## Introduction: Perpetual-horizon, Laser-cut Erasable Calendar

Create a perpetual-horizon planner from laser-cut plywood and acrylic using just wood glue and duct tape.

This is a rolling erasable planner which shows the right day and date for any 4 consecutive weeks in any millennium. The rotating and sliding mechanism means it can be reconfigured at the end of each week with the minimum of effort. You can see a 3d demo of a calendar working through the days of the year by clicking here.

From constructing prototype perpetual calendars for friends and family christmas presents I've learned some things to do and things to avoid with when fabricating this design in particular. For this instructable I wrote up some instructions and published our laser-cutter files under a creative commons license. To test-run, Clare had a go at following the steps while I took the pics you see here.

There is plenty of scope for looking at it as a reference design and cobbling the same thing from found parts or experimenting with your own fabrication methods to replicate the loops of numbered tiles.

If you like the project, please rate it or vote for it in the Gorilla Glue contest. Skip to the last step for a rough and ready demo video.

## Step 1: The Problem

The visible horizon of conventional monthly calendars shrinks as time passes. On the last day of the month, even tomorrow isn't visible! Waxed paper calendars aren't reusable, unless you're very patient, and they are destined to end up in landfill.

They are also covered in kittens, so you have to hide them round the side of your fridge so visitors don't feel ill from the kitsch intensity.

A SOLUTION

A perpetual horizon calendar always allows you to see a certain distance ahead into the future. New weeks slide into view as old weeks are retired. Since it lasts forever, you can make it from more expensive materials. You can invest in personalising the design to your taste and put it on show.

Replacing the tired old format of the paper calendar is something of an engineering and mathematical challenge. To achieve both correctly numbered day tiles and correctly labelled day columns a mechanism is needed with both a horizontal and a vertical rotation of carefully chosen tile sequences.

Rotating vertically allows you to slide old weeks out of view above, and new ones into view below. Rotating horizontally allows the day headers to line up with the numbers. It also permits the 1st of the following month to follow on from the 28th, 29th, 30th or 31st of the previous month, depending on what's needed at the time. In combination, these two axes mean the planner can represent any possible future date range in the Julian calendar.

[Credit: flickr's swirlingthoughts and artolog for CC-licensed calendar and kitties.]

## Step 2: About the Design

The magic of this design is that each week the owner can just pull out the oldest week and always fit it below as a new week. If they find the numbers won't fit, then fold it as a blank week and carry on.

Took a lot of trial and error to come up with just the right sequences. Each month has, on average, 30.5 days, so no whole number of weeks (multiple of 7) can achieve what you want. The trick is that the crossover week can either be blank, or not. This permits an average rate of 30.5 to be achieved.

THE IDEA

The idea arose from my partner Clare's deep dissatisfaction with regular calendars for the reasons above. We set ourselves the task of finding a satisfying solution as we zoomed about in our camper van in the UK winter. Finally cracked it over christmas at my mum's. You can see some of the thinking process in the paper mock-ups and the post-it layout for the laser-cutter plan. Even the dog helped out!

ORIGINS OF THIS DESIGN

The design is based on a public domain invention I published a few months ago as enigmaker #project3. It's been christened the 'Tacticalendar', because it's about making something tactile and physical whilst everyone seems to be rushing to digital calendars, though synchronization with Google Calendar is in the works.

## Step 3: Completing This Instructable Requires...

- A sharp craft knife and cutting board
- Some wood glue
- A roll of thin duct tape.
- 10 foldback clips (41mm or 50mm) or a similar number of workshop clamps.
- One of each of the SVG files, laser cut from the appropriate material

Given it's a Gorilla Glue competition, you could use this if you want. It can be less forgiving on light-colored wood than glue which dries transparently, but can be sanded back. I tried Gorilla tape but it is just too hardcore for the hinges - too thick and not bendy enough.

I was using a roll of the thinner Gaffer Performance tape which I had lying around - very sticky but also uber-flexible.

Foldback clips are cheap and work well as an alternative to clamps, although you need to be careful that they are gripping the walls straight when you clamp them.

SVG FILES

These instructions were tested using the latest Tacticalendar SVG files at the time of writing, v0.2 release candidate 2, although the general principles should remain the same between versions.

Fabrication of a single calendar from the laser-cut parts takes about a half hour of gluing and taping, once they've arrived. However, getting the parts cut can take from a month (Ponoko) to a day depending on your supplier. If you have your own cutter you skipped this sentence ages ago, so I don't know why I'm still typing...

