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Calling for expertise: Kreg jigging a rabbeted shelf joint

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Forum topic by Douglas Bordner posted 05-23-2010 12:43 AM 7455 views 1 time favorited 18 replies Add to Favorites Watch
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Douglas Bordner

3971 posts in 2818 days


05-23-2010 12:43 AM

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Hi y’all. I am building some shop furniture and would like to like to install some shelves in a leg set I made with a Kreg K3. I could use a plywood bit to route full width across the grain for the dado, but I want to be sure and have a tight joint, so I was going to cut a half-lap as shown above. What length of screw do I use, and do I set the pocket holes on the edge of the shelf, then cut the lap, or do I cut the lap and use the jig on the assembled joint, clamped down on the shelf bottom and butted against the upright. I have 1.5, 2 and 2.5 inch coarse thread Kreg screws

-- "Bordnerizing" perfectly good lumber for over a decade.


18 replies so far

View CharlieM1958's profile

CharlieM1958

15822 posts in 2973 days


#1 posted 05-23-2010 01:18 AM

I feel like this is one of those word problems where one train leaves New York at 8:00 p.m. going 60 mph, while another train leaves Chicago….etc., etc.

Seriously Mr. Bordner, I know nothing about pocket screws. But my coonass intuition tells me you’d have a sturdier shelf if you dadoed that whole 3/4” thickness of plywood into your leg. Then if you wanted to add a few pocket screws to keep things tight, go right ahead.

But my opinion and $2 will buy you an order of beignets. :-)

-- Charlie M. "Woodworking - patience = firewood"

View ROY53's profile

ROY53

77 posts in 1933 days


#2 posted 05-23-2010 02:29 AM

I nthink you could do it either way successfully. If it were me I’d do a mock up and play with it a bit just to see what combo worked the best.

-- Roy L, Arizona

View pmayer's profile

pmayer

609 posts in 1820 days


#3 posted 05-23-2010 02:39 AM

I’m with Charlie. I would just dado 3/4”, glue it and be done with it. Take your time and get a nice tight fit, and when your done you will have a solid joint that will never budge. Running screws out the end of cheap plywood will not add strength, it will degrade it. Reducing the thickness of plywood that is supported in the dado will degrade support further. I get where you going, but I wouldn’t go there, sir… :)

-- PaulMayer, http://www.vernswoodgoods.com

View chrisl's profile

chrisl

17 posts in 2475 days


#4 posted 05-23-2010 02:43 AM

I’d use the short screws and put the jig on the setback of the lap.

But that’s. Just me.

-- Chris L from Beatrice, NE

View Lee A. Jesberger's profile

Lee A. Jesberger

6704 posts in 2734 days


#5 posted 05-23-2010 02:44 AM

Hi Douglas;

Looking at the drawing tells me you should give up woodworking, at take up drawing!!! LOL

It’s a beautiful drawing!

If the drawing is to scale, you have no threads holding the parts together.

Also, as Charlie points out, you would have more strength with a full dado, however that does make the sides weaker. As drawn, with the thickness of the shelf reduced by 50%, you are also reducing the strength of the board equally. What is possible is the shelf board would crack in time, right at the dotted line you drew.

If you were to rabbit the top edge of the shelf board, you would lose no strength, nor would you risk splitting the shelf board from weight placed on the shelf.

But, if it were me, I would buy metal shelves, and be done with it. (just kidding).

Since I don’t use pocket screws, I don’t know if you can alter the angle of the screws, but I would be inclined, (get that little play on words…inclined, angle), to make the angle greater.

Hope this helps.

Lee

-- by Lee A. Jesberger http://www.prowoodworkingtips.com http://www.ezee-feed.com

View patron's profile

patron

13187 posts in 2096 days


#6 posted 05-23-2010 02:51 AM

sorry doug ,
can’t help .
i bought a pocket clamp on thing 12 years ago ,
the bit still has the rubber tip on it ,
i have never used it !

-- david - only thru kindness can this world be whole . If we don't succeed we run the risk of failure. Dan Quayle

View teenagewoodworker's profile

teenagewoodworker

2727 posts in 2523 days


#7 posted 05-23-2010 02:55 AM

remember a dado is for vertical strength. the pushing down force. it is the strength of the board you are dadoing into that gives it the strength. You would probably be just as well off with an 1/8” dado. then just screw from the outside into the end of the board and glue and you should be all set. using a pocket screw might not be the best option also since it has the drilling area at the end of the screw. you really won’t have many threads in the wood. you can also just toenail it with glue and it will be very strong as well.

