It costs $1.00/minute of burn time to use the laser so I wanted to simplify the design of my clock as much as possible (I have, in part, Scotch blood in my veins). So I omitted most of the text describing the stars, the lines linking the stars, and I converted to time to a 24 hour clock so I could omit burning 12 AMs and PMs. I also made my time window wider than those in the online PDFs I had found so that it my index didn't point right at an hour you would see part of the adjacent hour so you knew whether the time was after say 2100 or 2200. This I thought was important since the numbers run in reverse of a conventional clock. These are the sketches I created.
(N.B.: these are screen captures of the PDF I actually printed from and may not be the same size. If you have somehow found this page and want to reproduce my version of the clock you may have to scale one of the images up to get it to the same scale as the other. Alternatively you can wait until I get my files posted on Thingiverse.
To do this project I needed a graphics program like Adobe Illustrator. But since I have Scotch blood I wasn't about to pay $600 to do one project. So I downloaded Inkscape, which is a free, open source drawing program, and taught myself how to use it well enough to get through this project.
The hardest thing to do was getting my stars in the right place and making them the correct sizes. I ended up using a planetarium program set to midnight on 31December to get the star positions. I did a screen capture and imported it into Inkscape as a layer. Then I looked up the brightness of each star and decided to dummy that down by rounding the brightnesses to integers. That meant I only had to make four different sizes of stars and I knew from my GIS days that we can only perceive five differences in color or size so that made me safe. After that it was simple to create circles of similar sizes and drag them over the star locations in the screen capture.
Our instructor knew Illustrator better than Inkscape, and it was on the computer that is attached to the laser so we imported my PDF into Illustrator to do any last minute cleanup (little required) then exported that file into Corel Draw(?) which is what drives the laser, which is basically a printer. (The Makerbot is basically a printer. The milling machine is basically a negative printer. Everything it appears, is basically a printer.) Somewhere in those two imports something wanted to inflate my carefully scaled work to the maximum size of the "paper" in the printer. One of my design requirements was that the finished product be 3" in diameter or smaller so that it would easily fit in a pocket, and because that would minimize burn time and make the project cheaper. So we had to do some scaling to get it back to size.
Once everything was finished the job was sent to the laser. Anything that was a black shape was treated as a raster and was etched into the surface of the plastic. The thin lines were treated as vectors and were burned through the plastic. The laser printed the rasters first then went back and printed the vectors. I don't have a good sense of what my burn time was because it was included in my class fee and when I went back and made another copy by myself I forgot to delete the neat line around the project and burned a lot longer than was necessary. The little circle in the upper left corner of the diagrams was there so I could test holding them together with blind rivets (I can't get real rivets from Home Depot) and isn't really a part of the clock.
So here are my two disks.
When I tested the rivet to make sure it wouldn't put too much pressure on the disks and crack them or make them too hard to turn or not put enough pressure on them and make them slip too easily, the rivet broke before it grabbed the plastic and the tool wouldn't release the broken off shaft. So I have been cogitating about how to procede. I determined that I needed a #10 machine screw to fit the hole I had made and couldn't get that from Radio Shack or the model train store near me. I was down at Adorama talking to them about mounting my EZ Finder Deluxe on my Celestron C70 Mini Mak so stopped off at Home Depot on the way home to pick up some hardware for that project and to get hardware to finish this project. I ended up with #10-24 x 3/4" machine screws with nuts, #10 internal tooth lock washers and #10-24 cap nuts. I wanted pan head Phyllips screws because I figured they would be easier on pockets but all I could get was combination head screws.
To hold the disks with a constant pressure I wanted to use double nuts that will hopefully not come loose. I use the conventional nut that came with the screws for the bottom nut. The screws were bought longer than I thought I would need them since I didn't have my disks with me for sizing so I had to thread the screws through the disks and put two nuts on them then cut off the excess screw thread with a Dremel grinding blade. I put took off the extra nut, added a washer then a cap nut. I tightened the bottom nut to the pressure I wanted on the disks, held it with needle nosed pliers and tightened the cap nut tight against it with an adjustible wrench. The idea of the cap nut it that it too will protect pockets. All this hardware is putting more bulk on the clock than I wanted but since the blind rivets are a no go I will have to deal with it until I come up with a better solution.
I had built in a hole at the top of the clock with the idea that it could be attached to something. I got some jump rings and sealed one through the hole than threaded some thin climbing line that I had here through it for a neck cord.
To use the clock you hold it with the name of the current month at the top of the disc. Then you rotate the smaller disk until the star pattern of either Cassiopeia and the Little Dipper or the Little Dipper and the Big Dipper agrees with their respective positions in the sky. The index mark indicates the current Standard Time. I had wanted to figure out a way of having the time wheel rotate by one hour to offset it for Daylight Saving Time but time constraints forced my to add the "+1 hr for DST" line under the time window. Maybe that will get done on Version 2, if there is one. The picture below shows what the clock would have looked like tonight at 2115 EDT if it hadn't been cloudy tonight.
I tested it out on the night after I cut it and before it was totally assembled and one time it was off by about 3 hours and later that night it was dead on.
I haven't used it much but am noticing some scratching of the bottom disk at the top of the time window and suspect that there is a little roughness in the cut edge of the window that I need to file off.
The cost. Depending on how you calculate it it was either very expensive or about what you would pay for something of this size in a store.
| Item | Cost |
|---|---|
| Laser class | 77.82 |
| Acrylic | 8.87 |
| Rivet tool & Rivets | 19.51 |
| Laser burn time | 9.00 |
| Jump rings | 3.27 |
| Screws, washers, nuts | 3.90 |
| Total | $122.17 |
My next astronomy/laser project is to build the adapter for the Mini Mak. It has been designed and just needs a reality check by the instructor and additional instructions on controlling the scaling problem.