The current design has three separate files...

* Days+face.svg should be cut from a 300x600x3mm laser ply. It provides the front panel including the day headers, the side walls and all the numbered day tiles.
* Back.svg is cut from a 300x300x3mm laser ply. This is the backing plate with guide-lines for the walls, recesses for gripping the week-panels, string holes and a triangular wall-hanging bracket.
* Acrylic.svg should be cut from 300x300x2mm clear acrylic. It creates 7 erasable week panels, giving you a spare in case of writing on one with an indelible marker by mistake.

I sent off to have the parts laser-cut from a local supplier using SVG design files which I've shared under a creative commons license here

It costs me about £50 to have these cut as one-offs, but they're cheaper if you make a few in one go.

I'm told by the laser cutting people that the SVG files were fairly easy to import into Rhino3D through Illustrator, and matches the 'control colors' for their laser cutter but may not match yours.

You might want different day letters (French/Swedish instead of English), different fonts or different line colours (for example, the Ponoko service uses different colours and thicknesses for 'cut', 'raster burn' and so on).

Simple edits should be easy in Inkscape but I don't mind helping out, so if you're stuck just message me and tell me what mods you need.

## Step 4: Preparing Parts and Glueing Walls

FINISHING THE PLYWOOD

When the laser cut wood arrives from the fabricator, you should turn it over and look at the back - that's the blank side with cuts which aren't so blackened from the laser.

By inspecting the back you may find that some of the lines haven't cut right through. The laser ply is an irregular organic material and some parts of it have a different consistency of glue and wood fibres which turn out to be harder to cut through than other parts.

Carefully trace any uncut lines with shallow strokes of your craft knife applying increasing pressure to complete the cut. This should separate all the parts from each other cleanly. Small lumps of the woodgrain sometimes break off from one part and remain attached to another, but these can be whittled off with the knife.

If the plywood has a rough face, avoid moving it around on carpet, as the weave will pull on any exposed splinters and make them worse. It can help to use a fine sandpaper to smooth the finish, being careful of the text etched into the veneer of the face.

WALL PIECES

Identify the wall pieces. They should fit perfectly into the guide marks etched into the back plate. There is a top, left and bottom wall, as well as a three-section right-hand wall, providing an exit hole for old weeks at the top and an entry hole for new weeks at the bottom.

Decide whether you prefer the clean wood or the blackened finish on the outside of the walls and apply this policy consistently.

Before glueing the walls in place, find the smallest wall section, which fits the bottom right hand corner of the back-plate. Use the craft knife to chamfer/bevel a rounded shape on the inner facing corner so new week panels seat easily after sliding into the entry hole.

Leave the middle section of the right hand wall until last (special instructions below).

Take all the other walls one at a time and run a thin bead of glue along the bottom of the wall, smoothing the excess using a bit of paper into a continuous thin layer of glue. Don't forget to add glue to any end-faces which need to be attached to adjoining walls. Align the edge of the wall with the outside line of the guide-lines, leaning it outwards, then lever it inwards until its edge is flat on the backboard. This ensures that it is square and not bent along its length. It also ensures that excess glue (mostly) gets pushed out of the inside face of the wall where it will be hidden. If there's really too much glue, use the corner of a bit of paper or the tip of your craft knife to scoop up the excess. The wood glue I use dries clear anyway, but a lot of glue on the inside corner can obstruct the sliding movement of the week panels.

Make sure each wall is exactly square vertically, and aligned with the guides. Arrange a book or something which allows the backboard to be placed on top with its edges freely accessible all around. Then use a series of foldback clips to clamp the walls in the right position. If you allow the foldback clips to actually support the weight of the backboard, they bend inwards a bit, the walls get twisted and will dry crooked. If the backboard is on a book, the foldback clips are in free space and can apply a perfectly vertical pressure evenly along the wall length - really aids with squareness and adhesion.

Important: The middle section of the right wall needs to be glued with a small outward offset, else it holds the week panels too tightly for them to slide easily. Basically for this section alone, keep it aligned with the guide lines vertically, but move it horizontally to the right by 1 to 1.5mm to provide extra play.

## Step 5: Making Up the Loops

Whilst the walls are glueing, you can proceed to build the tile sequences.

GETTING THE RIGHT SEQUENCE

The tile simulation using the Javascript loops simulator is useful to see the proper sequences of the tile loops. Tip: click on the page and use left and right keys to simulate the precession of the calendar.

