Kerf-cutting perfect dados with TS55 and MFT?

[rdesigns]

The key is to make a perfectly-sized spacer that's exactly the thickness of the kerf of the blade you will use. I'll tell you how to do it without measuring or trial and error.

Then I'll explain how to use it in making perfect dados.

I started with a piece of 3/4"  MDF about 5" wide and about 15" long. The sides of the 5" width must be perfectly parallel.

Using the intended blade, I cut TWO kerfs into one end of the piece; the cuts run about 5" into the piece and stop. Eyeball them them to be about equally spaced across the 5" width--what you are doing is subtracting from the material the width of two kerf thicknesses. The cuts must be parallel to each other and to the sides of the 5" width. You will have what looks like three thick "fingers" at the end of piece.

Now, crosscut the piece to separate the three fingers from the main piece. These 3, ganged tightly togther, will be exactly two kerf thicknesses less than the remaining  piece.

Next, choose a wide piece of  solid wood (I used a piece of 1 x 12 pine about 6" long) that has been squared on one end, and whose sides are parallel. From this piece you will cut the thin spacer whose thickness will be exactly the width of the kerf.

Next, I set up the fence of the MFT to be 90 degrees to the guide rail, and placed the squared end of the 1 x 12 tightly against the fence, and under the guide rail with about 1/2" showing past the edge of the splinter guard.

Then, I placed the 3 fingers ganged tightly together against the other side of the 1 x 12 and brought the rail stop up tightly against those 3 fingers. Then, with everything tight against each other, and the 1 x 12 tight against the fence, I cut the 1/2" off the side.

Next, , I removed the 3 fingers and placed the 5"-wide original piece in that space where the 3 fingers were, pushed it and the 1 x 12 tightly against the rail stop. The 1 x 12 now protrudes exactly two kerf thicknesses past the splinter guard, one of which will remain after you make the cut. This thin piece will be equal to the kerf of your blade.

How to use it?

Say you have a cabinet side that needs a dado to accept a shelf. You place the cabinet side against the fence of the MFT with the splinter guard of the guide rail exactly on the line that will be the left side of the dado, and use your new kerf spacer to place against the other end of the cabinet side and bring the rail stop up tightly against it. Make the cut with the plunge depth set to the desired depth of the dado.

Now, remove the spacer and replace it with a piece of the shelf that needs the dado in between the rail stop and the end of the cabinet side. This will position the cabinet side further past the cut line of the splinter guard--this position will put the far side of the saw's cut line exactly on the the right side of the dado, and the width of the dado will match exactly the thickness of the shelf material.

At this point, all that is needed is to chew out the waste between the two exact cuts, using repeated kerf cuts.

 

[Ken Nagrod]

Ok.  I'm guessing this is a viable alternative for someone who doesn't own a router or a dado stack on a table saw? 

 

[rdesigns]

Ok.  I'm guessing this is a viable alternative for someone who doesn't own a router or a dado stack on a table saw? 

Right--and it's a way to match dado size to material that doesn't match standard dado stacks.

Also, I have done the same procedure with my table saw. In that case I use the spacer against the fence and then remove it before placing a short piece of shelf offcut between the fence and the workpiece to achieve a perfect dado without the need to set up the dado stack.

It's a time-saving and accurate technique for those times when you need just one or two dados, or if you're on a jobsite without all the equipment you'd have in the shop.

 

[VictorL]

Why not to use pencil marks for single dado? Cut one side, then rotate rail and cut second side, then remove everything in between with multiple cuts? Regular ATB blades a really bad choice for dadoes.

VictorL

 

[rdesigns]

Why not to use pencil marks for single dado? Cut one side, then rotate rail and cut second side, then remove everything in between with multiple cuts? Regular ATB blades a really bad choice for dadoes.

VictorL

Once you have the spacer, (I keep my spacer in the Systainer that holds my TS55) it's quick and more precise than relying on pencil marks, or the precision (?) of the splinter guard, or, in my case, struggling with less-than-perfect close-up vision.

An ATB blade will yield a flat-bottom dado if you make repeated passes, closely spaced.

BTW, the basic method of using a spacer equal to the kerf of the cutter is applicable to dado blades and router bits as well. For example, you can use a 1/4" router bit to plough out a dado of any width greater than 1/4" if you use a 1/4" spacer to stand the workpiece off the fence before using the shelf offcut as the final spacer. Undersized plywood is no problem with this method.

Spacers used this way are simply a means of subtracting the width of the kerf from the thickness of the shelf material.

If you're using a tablesaw, there's an even more direct method if the shelf material is at least 1/2" thick:  you take a short (about 3/4") offcut of the shelf material and cut in in half using the intended blade. The two remaining halves can be used to make a perfect spacer that is placed after you make the cut for the first side of the dado; place it between the fence and the workpiece; the result will be that the workpiece is repostioned exactly the distance needed.

.

 

[VictorL]

If you use pencil marks you don't need subtract kerf width. Place shelf then mark both sides. Place guide rail, cut bottom side, then ROTATE rail and cut upper side. All your cuts go inward, into the dado. Rail's splinter guard works as cutting guide.

VictorL 

 

[rdesigns]

If you use pencil marks you don't need subtract kerf width. Place shelf then mark both sides. Place guide rail, cut bottom side, then ROTATE rail and cut upper side. All your cuts go inward, into the dado. Rail's splinter guard works as cutting guide.

VictorL 

I got that the first time.

For me, the problem with pencil marks is that they require a certain degree of interpretation, both in the initial marking, and in cutting to them. Using spacers, guides, templates and jigs are all ways of avoiding such interpretations. If you like pencil marks, and are successful with them, which I have no doubt you are--otherwise you wouldn't be recommending them--then have at it.