So I'm in the middle of redoing/changing/painting/[pick your synonym] our interior doors, and at the same time figured our feline buddy could use an upgrade too. The door I worked on already had the spot cut out for his door (I had done that ages ago and just installed a vertical flap in it), but I'll be covering the preparation of that opening as well by using a scrap door.
The concept I was going for here was basically a miniature door that looked and felt just like the big one, down to the direction of the swing, the door handle, etc. The only difference would be that since Darwin did not allow for articulated hands on cats, the handle would just be for show and the door would swing both ways like restaurant kitchen doors.
To begin with, you need a door. And since you'll be cutting into it and removing a section (the mini door is 10" wide by 16" tall), you're better off starting with a flat door that has no designs on it (such as embossing) because the cutout will probably ruin the look and be challenging to seal around the new edges.
You'll also need material to make the small door (you could possibly use the cutout, but for reasons I touch on later on this might be too flimsy), and the thickness has to be the same as the big one. In my case I did not have anything that thick, so I resorted to gluing up two sheets of 0.25" plywood with oak strips (basically pallet material left from another project) glued in between.
I'm sure you can get a ready-made hinge for the door that lets it swing both ways and return to a closed position. I checked my local hardware store, though, and could not find anything. Checking online inventories of a couple more didn't really show anything either, so at that point I figured it was probably hard to find (and therefore expensive), so just made my own. All you need is a metal "L" bracket (we'll cut one side off), a nail, a screw, a washer and a couple of springs.
Finally, you'll also need a door handle. Pretty straight forward.
Tool-wise, I used a table saw, circular saw, hand saw, drill, and the usual basic hand tools.
Step 1: Cutting the Door and Making an Opening
I had already made my mini door opening a long time ago (and just had a vertical flap there until now), but am redoing that part on a scrap door for the purpose of this tutorial.
I made my opening 16" high by 10" wide (not sure where I got those numbers from; I think it was just eyeballed) and just cut it out with a circular saw. The cutout was then ripped open so I could extract the beam of wood at its bottom (which would later be used to seal up the top of the new opening).
The three new inner edges now had to be cleaned up and prepared for a piece of wood to be inserted, so the cardboard had to be trimmed back a little and the insides cleaned up and sanded. Once done, you can reuse the wood you removed from the cutout and glue it back in place at the top of the opening, and using scrap pieces of pine make similar pieces for the sides. Glue all three sides up, let it dry, and you're done.
Step 2: Making the Mini Door
So instead, I grabbed some spare 0.5" plywood and oak I had previously milled from a pallet, and made a new door. The concept is simple: you need to build one that fits the opening and has the same width as the big door. I positioned the two pieces of plywood on top of each other and sandwiched some of the oak in between to make up the missing width. The oak had to milled down a bit further to get it to the right width. If you manage to get material whose thickness matches that of the main door then you can pretty much skip this step.
To reduce the overall weight, I did not fill the entire interior with oak; rather, I just used it on the outsides of the frame. In hindsight, though, I would have filled the interior as well since I later had to trim the door size a bit and this left me with less material to drill into for the springs. I don't think the extra wood would have made much of a difference on the weight.
Start off by measuring the size of the door needed, and then cut/glue everything over-sized. This gives you a good margin of error and lets you make the final cuts to the right size when the whole thing is fully assembled and glued up.
Once the glue has dried up, pass it through the table saw to make the final cuts and bring it down to the proper size.
Step 3: Shaping and Hinge Installation
The hinge is pretty simple: the door pivots around point that is about 0.5" in from the edge, so I traced that spot on both the top and bottom of the door. At the top end I drilled a pilot hold, tapped a 3" nail about 1" down, and then clipped the top off leaving about 1" of the nail exposed. I put the mini door into place and marked the spot where the nail hits the top edge of the main door, drilled a hole there too (to accept this "nail hinge"), and that part was now done.
For the bottom hinge I used an "L" bracket with one side hacked off. The idea is to mount this metal support bar under the main door and have the mini door swing on it. The bar was about 3" long, and I did need to add a third hole to it so that I would have two screws holding it underneath the main door. Once the bar was mounted I just followed the third hole and added a pilot hole in the mini door. Final assembly was just to insert a washer between the support bar and the mini door and add a screw. Make sure you don't tighten it at all; I left about a hair's width, so just enough to let the door swing freely but also keep it as close as possible to the bar.
Once both hinges are complete, I started rounding the edges of the mini door so that it could swing without touching the main door. I started off by cutting off the corners at a 45 degree angle with a circular saw, and then going back and forth with a rasp and removing more and more material until the door could swing almost 90 degrees each way. Once that was complete, the entire thing just went through a good thorough sanding.
The final part to this was installing the springs. I had a hard time finding springs that were stiff enough, and will one day come back to this and look for better ones. The ones I ended up using were about 1" long and I actually trimmed them to half that length in order to increase the tension. The mounting is straight forward: I drilled two 0.75" wide/deep holes into the spine of the mini door, and added matching ones opposite them on the frame of the main door (although these were maybe half as deep). The springs were then just attached with a 0.5" screw at each end, and I did end up putting them on, removing them, trimming them, putting them back on (etc) about three or four times until I got a tension that was good enough to swing the door back into place when it is opened.
Finally all the was left was to give everything three solid coats of paint, mount the door in its frame, and voila! Our cat now had an official door to go through :)