You need to make up 6 sets of 14 tiles each, which correspond with the horizontal lines shown on the tile simulator.

The first line, for example, contains nine blank tiles and the number sequence 1,2,3,4,5. The second line needs two blank tiles followed by the number sequence 1,2,3,4,5,6,7,8,9,10,11,12.

The problem is that you have to place them face down. I find it's useful to make these up on a glass table, so I can check my working as I go by looking from underneath (I taped a load of backwards sequences by mistake one time).

There are two of each number, but don't jumble them all up if you can avoid it. It's nice to keep neighbouring numbers from the laser cut as neighbours in the week loops wherever possible, because then the wood grain matches up nicely, a design detail which is really visible.

ALIGNING AND TAPING

When you have a sequence that you're sure of, align the tiles with a straight edge like a long ruler or piece of wood and push them together to make sure there are no gaps.

All the duct tape I've encountered is too wide for the tiles, so I use the craft knife with the tape hanging downwards from a door-jamb to cut it into two half-width pieces.

You could experiment with book-binding tape as an alternative, which is available in narrower gauges. Run a section of the half-width duct tape along the back of the tiles, from the midpoint of the first tile, all the way over the fourteenth tile, leaving half-a-tile worth of extra duct tape hanging over the end.

Apply sliding pressure along the back of the duct tape to initially stick the tiles. Fold 6 of the tiles back over on top, leaving 8 below, and a half-tile of tape exposed upwards. Fold the 8th tile in the bottom row back over and attach it to the exposed tape, checking close alignment to the 6th tile on the top, to complete the loop.

Align the tiles cleanly on a hard flat surface, then press the entire loop with your weight to stick it well. Try refolding each loop into one or two different configurations, align the tiles and press again to ensure it's stuck. Repeat these steps for all 6 loops.

## Step 6: Glueing the Face On

With the front plate to one side, apply a bead line of glue to the exposed top of all the walls and smooth it out as before.

The main thing to look out for with the face plate is that it's aligned properly with the base plate and especially the lower wall.

ALIGNMENT

Align the face plate so that the bottom wall is flush with the inner edge of the face plate - no lip is needed at the front as the week panels are retained in the groove both left and right. If you have a square edge then use it to check alignment between top and bottom. It has exactly the same outline as the base plate, so you can judge this more or less by eye from above. Clamp the calendar as before. If you're using foldback clips or unprotected clamps to pressurise the glue joint then you may want to protect the face plate with some scrap material so it doesn't mark.

Now wait a bit for your glue to dry and...

## Step 7: Using Your Calendar

You've finished! Now to USE IT!

If you can't see the embedded video above go to http://www.youtube.com/watch?v=JWcAshJ585M for a snapshot of how it works.

Here's a description of how you use the calendar for readers with rich, visual imaginations..

INSTRUCTIONS FOR USE

Write appointments on the acrylic with a non-permanent fine marker.

When you want to rotate the weeks...

* Place the calendar face up on your lap.
* Grip the topmost panel through the diagonal slot, lift it and slide it out.
* Refold it so the numbers follow on from the bottommost week
* If they cannot follow on, then fold it to show a blank week.
* Grip the bottommost week panel through the two vertical slots, and slide up to make room.
* Slide in the new week panel until it is seated fully flat.
* Lift the calendar vertically to reseat the week panels.

The acrylic panels have probably come from the cutter with a protective matt coating so peel this off to reveal the gloss. You can tie string through the holes in the back plate or if you have a picture hook or nail, use the triangle shaped hole to hang it.

CHOICE OF PENS

I use Lumocolor Fine-tip non-permanent pens as high contrast wet-wipeable markers to mark my appointments. The dark colors work, the color contrast is quite visible and the lines are thin so you can write a lot in one panel. Orange and yellow are a waste of time. I found chinagraph pencils were nowhere near as good when I experimented with them, although I bought pretty cheap ones so YMMV.

CHEATING

For all the armchair makers out there, if you want one but without the fun of making it yourself, I can mail these out fully fabricated.

DESIGN IMPROVEMENTS AND FUTURE VERSIONS

If you have any problems with our design or any suggestions, there is an issue tracker for you to share criticisms and improvements. I'd love to hear your experiences anyway.

Future developments already on the tracker include a place to hang pens and erasers, and two-way synchronization with Google Calendar, although this will have to wait for version 0.4.

If you liked this instructable, then give it a rating, or vote for it in the Gorilla Glue 'Make it stick' competition using the controls in the corner of the page.

Happy planning!