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OK - How;d I mess up?

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  • OK - How;d I mess up?

    Picture a standard book case; 38" high, 20" deep and 48" wide. Material is 3/4" maple venered plywood - actual thickness measured at 21/32".

    My calculations on the shelf widths were as follow:

    One dado on the center line (width) means that the shelf widths ought to be:

    21/32 = .65625
    1/2 the center shelf is .328125
    .65625 + .328125 = .984375
    24" - .984375 = .23015625
    now add 1/2" of dado depth (1/4" both sides) and we get 23.515625

    I cut the shelves to 23.5" and the whole unit when assembled was 1/4" too long. The joints look tight. I've verified shelf cut width and dado depth (set with a gauge)

    So where did I go wrong?

    TIA for your help, Curt

  • #2
    Curt, I'm afraid I'm completely baffled as to what you refer?

    If your bookcase is intended to be 20" deep, the shelf width just about has to be 20 inches or less. If it is 48" wide, the shelf length should want to be a bit less than 48". I'm obviously missing something.

    If I may make a somewhat unrelated suggestion, regarding this:
    21/32 = .65625
    1/2 the center shelf is .328125
    .65625 + .328125 = .984375
    24" - .984375 = .23015625
    now add 1/2" of dado depth (1/4" both sides) and we get 23.515625

    I would like to suggest ditching the decimal equivalency table, thus:

    1/2 of that is 21/64
    Add them together is 63/64
    24" - 63/64 is 23 1/64"
    Add 1/2" is 23 33/64"

    This really is the beauty of the "Standard" system. I did all this in my head. Six digit decimals I can't carry in my head, and I certainly don't have a scale that measures in millionths of an inch.



    • #3
      I agree with Dave, ditch the decimals. And by
      all means stay away from the metric system, it
      was invented by the French!
      Just my 2ยข.
      Eastchester, NY
      "I would rather have a German division in front
      of me than a French one behind me." General George S. Patton


      • #4
        Sorry about the lack of clarity. Shelf unit is 48" across with a verticl at the centerline. I'm trying to figure ehat shelf width would be with a 1/4" dado. When glued up, the unit was 48 1/4" wide. Each shelf was 1/16" off ?? I cut the shelves to 23 1/2". My calcs are obviously wrong. I just need to know where I went wrong.

        Thanks, Curt


        • #5
          Divider in the middle? That's what I was missing.

          You are using 1/4" deep dadoes on 21/32" sides:
          21/32 - 1/4 (which is 8/32) is 13/32" remaining on each side. Doubling this, to account for two sides, is 13/16".

          In the divider, you have two 1/4" deep dadoes for 1/2" total. The remaining stock thickness is therefore 21/32 - 1/2 (which is 16/32), 5/32".

          5/32" plus 13/16" (which is 26/32) is 31/32", this is the total amount to be substracted from shelves.

          Overall you want 48", so 48" minus 31/32" is 47 1/32". Halve that to get two shelves,

          Half 47 is 23 1/2. Plus half of 1/32", which is 1/64". That is, 23 33/64" should be the shelf length.

          The exact same answer you got, except I used fractions. My guess is you are a victim of stacking tolerances. You were off by a quarter inch, where could that have come from?

          Suppose that, instead of 21/32", the plywood was actually 43/64". Don't be fooled by the router bit manufacturers, in my experience plywood is definitely not an exact 32nd measurement. Then, if your dadoes were a 64th shallow, 15/64" deep instead of 1/4", the variance in the width of the bookcase would be 1/4".

          Another way it could happen would be if the opening were not exactly 48". What was the measuring technique, tape measure. If the hook got stuck in the outside measure position, that's automatically a 16th (if memory serves). Add in that they are notoriously inaccurate, and it could be a significant factor.

          In short, my guess is that if you remeasure everything, the dimensions are not precisely as listed.



          • #6
            I would consider your problem more an issue of order of operations. I would have built the top, bottom, middle and sides first. Dry fit and then measure for the shelves. You could then allow for any minor variance in material thickness or cutting. Sure, you can design by math, but unless you have extremely exact manufacturing methods your math may not hold up. I know. I have built some pretty expensive firewood ;o)

            Best regards,



            • #7
              Dave & Henry,

              I think you guys got it right Dave, I checked the router bit depth with a Rockler gauge and the wood thickness with a Rockler gauge - both are analog and I could easily have been off 1/64" here or there - not sure I can see a 64th even with my glasses on.

              The 48" came from the sheet stock itself - checked with two tapes and hooks are ok. I'm also wondering if the gorilla glue could have added a 64th? I used a 21/32 router bit for the dados and had to use clamps to drive the shelves home.

              Regardless, Henry is correct: I should have done a dry fit. Expecting wood to not vary dimension over a 3 week period is probably wishful non-thinking.

              One more lesson learned - the hard way as usual. Oh well, at least I not a total ditz at math. BTW, my reason for using decimals is that my calculators don't do fractions. That, plus after 30 some odd years of programming, I just tend to do decimals. You can always tell who's messed with computer math by telling the following joke:

              2 + 2 = 5 For very large values of 2 and very small values of 5

              Thanks for the help guys! Curt


              • #8
                Perhaps try getting one of the ProjectCalc calculators that are set up to digest feet, inches, fractions, etc.

                Me, I use a *real* calculator HP32SII that does basic unit conversion, fractions, *and* RPN!




                • #9

                  I sympathize with your frustrations. Though I never use tapes for woodworking, (tapes are for carps and within 1/8" is good enough) I use to use aluminum rules and suffered some experiences same as you. I solved my problems by spending 35 bucks and ordered a set of stainless steel cabinetmakers rules. I use nothing else now. Here's the link:

                  John E. Adams<br /><a href=\"\" target=\"_blank\"></a>


                  • #10

                    I've HP RPN calculators most of my adult life. The current financial one doesn't do RPN (all my previous one's did), although my programmer's model does. I hate pressing enter and then remembering there is no stack.

                    All I need is another calculator for which I have no space and whose location I will not remember.

                    Later, Curt


                    • #11

                      Thanks for the link. I think I will buy myself a set of rules.

                      Yes, you are correct. I'm coming out of the "pound to fit and paint to match" crew.

                      Later, Curt


                      • #12
                        I could easily have been off 1/64" here or there - not sure I can see a 64th even with my glasses on
                        For a recent (large ) birthday I got a digital caliper, since I was having trouble seeing that fine, and have learned that the difference between a too tight and too loose tenon can be 1/128 inch. But, like Dave, I prefer fractions. So I posted a large-print conversion table in the shop, and have become very familiar with 1/128 being close to .008 (actually .0078125, but I am willing to work with rough measurements ). That $80 gift is proving wonderful - I recommend it.


                        • #13
                          I've spent the last 30+ years primarily working in metric decimal units. Since WW'ing came upon me last September have had to go back to fractions...not easy after that period of time. Bought a dial caliper, lasted a month converting, then found the fractional caliper at Woodcraft, and on sale for $25...this is the way to go. My joints are still too loose or too tight, but at least I now know by exactly how much in a fractional measurement!




                          • #14
                            thanks john at woodys workshop for the tip on the switch for my band saw, it's working great!