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If I build an exact width dado jig before an upcoming project, the effort will probably pay back on that single project... and that's before considering the benefit of the jig's precision. After examining different ways to make this jig, and I'm inviting your combined expertise to save me from pitfalls in my plan...

Additional context: my router is a Bosch 1619, and I'm working with a minimal set of tools right now. Saws are jigsaw, circular saw, and handsaws. No jointer or planer.

The first major jig design choice is how to guide the router along the jig. I see these options...
  1. Use a bearing guided bit
  2. Use a bushing
  3. Reference the router base against a fence

My current preference is #3, with #2 as second choice. Why?
  • I don't like being restricted to a bearing guided bit because it complicates the details of horizontal alignment with constraints on vertical alignment. That's fine when using the jig for the first project I'm planning, but there's more opportunity for surprises or limitations later.
  • I'm fine with the bushing option. It would be the first time I've used one and a good chance to learn.
  • I slightly prefer running the router base against something rather than using a bushing as it's stupid simple to build the jig that way. It also doesn't split the working edges between zero clearance and bushing parts, so a thinner base may be an option... perhaps 1/4" masonite or 1/4" BB rather than the 1/2" BB I'd otherwise use.

Are there any hidden surprises in this choice? I'll probably need to always orient the router the same way rather than assuming the bit is perfectly centered, and I'll definitely need to make the jig wide enough that clamps don't interfere with router handles. I don't see any other challenges here.

If my description of the options wasn't clear, here's a few videos that illustrate each type...
  1. Bearing guided... see 2 minutes into this video...
  2. Bushing guided... see 8 minutes into this video...
    . A slightly improved version of that jig is here:
  3. Fence guided... see 10:20 into this video...
    . Another examples is here
    .

Remember, my focus here is primarily deciding on the guide mechanism... though any other observations will probably be helpful too.
 

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I prefer using a bearing guided bit. It works with any diameter bit, you just need a short bit.

Reasons to not choose one of the other choices:

Using a bushing requires that you get a router that can accept bushings, preferably without having to spend big money to get a bushing to work. Lots of routers have compatibility issues with the standard Porter Cable bushings. Also, you have to keep track of the bushing and the router bit you used to set up your jig the first time so that you can find them the next time you use your jig.

Using a fence to guide the router requires that the same router be used every time, and that the router be exactly centered on it’s base, otherwise you can damage the fit of your jig if you don’t orientate the router to the jig the same every time.


In woodworking there is always more then one way to accomplish something.
 

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bearing guided...
IRL measures and and no bushing centering issues...
no MDF either...
 

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I use Bushing guided as in example #2. I've marked onto the jig, the bushing size and bit size for future reference. I like the track added (example #3) to use adjustable stops for stop dados. I also like (example #3) that the knobs are on the top...because the knobs on the top it allows you to place the jig under you work on the bench solidly, without having to hang off the bench. Using a bushing guided system allows me to use different length standard router bits, without the added expense of bearings.
 

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If you use a bushing you have to account for the offset in the bushing which could mean trial and error to get the gap in your jig right. Using the bearing guided bit means you can use a piece of the board or panel you'll be using for your shelf as your setting gauge which assures you a perfect fit the first try.

You can build a jig that is two tiered jig like a straight edge cutting guide for a circular saw where you take a strip of thin ply (1/4 to 3/8") and laminate a guide strip on it. When you make this jig you leave the bottom strip a little wide and cut off the excess on the first pass. The downside of this jig is that you have to use the same bushing and straight bit every time and they have to be centered every time so this method winds up being more complicated and time consuming.

Here is an article about the circ saw guide so just imagine two of them joined together and on a smaller scale. https://www.woodmagazine.com/woodworking-tips/techniques/sawing-solutions/circular-saw
 
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For most situations, I like a bearing. Ashley mentioned getting the bit centered. If it's not, then a slight movement of the base off dead center will leave a "wiggle" in the dado. Same is true using the base along a straight edge. Yes, it works but it can go off if you don't have perfect alignment. By using a bearing, you could, conceivably rotate the router 360 degrees and the bearing will still follow the edge.

To keep with full transparency, I've used all three methods and they all have worked for me. I choose which one I use based on the project requirements. For one or two dados I'd use a straight edge if it would work. For more, depending on the size, I'd make a jig then use a bearing. If the bearing would be a problem due to the depth of cut, I'd use a bushing. Whatever gets the job done.
 

