How Not to Cut Parallel Dadoes

Paper Sorter CompletedThe Project: Paper sorter for my sister-in-law’s 1st grade classroom.

The Challenge: Lining up a series of dado cuts for shelves on four different pieces.

She wanted to replace her cardboard sorter that was falling apart. Why not just buy a new one? I was able to build two sorters out of MDF, hardboard, and some fir (relatively inexpensive materials that would last a great deal longer) for roughly half the cost of buying one cardboard sorter.

Paper Sorter AssemblyI used 5/8″ MDF for the sides, and 3/4″ MDF for the inner partitions (so I had enough room for dadoes on both sides), then used 1/4″ hardboard for the shelves.  Since the shelves only had to hold paper, I didn’t need any kind of support other than the 1/4″ dadoes in the sides and partitions.  However, they all needed to line up well enough so the final product was square and the shelves didn’t run askew.

My initial idea was to build a jig similar to those I’ve seen in magazines, two parallel sticks placed wide enough for the router to slide along them with two other sticks affixed at right angles on the ends attached from below so My Dado Router Jigthey sit level with the surface receiving the dado.  Rigs like this make accurately repeating cuts easy. Theoretically it’s a simple jig to build, only four parts and minimal measuring.

Jig in place for test cutsI had given myself a firm deadline of one weekend to get all of the work completed.  While attempting to keep this deadline, I rushed to cobble together this theoretically simple jig.  In short, it didn’t turn out square.  Luckily, (because I begrudingly learned some years ago: Do Test Cuts!) I found out before it caused any real problems.

After running a series of cuts on both sides of some scrap MDF, I figured out something was wrong.  The cuts lined up perfectly First round of test cuts.on one edge and were anywhere from 1/16″ to 1/8″ off on the opposite edge.  I went through a mental checklist of all that could have happened and finally, even though my pride didn’t want to let me do it, I checked the jig with a square.  Yup, that was the problem.

I tore it down, used a square to line up the parts, reassembled it, and ran some more test cuts.  Again they were lined up perfectly along one edge and off on the opposite.  Only this time they were closer to 1/16″ or less.  I checked the jig again and it seemed to measure 90 degrees in all corners, but the results of my test cuts proved something was still wrong. I’ll admit it could have been wishful measuring (akin to wishful thinking and can cause just as many problems).

It could have been any number of things:

  • Some of the screws followed their original holes enough to pull it undetectably off kilter.
  • Too much play between the rails for the router to run straight.
  • The pine I used for the rails wasn’t as straight as I thought.
  • The edges of all the parts weren’t square enough for the jig to work properly

I was frustrated by this point because I had used up lots of my precious time building and rebuilding this simple jig and still wasn’t close enough with my cuts to actually use it.  I scrapped the whole thing and came up with “Plan-B” … The table saw fence in the shop where I was working could go out the length I needed in order to cut all the dadoes.  I could make each cut on all four pieces before adjusting the fence for the next, ensuring that each cut lined up.

I have seen people use this type of router jig to cut repeated dadoes with great success. So, I know it’s not the basic concept of the jig. The problems I had could have been any item on my list or any combinations of those items.  It even could have been reasons I didn’t consider.  Most likely, though, I can chalk the whole thing up to rushing.  And that is what can be the root cause of everything on my list of possible problems.  I know now, when I’m making up deadlines, it is just as important to factor in time to properly build any jig I might need.

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2 Responses to How Not to Cut Parallel Dadoes

  1. Amanda says:

    I need one of these for my classroom, and I was disappointed by the “cheap” versions available for sale. My boyfriend and I are planning to use your advice when making ours from scratch this summer. Any other advice?

    • Sorry for the late response. I’ve been away from the blog for several weeks, and since you left this comment back in May my advice might be late.

      The first piece of advise I can give you is to come up with what I call a list of procedures, simply a list that lines out the order in which you will cut and assemble parts. It will help you think through the process of building and also help you anticipate potential problems.

      After that I suggest building the case portion of the sorter first then cutting the shelves to fit. That way if there are any “changes” in measurements, you are only cutting the shelves once instead of twice.

      Lastly, don’t be afraid to assemble and disassemble to make sure all of the parts fit together the way you want.

      Hope this helps. Thanks for reading!

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