Nate Duxbury's Blog

Make all the things!


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Space Time Coffee Table

Finished Product First!

Finished Product First!


SPACE TIME COFFEE TABLE


Recently, I graduated from Ohio State, started a new job and moved into my own apartment. I had almost no furniture for the new place but this afforded me a great opportunity: I could make some of my own!

The first thing I decided to make was a coffee table. Months ago, someone on Reddit posted a conceptual design for a “space time table.” The idea behind the concept is to imitate the curvature of space time around a massive object (such as a black hole). I thought it looked awesome. At the time though, I was living in a shared college house and did not have the funds, reason or access to machinery to make such a thing, so I filed it away in my head as something cool I would look into in the future. Fast forward to May of this year, and I am now employed at a Water Jet manufacturer, where I am allowed to use the sales model for personal projects after hours! (possibly the best work benefit ever?!) With the tools now available and the reason to very clear, I began modifying the design to be manufactured.

If you’re more interested in just pictures, check out this Imgur Album. It’s not as thorough as the this write up, but it is a quick summary.


DESIGN PROCESS


It was super convenient that the reddit user who posted the concept images also posted the source files AND they were in SolidWorks format! Perfect for me to load up and modify to fit my needs.

Step one was downloading the source files (Source). Step two was deciding what size I wanted to make it, such that it would fit nicely in the space I had available for it. So, like any normal person, I laid it out in CAD!

Rough Floor Layout of my apartment to get sizing.

Rough Floor Layout of my apartment to get sizing.

 

In the center right of the layout is the couch I have and then the square is the table size I felt was comfortable. The size I drew was about 24″ x 36″, but as I was going to be scaling the dimensions off of the concept, I could only really choose one of the dimensions and the other would be based on the aspect ratio of the existing design. In the end I was able to get 34.125″ x 23.5″ as the top dimensions.

I used envelopes to mark the bounding box of where my coffee table would fit. This gave me a great sense of scale for the project.

I used envelopes to mark the bounding box of where my coffee table would fit. This gave me a great sense of scale for the project.

Knowing the final size I wanted, I could now start modifying the design. There were four main points to this part of the process:

  1. The size of the table. The original design was 45.5″ L x 31″ W x 17.4″ H and I had to shrink the width dimension down to at least 24″. Applying the same scale down to all dimensions yielded 34.125″ L x 23.5″ W x 13.125″ H.
  2. The height of the table after scaling. A 13″ Tall coffee table would be no good. The bottom of the table was also very fragile in the original design. It terminated in sharp ends, which i did not like from a durability standpoint. The solution here was easy, I simply added material from the sharps directly down 3″. This increased the height to a nice 16.125″ and made the base less fragile. This feature was a departure from the concept, but it was a practical change.
  3. The concept model was all 3D features. The concept had sloping contours on every member and would have been extremely hard to manufacture. Again I decided to make a change for the practical, by removing all 3D features and making each part only a 2D contour. Since I had the 3D contours already from the concept model, all I had to do was pick the side with the larger features and extrude it to become the thickness of the whole part. I did this process for each part, but since I was going to scale everything anyways, all I ended up taking from each part was it’s 2D contour. Using only 2D contours made manufacturing immensely easier and still made for a very good approximation of the effect I was after.
  4. Making it possible to assemble. The concept model was a multi-body part file. This meant that the model’s bodies intersected each other, and that they could not actually be assembled as is. The fix here was to make the parts have matching slots and tabs. The parts going in the long direction got slots cut in the top half of them and parts going in the short direction got slots in the bottom half. With these interlocking slots, the assembly became assemble-able and even very easy to assemble!
The original concept on the right and my model on the left after modifications.

The original concept on the right and my model on the left after modifications.

SolidWorks render of the coffee table after modifcations.

SolidWorks render of the coffee table after modifcations.

