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Typically, when I rout a mortise, I use a router bit the same diameter as the mortise is wide. I set up a fence and the depth stop on my plunge router and layout the mortise. I then make a series of full-depth plunges along the length of the mortise before cleaning up by guiding the router along the length of the mortise.
Now, I've been reading through back issues of Woodwork and in issue #9 from 1991, Pat Warner has an article on Router Template Mortising. In it, he states that "Single-cutter witdth cutting is high-demand routing that puts undue flex into the cuter and dulls it early from heat build-up. Often the stress on the cutter is such that its deflection produces a mortise wider than the diameter of the cutter." I'm not sure, but I think that is moreso the case with plunging a quarter inch, moving laterally, plunging another quarter inch, moving laterally again, repeating until the final depth has been achieved.
Pat's method is to use a samaller diameter cutter than the width of the intended mortise and start the cut at the center of his template and work his way towards the edges of the mortise.
Edit: Pat Warner (routerman), I trust you'll find this post. I'm curious whether your mortising procedures have changed since the article was published nearly 20 years ago. I haven't read much of your work before delving into Woodwork's back issues and am enjoying your articles very much.
The reason for this post is partially because I am curious about your mortising technique with a plunge router. I also, as with many of my posts, hope to educate and broaden the views of my fellow woodworkers. But most importantly, I want to analyse why I do things the way I do them (and why I don't do them some other way).
Sidenote: When I clicked on "Post new", I was brought to the page we are all familiar with a small text box. I resized it, typed, resized it again, then typed some more. Now, even though I have the two diagonal resizing lines at the bottom right corner of the text box, I am unable to resize the window - when I try, I just hightlight the text in the "Forum Status" column (see attached). I am using a Mac, running Safari 4.0.4
Note while editing: When I came back to edit this post, I was able to resize the text box once, and once only.
- Success is not the key to happiness. Happiness is the key to success. If you love what you are doing, you will be successful. - Albert Schweitzer