What is a "dust pass"?

Question

In a question about routing MDF & HDF, an answer suggested:

when routing conventionally use a second "dust pass"

What is a "dust pass"?

Answer 1

A "dust pass" is a second (or last) machining pass that removes just a tiny amount of material — sometimes literally producing only dust as the name suggests — to improve surface finish and/or remove burn marks.

In a router context I believe it's most commonly utilised for improving edges when pattern routing and when using a fence, either the fence on a router table or one attached to the router itself. But it can also be done to improve the bottom of a cut, or the surface left after router planing as the following tip mentions.

^{Tip #5 from 14 Fuss-free router-bit setup on Wood Magazine's site.}

This should probably be done more in routing since it can dramatically reduce the need for sanding, and on occasion may even prevent it entirely.

It's on the table saw¹ where this process is used more widely, where it is sometimes referred to a "skim cut" but more commonly a "dust cut". Despite it being more common I only became aware of its use in a table saw context relatively recently; I've been unable to locate the reference, so I suspect it may have been mentioned in passing in a video on YouTube (likely during an episode of Shop Talk Live from Fine Woodworking).

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This basic principle is quite widely used in woodworking, very light cuts (and occasionally a succession of them²) being used to give the best surface possible³. Here's a reference to the same idea on a thickness planer:

"Set the depth of cut for a 1⁄64 "-or-less “skimming cut” on the final pass for the same reason [to get a smoother surface]"

^{No-fail Routines for Jointing and Planing on Wood Magazine's site.}

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¹ Although presumably will work equally well on other rotary saws. But bear in mind this is only important if no other process will be used to finalise the sawn surface, such as planing to final length in a shooting board or running a cut edge over the jointer.

² Q.v. the related incremental routing.

³ Not unlike the final wispy shavings taken by a smoothing plane, or the even finer and wispier shavings taken by a scraper.