G'day Hilary. welcome to the forum.
If I understand your question, the centerline of the cutter to the edge of the guide bushing is simply 1/2 the size of the guide bushing. The offset between the outside edge of the cutter cutter and the outer edge of the guide bushing is (guide bush - cutter) / 2.
What size cutter and guide bushing are you using?
Do you need to make multiple passe to cut a 23/32 groove?

It also helps if you give us the router/model that you are using.

Another way to measure, is to run a test cut against the straight edge with a v groove cutter set vey shallow so that it just cuts pencil thin line...This will be the centerline....

 

Welcome to the forum Hilary.

 

Hi Hillary, are you cutting a dado, an elongated groove? I couldn't quite understand what you were trying to do. With that in mind, an exact fit dado jig can really make this happen easily. If you are certain the thickness of the piece fitting into it is 23/32nds (18mm metric) you can use a mortising bit with a top mounted bearing and a straight edge (a very straight board), and do it freehand. Use a marker to put a spot on the rim of the router base and keep that spot next to the straight edge. If you rotate it, the groove will wander slightly.

I'm assuming you're doing this freehand not on a table. Clamp the straight edge in place so you can make several passes of about 1/8th inch each. The bit I have in mind is fairly common and looks like this mortising bit. Mortising bits leave a very flat bottom to the cut.

If you are cutting close enough to the edge, you could also use an edge guide like this:

 

@jw2170 : Thanks for the good response and your simple solution -- easy to understand and easy to do. BTW, the router model is a Dewalt 618.

@old55 : Thanks for the welcome!

@DesertRatTom : You've opened my eyes to some new techniques -- easier and produce better results!

Gentlemen my appreciation and gratitude.

Hilary Smith
Pacific NW USA

 

Hi, I just realized I forgot to attach the edge guide picture. I added it above. The flat bottom mortising bit is good because it makes better glue contact with the edge of piece fitted into it.

Dados are grooves that go from edge to edge. A groove does not go all the way to the edge. Look up exact fit jigs. You can buy one already made, or make your own. If you make your own, it looks like this: If you are fitting a shelf, for example, you slide a piece of the shelf into that gap and snug the wider piece up to it, and use a bit with a bearing on it, and that is a little over half the width of thickness of the shelf. You make one pass riding the bit against the top or fixed side, then again on the movable side. You make it much longer than any thing you expect to make. The trick is to make certain the top cross piece is exactly 90 degrees to the side pieces. Being careful about that (using an engineer's steel square), will make it really easy to use and very accurate. One last thing, My gadget requires using what's called a spiral bit. The shop made jig uses a much cheaper mortising bit.

Here is an exact fit setup from Rockler woodworking. You fit the sliding part into a groove on the long clamp. The long clamp has something to grip the edge, but it doesn't take very much to make is shift, so clamp it down.

I have an old gadget called the Dado Wiz, now discontinued, that I use and it is really nice, but I have to make sure it's lined up just right using a tape measure. If I'm doing a shelf or any similar thing that has to be exactly parallel on each side, I will cut the big plywood sheet to the height and then a little wider that double the width of the side pieces (cabinet or bookcase), cut the dados straight across, then use my table saw to split the two sides and trim them to the final width. But I'm a fuss budget about such things.

 

Tom,

Great stuff! Thanks for taking the time to write it out and illustrate it. I'm just may build one of your Exact Width Dado Jigs -- it looks so very handy.

Hilary

 

I had my #2 son make me a special router bit (not hardened) with a sharp point. It has both 1/2 & 1/4" shanks, so fits in whatever collet or router that I'm using. When I need an exact center to edge dimension, I install this bit and with the router against the edge guide, I plunge a small dent into the wood. This is exactly where the center of the router bit will be when installed. Keep in mind that router bases aren't always perfectly round or concentric with the bit. To avoid problems here, I put a large arrow mark on the top of the router base with a felt tip marker, and always point this arrow in the same direction, so the same edge of the base will always be against the edge guide.

Charley

 

Do not take too deep cuts. My experience is debris "clogs" up in the track and causes your router to lose much speed. If possible, use vaccum to suck up debris while routing.

 

DeWalt makes an accessory dust collection attachment for the 618's plunge router base. This would be a nice accessory when cutting almost anything. Routers are notorious for creating and launching sawdust into the air. This would reduce almost all of that nasty sawdust. Compact Router Dust Collection Adapter for Plunge Base | DEWALT

This is a minor point, but I keep an extension cord hanging down from above my workbench so I can plug in but keep the power cable out of the way.

 

This is a minor point, but I keep an extension cord hanging down from above my workbench so I can plug in but keep the power cable out of the way.

My local Mens Shed also has this method of providing power to a few of its work tables...Very handy...