How do u cut matching dadoes in a tall cabinet’s sides, can’t use TS or router table
Hi. I am making two tall cabinets out of 3/4″ ply. The sides are 91 1/2″ long by 23″. There is a dado I need to cut 21 3/4″ down from the top, on the inside surface of both sides. I can’t use my table saw because the cabinet side is too long to be supported to the left of the blade (I don’t have any room in my shop to make a support table right now). And I don’t have a router table. All I have is a handheld router and a straight edge clamp.
How would you rout accurate (meaning both dadoes are exactly 21 3/4″ from the top edge) dadoes? Any tricks you can share to accomplish this?
In the past, whenever I try to align two pieces of whatever together being very very careful that both pieces are flush inevitably there is a slight mismatch despite me using a straight edge ruler, squares, knife lines, and the feel from my finger touch etc. to align tops and bottoms.
Clamp a registration block to your bench, push the sides against it, and clamp them down. Make a solid plywood or MDF panel that runs from the registration block to the router instead of using a free-floating guide. Small errors are usually an out of square guide. The full panel guide block should avoid the issue.
Make sure not to rotate the router as you go. If the base is slightly off center to the bit the rotation could undo your efforts.
My workbench can't handle two panels at 23" each. I'll have to do this on my garage floor with a straight edge clamp. See my photo. My only option. Just need to be real real careful lining the two up even.
The important thing to remember when doing something like this is that tiny errors can get multiplied if you are not careful to register from the same edge, a 1/32" error can become a problematic 1/16" real quick.
The simplest way to avoid this on a one time use jig is to rip a piece of ¾" MDF, plywood will work just make sure it is void free, 21¾" wide and 6" longer than your work piece is wide, then screw a 1¾" wide fence to the edge so that you have two ½" "hooks" to register against your work piece. If you have a top bearing pattern bit exactly the width of the needed dado you need this may be all you need. Cut one dado then flip the jig over and cut the second one. This insures that the dadoos are mirror images of each other and any minor discrepancies are not magnified.
If on the other hand the dado needs to be an odd size to get a perfect fit for undersized plywood. Take a second piece of the same MDF at least 2"wide and 6" longer than your work piece is wide and screw two 6" x ¾“ strips of ¼" plywood plywood to the ends. Now with both pieces lying on a flat surface squeeze a piece of the material that you will be placing in the dado between them and screw the strip into the edge of the larger part of the jig. You can now use any top bearing pattern bit smaller than the dado to clean out the waste and you will be left with a perfect fitting groove. Just remember to flip over the jig to cut the second piece.
In the event you don't have a top bearing pattern bit of appropriate dimensions you can use a combination of a router guide bushing and any straight bit smaller than the dado to achieve the same results, just adjust the dimensions of the jig appropriately.
Router with a guide. There are various dado jigs for routers but, a straight edge will do.
To work on your garage floor: get two speed squares, double-sided tape, and a straight edge scrap that is longer than 46" to accommodate the router base length. Use the tape to 'edge glue' the panels together so they become 'one' (clamps also work). Square up the straight edge using the two speed squares to line up the router bit to the dado cut line. Clamp in place or tape it to the panels. Double check the squareness. Plan on two passes with the first one being shallow and consider a little backcut on the outfeed side to avoid blowout. Hope that helps and I'm sure there are many more tips and tricks out there ..
I made a simple version of the jig GeeDub shows. More similar to this: https://www.finewoodworking.com/project-guides/shop-projects/how-to-build-a-dado-jig
There are plans for a million of these dado jigs out there. Every magazine shows a version every year or two.
There is absolutely no need to cut them both at once. Make a stop block to index the jig off the top of each side. They will line up perfectly.
I don’t understand why there would be a mismatch.
If you clamp them together, then scribe, you can line up a jig or straight edge accurately enough.
A foolproof method I’ve found is to route the dadoes before you rip the sides.
In a perfect world, yes, that seems to be feasible and would work perfectly. However it's not a perfect world, and slight imperfections in the long edges sometimes cause the gap between the two long boards to not stay closed when clamping them (and I was absolutely careful when ripping these long sides but the slight wobbliness in feeding the boards into the TS possibly caused this to happen). This is real world woodworking in action. I am abandoning the approach I showed in my photo because when I applied a clamp to them the gap between them opened up at the other non-clamped end.
Plus scribing them is only as good as how careful you are in scribing and I'm VERY careful to the point of being overly so. I use a knife and no pencils allowed too.
I suggest you start by correcting the crooked edges first. This can be done with a router and straight edge.
You can also clamp the sides together face to face, mark the dado location on the edges & scribe lines off of these marks, line up your guide and route.
Real woodworking involves adapting methods to what your capabilities are.
If you don't want to sit on the floor, make two saw horses and set the boards on top. If you don't want to make them, look around for chairs or something similar to set the boards on. Clamp the boards together as suggested above and use the fixture for your router. Woodworking is about solving challenges.
The problem with that approach is that the ply sags between the two saw horses or chairs, which introduces another error. Not enough of a solid flat support if you did it that way.
Clamp board on either end under the sides. This registers the wood. Move chairs closer to the cut area. Clamp another board under the cut area to the router guide. No sag. Route away.
I use a set of plastic sawhorses as my auxiliary work surface. I have two dedicated 2x4's with indoor/outdoor carpet stapled on one side.
I pull this mess out into the driveway when I bring home sheet goods and do the rough break down. I use it inside the shop as simply another place to work and spread out larger projects. I attach plywood pieces to the 2x4s with screws to really stabilize something, or as outrigger supports for larger face frames, etc.
Saves my bad knees and horrible back.
Now, onto the original topic.... I use a jig like GeeDubBee shows. It is an exact size dado jig and works perfectly with a templating bit of the proper length required.
I made mine out of some scrap Baltic birch ply, 1/4-20 bills and some shop made knobs. Works perfectly.
The first thing that occurred to me when you talked about having a tough time getting them aligned on both boards was to cut a spacer for MDF or something of the correct size to keep my dado jig perfectly placed.
Use a third saw horse.
JC2's suggestion that they can be done one at a time is correct. Use the same setup but mark the router where it makes contact when you rout the first one to take the rotation error out of play on the second one. Perhaps your benchtop is large enough for them one at a time.
I recently had a similar issue with making a series of dadoes in large dresser sides. I made this jig from FWW. I heartily recommend it. You can adjust the overall dimensions to fit your project.
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