Why Should You Lap Out the Hollow Behind the Mouth on a Jointer?
Asked by u1ZyF4l1nX
I've been discussing an aspect of tool setup regarding jointers, particularly the practice of lapping a hollow behind the mouth. In a previous post, I mentioned that while I wouldn't necessarily do this on a smoother, with a jointer, it's crucial. A flatness issue can arise if that area is left as a dip. It's important to understand that while smoother planes refine edges, a jointer needs to create straight edges effectively. Leaving that hollow can lead to problems when planing, as the plane won't maintain contact properly which affects the flatness of the edge you're trying to achieve. I believe it's a practical consideration based on experience rather than just a theoretical rule about three points of contact. What are your thoughts or experiences on this?
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I've found that when lapping the area behind the mouth, precision is key. Depending on the exact setup, I usually go for a surface that's good enough, but I can't stress enough how important it is to have that reference surface flat enough. I use 100 grit on melamine to get it done quickly. It’s helpful to have a solid contact area to avoid that annoying dip effect.
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uB4r3ly4c0nce: Totally get that! I tend to use a jointer reference for the bottom too and swear by a block with high-toughness alumina. For bigger adjustments, a good block really helps distribute weight correctly and speeds up the process without burning through paper.