Why use a buried blade set on a Table Saw?

The blade you see buried in that picture is a dado set and is set up to do a rabbet along the edge of a piece. Among other reasons, burying the blade allows you to adjust the width of the rabbet. For example, you can adjust the fence so that the rabbet is only 3/16" instead of the full 1/4" of the dado set.

The wood attached to the fence is called a "sacrificial fence". When using a wider blade set than than the rabbet cut it allows you to run the wood up against the fence to make the cut. If you watch this video you will see how it is used. The use of the wood fence starts about 3 1/2 minutes into the video.

Purposes are rabbets and sometimes tenons.

Reasons are safety and precision.

Safety, because the blades stay buried under the wood being cut and the sacrificial fence. However, I still don't use my hands to move the wood or hold it down; I gots enough push pads and blocks and other accessories that I don't need my naked hands near even a covered blade.

Precision, because it lets you "fit" the rabbet. I often use rabbets on plywood edges for cabinets. Plywood ain't NEVER 3/4". So I set up a 3/4" dado and bury a 1/16 under the sacrificial fence. Make a test rabbet and try to fit another piece of plywood in it. The new piece will be proud (usually about 1/32"). I scooch the fence over a little less than that and recut the test rabbet. Try the fit again. Usually just barely proud. Scooch and test again, until I've "snuck up" on a perfectly flush fit.

Always fit, sneaking up on the exact right size. Never measure.

Thank You all for the replies. I appreciate it greatly. I still have a lot of learnin to do so I'll still be around asking questions to what seem like Common sense answers so please bare with me from time to time. What I can't find in a search, I'll be asking.

I was surprised I really couldn't find answers via Google. My Google-Fu is usually pretty good.

The principal reason for the sacrificial fence is to protect the normal fence from being cut into when doing rabbeting.

John McD,

Than you for asking the question. I thought I knew the answer but wasn't sure…

So with rabbeting and creating tenons, how do you use the measure since, the sacrificial fence increases the width and throws off ruler on the table or whatever you're using to measure with?

Would you make your first cut from the inner part of the tenon or rabbet? Do you setup some sort of stop on your miter?

I was just cutting some rabbets with a sacrificial fence and dado stack yesterday. You're right, the measurement of the table saw ruler may not be accurate under these circumstances.

For rabbets, I measure with a combo square and then make several test cuts to "sneak up" on a snug fit. As mentioned previously, especially with plywood, the width of the piece that will fit into your rabbet is usually not exactly 3/4" (for example.)

As for your second question, I'm not sure if I understand entirely, but I set up the dado stack/fence to cut the full width of the rabbet on one pass. I typically do tenons a little differently using the miter gauge. In that case I'll set up the fence as a stop and cut it in two passes, the first cut being the one closer to the end of the workpiece. [Edited for clarity]

I don't measure.

Take a rabbet as an example. I actually use a marking gauge, set on the board that's gonna fit in that rabbet, and mark the cut line on the board that's gonna be cut. Lay the board to be cut mark side down on the table, and BY EYE set the fence to intentionally give a cut that's a bit too small. From that cut, I "inch up" on a perfect fit.

In your terminology, that's the OUTER part of the tenon / rabbet. That is, after the first cut, the tenon is too short or the rabbet is too narrow.

When I first started "inching up", I occasionally would over-shoot - due to impatience to get to the right setting. With practice, I'm at the point where I can usually get the setup right-on in three cuts (and I usually do this setup on scrap).

Its a lot harder to describe than it is to do. In practice, its quite intuitive.

I've always had a question about this as well:

If you want a 3/4" rabbet, why not set up a 1/2" dado stack, make the first cut at the very edge of the board (so you have a rabbet slightly less than 1/2") then use the regular fence as a stop to sneak up on the 3/4"? This way you don't need to use a nice, long, straight piece for a sacrificial fence.

I've always had problems defining a 2 - 3 foot long by 4 inch piece of straight wood as "sacrificial".

Another benefit is that you can cut multiple rabbets with one set-up. Install the blade set for the largest one and you can adjust the fence without having to change throat plate of the number of chippers and spacers. With the varying thickness of plywood, domestic vs. import you can sneak up to the correct width with a steel ruler and test pieces.

By inner, I mean the point that the tenon would sit after inserted into the other piece. Would you start off with your cut at that point and then work your way out toward the end of the board? Or, would you start at the end of the board and work your way down toward the point you want the board to insert? I would assume, dangerous I know, that you would setup your initial cut using scrap to do the initial cut and work out toward the end of the board from there.

Thank You for that photo. It helps me understand it much better now. I was trying to come up with something to better explain what I was trying to say with the previous post but failed horribly.

Regarding measurements I NEVER use the ruler on the saw as the final word! To me it is only a suggestion that I am fairly close to the measurement I require. I ALWAYS do my measuring from the blade itself making sure I am doing it from the correct side of the blade for the cut I am making.

Dang, I think that is a pic of my saw and temporary fence. Regardless, that is what I do when when building cabinet carcases. I set up the dado set for the 3/4/" plywood I am using. Then I cut the dado into each side for the cabinet bottom and top. When I am through with that, I adjust the fence and cut a rabbit on the rear of each side for 1/2 inch plywood, which is the thickness of the backs I use.

BTW, I attach the sacrificial fence with the clamps sold by Rockler specifically designed for this purpose.
http://www.rockler.com/universal-fence-clamps
Hope this helps.
Mike

So how do you cut a rabbet in a wide panel?

Blade cuts are ok for narrow stock but are a bad way to rabbet larger stock.