dado depth

[pilotlight]

I was just trying to plan out my first real project - router table - and had a look at some of the old NYW router table plans. I was under the impression that typical depth for dadoes was half the thickness of the wood (3/8 in. deep for 3/4 in. wood). However, Norm's plans have 1/4 in. deep dadoes. Norm obviously knows a little bit more about this hobby than I do, is this a personal preference thing or what? I was under the impression that this was sort of a standard. Learning as we speak, I guess!!! Thanks for the help.

Jamie

[offthemark]

I'm sure you'll get more official advice from some of the others but my guideline was to limit dadoes to no more than 1/3 of the wood thickness to avoid weakening the wood/MDF/ply. I always do 1/4" cuts in 3/4" MDF and they are plenty deep.

[Ken Weaver]

What Mark said, although I have seen plans that call for a 3/8" dado in 3/4" stock.

[LarryG]

I have seen quite a few 3/8" dados in 3/4" stock, but I've seen a lot more that were 1/4". Also a few that were only 1/8". It doesn't take much: it would be an unusual circumstance where 1/4" was inadequate.

My workstation here at the office was built by a very fine and very experienced millwork shop; on it, the 3/4" members have 1/4" dados and the 5/4" members, 3/8".

On the half-the-thickness thing, you may be thinking of mortise and tenons, where tenons cuts to 1/2 the total thickness is a commonly-used rule of thumb. (But with a low-stress joint, 1/3 the total thickness is often used.)

[pilotlight]

Thanks everyone, like I said, I've just started learning.

Jamie

[JimD]

I think that the dados are mainly in the piece to help align things during glue-up. A deep dado also adds some glue area but it is in a cross grain situation and I don't think you need the additional area with modern glues. I limit dado depth to no more than 1/8 inch for typical cabinet construction. The shallow dado is very helpful to locate the parts and provides some added strength to horizontal pieces let into a vertical piece because the edge of the dado is in shear where it is strong. Shallow dados are easy to cut and they dosn't weaken the piece the dado is cut into (typically the vertical piece for me). I can't imagine a situation where I would use one 1/2 the thickness of the material (that's a lap joint for me).

Jim

[LCHIEN]

the compromises:

For example on a bookshelf:
Too deep and the sides will become weaker and flex or break when loaded or stressed
Too thin and the shelf holding power will be limited as the thin lip will either break off or the side wall will flex and the shelf will literally fall off due to no support.
Too thin and there's no room for dimensional tolerances. e.g. suppose you cut 1/8" deep dadoes and the board length was 1/16" short then one side will ride on long 1/16th inch

[crokett]

I've always cut 1/4" dadoes in 3/4" stock, be it MDF or ply. That depth has always been plenty strong for my projects. As Loring said - it give s me a little more margin for error than a 1/8" dado.

[don_hart]

I almost always cut mine 3/8" in 3/4" stock. When the piece is glued up correctly you do not have to worry about waekening of the piece that dado was cut in. But you will get a weaker overall joint with a shallower dado.

[ErikS]

I always did 1/2 the material with dados......until I ran some 3/8" deep in 3/4 & picked the sheet up on the ends - kkkkkrrrrrraaaaaaccckk.

Of course I should have known better AND it wasn't anything that a bit of glue & scrap wood wouldn't fix. Stuck with 1/3 removal ever since.

[JimD]

I wouldn't fault anybody for 1/4 deep dados in 3/4 material but I think the piece would be stronger if you have tight fitting 1/8 inch dados than deeper dados that do not fit tightly. I count on the joint fitting tightly so that the glue is really enough to hold the piece together - the dado adds margin but is probably unnecessary. If you can't ensure the joints will be tight, it is probably a good idea to make the dados deeper.

Jim

[ChrisD]

I've read that the rule of thumb is 1/8" for hardwoods and 1/4" for sheet goods. Of course, Loring makes a good point about tolerances. I also agree with Jim that the goal should really be to ensure that the fit is airtight.

[Jim Boyd]

quote:Originally posted by don_hart

I almost always cut mine 3/8" in 3/4" stock. When the piece is glued up correctly you do not have to worry about waekening of the piece that dado was cut in. But you will get a weaker overall joint with a shallower dado.

I guess we are in the minority on this.

[AAJIII]

Well, add one more to the minority. I always cut my dados 1/2 of the material. You end up with alot more glue surface and IMHO a much stronger joint

[drumpriest]

Until recently I have always done 1/2 matrial. Note that 3/4" plywood is really 23/32", so I cut 23/64" dados. I recently started to cut those at 5/16, because it's much easier to setup. [:I]

They key is to have a nice tight joint.