How to attach case back?

[Les]

Hello,

I am planning to build a shelf unit using 3/4" plywood on the sides. I have 1/4" hardboard / masonite for the back. Here's my question: is it better to simply screw the hardboard to the sides or should I cut a rabbet to drop the hardboard into? Is one approach better structurally? Is one approach easier?

Thanks in advance,
Les
[]

[Tom Miller]

I usually cut a rabbet so the back is not visible from the side -- it looks cleaner. There's structural advantage as well in that shearing forces (on the shelf unit) will be transferred to the backer board directly, rather than through mechanical fasteners which may give way.

Regards,
Tom

[JR]

Les,

This is really a question of style. Screwing the 1/4" panel to the back would work ok.

Putting a rabet in the frame, sized to fit the back panel would give you a very stiff assembly that would look great. If it's a nice shelf, this is the way to do it. If it's a quick and dirty project go with option a.

JR

[Tom Hintz]

I also would rabbet it and set the panel in. Rabbets are quick and easy and the job turns out looking way better and you get the anti-racking strength to boot. I usually glue the panel in and shoot some staples in also. (Just to tick off the anti-staple/brad crowd).

[RodKirby]

rabbet, rabbet, rabbet +glue +staples/brads

[monte]

rabbet and brads

[silverfox]

Rabbet, glue, brads or staples.
Mike

[Les]

Thanks for your feedback! Since the consensus is for rabbetting I will have a chance to try my new dado set. But first I'll need to set up a sacrificial fence. Any advice for the quickest/easiest way to do this? I have the BT3000 with the accessory kit and not much else...

[Les]

One other question, I wasn't planning to put on trim on the case so unless I do a stopped rabbet (probably beyond my current skill level), the rabbet will show either on the top or sides. Any ways to avoid this...?

[LCHIEN]

quote:Originally posted by Les

One other question, I wasn't planning to put on trim on the case so unless I do a stopped rabbet (probably beyond my current skill level), the rabbet will show either on the top or sides. Any ways to avoid this...?

Use a router and bearing-rabbet bit on the completed cabinet,
or use the dado blade before you cut the sides and top. THen you'll have to use a miter or (can't remember the name) where the two ends are both cross rabbeted halfway deep. Its easy to make. Only half of the end of the wood shows in this one, and you can rabbet the back and the rabbet won't show on either top or bottom.

[Les]

Another "devil in the details question": how far into the 3/4" plywood should the rabbet extend? I'm thinking 1/4" but that doesn't seem like much overlap with the edges...

So many questions, so little time for woodworking!

[don_hart]

I would make the rabbit 3/8" this hould give you more than enough area to clue and staple in.

[Les]

Thanks for the advice. I will have to make the rabbets with my BT3 since I don't have the correct router bit (and I am determined to finish this project this weekend!). I must admit I am confused by how the sacrificial fence works...

Am I supposed to cut out the arc of the dado so I can partially bury the dado in the fence? Or should the sacrificial fence snug up to the side of the dado stack but not touch it??

Sorry for so many questions...but I did make a lot of sawdust today!

[LCHIEN]

quote:Originally posted by Les

Thanks for the advice. I will have to make the rabbets with my BT3 since I don't have the correct router bit (and I am determined to finish this project this weekend!). I must admit I am confused by how the sacrificial fence works...

Am I supposed to cut out the arc of the dado so I can partially bury the dado in the fence? Or should the sacrificial fence snug up to the side of the dado stack but not touch it??

Sorry for so many questions...but I did make a lot of sawdust today!

For a sacrificial fence, set the dado a little bigger than the rabbet you want to make, then put the excess buried in the fence, you can run the fence right up to or even over the dado (which you would not do with your real rip fence). Even if there's a sliver of light between the fence and dado, you can always use a chisel to clean the sliver of wood right off, or just peel it off.

As for the depth of the rabbet, I would make them boththe same... make it 1/2 the thickness of the plywood which will be 1/2 of 23/32.

Easy to do - No measure method.
Cut off a 2 scraps of plywood, Set up the dado, cut as close to halfway as you can. Flip one over and butt up rabbet to rabbet. If its too deep, you can see light between the two edges. If its too shallow, they won't be able to overlap. Adjust until ist right.

Of course, I just measure with my digital calipers until its where I want it, thickness wise.

If you set the width of the rabbet to the same as the depth, then the
side and top will overlap perfectly, like this:


That is looking edge on at your sides. I did not show the rabbet for the back of the bookcase. If you rabbet the edge to the same width as you did the corner joints, then the rabbet won't show on the outside and it will have perfectly square corners for receiving the back.
So don't change the rip fence setting bewteen operations!
The depth obviously should be the same as the thickness of the back.

BTW, this is a pretty strong joint because it has lots of surface area for glue and the glue in in two directions. If you did it right the edge will be perfectly flush with the side, if not, you have a 50-50 chance of cleaning it up with a flush trim router bit!

The only bad thing about this joint is, if you're not careful, you can assemble it backwards of how you want it and then your cabinet will be shorter and wider than you want it (by 3/4"). Plan carefully and use a pencil to put X's on the glue surfaces, when you dry fit it.


HTH.

[Les]

Loring,

Thanks for the detailed description! Very helpful! I particularly like how rabbeting the case top and sides fixes the rabbets showing from the case back. However, I don't have any big clamps so I was planning to assemble the case using pocket hole screws on plain ol' butt joints. Is it a good idea to run screws through the type of joint you are describing?

And, one more really basic question...how do I make a sacrificial fence? I have seen pictures that show three pieces of ply cut to fit snugly over the rip fence. Does it need to be bolted on? Also, to make the cutout to by able to bury the dado blades, do I just turn on the saw and raise the blade into the sacrificial fence? Seems like I might risk chewing my real fence...

(Clearly not faking that I am a newbie...!)

Les