We needed a small cat door so our cat could go in and out of our basement while keeping the (human) door closed so our kids wouldn't fall down the stairs (they are clumsier than the cat).

I looked into pet doors that you install into existing doors, but they seemed a bit over kill for an interior door and none of them would have looked very nice in our woods doors.

We have 6 panel solid wood doors and the thought occurred to me, why not just cut out one of the lower panels and use it as a swinging pet door?

The current version is kind of a beta project just to check if it was possible and that the cat would actually use it. It is possible and the cat does use it. I'll update this instructable or create a new one with any changes. This is my first instructable so I'm not really sure what is appropriate.

Step 1: Assess Your Door Situation

First, you should think about if this is the right solution for you.

The door we are working on is an interior door. This is important because this door modification does leave a small gap between the swinging pet door section and the main door that could allow air flow through the door.
Your door material and construction - The door we are using is solid wood so cutting out a section will give us a solid swinging "flap". It is theoretically possible to do this with a hollow core door, but I would (wood?) expect some extra steps are required.
"Flap" location - We cut out a panel because it just made sense for our door layout. The panel is thinner than the overall door so there is less swinging weight and we didn't have to worry about rounding edges to compensate for a thick section swinging. This was just lucky for us. If you do not have a panel or you do have panels but they are too high for your pet, it may still be doable but it may require a little extra thought.
Appearance - I'm going to be honest here... this is a basement door, I didn't give much care to how the inside of the pet door looked. It looks good from the outside of the basement, but the hinge is kind of experimental on the inside. This is something I hope to work on but for now if you need a work of art on both sides, then this instructable is just a start for you. Please please please share if you make any improvements on this.
Before making any modifications this door was structurally solid and in working order. If you are taking out a section of a door that has any sort of give or wobble to it, you are probably going to make it worse. I'd suggest that if you really want to do this to an iffy door then you should really reinforce it and make sure it operates perfectly as an unmodified door first.

Step 2: Tools and Materials Needed

The tools and materials needed may vary a bit so I'd suggest using this as a guideline and make any tweaks you'd like after reading through once.

Something to remove the door from the door frame - This can either be an awl and hammer for removing the hinge pins or a screw driver for removing the hinges depending on your door situation.
A saw for cutting out the panel - So I employed a really fancy saw for cutting out the panel, but it is not absolutely necessary. I used a Festool track saw (its not mine, but if Festool happens to read instructables, which they should, feel free to send any spare ones my way). I'll discuss alternatives in the appropriate step.
Hand or pull saw with a thinner blade than the previous saw
Wood working chisel and sandpaper
Drill with various drill bits
Wooden dowel - at least a couple inches wider than the pet door opening
Copper tube strap - should fit the dowel, but not too wide or the door will flop around
Wood screws - types determined by your door situation
Small wood sheets - these are needed if the panel you are cutting out are recessed from the rest of the door
Nuts, bolts, and large washers

Step 3: Remove the Door From Frame

I chose to tap the hinge pins out of the hinges to remove the door, which worked perfectly... on 2 of the 3 hinges. The bottom hinge was a little corroded and the pin would not budge. If this happens to you do not fret, I removed the screws from the hinge and proceeded. Once the door was off I was able to remove the pin and clean up the offending hinge pin with some oil and a scotch brite pad.

Once you have the door removed, lay it down on a nice safe work surface.

Step 4: Almost Cut Out the Pet Door Opening

Let me explain why I say "almost" cut out the opening. If you cut the panel out completely you will then have to line it back up in the opening and hold it perfectly in place while putting the hinge in place. And if you put the hinge in place without making any cuts first you will have to work around the hinge while cutting. So you want to make as many cuts as possible without removing the panel. My suggestion is to cut along the top, bottom, left, and right, but do not cut all the way into the corner. If you are using a circular saw like I did you won't want to use that saw in the deep corners anyways.

For the door I am cutting into our goal was to get as close to the edge of the panel opening as possible and keep it as straight along that edge without cutting outside the panel. The Festool track saw made this really really easy, we just used a square to set the cutting edge of the track and made sure the blade depth was enough for the recess. However, you can do this with a regular circular saw but I highly recommend you set up a guide for the saw using at least a straight edge and some clamps or else your opening may not look very nice.

Step 5: Attach Your Hinge

Before you complete cutting the corners freeing the panel, you should attach the hinging mechanism.

First lay the dowel across the opening and cut it to length if you have not done so yet. I chose to put the hinge about three quarters of the way up the panel. I felt that this would be an ample opening for the cat, swing well, and look nice but I didn't really put much thought into it past that.

I attached the dowel to the door with screws after pre-drilling to prevent splitting. Make sure the dowel is level. I measured it relative to the top of the panel opening.