I also did something really clever when I re-modeled the whole thing. I linked all the dimensions together with dimensions. This way, I could specify any thickness of material, any slot tolerance and any table height that I wanted by conveniently changing the value in a text file! I was unable to link a scale feature to the text files (not sure why…) but the rest of the features were now all linked and very easily changed. This allowed me to compensate for material thickness (since wood is rarely true dimension) and adjust the tightness of fit in the slots. It’s the first time I’ve done equations in SolidWorks, but I can see it being very helpful for projects like this. I could have used 1″ thick wood if I wanted and it would have been only three keystrokes to make the change.

I put together a quick explosion animation to check out how it would all assemble.

 

Here is the link to my  Design Files. The files are available in STEP, IGES, DXF and SolidWorks formats. Even if you don’t have a water jet, you could still use a large format CNC router or even print them out 1:1 and cut them with a jig saw.

 


PROTOTYPING


 

After completing the design, I wanted to make sure everything would fit together correctly. When I had the time one weekend, I fired up my Shapeoko 2 and got to cutting! I planned to make the final table out of a 4’x8′ of .5″ plywood, so going to quarter scale translated perfectly. I had a 1’x2′ of .125″ HPDE, so I could even check my nesting. Not much to talk about here, the shapeoko cuts through HDPE like butter, and I was able to get all the parts out of just the one sheet!

1/4 scale model of the space time coffee table made from HDPE on a shapeoko 2 CNC router.

1/4 scale model of the space time coffee table made from HDPE on a shapeoko 2 CNC router.

Doing the scale model proved to me that the interlocking slots worked just fine and that I could fit all the parts onto one sheet of plywood. Plus I could bring it with me to work and explain to my coworkers what I was planning to make.


WATER JET CUTTING


This part was a lot of fun. It was not the first time I got to use a water jet for something, but it was the first time that I ran the machine (mostly) my self. I leaned heavily on one of my coworkers for setup help, speeds/feeds, and a little bit guidance (shout-out to Ben Adams, thank you for all the help Ben!).

This section is best explained with pictures, so I will do my best to annotate each of the steps.

The first step was nesting the parts to make the part program. I used a 5'x5'x0.5" (actually real dimensions!) sheet of baltic birch plywood

The first step was nesting the parts to make the part program. I used a 5’x5’x0.5″ (actually real dimensions!) sheet of baltic birch plywood, and laid it all out with SolidWorks. Everything fit within a roughly 58″x 58″ bounding box, leaving me a 1″ border to clamp on. I exported this assembly first as a part file, then as a .dxf to feed into the water jet CAM.

Drilled Pierce Points

Kind of hard to see, but the little piles of wood are points where I had the water jet do a drill operation. The second head on our water jet has a pneumatic drill mounted on it that can be set up to drill holes in parts, or in this case, drill the pierce points for the water jet. A pierce is where the water jet initially punches through the material, and can result in delamination in woods, or just sloppy starts to cuts. Using a 1/8″ drill bit was an easy way to get clean pierce points, by both being big enough for the water jet to hit the target, and small enough to not ruin any of the areas of the parts that I wanted to keep.

Water jet cutting in progess

The water jet cut was going very nicely. The water jet uses super high pressure water (~50-60ksi) and abrasive (basically a sand made from garnet) to cut. As the company’s website says, its not actually cutting, but accelerated erosion. Basically, it is sanding it like crazy until it is through. Side effect of this is a super nice edge finish (well, this depends on feed rates, but I went slow enough to get a very smooth edge finish).

More water jet cutting

You might be thinking, “Isn’t water bad for wood? Won’t that make it warp and be generally annoying to work with?” While this is true, I did elevate the good birch plywood with a 4’x4′ of cheap particle board, which served two purposes. One, it elevated the good wood out of the water, so it wouldn’t just soak up all of the water. And two, it minimized blowout on the back side of the birch by supporting the drill bit when it plunged through to the other side. The birch still did get quite wet, but I paused and pulled it off as often as was convenient and then rinsed off the abrasive and did by best to dry it with a blow gun.

Almost done water jetting!

I could say the program ran without issue, but that would be incorrect. I ran into an issue with some of the consumable parts of the water jet that had worn out and needed to be replaced. Luckily, the night shift production water jet operator was around and helped me out. Thanks a lot Alex!

