Mortising Jig

[PhilD]

I would like to make the mortising jig but I am having more trouble than I should have determining the length of the the un-routed portion in the bushing slot.  I would appreciate some advice in what is probably a ridiculously simple thing.  It is just not coming through my thick skull.  Also, the sketchup diagram appears not to be working.
Thanks.
PhilD 

[phil (admin)]

First, sorry for the broken link, I fixed that.

On wrapping your head around that measurement...

Consider that the bit is 1/4" wide.  Let's assume the distance between out two spacers on the bottom of the router were 2" apart (measuring the gap between the two spacers).  This is just an example, to illustrate how to think about these things.

So if we made the "unrouted portion" 2" wide, so the spacers were trapped and couldn't move side to side at all, then the width of our mortise would be 1/4" (we'd basically be plunging a perfect circle).

For every bit we router away from the "unrouted portion," we add that to the width of the mortise.  So let's say we now make the "unrouted portion" 1-1/2" wide, we subtract that from the 2" spacing between the spacers, and add it to 1/4" (the width of our bit), and have 1/2" + 1/4", which is 3/4", right?

So you need to accurately measure the distance between your two spacers, and use that as a starting point.

Does that help?

[PhilD]

So, if I want the mortise 2" wide and the distance between the screws (I don't have bushings yet so I can't measure that distance but I know that is the actual measurement I need) is 4 5/16 I would substract 7 quarter inch units from that measurement and make the unrouted section 2 9/16?  Since this is not the actual distance between bushings, I know this is not the correct figure but just to confirm the method. 
For some reason I still can't get the sketchup to open.  I get an error message saying the requested URL is not found on the server.  I think I can follow your excellent pictures, though.
Thanks (and hoping I followed your method and math correctly)
PhilD

[phil (admin)]

You have the math right.

I promise I fixed the SketchUp drawing link now.  If you refresh the page, you will be able to download it.

[PhilD]

I appreciate the help.  The SketchUp link works, thanks for that, also.  Now I have to get the components and put the baby together.

[phil (admin)]

I appreciate the help.  The SketchUp link works, thanks for that, also.  Now I have to get the components and put the baby together.

Let me know how it works for you!