# Calculating template sizes



## ceetee (May 10, 2008)

Hi everyone - I'm new to Router Forums but wanted to see if someone will give me some needed info. I want to make a circular jig to put on top of a 12" round board that will allow me to leave a 1/8" wide lip on the board, how to I calculate the diameter of the round jig? Like my memory, the formula I once knew is long gone! What I'm attempting to do is make some lazy susans with the lip idea. That should prevent items from falling off the top surface - especially if it should get spun too fast. I would like to use a 1/2" straight bit and corresponding bushing. After routing the circular groove, I'll find a way to remove the rest of the top surface - leaving only the lip sticking up. Help!


No one is ever too old to learn or practice good safety!


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## bobj3 (Jan 17, 2006)

Hi ceetee

Here's just one way to get the job done quick and can be used over and over..
Just chuck up a 1/2" router bit and put the 3/4" brass guide in the router base, turn the jig with the router plunged down, put in the slot outside slot then use the other holes or slot to plow out the center...



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ceetee said:


> Hi everyone - I'm new to Router Forums but wanted to see if someone will give me some needed info. I want to make a circular jig to put on top of a 12" round board that will allow me to leave a 1/8" wide lip on the board, how to I calculate the diameter of the round jig? Like my memory, the formula I once knew is long gone! What I'm attempting to do is make some lazy susans with the lip idea. That should prevent items from falling off the top surface - especially if it should get spun too fast. I would like to use a 1/2" straight bit and corresponding bushing. After routing the circular groove, I'll find a way to remove the rest of the top surface - leaving only the lip sticking up. Help!
> 
> 
> No one is ever too old to learn or practice good safety!


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## ceetee (May 10, 2008)

bobj3 - Thanks for the quick reply. Maybe I should have been a bit clearer. I know there is a woodworker's formula that involves the diameters of the bushing and bit. So, I have a piece of wood that's been made from several pieces which have been biscuited and glued together. The finished size is 12" in diameter. I would like to cut a circular jig out of some 1/2" thick ply I have. I'll clamp the circular jig on top of the 12" round finished piece I have and I want to route around the jig using a 5/8" bushing and 1/2" straight cutter. I'd like to calculate the diameter of the jig before cutting it out. My intent is to leave a 1/8 to 1/4" wide lip around the entire top of the finished round board. I used to know how to calculate the diameter of the jig I need to cut out with a jigsaw - but my memory is on vacation. Thanks again!


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## Hamlin (Dec 25, 2005)

Hi,

If you're wanting to leave an 1/8" to a 1/4" lip, do as Bj suggested. Use a 3/4" bushing/guide or larger with your 1/2" bit. For routing out the center part, I'd also suggest using a dish cutter bit, would leave a smoother finish.


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## bobj3 (Jan 17, 2006)

HI

Got ya, but with the jig, it will do all the work for you ,, no need to calculate the diameter , use the jig to cut the circle out,plus put the trim lip in place all with one jig, just mark the center point, screw the jig down , to get the job done..


But here's a link that may help
http://www.routerforums.com/email-router-tips-members-only/4348-guide-bushing-chart.html


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