How to cut rabett in back of carcass

[paulhtremblay]

This question should be easy to answer since it involves a very standard procedure: how do you cut a rabbet in the back of a carcass?

I am making my cabinets out of 19mm MDF. For extra strength, I want the back to also be 19mm. The sides should have no seams. That means the back has to be completely hidden.

First, my current method and why it doesn't work. I cut all my parts to my cabinet to width and length. The width for each piece is 610 mm exactly. I use the edge guide on my OF 1400 router to create a rabbet 19mm wide and 10 mm deep. All my measurements are exact to .1 mm.

I use a Jessem dowel jig for the joints. Jessem creates very accurate joints. However, when I drilled the holes for my first joint, I noticed that the front edge of the cabinet was off maybe by as much as .1 mm. Perhaps I didn't align the Jessem properly. I can easily sand the front edges smooth.

But the rabbet in the back no longer aligns because of the slight mis-alignment on the front. I don't care about slight imperfections on the back, but the glue joint will be a weaker, and you might be able to see a gap in the inside of the cabinet.

I should bring up one more factor. I used the parallel guides to rip my stock to width, and some of the widths on end came out 1 mm wider than the other. I fixed the mistake by ripping one piece exactly 618 mm wide, placing the other pieces on top, and using a flush bit to match that exactly.

This long introduction is to say that I am going about this problem the wrong way. I am trying to get the pieces too exact before I assemble them. I think I should assemble the carcass, and then use a smaller router and a rabbet bit to cut the rabbet? Or, if I choose to do a dado, I should make the dado somewhat loose fitting?

 

[mouppe]

Are you asking how to fix your current problem or how to do it in future?

To fix your current problem you could use a bullnose plane or chisel to remove the step and level the rabbet.

Normally I cut my rabbets in the carcase first, assemble it and then cut the back to fit the case precisely. This allows me to fine-tune the back if the carcase is slightly out-of-square etc. It's virtually impossible to get your sides glued up to within 0.1mm which seems to be your target.

I don't like loose-fitting backs as a good back will give strength and rigidity to the whole carcase. You can use a sliding bevel to set the exact cutting angle for each side if you are not square.

 

[tjbnwi]

I use a groove/dado. 1/4" inset for scribe, 1/2" cabinet back.

Tom