Flush 90 degree joints in plywood boxes

[DrPcFix]

I'm building some plywood cabinets and am having an issue when joining two pieces of 3/4" plywood at a 90 degree angle (such as the back to the sides or the top to the sides/back).

I use two Kreg right angle clamps to pull the boards together and as I clamp them up I make sure that they are perfectly flush.  However, as I drive in a Kreg 1-1/4 course screw, (into the hole between the two clamps) the board moves inward a small bit resulting in a joint which is not flush.  I've set the clamps as tight as I can before they start to damage the holes into which they are set.

The only way I've found to get a perfectly flush joint is to line up the board slightly proud of where it should end up so that when it moves inward it ends up flush.  But doing so is not the way (IMHO) that this should work.

I've resorted to using a pair of bar clamps to keep the boards flush but this negates the value of being able to quickly assemble cabinets using pocket screws -- I might as well just screw them from the outside with regular screws.  I've also had success shooting in a few 1-1/2" staples first and then putting in the screws but that also should not be required.

The material I am using is 3/4" two sided pre-finished maple which I obtain from a local high-end lumber shop.

[Keith Bryan]

Yeah that can be a problem at times.  Are you using glue?  Seems to make it worse.  Lot's of pro's & con's about using glue with pocket holes, edge grain, etc.  I usually make sure that I have two holes close together where I plan to clamp (each end, in the middle  -  depending on distances).  That way I can clamp in one hole and then screw close to it.

[Suzy]

I recently ordered these clamps from Rockler for my husband to help him with drawer making.  They don't work well for drawers but he said they would definitely work for large boards like plywood for cabinet making.  You need a minimum of two each of the items in the first two links below but I would recommend four of each item.

I ordered four of each and hubby said it was good I did because he said it would make assembly of a cabinet much easier with one for each corner.  It didn't work on the drawers because the materials are too short, and only 5" high so they did not stay square.  Also, the drawer boards weren't totally straight and that played a part in them not staying square even with the clamps.  He eventually works it out but I see him struggling with that each time he makes a drawer using the pre-cut and dadoed drawer boards.  He is happy to have the new corner clamps though. 

http://www.rockler.com/rockler-clamp-it-corner-clamping-jig?utm_sou...

Then you have this you have to order separate for the clamps above

http://www.rockler.com/clamp-it-assembly-square

Kreg has a corner clamp too, we don't own any but I thought I would toss that out there for you.  good luck on your project.

http://www.homedepot.com/p/Kreg-1-1-4-in-90-Corner-Clamp-KHC-90DCC/...

[DrPcFix]

I'm not using glue, because the UV coated plywood does not stick to any glue that I've tried.  I think that the movement occurs because the wood is so slippery due to the coating.  I will continue to use my method of using the Kreg clamps to get everything lined up and square and then using one or more bar clamps right next to the pocket hole to keep the joint extra tight and prevent the movement.

The 1/16" movement that I get in the construction of a cabinet really does not make the cabinet unusable or even noticeable once it's all put together, but it hurts my pride in wanting to build it "right".

That's why I have resorted to using epoxy coated staples (they are nearly impossible to pull out once put in) and also 1-1/2" or even 2" coarse wood screws put in from the outside (where they don't show).

I've been to professional cabinet making shops and they all use rabbits on the back and dadoes for tops and bottoms to build strong cabinets.  I was hoping that the Kreg system would allow me to build equally strong cabinets without having to make these advanced cuts.

Certainly having the most shallow rabbit on the sides would prevent the back from moving when screwed.  It would also allow me to use glue since the cutting of the rabbit would remove the slick finish from the mating surface.  The same will hold true for the tops, bottoms and any fixed shelves.

[Jay Boutwell]

 

 

Gentleman

I build butt jointed cabinets and have for years with out any problem.   I always use glue as I know from expereiced that glue will insure and provide the strongest joint possible in any type of construction.  To get away with out the slipping of the pieces due to glue is to appy a screw attaching and setting the position of the material and then remove and apply the glue and drive in the same screw first.  This sets the pieces in the position that you want them.   Kreg does provide the clamp, pictured above however it does the job of holding pieces it does not work as well as the method I descrived above.  When clamping you have the same issue with the material slipping while clamping.  In the pre-coated material the best method is to avoid using it and applying your own finish after completion of the the build.  For a high end finish use the sun light resistant catalized lacquer .  The only glue that I know of that stands a good change is using the one called :Roo: which is made for bonding surfaces like melamine and plastics.    You will also have to consider that the bond will be only as strong as the finish coat as that is what the glue will bond to.

 

When I first starte building cabinets for a living over a quater of a century ago I used to use the dado joinery but when I found the Kreg Jig that was its purpose, to speed up the process of building butt jointed joinery.  The irony of the thing was that back then the only screws available was the fine threaded screw and these were screwed into not only hard wood but soft wood as well.  Most of the cabinet building was using both plywood and melamine and so the fine thread screw was used.  I have many large builds made of both plywood and melamine that have been installed over a 1/4 century ago and are still in tact and providing excellent service to day.  The thing then was to use glue on every joint and perhaps that is why the cabinets stayed together and in service today.  Many of these were installed in apartment rentals where they had to withstand alot of abuse.  I think that most of this suscess is not only well build cabinets but also the use of glue in each joint.

