Dado stacks from Holbren

[pelligrini]

I'm looking to get myself a new dado stack. My old craftsman stack is getting dull, and it was never made to cut flat bottom dados.

I'm leaning toward getting the 6" CMT 20T or possibly the 8" Oshlun 42T. I like the higher tooth count of the Oshlun and the full body 6T chippers. I'm not sure if the CMT will produce flat bottom cuts though. From the detail images the outer blades have an ATB grind. I wonder if there are also FTG teeth on them.

I've never really had problems with my 8" craftsman on my 21829. I could put on a 3/4" stack with adding a narrow 5/8" washer to the OEM arbor washers.

[Knottscott]

Erik - Perfectly flat bottoms from a stacked dado set is a myth. Some are better than others, and the very best are really, really good, but nearly all the stacked dado sets I know of use beveled teeth on the outside cutters to help minimize tearout (Freud, Forrest, Infinity, Ridge Carbide, Systimatic, Oshlun, CMT, Amana, etc). In order to be of any benefit, the beveled teeth must protrude slightly above the flat teeth of the inside chippers. The protruding beveled teeth leave tiny grooves at the outside of the cut…a trait often called “bat ears”. The better sets tend to stagger some flat teeth in between the beveled teeth to minimize the depth of the bat ears, but they’re there. It’s minor enough that a lot of people don’t even realize it, but it’s not realistic when manufacturers or owners claim that these sets leave truly flat bottoms, when indeed they don’t. It’s likely that there are sets that don’t have any beveled teeth on the outside cutters and use all flat top ground teeth, which will leave a flat bottom, but those sets will also tend to exhibit more tearout in cross grain cuts than the better sets that use beveled teeth. The other option for truly flat bottoms is from a router bit, but there are other drawbacks to that method as well… there’s never a free lunch!

This graphic from “Woodhelp” illustrates the “bat ears” effect and the cause:




Here’s another pic from Forrest’s website that shows the correct orientation of the cutters…it also shows the bat ears left by the beveled teeth:


I'm sure that either the CMT or the Oshlun set would serve you well, but if you're going to go with an 8", I'd go with the Delta/DeWalt 7670 set over the Oshlun....Grizzly sells it for < $100, so it's a few bucks more, but is the best of the < $100 sets I know of, plus it's got a great carrying case and excellent shim stock with their thickness imprinted in them. The design is similar to the Forrest Dado King, which puts the mass closer to the center of the hub and as a result is actually easier to spin than the Oshlun, which has a lot more mass at the outside of it's diameter.

[pelligrini]

OK, thanks for the help. I thought some of the higher end sets used some FTG raker teeth to achieve the flat bottoms. My craftsman stack has really pointy ATB teeth on both cutters. If the others leave bat ears, my stack leaves 2 pair of jackrabbit ears.

I think I'll probably go with the CMT stack.

[Holbren]

Thanks Scott and Erik I just replied to your email.

Problem with setting the FTG raker at the same height as the ATB teeth is if the enter plywood first when crosscuttign, you'll tear it up. You need the point of the ATB to make the clean cut then the rakers will flatten it up.

That little bat wing can help capture glue squeeze out too.