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My jig looks like the one in Mikes last photo, except my jig has a strip along the outside edge on top of what Mike shows in the last picture. These strips guide the base of the router to keep the bit centered, and when making the jig, you trim the edges of the slot using the router bit while the base of the router rides along each of these strips.

In use, the jig needs to always be used with the same router, router base, and bit, but it will cut a dado the exact width of the gap in the jig every time. So you can set the jig by putting the board to be inserted in the dado in the slot of the jig and move the two halves of the jig until it is tight against the board. Then lock the jig setting and remove the board. Place the jig on the board to receive the dado, position the jig where you want the dado and then clamp the jig into place. Set the bit depth and rout down along the left side of the jig and then back on the right side. The dado will be the exact width of the board used to set the gap width in the jig. No collars or bearings are needed. The router base and these upper rails of the jig do the guiding instead.

Charley
 

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This one requires no external clamps. Unlike yours, the boards at the top and bottom sandwich the board being dadoed. Since the top one moves and the lower one has a built in clamp, they grip the board tightly.

It relies on a bearing for dead accurate positioning and cuts, regardless of how well the base fits.

http://www.routerforums.com/jigs-fixtures/44539-kellys-new-router-jig.html
 

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If you choose #3, use a square router base and mark the edge you used to make the zero clearance.

All three methods have value, depending on the individual project...
 
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Kelly, Looks like you've worked out all the bugs. Nice solution.
Thanks, gmercer. The bearing guide system, like others suggest, makes it a "what you see is what you get" jig. The fact I just back off the one clamp, move it to the next position, turn the clamp and go makes blowing out multiple dadoes very quick.
 

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I use a Dado Wiz for certain dados, or a dado stack. The standard jig with a bearing guided bit is my alternative approach.

If you had a dedicated square base with the bit centered on it, you could build s similar exact fit jig, but with a small fence on the outside of the fixed and movable parts. The bit need not be centered, if you always use one edge to set the width. You would fit your work piece into the space between the fence and the router's straight edge. Close it down tight and you would be able to cut an exact fit dado without a bearing guided bit. You could use, for example, an up or down spiral bit. By not changing the base, you have a reliable distance for the beginning and then the second cut.

So I'll stick with my own methods and skip bushings and edge guiding, thank you.
 

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The best method for dado cuts using a jig is to use a template guide of KNOWN external diameter and known offset.

e.g If you are in America and use the imperial system, it is easier.
A 3/4" template guide + a half inch bit (1/2") will have offset of 1/8" always.
A 3/4" template guide + a quarter in bit (1/4") will give an offset of 1/4" always.

Template Guide is best suited for Dados because unlike the pattern bit (bearing guided) you can vary the depth of cut just by plunging which is impossible with a pattern bit (bearing guided) unless you buy a pattern bit with a very short cutting edges and a very think template.

BUT you have a BOSCH and I think their simplified optional dust chute for your router will not work with template guides (?).
So for Bosch users I can only say use it would be better to use SHORT pattern (bearing guided) bits.
An annoying thing is that a 1/2" pattern bit (most useful size) with a 1/2" bearing will by physical limitations only come in a 1/4" shank.
FYI: Pattern bits + jigs were designed for through cuts & trims not dados of varying depth.
 
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A suitable pattern bit for dados

Note
1. Short blades cutting edges
2. 1/4" shank if 1/2" bit
3. Require thick templates.
 

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Mortising bits like that cut nice flat bottomed dados. Also useful for installing hinges flush with a door or jamb. I used one of these with a 1/4 inch shaft to slightly deepen the rabbet for a thicker aluminum router plate, and to install hinges on three interior doors that replaced the old, teenager abused doors. You definitely want the half inch shank for dados and grooves.

The illustration is how I set up a pattern to fit the new mounting plate. Fit the 4 pieces around the edge of the plate and clamp them down. I'd add either a card or painter's tape to make the opening slightly oversized (or it will be hard to lift out).
 

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Stick,

1...what's IRL?
Also, does "No MDF"
2... include hardboard?



Sent from my iPad using Tapatalk
1... In Real Life...
2... Yes...

if you add a enlarged hole at one end it will facilitate plunging a top bearing mortising bit..
the top bearing guided bits are never off center and no math needed...
use a bit that's dia is approximately ¾ that of the width of your dado or grove.. clean cuts w/ two passes..
 
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