Tuesday, June 8, 2010

Murphy Table

Recently I finished a fun project I'd like to share with everyone. I call it a "Murphy Table" - not because it follows Murphy's law, but because it can be folded away into a wall and is supported solely by a wall to which it is attached.
The design of this project was a great engineering and math challenge, and would be a great way for teachers to show students how principles of science can be applied in real life. I've been a mechanical engineer for 8 yrs. and this is the first time I actually got to use most of the stuff I learned in college. Also, trigonometry was a big part. Mechanical engineers may be familiar with the four-bar mechanism that is a simplification of the operation of this table.
So, what is it?
I’m glad you asked. It is a 3’ x 7’ table attached to a wall. It also has 2 other working positions, plus the stowed position, for a total of 4 positions.
How much did it cost?
The closet door was free basically, since I replaced it with a new one and was going to throw this one out anyway. The aluminum angles I used on the side were the most expensive. I purchased 2 6’ sections from lowes for about $23. However, they are optional, depending on how strong your table needs to be.
The pulleys were about $5 each. Rope was about $7. Then there was miscellaneous. The total cost came to about $60 plus 20 man-hours.
Would it be cheaper to buy one instead? Absolutely! But they don’t sell them. And, it wouldn’t be as much fun!
I’m generally pleased with how the table functions and looks. I was originally trying to get it to function as a “floating” mechanism. Where you would be able to manipulate the table into different positions, all with the same center of gravity. It would basically roll around inside the wire loops on the ends. That did not happen. The resulting table can be manipulated by one healthy adult however.
Can you fix it?
I went through extensive calculations to try and find just the right geometry so that the wire loops would be the perfect length for the table in any position. That did not happen either. Please download and check out my equation sheet and working notes and tell me what I did wrong! My hunch is that a small variation from theory to real-life lead to a major difference in the actual results. For instance, I decided to use a pulley on the top attach point. I did this because the wire rope does requires gradual bends so as to not kink. But my equations did not take this into account. I was therefore about 6” off from one position to the other. I made up for this by installing a turnbuckle into the loop. I like turnbuckles, so that was fun. It does add a minute to the adjustment time, but its not really a big deal.
I also had to add a loop into the wire, for the wire to attach to a hard point that would also take out some of the slack. I was always planning on having a hard attach point, but not for it to have to take out slack.
I think that if I had made my mathematical model more realistic to actual dimensions and geometry, it would have been much closer. The floating thing was kyboshed because of one end of the table being fixed to a wall. In order to get the c.g. to remain at the same height through the range of positions, all ends would have had to have floated.
How do I make one?

Look at the pictures, look at my working notes (I have step by step instructions in there if you can read my handwriting), and then make your own plan. Its not rocket science, but let me give you some advise to make your project even better than mine.
Don’t skip the primer. This is a table, it will take abuse. If you skip the primer, you’ll scratch off the paint in no time.  Better yet, use Alkyd (Oil-based) paint.  I wish I had. 
I originally designed my hardware to be able to support a 200 pound force in the middle of the table when it was fully extended. Not that I wanted to stand on the thing, but I have little kids in my house, so who knows! I’m not sure how much these door would support before they broke. They are hollow panel – so basically you have a 1” wide wood frame with laminated cardboard glued on either side of it. The frames are so narrow that I couldn’t drill a hole to insert the lag bolts in the ends. I had to insert another 1” x 8” strip of wood next to the frame and then drill into that. I used a ton of glue to hold those in place. I also put a c-channel of aluminum on the edges of each door panel. This helps spread out the point load from the lag bolts over the entire surface of the door (yes, the lag bolts are a tight fit and I cold-worked their holes).
Adding the angles to the door panels made these light weight doors much more robust, but it also added many hours and considerable cost to the project. I’d avoid that if you could. The inserts were a necessity though. If I had it to do over again, I’d probably just use butcher block wood, like the stuff you can buy through Ikea. Then you could eliminate paint, sand, inserts, liquid nails, and lots of time. It would also allow you to use the door as a bench.
I used the really strong 3/8” thick cable – which I regret now because it is too bulky for what I needed. I’d only recommend that gage if you were going to also use it with butcher block and/or as a bench.
Test your table out with a rope or something if you want to make sure your geometry will work.
Buy wire rope ends, or dip yours in plasti-dip to avoid injuries.
If you are installing your door next to a wall like I did, put all the hardware on first. You can’t remove those lag bolts in a tight space.

Bill of Materials

Stress Calculations

Rope length equations

Working notes
All pictures (.xps file)
Link to another Murphy table (countertop)

IF you liked this post, then please try and find an ad on my site and click on it.  Here should be one (no guarantees, i can't seem to figure out how to get adsense to makesense!) 

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