The compasses are inlaid into a custom table top underneath a piece of glass. The coasters were made to match - they also provide an inconspicuous home for the powerful rare earth magnets that affect the compasses.
Now I always know which way is north, and exactly where my cup of coffee is...er, so long as it's always on my compass table.
**Note, the reliability of compasses in mass quantities and close proximity decreases somewhat since compasses themselves are magnets. It still works, and actually results in some pretty cool patterns, but if you're looking for the table to be 100% accurate, I'd recommend not designing a table that places them so closely together. I'm a big fan of the way the table came out, as it's a toy just as much as it about the science of magnetism.**
Step 1: Materials
- 12mm sanded plywood
- table legs - I used 4 Ikea Vika Oleby prefab legs
- 16 7/16" x 1/8" thick glass circular top (I had this custom cut)
- a couple square feet of 6mm sanded plywood
- 2 cork coaster inlay
- 2 rare earth magnet
- approximately 500 "mini compasses". I got mine for around 20 cents a piece off Ebay. It takes 500 20mm compasses to cover a 16.5" circle - crazy I know!
- circle jig
- wood glue
- polycrylic - water based clear satin wood finish
- speed square
- random orbital sander
Step 2: Modify the Legs
Using a speed square I marked and cut two 45 degree lines on the end of the table leg where it mounts to the the table top so that I could fit 4 legs on my 18" table top.
I removed about 1.5" of material off the corners...no need to be precise here so long as you don't affect the mounting bracket.
Step 3: Route Out the Tabletop
Two 18" circles were routed out using a Jasper Circle Jig (a tool that I'm quite fond of). A third circle of the same size was cut, and then had a slightly smaller circle inscribed inside of it to create the rim for the table. The rim turned out to be 1/2".
Step 4: Glue and Clamp
Step 5: Sand and Finish
Then, I brushed two coats of polycrylic, a Minwax brand water based clear wood finish, that's a whole lot easier to clean up and is more forgiving than Polyurethane onto the table top.
I sanded lightly with 220 grit paper in between the two coats for a nice smooth finish.
Step 6: Attach Legs
Using the supplied mounting brackets as a guide, I marked and drilled the mounting holes for the legs into the top.
The supplied Ikea allen wrench tightens the bolts into place.
Step 7: Arrange Compasses
Arrange the compasses as tightly as possible working from the outside in, getting the final few in place takes a bit of a massaging, but once they all go down, it's a nice symmetrical tight fit that can adjust to whatever size circular table that you have.
Step 8: Coasters - Route Out
One base circle was cut from the thinner 6mm plywood and then one ring was created to become the coaster lip. The ring is a bit more tricky to route then it's big brother was with the circle jig because it doesn't have enough friction to hold it self in place as the router passes by. A little tape does the trick so that you can complete the cut.
The rare earth magnets that I bought were 3/4" by 1/8" thick, so I routed out a small inset for them in the coaster base using just my eye, since that hole was going to get covered up anyway in the next step with the cork coaster pad.
When you lay the magnet down in its pre-cut hole, make sure that you've got the pole facing the right way, we want to attract the north end of the compass, so flip it around until you've got it the right way.
Step 9: Coasters - Glue and Clamp
Clamp them between some of those pieces of maple from before and allow them to dry.
Step 10: Coasters - Sand and Finish
Finding things to dry them on is easy since they've got a magnet embedded in them...
Step 11: Glass Top
I got the glass cut at East Bay Glass at exactly 16 7/16" so that would perfectly fit inside the rim of the table. It sits directly on top of the compasses nicely and has virtually zero play. As a result, I'm not worried about scratching the tops of the compasses, but if whatever you were inlaying was more vulnerable for some reason, putting in some kind of supports for the glass could be a good idea.