# Dado jig vs. exact size bit?



## jbroweleit (Dec 16, 2014)

I notice that many of you build an adjustable dado jig to provide the exact width for any size board to fit into the dado cut exactly, like for a shelf in a cabinet. 
Here is my question: Why use a jig? I will be building shelves all with 3/4" plywood with a 3/4" router bit. Why do I need the jig? I could just clamp down a straight piece of wood to the top of my project piece and run the straight edge of the plunge router along that side. (all carefully measured of course) I am wondering if that will be inaccurate, like maybe the plywood width varies or something and the 3/4" bit will make for a sloppy fit. I want to do have an exact fit. Thanks for your help. Jan


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## BrianS (Nov 7, 2004)

Measure any 3/4" plywood you like... if you find a piece that actually IS 3/4" today, then you've found something akin to the holy grail. All plywood thickness is undersize today. That's why there are bit sets sold specifically for cutting dadoes for plywood. Also why some people use dadoe jigs.. they don't want to buy a set of bits that only get used once in a while.


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## Nickp (Dec 4, 2012)

You're exactly right, Jan...the plywood, or other piece, will not be exactly 3/4 and the bit will be...

Jigs for such a job are pretty easy to make...2 short pieces and 2 long pieces. set the distance between them to satisfy your router base and use a 1/2 bit so that you can make couple of passes and get the exact width of the dado/groove you need. You can use the board that will go into the dado to size an adjustable jig and then cut away...

Welcome...you'll have fun here...Nick


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## PhilBa (Sep 25, 2014)

You can get plywood bits that are slightly undersized. I believe that there is no real standard for actual thickness of plywood. But why bother with highly specialized bits when you can make a jig that will cut a perfect dado, no matter what size needed?


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## fire65 (Oct 29, 2008)

Plus I bought a set of undersized plywood bits and they still do not fit properly, the dado is still a little small making assembly very difficult. 
They exact width jig is IMO the best way to go and you can use whatever bit you have.


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## Al B Thayer (Jun 2, 2014)

The best exact width jigs use a router bushing to guide the router. This way you set the dado width with the actual board going in the dado slot. It's set without ever needing to know the width. 

If you try to cut a dado with just a straight edge. You run a very high risk of messing it up with just the slightest error.

Find the jig on line and build it. You won't regret it. It will also be much easier to get all the shelves exactly where they should be.

Al


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## Al B Thayer (Jun 2, 2014)

Nickp said:


> You're exactly right, Jan...the plywood, or other piece, will not be exactly 3/4 and the bit will be...
> 
> Jigs for such a job are pretty easy to make...2 short pieces and 2 long pieces. set the distance between them to satisfy your router base and use a 1/2 bit so that you can make couple of passes and get the exact width of the dado/groove you need. You can use the board that will go into the dado to size an adjustable jig and then cut away...
> 
> Welcome...you'll have fun here...Nick


Umm well kinda. It's a little more involved than that. 

Al


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## Nickp (Dec 4, 2012)

Al B Thayer said:


> Umm well kinda. It's a little more involved than that.
> 
> Al


Right on, Al...maybe a picture might clear up the words... I made this adjustable so I can cut a dado or a groove. At the time I made it to cut out for a closed stringer where one side of the groove needed to angle away from the other. Being adjustable it allowed me to fit my step and wedge, on end, to measure and then used my router to hollow out. My jig is zero clearance for a specific router and straight bit so I just line up my wood to the line I need to cut and it's been right on every time. I square it up whenever I make dados for shelves, etc... I put my piece in on end, squeeze it to desired pressure, tighten it up and cut away...

Hmmm...I guess you're right...a teensy, weensy bit more involved. I should have added the picture on my first reply...

Thanks for pointing that out...Nick

EDIT...click on "View Nickp's Uploads" on the left for better pictures...last page...earliest pics...


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## Al B Thayer (Jun 2, 2014)

Unfortunately your picture is only coming up as a thumb nail to me. Clicking on orignal size just gets me a thumbnail pic in the corner. But you did get the now standard double pic post.

So if you can register the wood in the fixture to determine the size of dado needed. and make repeat cuts. I'm all for it. 

On my fixture the bushing runs against both sides after setting the width. Set it forget it. Cut it and slide it down to the next dado needed.

Al


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## chessnut2 (Sep 15, 2011)

Great question, Jan. In addition to variances in the thickness of available wood, I have found out the hard way that there can be variances in the router bits themselves. 

