A challenge that came up frequently while I taught was helping students with their case good/cabinet joinery. This is about using French dovetails where appropriate.  Making a dovetail slot is fairly easy. Making the dovetail side of the joint a good fit takes some fuss.  Too loose and you get a loose joint all along the length. Too tight and it won't slide together. A good fit on wide boards may require a bar clamp or wood mallet to close the joint.  Friction increases between parts the farther the joint is closed.  A slight taper in the joint can let you slide it together easily while ending up with a joint snug all along its length.

 

 

I've written this all up in my blog: HERE.  It includes the steps and strategies I use to do this with and without a CNC.  For using the CNC I've included photos of the vectors I use for both sides of the joint and explain how one is created from the other.

Thanks!

Have always done this on router table but never thought about a tapered joint. Thanks for the info.

1 hour ago, Gerald said:

Have always done this on router table but never thought about a tapered joint. Thanks for the info.

You're welcome, Gerald and Lew.   

 

Tapering sliding dovetail joints came to me one day watching a student struggling to get her board to slide down the long dovetail slot.  She had used two passes to make the dovetail slot and apparently not gotten them perfectly parallel. Her dovetailed edge would slide in easily but get stuck roughly half way down the slot.  No bar clamp or mallet could get it more than another 1/2" down the slot.  Discussion among professors. deduced that the slot must have a slight taper to it, so a careful measurement was take of both ends.  Sure enough there was about 1/40" difference.  I realized I could taper off a bit of her dovetailed edge (using the  trick I mention in my blog) to help it fit down the tapered slot.  This was several years before we had CNCs to play with.  Even when CNCs first arrived we didn't have any way to clamp boards vertically under the spindle.  It took me roughly a year after getting my own Probotix CNC that it came to me that I could open up the bed and make a vertical/angling clamping jig.  Doing the tapered French dovetails on the CNC followed.   

4D  

Heck ya, great topic 4D, very nice, I love the similarities in cnc and hand tooling, the tapered sliding DT is a wonderful joint, done either way.