# Problems with measuring offset of cut



## Art Hackett (Aug 21, 2014)

I'm building some bookcases and am at a stage where I need to make three quarter inch square cuts at the ends of the top of the case so it will fit down over the sides of the cases.
I'm using a Craftsman router that I haven't worked with for a while. The offset from the right side of where I want the cut to be is three and a half inches. I've calculated it three or four different ways. 
I make a mark three and a half inches from the end of the board and clamp a square securely flush with the edge of the board to use as a guide. 
When I make the cut it winds up being off anywhere from one quarter to one eighth to the left of where I want it to be. I cannot get the same result two times in a row.

Any suggestions?


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## oldwoodenshoe (Nov 28, 2011)

Art,

From your description it sounds like you are intending to use the outside of the router base along a clamped board as a guide. The distance from the center of the bit to the outside of the router is 1/2 of the diameter of the router. So if you are figuring this to be 3-1/2" this would mean your router base is 7" across on its diameter (7 divided by 2 = 3.5). If your router base has a different diameter than 7 inches, then the offset to the center of your bit will be different.

When calculating the offset to the edge of the bit you need to subtract 1/2 of the bit diameter form the offset to the center of the bit. Example: If you are using a 3/4" bit, 1/2 of the diameter of this bit is 3/8". So the offset would be 3-1/2" - 3/8" = 3-1/8" from edge of router to near edge of bit.

Another consideration is how thick of stock is the wooden guide you are clamping to your work piece. Some Craftsman routers have a casting that sweeps out to the handles. If your stock is too thick (greater than 1/4") then this outward sweep of the casting will push our router away from your guide if your handles move toward the guide.


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## Cherryville Chuck (Sep 28, 2010)

Welcome to the forum Art. You are able to post pictures and diagrams if needed as long as they are in the hard drive of your computer. You would click on the Go Advanced button and then click on Browse Files where you will go to find the file you want to upload.


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## Quillman (Aug 16, 2010)

Stop.
No matter what router, routing, using the subbase, will be ambiguous. Don't use it. The subbase, unless a square or rectangle, cannot be counted on to locate a cutter position.
A collar system is a better choice but only if that collar can be centered to the cutter.


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## Nickp (Dec 4, 2012)

Art Hackett said:


> I'm building some bookcases and am at a stage where I need to make three quarter inch square cuts at the ends of the top of the case so it will fit down over the sides of the cases.
> I'm using a Craftsman router that I haven't worked with for a while. The offset from the right side of where I want the cut to be is three and a half inches. I've calculated it three or four different ways.
> I make a mark three and a half inches from the end of the board and clamp a square securely flush with the edge of the board to use as a guide.
> When I make the cut it winds up being off anywhere from one quarter to one eighth to the left of where I want it to be. I cannot get the same result two times in a row.
> ...


If you are using the base of the router it may not be concentric with the bit. If the base is square, one edge may be shorter/longer than the other. If base is round it may not be centered on the bit. If you might be using a different part of the base as you pick up and use the router each time it could account for inconsistent cuts with the same router.

If this is the case, center the base if round or use the same edge if square base.

Good luck...Nick


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## 64 ford (Apr 21, 2013)

I agree with Pat. As said ,just make sure the collar is exactly centered.
Dennis


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## Willway (Aug 16, 2012)

Hi Art, All of these are good suggestions, but not the easy way. My way is to take a piece of scrap and clamp a strait edge to it and make a cut. Then measure the distance from the strait edge to the edge of the dado you just cut in the scrap. From that point on all you need to do is mark one edge of the cut plus that measurement, and clamp your strait edge in place. You do need to make sure you always keep the router base turned the exact same way toward strait edge. Just mark the router with a Sharpie, and write the measurement down to remind yourself.

Dick


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## old55 (Aug 11, 2013)

Welcome to the forum Art.


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## jw2170 (Jan 24, 2008)

Willway said:


> Hi Art, All of these are good suggestions, but not the easy way. My way is to take a piece of scrap and clamp a strait edge to it and make a cut. Then measure the distance from the strait edge to the edge of the dado you just cut in the scrap. From that point on all you need to do is mark one edge of the cut plus that measurement, and clamp your strait edge in place. You do need to make sure you always keep the router base turned the exact same way toward strait edge. Just mark the router with a Sharpie, and write the measurement down to remind yourself.
> 
> Dick


+1 for a quick and easy way to make the cut without having to make a jig.

Saw this method also on youtube, but can't seem to find it....


Maybe this will explain better.

http://shar.es/1nSAjG 

The first part applies in your case.


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## timbertailor (Oct 4, 2009)

Willway said:


> Hi Art, All of these are good suggestions, but not the easy way. My way is to take a piece of scrap and clamp a strait edge to it and make a cut. Then measure the distance from the strait edge to the edge of the dado you just cut in the scrap. From that point on all you need to do is mark one edge of the cut plus that measurement, and clamp your strait edge in place. You do need to make sure you always keep the router base turned the exact same way toward strait edge. Just mark the router with a Sharpie, and write the measurement down to remind yourself.
> 
> Dick


I agree. I have templates I have made for the different diameter bits I use. Keep that in mind.


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## Art Hackett (Aug 21, 2014)

Thanks. The problem was a combination of accumulating measurement error plus the aluminum square I was using as a guide was flexible enough that the router base was slightly riding up over it. I switched to an old steel square that was thicker and that solved the problem...along with being more careful in measuring.


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