What I Screwed Up This Week – October 20, 2010


I mentioned that the outfeed table should provide plenty of food for thought for this column, and this proved true. The first problem occurred while assembling the plywood frame for the table top.
 
The Problem:
 
As I drove the first pocket screw to assemble the frame, it went through the bottom of the hole, and split the end of the plywood apart.  I was puzzled – the screw had worked just fine when I did some test joints in pine.  Obviously the torque setting was too heavy for plywood, and had spilt the plys apart rather than stopping when the screw bottomed out..
 
The Solution:
 
I backed the screw out, glued and clamped the split plys back together again, and redrove the screw with a correct torque setting.
 
Lesson Learned:

Always make a test run of joinery techniques using scraps of the same material as your project.  This will ensure that everything works correctly, and you don’t get surprised on that first connection like I did.

The second problem was noticed the morning after I had attached the frame to the top.

The Problem:

The plywood top had developed a pronounced bow during the night.  This was despite the fact that the support frame had been screwed into place. 

I had tried to save some money by using a B-C exterior plywood. I reasoned that only the legs would be visible, an the frame would provide plenty of rigidity.  I was wrong – the plywood was not stable enough for this type of light support, and bowed 3/4″ in the middle.

The Solution:

There was only one thing to do – replace the plywood top.  Accordingly, I went back to the lumberyard and got a sheet of cabinet-grade maple.  This stayed obligingly flat and allowed the project to continue.

Lesson Learned:

Construction plywood is for just that – construction.  It rarely has the degree of flatness required for furniture.  In construction, a certain amount of warp is acceptable since the plywood sheets will be firmly nailed to a sturdy frame such as a stud wall or roof rafters.  In furniture, rather than being attached to a structure, plywood often IS the structure and flatness and stability are paramount.  A sheet of cabinet-grade maple was only $11 more.  Spend it.

That’s it for this episode.  Stay tuned for more goodies in the future.

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2 responses to “What I Screwed Up This Week – October 20, 2010

  1. So are you finished with the project now?

    yaakov….

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