The copper straps then get put over the dowel and attached to the panel. As you can see in my case I had to put a wood spacer in because the panel is recessed from the main door. In either case make sure you use short enough screws that do not go through the panel but still secure it. If your panel is recessed like mine hold a straight edge across the opening and measure the gap between the panel and edge, then double that and subtract it from the door depth to approximate the panel depth.

When attaching the straps make sure that if there is any slack to rest the dowel on the topside of the strap. This is so that when the panel is freed it won't be pulled down by gravity and hit the bottom of the opening.

Step 6: Free the Panel!

You can now finish the cuts in the corners and let the panel swing! Use a pull or hand saw, they should be much skinnier that the opening the circular saw blade made.

After the panel is free you can clean up the corners and edges with a chisel, sandpaper, and/or block plane. Particularly in the corners I felt like it stuck a bit while swinging if you don't clean it up.

At this point, you may notice that the panel can move side to side along the dowel. Don't worry we will take care of that.

Step 7: Put the Door Back Up

Now it is time to put the door back into the frame. Be careful when moving the door as the newly freed panel may swing around a bit and you do not want to damage it or yourself. I didn't feel that it was enough of a problem to secure it but results may vary.

Once you have your door back in place you will want to shim the panel with either shims or folded up paper so that it is centered in the opening. Once you have it in place, you should place screws in the dowel to the outside of the copper straps to prevent the panel from moving left and right but not so tight that it won't swing.

Take the shims out and test how it is swinging. If there is a little too much slack vertically you can try adjusting the copper straps by giving them a slight squeeze with some pliers.

Step 8: Balance the Panel

Your door should now be in full working order! Except for one problem...

It doesn't hang in a closed position. However, I left it this way for a little while so the cat knew that it was an opening it could use. So think of it as a bonus learning step.

While planning this door I tried to explain why this unevenness would happen but no one seemed to believe me that it would be a problem.

You can think of the dowel as a playground see-saw, but with the only a fulcrum. You want the see-saw to be perfectly flat when no one is touching it. To do so you must put equal weight on each side of the fulcrum (or dowel). You may be thinking, "Well then why didn't you put the dowel halfway up the panel instead of three quarters of the way up?," and I would respond because the panel is one of the weights (or people in the see-saw analogy), not the long flat part of the see-saw. Putting the dowel half way up would guarantee that the door was open at all times because the panel would swing down and level out and that is the complete opposite of what we want in this little experiment.

Another argument is to put the dowel as far up on the panel as possible. This would get us close to even but still a bit off. The position I chose makes it a little easier to mount the dowel to the panel because it is flat rather than tapering off near the edges and I think it looks better. You may choose to do it differently, if you do please share your results.

If we were to put the dowel straight through the middle of the door and panel, at a height anywhere higher than half way up the panel, then we would be perfect. I would have loved to do that but it seemed a little too difficult to do right. So my solution is to put a weight on the other side of the seesaw that equals that of the panel. To do so we are going to use some washers, but we have to make sure it is on the opposite side of the dowel as the panel.

Step 9: Adding a Counter Weight

For the counter weight we used long bolts with nuts and washers.

Drill holes slightly smaller than your bolt diameter. I used two bolts but it is very possible to use one and just use heavier washers.
The distance the weight is away from the door will change the panel's resting location, so I opted for longer bolts and sandwiched the washers between nuts so I could fine tune it. This is also handy if you later find that your pet needs to be eased into it being completely closed.
With the washers on the bolt, then screw the bolts into the holes. If you did not need spacer blocks like I did and do not want to put more holes in you panel, you could 3M tape a block on to secure your weights to. Just remember that anything you put that is against the panel and not on the opposite side of the dowel will need more counter weight.
Now adjust the location of the washers so the panel hangs as desired. Moving washers closer to the dowel will make the bottom of the panel hang towards the hinge side of the panel. Moving the washers away from the dowel will make the bottom of the panel hang away from hinge side of the panel. It may take some trial and error to get it just right.

If you use all of your washers and they are right against the bolt head as pictured and the panel still doesn't hang at or on the other side of the door, then you need more or heavier washers.

One question my wife asked as I was doing this was, "Will adding more weight make it more difficult for the cat to move the door?" The answer is no, it will actually make it easier. If you think of it like a see-saw again, when un-balanced it is like having a person on one side and no one on the other side. If you want to move the see-saw it will take some effort (a lot of effort if that person is quite heavy). However once we balance the door it is like a see-saw with a person of equal weight on each side. If they keep the see-saw even and still it should be considerably easier for you to go over to one side and push or lift it.

You could get a bit more scientific with this step and weigh your panel to figure out how many washers you really need, but do whatever you think is most fun.

Step 10: That Is It!

I hope you enjoyed my first instructable. I feel like I've said it a million times now but this was kind of a trial project so if you have any improvements or tweaks, please share it where everyone can benefit from it. I hope to make some improvements myself.

For starters I'm going to try and get a video of the cat using the door, she loves using it but she is very sneaky about it. If I do get one and I can edit the instructable, I'll post it right here.