I'm trying to wrap my head around why a jointer alone doesn't give you S4S (surfaced four sides) lumber. I get that a table saw and a planer make the process easier and more precise. From what I've read, a jointer can only create flat surfaces at 90 degrees to each other, but it can't make opposing faces parallel. I understand the general workflow involves jointing one face, then using that as a reference to joint an edge, followed by planing and sawing the other edge to be parallel. My question is, why can't I use the same method for both edges by flipping the board? Also, I've seen mentions of compounding errors when using a jointer—how does that play into all of this when moving to a table saw?
3 Answers
Think of it like this: when you run a board that's trapezoidal through a jointer, it won't magically turn into a perfect rectangle. A jointer straightens edges but doesn’t make them parallel. That's why you need a planer or a table saw to even out the other sides afterwards. So, while it seems like you might be able to get the same results on all sides, that just doesn't happen with a jointer alone.
You can't use the jointed edge to get a straight face on the opposite side; it’s usually too risky when the board is narrow. It’s safer to use push blocks when working with the jointer, and that goes for the other faces too. It’s just not a stable way to get a square piece. So, while S4S might sound simple, it really requires both tools to do it right.
A jointer creates flat surfaces but won’t align edges to be parallel. For example, if you joint one side of a board that's thinner on one end, you’ll just have a flat but not uniformly wide piece afterwards. The table saw fixes this by using the straightened edge against a fence, ensuring both edges are the same width.