Originally Posted by
John Gornall
For half blind dovetails only one router and one bit required.
This is generally correct. The D4, however, is often used for a variety of through dovetails as well as other kinds of through joinery where two cutters are used. That's where the two router scenario comes into play for folks who want to adjust both cutters and test fit prior to cutting the production workpieces. Through joinery is popular for casework which the D4 is nicely suited to in some cases because of the width.
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