Exact Width Dado Jig - Router Forums
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post #1 of 18 (permalink) Old 10-23-2012, 07:42 PM Thread Starter
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Default Exact Width Dado Jig

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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post #2 of 18 (permalink) Old 10-23-2012, 08:33 PM
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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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post #3 of 18 (permalink) Old 10-23-2012, 08:35 PM
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MLCS Dado Clean Out Router Bits

Or

MLCS Straight & Plywood Straight Router Bit Sets

==

Quote:
Originally Posted by jaytosh522 View Post
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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Last edited by bobj3; 10-23-2012 at 08:42 PM.
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post #4 of 18 (permalink) Old 10-23-2012, 08:42 PM Thread Starter
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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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post #5 of 18 (permalink) Old 10-23-2012, 09:10 PM
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Quote:
Originally Posted by jaytosh522 View Post
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

John Schaben

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post #6 of 18 (permalink) Old 10-23-2012, 09:26 PM
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Quote:
Originally Posted by jaytosh522 View Post
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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post #7 of 18 (permalink) Old 10-24-2012, 09:43 AM
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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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post #8 of 18 (permalink) Old 10-24-2012, 03:08 PM
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Here is one good example of a jig.

139 – Exact-Width Dado Jig | The Wood Whisperer

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post #9 of 18 (permalink) Old 10-24-2012, 04:41 PM
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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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post #10 of 18 (permalink) Old 10-24-2012, 05:14 PM
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Jay. these two jigs may help you.

http://www.routerforums.com/jigs-fix...many-jigs.html

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