How to Calculate Jig Size For Router Guide Bush

[PeteG]

Hello I need some help, as my mind has gone blank! I need to make a circular jig to use with a 30mm guide bush on the router. The finished hole size I need is 59mm.
How do I calculate what size hole I'll need to make in the jig to come out with the 59mm? Thank you

 

[Jamesc]

Firstly you need to know what size of router bit you will be using.

Then subtract the router bit size from the bush, this will give you the offset that you need all around the hole.
As it is a circular hole you want you need to add 2 x the offset to the hole diameter

James

 

[PeteG]

Thank you James So with a 30mm guide bush and a 10mm cutter, the hole in the jig would be 79mm and that should give me a 59mm finished hole in the work piece?

 

[Jamesc]

Yes that is correct.
The offset between the cuter and the bush is 10mm.
As you have a circular hole the size should be 10+59+10 which is 79.
I always find it easiest to sketch this sort of thing out

James

 

[PeteG]

You're a star Jamie, thank you ever so much

 

[JonnyW]

Thank you guys for this post. A huge light has gone on!!!

For years I've struggled setting up templates for routing. I don't know why, but my mind goes blank.

Thanks again

Jonny