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Getting Dadoes and Plywood Thickness to Match
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undersized router bit profileHave you ever used a 3/4" router bit to cut a dado for a plywood shelf, only to discover that “three-quarter inch” plywood isn’t all that it claims to be? It can be a shock the first time; you’re left a plywood-to-dado fit that has more than just a little “slop”. In fact, the typical sheet of 3/4" plywood actually measures close to a hefty 1/32" less than its nominal thickness would lead you to believe.

So, how do you get your dados to match the actual thickness of common sheet materials? Using “undersized” plywood router bits is one of the simplest routes. Undersized router bits come in odd sizes that reflect the usual actual thickness of a standard sheet of hardwood veneered plywood, the most useful being 23/32" and 15/32" for plywood commonly sold as 3/4" and  1/2" respectively.

Undersized router bits aren’t the perfect solution. They’ll give you a perfect fit sometime, and they’ll almost always get you close, but no sheet of plywood is actually guaranteed to be any exact thickness. As we all know, wood expands and contracts with fluctuations in humidity, and a piece of veneer core plywood is no exception. Also, plywood is manufactured to standards that allow for variations in thickness of up to 3/64" for plywood sold in nominal thicknesses of 1/4" and more. If you want a perfect plywood-to-dado fit, you’ll have to take another approach.

incra router table fenceOn a router table, the “two pass” method with a few test cuts and some skillful fence positioning will get you a dead-on fit, but the process can be maddeningly slow. You can speed things up (with dadoes and in general) by treating yourself to a fence with an accurate micro adjustment system, like the Incra Lead Screw Router Table Fence. And while it’s not the kind of thing most of us would pick up on a whim, a precision fence system is worth some thought, especially if you’re planning to take up precision router table work on a regular basis.

The same accuracy can be achieved on a table saw using a dado blade. But with a typical stacked dado set, you’ll still be looking at two passes – one to hog out the majority of the material, and a second to fine tune the width of the cut. The tricky part, again, is getting set up for an accurate second cut. Most table saw fences won’t make the necessary fine adjustment a simple matter: Typically, you’ll have to whack it around with the heel of your hand and make test cuts until finally it lands in the right spot. But there’s a simple solution: the Magnet Micro Adjuster. It’s an affordable add-on, and with it you’ll be able to dial in the perfect fence position in a couple of seconds.

freud dial a dadoFinally, if you really want to laugh in the face of vagaries in plywood thickness, get yourself a dado blade with true micro adjustment capabilities, like Freud’s Dial-A-Width. That’ll make matters about as easy as they get. Here’s the refreshingly simple procedure: Stack up the appropriate number of chippers, set the micro adjustment dial to the desired width, make a test cut if you feel it’s necessary, and start cutting.

posted on June 1, 2007 by Rockler
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4 thoughts on “Getting Dadoes and Plywood Thickness to Match”

  • John Dickinson
    John Dickinson June 8, 2007 at 6:14 pm

    Well, a less expensive solution is to use the Rockler router table shims. I like them for making dados that will accomodate MDF which tends to be a bit thicker than spec. The one caveat is that you need to make the shimmed pass first or else your workpiece will travel. An alternative is to make the unshimmed pass first, then make the shimmed pass backwards. Anyway, they do the job nicely at my shop.

  • You have some great products and I love what you sell. The freud dial a dado blade is a great invention but it does take some getting used to. If for example you would like to cut a dado 1/2 inch wide you must put in the appropriate number of chipper blades and dial in a certain number of clicks according to freud's instruction sheet. The problem is there is no marked zero point on the dado blade. So when the directions say dial in 6 clicks the first question that comes to mind is 6 clicks from where? You must first guess at what the zero point is and verify your guess by laying a straight edge across the adjustment screw and blade. Tell freud to add a zero point on their next gerneration dial a dado and watch your sales increase! If the instruction sheet is lost so is the customer. Standard thicknesses cannot be easily made. Have freud engrave the settings on the dial or blade.<br /><br />

  • I stumbled upon a great way to cut an odd sized dado on the table saw using a traditional stacked dado set and a quality shim set. You will get the perfect dado on the second try every time. If your are cutting for undersized 3/4" plywood, you know the 11/16" arrangement will be too small. Therefore, load 11/16" of blades into the saw and most of the shims in the set. Make a cut. It should be too big. Dismantle the dado stack and shims. Place the plywood into the ill-fitting groove and using the stack of shims that you took from the saw, place enough to snugly fill the gap that is between the wood and the side of the groove. Some shims will be left over and some will be in the groove. The ones that are left over go back on the table saw combined with the 11/16" blade stack. The ones in the groove are left out because they are the ones that created the gap. Make the next cut. The fit will be perfect. Rockler should make their your own set of shims and include this direction.

  • Blog Editor

    Good points, all. Better stacked dado sets usually come with a set of shims. The <a href="http://www.rockler.com/product.cfm?Offerings_ID=17335" rel="nofollow">Oshlun 8" Dado Set</a> comes with 8 shims, for example, the thinnest of which is .005" - less than 1/128". Using the method Nick describes, you'd end up with a perfect fit with only one test cut. A similar approach could be taken with the <a href="http://www.rockler.com/product.cfm?page=17189" rel="nofollow">Rockler Router Table Shims</a>, if you prefer to do things that way.<br /><br />As far as the Dial-a-Width dado blade and the “six clicks from where” question: Interesting point. We’ll get back to you on that.<br />

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