# Exact Width Dado Jig



## jaytosh522 (Oct 23, 2012)

I'm making an exact width dado jig that will use two straight pieces of 3/4-inch plywood and a straight bit with a bushing on top. I want to cut dados for shelves in bookcases and kitchen cabinets. The current shelves I am installing are 1/2-inch plywood, about 1/4-inch deep. What is the best bit to buy, a flush trim router bit? What size - length/thickness would you recommend?


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## jw2170 (Jan 24, 2008)

welcome to the forum, Jay.

1/2" ply is not always 1/2" thick.

I would use a 3/8" straight cutting bit.

Does the dado jig allow for the bushing? Some jigs that use a bushing have a set bit to use with that bushing.

Can you post a picture of your jig as this may change the answer.l


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## bobj3 (Jan 17, 2006)

MLCS Dado Clean Out Router Bits

Or 

MLCS Straight & Plywood Straight Router Bit Sets

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jaytosh522 said:


> I'm making an exact width dado jig that will use two straight pieces of 3/4-inch plywood and a straight bit with a bushing on top. I want to cut dados for shelves in bookcases and kitchen cabinets. The current shelves I am installing are 1/2-inch plywood, about 1/4-inch deep. What is the best bit to buy, a flush trim router bit? What size - length/thickness would you recommend?


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## jaytosh522 (Oct 23, 2012)

No picture yet, as I am in the process of making it. I am just using two pieces of 3/4-inch plywood, separated by the width of the plywood I am using for a shelf. I will clamp the plywood pieces down using square to the piece I am routing and parallel to each other. I was hoping to use a straight bit with a bushing to ride the side of the jig. I see your point that a 1/2-inch bit might be too wide. In that case I should just use a straight bit and set up a straight edge to guide my router. 
Thanks, you got me on the right track!


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## jschaben (Jun 21, 2009)

jaytosh522 said:


> I'm making an exact width dado jig that will use two straight pieces of 3/4-inch plywood and a straight bit with a bushing on top. I want to cut dados for shelves in bookcases and kitchen cabinets. The current shelves I am installing are 1/2-inch plywood, about 1/4-inch deep. What is the best bit to buy, a flush trim router bit? What size - length/thickness would you recommend?


Hi Jay - Welcome to the forum
If you are using an exact width dado jig that uses a bushing, the bit size is dictated by the width of the dado and the bushing size is dictated by the offset the jig was designed for. For instance, if you build the jig around a 3/4" bushing and a 1/2" bit, you can use any bit/bushing combination that yields a 1/8" offset. All exact width jigs usually require two passes so I pick a bit that is greater than half the width of the dado and then go with a bushing that will give me the required offset. In your case,with the described jig, a 1/4" bit and a half inch bushing would give you an exact width for whatever the plywood happens to be. A 3/8" bit with a 5/8" bushing would also work exactly the same.
Using a jig that hasn't been designed around using a bushing would require a pattern (shank mounted bearing) flush trim bit. That would be hard to come by in cutting diameters less than half inch.
Good luck


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## jw2170 (Jan 24, 2008)

jaytosh522 said:


> No picture yet, as I am in the process of making it. I am just using two pieces of 3/4-inch plywood, _separated by the width of the plywood_ I am using for a shelf. I will clamp the plywood pieces down using square to the piece I am routing and parallel to each other. I was hoping to use a straight bit with a bushing to ride the side of the jig. I see your point that a 1/2-inch bit might be too wide. In that case I should just use a straight bit and set up a straight edge to guide my router.
> Thanks, you got me on the right track!


Remember to allow for the offset between the bit and the bushing.


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## Mike (Nov 22, 2004)

Jay, I would use a guide bushing with a straight bit. In the example above you would use a 1/4" shim with your shelf board to get an exact fit.(1/8" offset on both sides of the board) If you are cutting plywood a spiral downcut bit is the best choice; in solid wood a spiral upcut does the best job.


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## Leifs1 (Dec 16, 2009)

Here is one good example of a jig.

139 – Exact-Width Dado Jig | The Wood Whisperer

Regards
Leif


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## JCJCJC (May 15, 2012)

To me, some confusion arises when you mention exact-width and a bushing together. If it's an exact-width jig, a top-bearing pattern bit should do the job for you without a bushing, with the bearing riding on the edges of the jig. If there's a bushing involved, I can't see how the jig can be exact width- it'll have to be bigger than the dado to take account of the offset between the bushing and the plain cutter.

I would also screw & glue your plywood guide pieces to at least one batten so that you need only set up accurately once - the batten will run along and push against the face of the shelf upright - you end up with a one-piece jig then, much easier to use and less risky than setting up every time. For fine-tuning the width of the jig, use masking tape or duct tape to reduce the size of the finished cut by tiny amounts until it's perfect.

I use jigs and bushings on the same basic principle to make my guitars - this rear-mounted shop-made 10mm square tone bar fit shows the kind of accuracy I'm shooting for and hitting regularly and easily.










Hope this helps.


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## jw2170 (Jan 24, 2008)

Jay. these two jigs may help you.

http://www.routerforums.com/jigs-fixtures/30181-can-man-have-too-many-jigs.html


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## Mike (Nov 22, 2004)

John, read post #7. When you know the offset it is easy to cut a shim for an exact fit using guide bushings.


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## bobj3 (Jan 17, 2006)

Hi

No need for shims with the jig below,just put the 1" guide in place put the Right plywood bit in place and you are set to put in dado's all day long >>>>no need to adjust the jig,it's always dead on 

http://www.routerforums.com/jigs-fixtures/2711-dado-jig-plunge-router.html

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## Mike (Nov 22, 2004)

BJ, that jig works fine for off the shelf material but for using other thickness boards you would have to reset the jig for a second cut.


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## Chris Curl (Feb 13, 2012)

i'm thinking he probably means a bearing, not a bushing.


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## bobj3 (Jan 17, 2006)

Hi Mike

No reset needed just pop in a new guide like a 7/8" or 3/4" or what every it takes.

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Mike said:


> BJ, that jig works fine for off the shelf material but for using other thikness boards you would have to reset the jig for a second cut.


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## JCJCJC (May 15, 2012)

Mike said:


> John, read post #7. When you know the offset it is easy to cut a shim for an exact fit using guide bushings.


Thanks Mike. My table saw is good to about half a millimetre at best, so that's how good my shim would be. By tweaking my templates with tape, I can get a dimension to within a tenth of a millimetre. There are times when I need it to be that good.

@Chris - what you say makes sense. If the op said 'bushing' when he meant 'bearing' the whole thread makes sense - an exact-width template sized off the shelving material would do his dados perfectly then.


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## roofner (Aug 1, 2010)

The idea behind an exact width dado you would have to use a bit smaller than what the width of your dado. Because you are actually cut the dado in 2 passes and the actual piece
you are cutting for is used to set the width. The halfs are made by putting your router against each individual fence trimming the piece flush. Then one mount solid to the end and the other slides in the grove to set your width.


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## jw2170 (Jan 24, 2008)

The problem in using a bit with a bearing on the shank is that you have to adjust the thickness of the fence to allow for just 1/4" (or whatever the depth of cut) to cut into the workpiece. 

I have a bearing guided bit from Wealden Tools with only 1/2" of cutter depth.

If you use a guide bushing or the router base then the bit can be set to any depth.


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