I have four 3/8" (.375) straight bits made by a major manufacturer with a great reputation. Same model number and everything. They mike out at .371, .374, .378, and .379. I found this out while making box joints on the router table. I thought that maybe my measuring ability, or my cheapo micrometer might be the problem, but I took them to a machinist friend and he got the same measurements. So save time, aggravation, and money in the long run, and make an exact width dado jig. Jim


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## Al B Thayer (Jun 2, 2014)

*exact width dado fixture*

This is the method I use. After cutting over 400 ft. of 3/4" dados in walnut plywood is has served me well and still produces an accurate exact fast dado.

The bushing in the router can be any size that is shorter than the fixture board is thick.

The fixture has one side fixed square to one of the rails. The other side is adjustable. 

The last picture it the board setting the fixture to the exact width. The slot below it is where the bit depth is set. The offset is the place the bushing rides in. The first time you build the fixture you make this offset wider than the bit is from the bushing. Then run the bit down both sides to cut the offset to the exact size. 

Difficult to find a better way to cut exact with dados.

Al


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## mmalinco (Jun 12, 2012)

jbroweleit said:


> I notice that many of you build an adjustable dado jig to provide the exact width for any size board to fit into the dado cut exactly, like for a shelf in a cabinet.
> Here is my question: Why use a jig? I will be building shelves all with 3/4" plywood with a 3/4" router bit. Why do I need the jig? I could just clamp down a straight piece of wood to the top of my project piece and run the straight edge of the plunge router along that side. (all carefully measured of course) I am wondering if that will be inaccurate, like maybe the plywood width varies or something and the 3/4" bit will make for a sloppy fit. I want to do have an exact fit. Thanks for your help. Jan


I am sure that I am not the first to reply to this posting. The answer is that 2/4" plywood is, in fact, 23/32". So, if you use a 3/4" bit, your dado will be 1/32" too wide.

However, you can buy router bits sized specially for plywood. For example, Freud sells set of bits for common plywood thicknesses (Freud 89-650 Four Piece Undersized Plywood Router Bit Set, available from Amazon, $66.01). Individual bits are also available at a lower price, e.g., Kempston 104479 Straight Bit, 1/2-Inch Shank, 23/32 Cutting Diameter, available from Amazon.com, $14.25.


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## Nickp (Dec 4, 2012)

Al B Thayer said:


> Unfortunately your picture is only coming up as a thumb nail to me. Clicking on orignal size just gets me a thumbnail pic in the corner. But you did get the now standard double pic post.
> 
> *View my uploads for better pictures*
> 
> ...


Thanks...it sounds like the basic difference is being able to cut grooves at angles and varying sizes. For quick dados for shelves I think I like yours better...


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## neville9999 (Jul 22, 2010)

jbroweleit said:


> I notice that many of you build an adjustable dado jig to provide the exact width for any size board to fit into the dado cut exactly, like for a shelf in a cabinet.
> Here is my question: Why use a jig? I will be building shelves all with 3/4" plywood with a 3/4" router bit. Why do I need the jig? I could just clamp down a straight piece of wood to the top of my project piece and run the straight edge of the plunge router along that side. (all carefully measured of course) I am wondering if that will be inaccurate, like maybe the plywood width varies or something and the 3/4" bit will make for a sloppy fit. I want to do have an exact fit. Thanks for your help. Jan


jan I don't bother with that, just get a cutter that is smaller than the thickness of the ply say 70 to 80% the thickness, run the groove off your straight edge, level at the top or the bottom as it does not matter, then when all the groves are done set up to run a rebate, run the rebate exactly on the shelves so that what is left fits into the groove and that the shoulder is the correct depth so that the shelf will reach the bottom of the groove, this way your tenon will suit the router cutter, every time, if you want to go to the trouble then you can stop the grooves at the front. and when the rebates are all done then drop the cutter lower and use the same fence set up to take off the first small bit of the tenon at the front, doing this will give you a stopped rebated joint so a square shoulder at the front, I would not do this if you were going to put an edge on later as there would be no point. If your shelf was only a very small amount thicker that the cutters is then there is no reason why the rebate cannot be very small so you could do this very small depth cut the get it to fit. N


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## Al B Thayer (Jun 2, 2014)

Nickp said:


> Thanks...it sounds like the basic difference is being able to cut grooves at angles and varying sizes. For quick dados for shelves I think I like yours better...


My fixture will cut any size dado on any angle. It's only limited by the width of the stock your putting in the dado. But I think 3" wide is more than anyone needed anyway. The fixture is 36" long. 

I guess I'm going to do another video demonstration.

