Dado jig vs. exact size bit? - Router Forums
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post #1 of 23 (permalink) Old 03-14-2015, 10:28 AM Thread Starter
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Default Dado jig vs. exact size bit?

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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post #2 of 23 (permalink) Old 03-14-2015, 10:46 AM
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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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post #3 of 23 (permalink) Old 03-14-2015, 10:49 AM
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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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post #4 of 23 (permalink) Old 03-14-2015, 11:32 AM
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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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post #5 of 23 (permalink) Old 03-14-2015, 11:47 AM
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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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post #6 of 23 (permalink) Old 03-14-2015, 02:47 PM
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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.

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post #7 of 23 (permalink) Old 03-14-2015, 02:48 PM
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Quote:
Originally Posted by Nickp View Post
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.

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post #8 of 23 (permalink) Old 03-14-2015, 05:26 PM
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Quote:
Originally Posted by Al B Thayer View Post
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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post #9 of 23 (permalink) Old 03-15-2015, 10:49 AM
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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.

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post #10 of 23 (permalink) Old 03-15-2015, 11:40 AM
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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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