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The Thien Router Table Circle Jig
Design Copyright (c) 2005 Phil Thien
If you want to make your own Thien Cyclone Separator Lid, you're gonna need to make circles.  Of course, there are all sorts of reasons you may need to make circles (or holes, this jig makes those, too).  And there are all sorts of ways to make both.  But this jig is easy and foolproof.

Using the jig to make a circle
(1) Drill a 1/4" pilot hole as the center of your circle.  (2) Adjust the jig's pivot to the correct radius for the desired circle.  (3) Use the jig as a compass and (using a pencil) draw an outline of your circle on the wood.  (4)  Rough-cut the circle (leaving about 1/16" of material) with a jig saw or (if you have one) a bandsaw.  (5) Finally, using a 1/2" (diameter) flush-trim bit in your router table along with the jig, perfectly finish-trim your circle.

Using the jig to make a hole
(1) Drill a 1/4" pilot hole as the center of your hole.  (2) Adjust the jig's pivot to the correct radius for the desired hole.  (3) Use the jig as a compass and (using a pencil) draw an outline of your hole on the wood.  (4)  Drill a 1/2" starter hole just inside of the pencilled outline.  (5) Finally, using your router table with a 1/4" flush-trim bit (inserted through the starter hole), along with the jig, cut your hole.  NOTE:  As you finish the hole, the center pivot point will be unsupported which can lead to a ragged finish to your hole.  I suggest inserting pieces of 1/4" dowel in the bit's cutting path as you progress to keep the middle (waste) centered in the hole.

I realize this page is short on details but it should provide enough information to make your own jig.

Detail shot of the pivot.  This was made using:  (1) Knob with a 5/16" threaded hole, (2) 5/16" t-nut (from Woodcraft), (3) 5/16" brass all-thread that was turned down to 1/4" at the end (I just chucked it in the drill press and filed the threads off), (4) Some washers.  I will try to add better pics of the pivot shortly.

 

The business end of the jig.  There is a 1/2" diameter radius in the end to catch the larger flush bit for making circles, and a 1/4" radius on the inside to catch the smaller bit when making holes.

 

Making a circle for one my of Thien Cyclone Separator lids.

 

Detail shot:  The wood is pushed against the bit and spun.  The bearing rides in the jig and creates a perfect circle.

 

--Phil
phil@cgallery.com

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I consider this design my unique intellectual property, and reserve all rights.  Please do not take credit for my work by submitting this design to magazines as a "reader's tip."  I realize that this design will appeal to woodworkers and is something that can be made in your shop.  If you want to make one for your own use, please be my guest.  If you want to mass-produce them, please contact me first.