Some tropical hardwood types contain higher amounts of silica which will dull the HSS quickly, and thus it’s advised to used the M4 or carbide on those woods. ?Teak has high silica and is best matched to the carbide blades. ?I run M4 on my J/P and have and zero “quick dulling” issues with that grade when used on all the common American hard and softwoods including Ash, Oak, Maple, etc, as well as with European Beech, as well as Cocobolo, SA Rosewood, Bubinga, Wenge, etc.. ?I only switch to carbide when machining Teak, although there may be other tropical hardwoods that also contains higher amounts of silica - others here may have broader experience with the exotic varieties.

I just changed blades on my Dual 51 4-knife Tersa head and it was under 10 minutes start-to-finish. ?Turning all the cutters on a spiral head to get a fresh edge is often an hour or more depending on sap buildup and the amount of cleaning required. ?The primary advantage of the spiral heads are lower noise, and more compact chips in the dust extractor. ?I love the Tersa system - great finish, easy knife changes, and extractor bags that fill more quickly but are not as heavy and easier to manhandle. ?