For both the doors and windows on my projects I first have to make the lumber that creates the frame pieces and then put grooves into those pieces to receive the glass or wood panels. This time around I am using poplar wood for the window and doors. Poplar is nice and hard and holds crisp details and best of all it is available at most any lumber yard. Sometimes the poplar pieces look green in color but that green will eventually go away and they will be the normal browns you see in most wood.
To cut the grooves into the edge of the lumber I use an 9/32" diameter, 1/8" shanked router bit. This bit cuts only on the sides, no cutting edges on the top of the bit. It cuts much like a blade on a table saw does. The advantage of using a side cutting bit in a router table is you are not trying to balance an upright thin piece of wood over the top of a table saw. You will have more control with the router table. The bit I use was sold by Dremel as a #198, Dremel has now discontinued making this 9/32" bit but it is still available online from Widget Supply as Item: D-AL21.
Link Click: Groove Cutting Bit
You will need to make several passes to cut the grooves to full depth. The reason I like the #198 bit is the depth is always the same, you go as deep as the 1/8" diameter shaft allows. When you hit the shaft you can cut no deeper. Another huge bonus of this bit is you can make arched windows by following along the edge of the curved window frame using the shank of the router bit as a pattern follower. The depth of the groove is just enough to hold a piece of glass securely and hide any minor chips on the edge of the glass. The width of the groove can be adjusted by doing a flip cut.
To determine the height from the router table to the top of the router bit first figure out the width of the groove. Now subtract that dimension from the thickness of your framing member. Divide that number in half and subtract it from the thickness of your framing member. Now set the height of the top of the bit to that calculation. I use a digital caliper to check the height of the bit. If you are using standard window glass or art glass you will need to flip the framing piece over and cut again to get the full width of the groove as the bit is not thick enough. With this method the groove will be perfectly centered in your framing member.
My lumber thickness is determined by the thickness of the glass I am using as is the width of the groove. I am not using micro thin glass this is window glass meant for real houses. Using this glass does push my pieces just above true scale dimensions as to the thickness of the door but it is not something viewers pickup on. The illusion is still there and that is what really matters. When I first began to make structures I was all worried about exact precision of dimensions for doors and windows. What else would you expect from a woman who for many years created aircraft parts where the tolerance of parts was measured in thousandths of an inch? But I learned to loosen up my thinking and my miniature work and instead of worrying about exact dimension I create that which is believably realistic and that which works visually with the project I am making and the materials I am using.
Creating that which is believable requires that you educate yourself by putting thousands of images of the real thing into your brain. Seattle is a long ways from the old buildings in Europe so I still start and end most days by looking at images of old buildings as well as reading about the history of the design and building of them. My bookshelves are located right next to my bed and there is always a stack of books by the table where I take meals and I take books along in my car as well for times when I sit waiting for a bridge to open or want something to read at dinner or lunch while out running errands. Many days of the week I browse the internet collecting images for the reference files I keep on my computer. Twenty years of this daily habit of ingesting images has been a real education in architecture and structural details. My mind now knows what feels just right and I can design original buildings in the style of old buildings from many areas of the world and from many eras.