile tool to cut inside of the dog-bone shaped holes that hold the top on.



Setting up the Shopbot to cut inside of the dog bone shaped slots.



4. The remaining geometry was cut on the outside using the Aspire profile tool. Notice that tabs were configured so that I would not have to screw down the parts that would be completely cut loose from the main board.



Outside profile cuts with tab locations defined.



5. I used Aspire to simulate the programmed Shopbot operation. Everything looks good.



Preview of program. Red lines represent non-cutting rapid moves above part. Blue lines represent cuts at feed rate.






Shopbot machining out the parts for the side table.



6. I used dog bone shaped holes since the 1/4" diameter round tool would not normally remove all the material from the inside corners of a hole. Since I didn't use tabs inside the dogbones, the cutout parts came loose and were sucked into the dust collector.



Dog bone shaped hole used to hold table top. In the upper left corner of the picture, an example of one of the 3D machined holding tabs can be seen inside the slot.

Cleaning up the parts and assembly

1. When the Shopbot was finished cutting, I used a cordless screw gun to remove the hold-down screws. It was then very easy to use a hand saw to cut the thin 3D machined holding tabs. I sometimes just break these tabs loose but I didn't want to risk splintering or peeling the plywood surfaces of the finished parts.

   

   Holding tabs as seen from the underside of the board.

   
   

   

   It was easy to remove the remaining part of the tabs using the bench sander.

   
   

2. The cutting precision of the Shopbot was impressive. The pieces fit together perfectly.



The assembled side table.






Everything fit together nicely.






If the bigger top piece with rounded corners is not used, you have a tray table with an inset surface and about a 1/2" lip all around the table top.






The angled legs cause something like an optical illusion. Looking straight at one of the sides, the profile of the legs looks perfectly straight. From an angle, the legs appear to flare out wide on the bottom. While from another view point, the legs appear to angle into the center of the table.






While I was making the side table, I realized the design could be fairly easy to modify to make a matching chair, with or without arm rests.

Related files

• The overall side table design in dxf format - sztable.dxf