The best of the jigs I've found is the Wells/Karol type, which has a range limited only by the length of the guide rods used...I've used mine on furniture projects as well as instruments. Accuracy in setting is excellent, as the adjustment thread is a 1/4-20 (0.050" for turn). Sylvan Wells has the plans on his website (paid access), but the video from Chris Paulick linked below covers all of the essential measurements, bill of materials, and process to build one. Much more accurate than any of the Dremel-based jigs, and on par with the $$$ solutions from MicroFence.
Chris Paulick shows installation of a PC 7301, but the less expensive $99 Ridgid 2401 has much better depth setting repeatability, variable speed, soft start, and a lifetime warranty - we use these in most of our jigs that require a smaller router. Besides all the standard modern router features, the Ridgid has decent LED lighting, comes with a functional edge guide that can be kit-bashed into a number of useful jigs, and has outlasted my Bosch Colts and PC's in use. We use the brass StewMac 1/8" shaft adaptor for 1/8" shank bits.
Here's the first of Chris Paulick's three-part series on building and using the base:
https://www.youtube.com/watch?v=9oRqUK-CMIo
A few observations:
- Plunge bases are not needed...the cut is so shallow (0.050"-0.060") that even a cut starting in the field will not show plunge irregularities with sharp bottoming end mills or downcut router bits. Start and stop any cut under the area to be covered by the soundboard extension, or on the upper centerline where the purfling ends will meet for Weissenborn or J-35 style rosettes where the upper ring butt join is not covered. A scarf can minimize the visual noise here as well, but I find an accurately cut butt joint exactly on the centerline looks purposeful.
- Three ring rosettes need graduated spacing between inner and outer rings...the spacing on the outer ring needs to be about 15% greater than the inner, so a 0.100" inner gap generates a 0.115" outer gap. In practice, this is easier to do the uniform ring size, as the inner ring channel is routed, then center, then outer, and a minor error - up to 0.005" either way is not noticeable (or likely, given below).
- Make up a dummy rosette from 1/2" - 3/4" MDF or scrap ply and 1/4" rod stock. Do all jig setting and trial cuts on the dummy, then mill the top. Most of my rosettes are one-off efforts, but saving the dummy with pin pulled and measurements noted in Sharpie can save some time on identical guitars.
- A 1/8" downcut spiral is great for milling out wider channels, while decimal and fractional inch bottoming end mills (about $6 or so each from various suppliers) work for 0.022", 1/32", 1/16", and 3/32" channels
- Ditto other recommendations on always bordering a trim element with a defined border, such as 0.010"-0.020" black fiber or purfling line...shell or wood set in the top looks unfinished - your eye needs help with seeing the transition, and those scant few thou of edge tells the eye where the rosette is intended to end...intent is important!
- Multi-piece wood rosettes are best milled separately after assembly on a backer of 1/64" aircraft plywood...fiberglass double-sided carpet tape holds everything on a sacrificial MDF milling board and releases with naphtha once the job is done
- For wood rosettes, mill the channel a few thousandths oversize, glue into play, then after that dries, rout the purfling channels and glue them in place...much cleaner than trying to complete the rosette off the guitar.
- Wide shell is at best an acquired taste - most shell rings look wider than they really are due to the surrounding black purfs. I don't go any wider than 1/16" theses days, as wider reminds me of other bad fashion decisions, like super-wide disco shirt collars and wide-wale corduroys.
- Sharp bits are cheap...most downcut fractional bits will mill cleanly for a couple of tops...after that, reserve them for wasting large areas of inlays or toss...a fuzzy edge on a narrow rosette channel cannot be easily fixed
- The 1/4" holes in the jig benefit from a 0.2505" chucked straight shank/flute reamer - the same one you'll want to finish up 1/4" shaft tuner holes...about $20 for a good one from MSC or Amazon Prime