Calculating guide bush sizes

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Established Member
20 Feb 2004
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In the eternally wet North
Just starting to work with these. I have a template jig with various size holes and both even and odd guide bushes. Trying to work out what size template hole and what size guide bush and what size router bit to produce a hole of specific diamater is a bit of a tease.

I think the formula is template hole size - guide bush diameter + cutter diameter = final hole size. But then I look at the Trend site and their guide bush calculator starts talking about offsets and doesn't seem to help me much.

Am I right? Trial and error cuts suggests so but that could be by more luck than judgement!
You're right. It's just for straight line work the offset is a more appropriate measure whilst for circles the machined diameter = templete diameter - (2 x offset) Or does that confuse things again? :shock:

Hopefully, this pic will make expalantions easier:

So, if you're using a 9mm bit inside a 15mm guide bush, the offset is half the difference = 3mm.

The offset of 3 mm in the diagram above is correct for straight work like scrit says , if you are doing shapes you need to double the offset as you are comming in on the cut another 3mm on your return cut .
I am being really thick here as I can't see the difference between a straight cut (since the template has two edges) and a circle that also has to (curved) edges...or is it the case that when using a template to cut a straight line then the gap in the template is the same as the external diameter of the guide bush?

If you're using a template that is "outside" the cut, then the offset is as shown. If you're using a circular template where you're cutting inside the circle, then the width of the cutter must be added to the calculation, if you see what I mean.


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