plywood wiggle room

[tombo]

How tight should dados be in plywood? i am building a cabinet out of 18mm birch plywood and will fit the shelves into the sides with dados. I dont actually have an 18mm router bit but i do have a 17.5 plus the plywood is a shade under 18mm. Should i get an 18mm cutter or will i have super tight joints with the 17.5

tom

 

[superunknown]

I would do a test cut and see how it works out. You dont want it to be a sloppy joint and at the same time you don't want it too tight.

 

[Steve Maskery]

It really is a case of try it and see. If they are too tight you will find that the shelf forces open the dado and your side becomes bowed. Too sloppy and you will have a weak joint, of course.

You want them snug. But 17.5mm does sound rather tight for 18mm, I have to say.

One option would be to rout the 17.5mm slot, then turn your router base a bit and do it again. If your bit is anything but perfectly central to the base, you will be able to get a bit of clearance. Or you could stick a bit of masking tape on the router base side to give you a few extra thou.

HTH

Steve

 

[Sgian Dubh]

Where are all these Americanism's coming from here in the UK, along with American styles of woodworking? Dado's, full dado's too? The malign and pernicious influence of that nail gun wielding smiling hack Knock-it-out-Norm on the box must be to blame, ha, ha-- ha, ha, ha.

Another way around might be to create a housing using something convenient like a 6-- 12 mm bit (whatever you've got to hand) and a T square, then make a tongue on the end of the shelf to suit using the router and a side fence.

It's quite easy to do if you're using a router, and also stronger because of the extra returns in the joint along with the increased glue line length. Slainte.

 

[Barry Burgess]

Tom if it is the B&Q birch ply the 17.5 should do. I usually use a 1/2"(12.7mm) and run it through the router table twice with scrap and check the fit - must only need a hand fit no hammers

 

[mailee]

I don't know if this is any help but when I cut Housings in a board I use a jig I made which makes the job a piece of cake. I can't remember where I got the idea but it was off one of the forums, this or one of the others I go on so many! Take two strips of ply, hardwood or whatever is straight and rout slots in them. Glue and screw them to another straight batten at right angles on top. This latter will butt up against the side of the board. Drill holes in another straight piece to align with the slots and then glue a piece of thin MDF, ply to them both. Attach bolts and wingnuts through the holes and slots and then rout awaythe MDF, ply with your router bit combination running down each straight edge. Now slide the two pieces up to your 'shelf' and sandwich it between the MDF, Ply this is then set for the width of your housing! It is much easier to show than to explain but I am afraid I havent got a photo of it at the moment.

 

[mailee]

Here we go, I knew I had seen it somewhere. I just hope the link works. http://www.newwoodworker.com/autodadojigpln.html

 

[tombo]

thanks for the replies, barry how did you know yep tis B&Q ply, pretty good stuff too I have paid as much for lesser quality in the past and it is convenient to get it chopped up rather than wrestling with the heavy sheets back at home.

I guess i will try a couple of test cuts with the 17.5, but i am definitely going to make a jig of some type i don't have much luck with straight edges

tom

 

[Barry Burgess]

thanks for the replies, barry how did you know yep tis B&Q ply, pretty good stuff too I have paid as much for lesser quality in the past and it is convenient to get it chopped up rather than wrestling with the heavy sheets back at home.
tom

I
ts the best quality ply I have seen out of B&Q and at £32 a sheet. Its a good weight not like some of their other sheets but is not 18mm but about 17.5mm
I am making 3 workshop cupboards 1220X570X900 and have used the birch ply and made many rabbits and dados using my Triton router table with no problems but I always run them through a second time if the sheet is large.
With B&Q I always make them cut the sheet as the weight of a full sheet of birch ply or 18mm mdf is too heavy although my saw handles a sheet on the sliding table but I don't