Al


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## Al B Thayer (Jun 2, 2014)

mmalinco said:


> I am sure that I am not the first to reply to this posting. The answer is that 2/4" plywood is, in fact, 23/32". So, if you use a 3/4" bit, your dado will be 1/32" too wide.
> 
> However, you can buy router bits sized specially for plywood. For example, Freud sells set of bits for common plywood thicknesses (Freud 89-650 Four Piece Undersized Plywood Router Bit Set, available from Amazon, $66.01). Individual bits are also available at a lower price, e.g., Kempston 104479 Straight Bit, 1/2-Inch Shank, 23/32 Cutting Diameter, available from Amazon.com, $14.25.


But I think there is more to it than just trusting the thickness to be what they say they make it. It changes from one sheet to another. And then there's the bit cost and availability. Can't just bop down to the box store and pick them up.

Al


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## Al B Thayer (Jun 2, 2014)

*Sure easy peasy.*



neville9999 said:


> jan I don't bother with that, just get a cutter that is smaller than the thickness of the ply say 70 to 80% the thickness, run the groove off your straight edge, level at the top or the bottom as it does not matter, then when all the groves are done set up to run a rebate, run the rebate exactly on the shelves so that what is left fits into the groove and that the shoulder is the correct depth so that the shelf will reach the bottom of the groove, this way your tenon will suit the router cutter, every time, if you want to go to the trouble then you can stop the grooves at the front. and when the rebates are all done then drop the cutter lower and use the same fence set up to take off the first small bit of the tenon at the front, doing this will give you a stopped rebated joint so a square shoulder at the front, I would not do this if you were going to put an edge on later as there would be no point. If your shelf was only a very small amount thicker that the cutters is then there is no reason why the rebate cannot be very small so you could do this very small depth cut the get it to fit. N


And to say nothing of lining up the dado cuts and never slipping off the straight edge. Then cutting a perfectly flat and true rebate on a shelf of any length right? Sure seems like your going to a lot of bother with this method and very risky. 

Al


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## jbroweleit (Dec 16, 2014)

Nick and Al, thanks for your discussion. Your comments are so helpful. Nick I would love to see the photo you sent but it is so small. Any chance you can resend it larger? Jan


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## Nickp (Dec 4, 2012)

I don't have my photos with me but if you click on "View Nickp's Uploads" just to the left under my user name and Posts you will get all my uploads...go to the last page and you'll see a few of the jig...


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## neville9999 (Jul 22, 2010)

Al B Thayer said:


> And to say nothing of lining up the dado cuts and never slipping off the straight edge. Then cutting a perfectly flat and true rebate on a shelf of any length right? Sure seems like your going to a lot of bother with this method and very risky.
> 
> Al


One day we may talk about how easy it is to do perfect small rebates, I use a Makita laminate trimmer to do them as they are only small, Oh, as a professional cabinetmaker then I never slip off a fence. N


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## Al B Thayer (Jun 2, 2014)

neville9999 said:


> One day we may talk about how easy it is to do perfect small rebates, I use a Makita laminate trimmer to do them as they are only small, Oh, as a professional cabinetmaker then I never slip off a fence. N


Right. But we are not always giving advise to professional cabinet makers. Best off to give them the safest possible "route" don't you think? My guess is the OP has never cut the first dado due to the nature of his question.

BTW Im a professional woodworker specializing in furniture and high end cabinets. I wouldn't dream of cutting a dado with a single straight edge for guidance. 

Al


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## Nickp (Dec 4, 2012)

Jan...in case you did not get to see the pics in my uploads...

Sorry about the double pictures...web-site thing going on...


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## AlAmantea (Feb 27, 2015)

Plywood is sized by the nominal thickness (in 3/4,1/2,etc.) minus one half of the allowable tolerance level allowed by the APA. This is to allow for marketplace variations during structural engineering and planning in order to insure fitment within drawings and plans to actual field conditions. 

They are not trying to cheat anybody, nor are they "scrimping to save costs" as some uninformed woodworkers would have you believe. They are following the standards and practices set forth by the APA and building codes everywhere. (IBC) 

Allowable tolerances are normally 1/16th inch, hence the 1/32nd difference in thickness. This can and does vary by Manufacturer though, sometimes greatly. 

This is the reasoning for using a jig to adjust your Dado or groove or rabbet to actual board thickness instead of relying on measurements or specs. 

Most seasoned professionals will hardly use measurements at all when it comes to joinery as it slows them down. That's why it is common to see tendons and such at one third the thickness. 

Measure twice, cut once, BUT don't measure at all if you don't have to! 

Al


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