# Helpwith the math



## whimsofchaz (Jun 6, 2013)

I am making bookcases for my wife and I can't remember the math to determine the offset of a guide bushing to the actual cut going to be made by the bit. I know it's simple and I could struggle and figure it out but I thought one of you could tell me how to figure that out. Also once I have the math will that help me set exact width dado's for the plywood shelves? Thanks

Chuck


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## Cherryville Chuck (Sep 28, 2010)

The OD of the guide minus D of the bit divided by 2. A 3/4" guide using a 1/2" bit will give a 1/8" offset.


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## Mike (Nov 22, 2004)

This may help.


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## DaninVan (Jan 1, 2012)

Thanks, Mike! I had to keep going back to my router book. Now I can just print it off.


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## whimsofchaz (Jun 6, 2013)

Thanks Mike I had seen the table before but couldn't remember where it was. Thanks Chuck


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## Gaffboat (Mar 11, 2012)

Thanks, Mike. I'm going to print that and stick in my router bit drawer for future reference. It will save doing the math.


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## Roy Drake (Feb 10, 2014)

Thanks, Mike. I've saved it.


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## papasombre (Sep 22, 2011)

Roy Drake said:


> Thanks, Mike. I've saved it.


Me too, Roy.


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## Dimitri M (Oct 4, 2011)

Lovely! BUT ... if you try to convert inch fractions to decimals for mm measures, prepare yourself for hysteric breakdowns, workpieces to the stove, and finally start knit work rather than woodwork. At least, this is my opinion. I take things the other way: NO bushing, no beating about the bush, and no bu(ll)sh.. I use a straight bit with a proximal bearing of equal width (suitable for template work), and put the straight guide / template where the rim of the dado cut should be. I save myself a lot of trouble, I think. Another thing is, book shelves are constructed for adjustable height. (I don't know why, after they are first used, they are seldom - if ever - changed, mainly because books in a home are deposited like bodies for burial: how many times have you reopened an old book to re-read it?). So, dados are not the way your bookshelf sides should look for adjustable height shelves. Anyway, in my opinion, dados will loosen with the passage of time, either because of shrinking of the shelf (due to moisture loss) or because of movement due to the extra weight of books.

Best wishes

D


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