I recently received the following question from a reader, and as it is asked often it makes sense to post the answer here. At first glance it is a technical question, but my answer veers off into philosophy, and bounces back as a question.
“In one of the video series, you show how to build a small cabinet with a drawer. When you built the drawer, you used the cabinet drawer opening (from the rail-stiles). As a result, the drawer dimensions are exactly the same as the opening. In real life, the drawer would need to be smaller in width and height to properly fit into the cabinet opening.
One option is to adjust the dimension during the construction, but then there is the issue of using the cabinet as the frame of reference. The other alternative is to adjust the dimensions after completed, but that can also interfere with drawer component. The last alternative is to leave it as you have drawn it and then adjust/fudge the sizes when constructing the furniture. That also has issues if you are constructing from a cut-list.
I have a couple of reasons for modeling the way I do, but I don’t always do things the same way. There are two important questions to be answered; what is the purpose of the model? and how much time to I want to spend modeling vs. building in the shop?
These days, most of what I build is solid wood, with inset doors and gaps as small as practical. When I model, I construct the box first, then model the doors and drawers from the openings in the box. It’s dirt simple to use the rectangle tool and snap from corner to corner, or corner to mid-point in the opening. That sets the size and I move that rectangle off into empty space to make the door or drawer. When that part is finished, I make it a component and move it back.
As Rick points out, this is expedient for modeling but complicates real life. For doors and drawers to open and close there needs to be a slight gap. That itself can be a subject for debate, but I belong to the “dime in summer, nickel in winter” school of thought. When I work in solid wood, I really do cut the drawer fronts and build the doors to the exact size of the opening. Then I trim them to fit. That gives me more control over what the finished cabinet looks like, and a bit of insurance if things don’t go exactly as planned. It really doesn’t cause a lot of problems; trimming a door is a few swipes of a hand plane (or a pass over the jointer) and I don’t build the drawer box until the front fits in the opening.
The alternative isn’t always out of the question. If I were modeling for factory production, and sending a cutlist to computer controlled machines, I would want the model to reflect the exact size of every part. In the past, I’ve done shop drawings where the plastic laminate was added in at actual size, so that the particle board cores could be cut precisely. Working to the exact finished sizes takes extra modeling time, and it also does two other things.
It makes the model difficult to read, and hard to work with. Where ever there is a gap, there will be two lines next to each other instead of a single line. When you print, these double lines get blurry. More important is that it forces you to zoom way in to snap on a specific point when you add dimensions or move something.
It also forces you to work at an extremely precise level in the shop. If you’re aiming to make a solid wood part the exact size of the opening, and you’re off by 1/64″ that creates a problem that can easily be corrected. If you’re setting up the saw to cut hundreds of parts for a kitchen’s worth of cabinets, that same error could lead to dozens of edge-banded doors that won’t fit and can’t be easily trimmed.
So the answer is; “it all depends, you need to know what you’re working on, how you work, and how intolerant you are about tolerances.” I try to take the easy way out.