Done Water Jetting!

After all the cutting and fixing, it was done! All told, it took be roughly 4 hours of time after work one night. The above picture is how much I cut out from the 5’x5′ of plywood. I used a lot of it! There was only one minor/huge problem at this point in the night. I was locked out of the office, and I sort of left my bag and car keys in there. Not a smart move, and I had to call Ben to help me out. (seriously, thank you Ben).

Dry/Wet Fit

 

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After that small problem, I drove home and immediately tried putting it together. I couldn’t resist! It all fit perfectly, it even had a slight friction fit to the joints. I designed in tolerance in the slots, but the wood was damp and slightly swollen, so it fit together very tightly. I really liked the result. The effect was awesome and it fit in the space I designed it for great. The next step was going to be lots of sanding and staining.


SANDING, STAINING AND ASSEMBLY


 

Ugh. This part was very challenging to get through. It took me a full weekend to sand everything up to 220 grit and then stain (about 4 hours per process). Again this step is best illustrated with pictures.

Parts Laid out of sanding

There was a considerable amount of surface area to sand on this project. (a little less than 50 sqft based on the starting piece of plywood). I purchased a random orbital sander for this job (oh man if I did not) and got to sanding. I started with 120 grit and then went to 220 grit. Very uneventful work, but very necessary to get a nice stain. When I finished, every surface in my work shop room was covered in a nice layer of dust.

Staining Step

 

The next step was staining. Being a space themed thing, I wanted to use a dark stain. I chose “Dark Walnut” stain. I think  I went too dark with the stain, but it did still look quite good. As you can see in the picture, what you need for this step is one pile of sanded table, wood stain, and a brush. I brushed on the stain, let it sit for a few minutes, then wiped off the excess with a shop cloth. I chose to stain before assembly because of the difficulty involved in getting inside all the cubbies once it was assembled.

Close up of the staining

Assembly Step

The next step after letting the stain dry was assembly. Take one pile of stained table, wood glue, and clamps, lots of clamps. I also used a pneumatic brad nailer to connect some parts.Clamping for assembly

 

Assembly was a little complicated, but once I got one part of it set up, it was just a matter of slotting in the parts to their appropriate receiving slots. I either used glue or brad nails (in hidden locations) to assemble the whole thing. The top perimeter employed only glue, as I did not want to show any brads. The result of wanting to do that meant lots and lots of clamps. (about 15 in the above picture). I let it dry over night to ensure the bonds were good.

Top test fit

When it was finished drying, I put in the 6″ diameter mirrored sphere (http://amzn.com/B0019ID360) and put on the acrylic top surface for it. Another problem was encountered here. The Problem was two fold: I had already cut the top for the un-glued table, so it was incorrectly sized, and I neglected to square the top of the table when I glued it, so it was slightly a parallelogram. I was able to fix these problems by setting up my dremel as a plunge router and cutting it to the correct size.Top clear coat

 

After sorting out the table top problem, I applied a clear gloss polyurethane to the entire table to protect it. I only did one coat, because I wanted to have the table ready in time for a dinner I was hosting, but i do plan on revisiting that in the near future for more coats. Additionally, the acrylic top is not the final plan. Sooner or later, I plan on getting a nice big piece of tempered glass for the surface. The original plan was to fit it down in the perimeter of the top, but since it is now a parallelogram and not a rectangle, I will place the surface on top of everything with a slight overhang.

I am very happy with how it turned out in the end. It provides functional space and at the same time looks awesome.

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PROJECT COSTS


 

This project was not terribly expensive to execute, but my free access to the water jet does skew that slightly. I won’t include tool costs, I already had some and needed others. The cost break-down is as follows:

5’x5’x0.5″ Baltic Birch Plywood – $35

6″ Diameter Mirror Sphere – $19

2’x4’x0.25″ Clear Acrylic – $50

Stain and Clear Polyurethane – roughly $25

{(planned) 2’x3’x.375″ Tempered Glass – $130}

Total Cost of the Project: about $140-220.

 

Perhaps not cheaper than just buying a coffee table at a store, but definitely way cooler.