 

Sometimes technology gets ahead of its self and practices quick but products flawed but the misuse of material and or building practices.  Now in knock down furniture, which is similiar to what you build if your building of a cabinet is a dry joint, expect failures.  Notice that in knock down furniture they provide a totally different type of hardware in the joints.  I saw here where kreg says that glue is not needed in a pocket hole joint, but I am here to tell you that experience tells me different.  The argument about glue in end grain against that of flat grain does not produce a strong joint.  True applying a single coat of glue on each surface does not provide a very strong joint.  The correct method of applying glue into a butt joint is to apply a coat on the end grain and allow it to soak in (5 or so minutes) then apply the second coat and then apply glue to the flat grain and clamp and drive the screw(s)  This will provide a much stronger joint than just a dry screwed joint.  

 

In my years with the trade doing it professionally, I have noted that most cabinet and furniture failure occurs in the joints.  This is why I am saying that glue is important in every joint.  In joinery including plywood you have moisture changes in wood.  What happens when you build a cabinet in a humid time of the year and its attachments in the joinery is only a screw.  The screw may be inserted and tight today but in time it may become loose because of the aging of the wood. Aging of wood with the absence of moisture does not swell up making the screw tighter but it shrinks instead making the screw loose.  It cracks and splits and becomes mush different that when it was built.  Plywood de-laminates due to the aging of the plies.  Glue on the end grains of the joint manages to hole the plies together at the joint where the screw in inserted.  In solid wood the most apt location for moisture to enter the wood is the end grains.  Glue covers the end grains and makes it much harder for moisture to enter.  It also will enhanse the strength of the wood end grains as it helps to hold them together.   i would think if a person wanted to build something he or she would want to add a glue bond to the screw strength as that would make the strongest possible joint in pocket hole joinery.

 

If you want to build something to take apart that is different but pocket hole joinery screw is not the right hardware to be using here and the repeated removal and insertion of a screw in wood will result in a weaker joint each time it is done.  One of two times might be alright but not repeated times.   Wood threads are much different than threads in metal as each time a wood screw is removed it weakens the thread in the wood and each time it is inserted the screw thread will cut into the wood until you have the results of a stripped screw.

A machined metal hole and bolt or screw of the same pitch will thread in and out many times without damage to the thread unless it is cross threaded and then it will begin to act like a wood screw cutting new threads and giving the same result of a weaker joint and or a stripped bolt or screw.

 

 

DrPcFix said:

I'm not using glue, because the UV coated plywood does not stick to any glue that I've tried.  I think that the movement occurs because the wood is so slippery due to the coating.  I will continue to use my method of using the Kreg clamps to get everything lined up and square and then using one or more bar clamps right next to the pocket hole to keep the joint extra tight and prevent the movement.

The 1/16" movement that I get in the construction of a cabinet really does not make the cabinet unusable or even noticeable once it's all put together, but it hurts my pride in wanting to build it "right".

That's why I have resorted to using epoxy coated staples (they are nearly impossible to pull out once put in) and also 1-1/2" or even 2" coarse wood screws put in from the outside (where they don't show).

I've been to professional cabinet making shops and they all use rabbits on the back and dadoes for tops and bottoms to build strong cabinets.  I was hoping that the Kreg system would allow me to build equally strong cabinets without having to make these advanced cuts.

Certainly having the most shallow rabbit on the sides would prevent the back from moving when screwed.  It would also allow me to use glue since the cutting of the rabbit would remove the slick finish from the mating surface.  The same will hold true for the tops, bottoms and any fixed shelves.

[KregRep]

The issue you are experiencing is due to the angle our screws enter the plywood at. It will naturally want to move the other piece. Feel free to contact Technical Support directly at 800-447-8638 or [e-mail] for further assistance troubleshooting. 

[Jay Boutwell]

Gentlemen,

Any time a screw or fastener goes through one piece of material it will push outward on the piece it is entering and unless it is held tightly together it will move and will move in the direction of the screw angle.  I prevent this by raising the piece that the pocket hole is drilled into about 1/16 of an inch.  Once the screw is tightened the two will be in alinement.  You can also clamp a straigh piece of material onto the piece that does not have a pocket hole drilled into it.  Clamp the piece the same depth as the thickness of the material that is being screwed to the mating surface.

 

This is what I am meaning about technology as it is trying to take the common sense out of woodworking with tools to sell that are supposed to make everything automatic.  Well sorry it does not always work that way and sometimes it takes patience.  That clamp that is pictured does the same thing as the screw that I mentioned about driving into the mating piece.  I have two of them and seldom use them as I do not need them.  I will be very interested in hearing what you get for an answer from the "Technical Support" people .