The dado blade (or dado set) can be a particularly accurate tool when it is understood correctly. With a combination of spacers and shims, a dado (or groove) of very precise width and depth can be cut in a single pass. Unlike a router bit producing a groove, the size of the stock to fit that groove does not have to match the router bit, nor do you have to make multiple passes to get to the required width, or depth.
The tablesaw is also much more suitable for processing large amounts of stock, and long lengths.
The dado set I have been using recently is this one from Amana Tool (through Toolstoday.com) It is an 8″ dado set, with a 5/8″ bore. The bore accurately matches the arbor of my tablesaw, so I don’t have to try to juggle washers for each blade and spacer, and means the blade set will produce a more accurate and flat-bottomed groove. It has most recently also been upgraded to have the environmentally-friendly Electroblu coating, which help mitigate heat buildup during the cut (which has an adverse affect on accuracy – the more heat, the more the blade can warp. This then results in more runout and therefore a change to the effective width of the cut.)
Click here to see the blade at Toolstoday.com Don’t worry that the blade is not blue – the photo has not been updated yet to include their latest coating that is now standard on their blades.
It is a particularly nice set, with four 1/8″ spacers and one 1/16″ spacer (or chippers), and a set of shims of various widths. The outside blades (which must always be used for every cut) are dedicated left and right (as is normal for dado sets). The blades are not ATB – they are either bevelled left, or right only (depending on which blade it is), with every 6th tooth being flat ground. They each have two teeth missing – this allows a place for the chipper blade to rest, so that the carbide teeth can overlap without knocking into each other. The flat ground teeth result in square corners at the bottom of the groove. Many other dado sets leave a telltale triangle cutout, where the bevel cuts deeper than the chippers.
An interesting feature of the outside blades is a raised section on the inside, effectively increasing the thickness of the blade, and when not using any chippers or shims means the two outer blades can rest against each other, again without the carbide teeth impacting.
This particular dado set has only 2 tooth chipper blades. This may seem a disadvantage (the general principle is the more teeth a blade has, the finer the cut), but the chipper blades don’t impact the side of the cut, where smoothness really counts. Chippers only remove the material in the middle of the groove, so they only touch the bottom of the cut, and with a combination of the quality of the carbide, the angle of the grind and these chippers leave a very smooth finish. The other really important aspect is the chipper blades have to be exactly the same diameter as each other, and the outside blades. This leaves a flat-bottomed groove. If one is over (or under) sized, it leaves a step in the groove base.
As you stack the dado set, you stagger the individual blades so that the carbide teeth have no chance of pressing on each other. Other than risking damage to the teeth, if they are pressing on each other, they will distort the effective width of the dado stack. When using shims, you need to space them out between each of the blades used. It is a good idea to put the thinnest ones nearest the arbor washer & nut. That way it is easier to change the finest shims to fine tune the effective width of the stack. When measuring the width of the blade stack, you cannot simply add up all the width of the components. The amount of runout of the two outside blades (in particular) (and to a lesser degree, the runout of the chippers) needs to be added to the final width, as does the runout of the tablesaw itself.
This can be determined by trial and error. Set up the stack for a certain width of cut (adding together the kerf of the outside blades, the chippers and spacers used). Perform a cut, and measure the result. This will give the actual width of the cut, and the difference between the two is the runout of the stack and tablesaw combined.
Despite the fact I have a 10″ tablesaw, an 8″ dado blade set is more than enough. There are a number of reasons for this: Unlike a normal blade, the dado set is only ever for cutting a groove (and more often than not, no more than 1″ deep). There is therefore no need to incur the extra expense of a larger blade. A dado set is already much heavier than a single blade, and can really push the limits of the saw motor just to get it up to speed, let alone maintain that speed during a cut. A larger blade has significantly more bulk and mass, and can exceed your saw’s capacity to spin it up. The benefit of this set, is those 2 tooth chippers have a lot less bulk than other chippers, so this also helps deal with the overall bulk of a dado set. The other thing is that a dado blade really does not need anywhere near the depth of cut of a standard blade. It is for cutting grooves, not cutting workpieces in twain.
As far as how this blade performs, it is excellent – but you’ll have to see the associated video (which I am working on) to see the result. Needless to say, I had a precisely cut dado that absolutely matched the board I was inserting, so the friction fit was beautifully tight.
To get that precision, I took the typically supplied notes on the various combinations of blades, chippers and shims to achieve a standard set of widths, and threw them out the window. Instead, I came up with a comprehensive set of every single combination possible, in order of increasing width, so I can exactly choose what combination to start with to match the board I am inserting into the resulting groove. Instead of having about 24 combinations that were provided, my list is just over 3000 combinations. And because I have it in Excel, it is easy for me to add in the runout of the blades and tablesaw to end up with even more precision.
Out of interest, I have attached the full list below (and below the smapshot of part of the table)
The Dado set by Amana Tool, from Toolstoday.com is a worthy version, and well worth considering when looking for a dado set for your workshop. When the video comes out, you will see just how precise a dado set can be!