View scottb's profile

scottb

3648 posts in 3082 days


#8 posted 05-23-2010 02:55 AM

I haven’t used mine a lot, but am always impressed with how tightly it pulls wood together, and how long a screw you can use without it poking through. The length of the pocket screws is determined by the thicknesses of the wood used. The jig should have a guide that came with it. I think the 1.5” screws are for 1” thick material, so you’re probably OK, but if you’re gonna use it with 1x or ply, your gonna need some shorter screws.

I’m currently using a couple of them in lieu of clamps (never enough of the right size) for a panel glue up, on the sides that will be cut short. works like a charm. If i could hide them better, I’d have put a few in the middle of the panel – a big box lid, and wouldn’t ever worry about that glue line breaking!

this might help:
http://www.kregtool.com/products/ka/ScrewChart.pdf

-- I am always doing what I cannot do yet, in order to learn how to do it. - Van Gogh -- http://blanchardcreative.etsy.com -- http://snbcreative.wordpress.com/

View AaronK's profile

AaronK

1399 posts in 2219 days


#9 posted 05-23-2010 03:07 AM

i agree – seems like if you want to rabbet, do the top edge, then pocket screw the bottom.

View Gene Howe's profile

Gene Howe

6058 posts in 2183 days


#10 posted 05-23-2010 03:21 AM

Your dado arrangement offers plenty of glue surface. Screws probably would add very little strength to the joint.
I wouldn’t use pocket screws in that type of joint, at any rate.

-- Gene 'The true soldier fights not because he hates what is in front of him, but because he loves what is behind him.' G. K. Chesterton

View GregD's profile

GregD

637 posts in 1891 days


#11 posted 05-23-2010 03:37 AM

I would agree with teenagewoodworker’s point that the dado does not need to be very deep to have significant strength holding up the shelf, although I would still be inclined to make it deeper than 1/8”, particularly given that the side is 1-1/8” thick.

On the other hand, I see no particular advantage to making the dado narrow, and no problem making the dado the full width of the plywood. If you want to cut the dado with a router bit, and aren’t confident that you have a bit that will give you a tight fit, I’d suggest using the bit that will give you the widest dado that is narrower than the thickness of the plywood and cut the rabbit to fit the dado.

The resulting rabbit/dado joint would, I expect, be strong enough without the pocket screws.

Also, the idea from Charlie and Lee to put the rabbit on top makes sense, and If you still wanted to use pocket screws that would give you a bigger target in the side piece to hit. I would plan the pocket hole so that it exits the plywood at the corner of the rabbit.

-- Greg D.

View CharlieM1958's profile

CharlieM1958

15822 posts in 2973 days


#12 posted 05-23-2010 03:35 PM

Greg, don’t give me credit for Lee’s idea. lol!

Actually, teenagewoodworker made a good catch when he said the dado didn’t need to be that deep. I also agree with him about screwing from the outside of the board into the shelf. If I were doing this, I’d use that as an opportunity for some decorative plugs or caps.

-- Charlie M. "Woodworking - patience = firewood"

View Douglas Bordner's profile

Douglas Bordner

3971 posts in 2818 days


#13 posted 05-23-2010 06:25 PM

Guess I will try to sneak up on the full width joint…
Lee — I gotta tell you cleaning up the pixel dust after drawing is way easier than cleaning up the shop (which is just a veiled way to say my desk is slightly less messy than the garage).

-- "Bordnerizing" perfectly good lumber for over a decade.

View a1Jim's profile

a1Jim

112943 posts in 2332 days


#14 posted 05-23-2010 06:38 PM

I would forget the screws and have a back on the case rabbited the top and bottom and dadoed middle shelf and make the rest pf the shelves adjustable.

-- http://artisticwoodstudio.com Custom furniture

View Michael Murphy's profile

Michael Murphy

448 posts in 1760 days


#15 posted 05-23-2010 08:39 PM

Don’t make the dado into the uprights so deep. Make it about half the depth you show so the screws actually tighten the joint. The way it’s drawn, the screws won’t do much of anything. Or flip the shelf over and put the screws into the bottom.

-- Michael Murphy, Woodland, CA.

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