# Total noob: jigs, bushings, calculations ...



## P.W.H. (Feb 16, 2018)

Ok, this is something I have never bothered with before ... my router came with what I assume you 'd call a bushing,
attaches to the router base, pokes out of the bottom for a router bit to stick through and provide guidance along
the edge of a jig. 

I want to cut a rebate around the holes in the top of my assembly bench so I can get a friend to do some 3d printed
plugs for it, to stop things from falling into the holes. 

I am completely baffled by how to calculate anything here. 
So I decided to resort to empirical data:

I have a bushing with an outside diameter of 18mm.
I am using a 1/2" router bit.
I cut a 32mm hole into a scrap of plywood to use as jig for an experiment. 

The hole I ended up cutting below the jig turns out to have a diameter of 28mm. 

Which happens to be more or less exactly what I needed. But: I have no idea 
how that number came to pass. 

12.7mm inside 18mm inside 32mm gives a result of 28. 
Would the size of the router bit change this ? One would intuit it should, but ... 

If anyone has a link to a website that explains this, or a youtube video, I'd be grateful.
Or .. explain it here ;-)

T.I.A. -Peter


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## Semipro (Mar 22, 2013)

Sorry here is a utube video but not in metric


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## Stick486 (Jan 4, 2013)

see if this helps..

...


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## P.W.H. (Feb 16, 2018)

Thanks John and Stick. Delved into the tables, and watched the video. All that makes perfect sense. 

The thing that boggles my mind is that in my experiment I should have ended up routing a hole with a diameter of 26.7mm
but my calipers told me that the hole was/is 28mm. A discrepancy of 1.3mm. 

That was why I thought that I was doing the math wrong, some kind of gigo happening here. But no, the info you gave
is exactly what I thought _should_ have happened. 

Heh. 

Regardless, I feel a lot more confident now about not being a total dipstick, thanks a bunch guys! -P.


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## Stick486 (Jan 4, 2013)

gigo???
is that like mojo...

glad we could help...


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## P.W.H. (Feb 16, 2018)

Stick486 said:


> gigo???
> is that like mojo...
> 
> glad we could help...


"garbage in, garbage out". Erstwhile computer programmer slang. But really it applies to every 
data processing, including humans 'thinking'. 

:grin:


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## Stick486 (Jan 4, 2013)

P.W.H. said:


> "garbage in, garbage out". Erstwhile computer programmer slang. But really it applies to every
> data processing, including humans 'thinking'.
> 
> :grin:


ahhh.
where in NZ are you??


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## MEBCWD (Jan 14, 2012)

One more thing to consider is centering the bushing. If the bushing is not perfectly centered then the resulting cut may not be what the calculations would make you think it should be.

Also you need to remember different wood cut differently so that can add to the mix.


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## P.W.H. (Feb 16, 2018)

I'm on the outskirts of a (for New Zealand) mid-sized town called Whangarei; which is on the east coast of the skinny bit at the top.

Oh, centering the bushing .... I should see if I can pick up one of those centering cones, for sure! Good point.


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