What does an engineer do, really?

If I had a nickel for every time I was asked this, I could buy a double stack at Wendy’s.  I could describe what engineers do, but that would be boring, and this blog is boring enough.  So I thought “hey let’s use an example”.  Examples are great. just about any time I have a problem, I look into a book for an example that’s close, and already thought out, and use it.  So here’s the problem we have.  I want to design a table for the Rockwell drill press.  Now I could just get some big honking steel beams, and slap it up, but steel ain't cheap, and it is heavy.  Once we have a problem to solve, we try to bound the problem with requirements.  I made the following requirements:

1)      Must be fastened together (no welder, and easier to fix/change bolted parts)

      2)      Must hold drill plus 500 lbs (1000 lbs total) with less than .025 deflection.

      3)      Max material length 12ft per piece

      4)      Max material weight 150 lbs per piece

      5)      Must have a hole pattern  same as the pattern on the bottom of the drill (so drill can bolt on)

      6)      Finish height of table = to other drill finish height (makes the entire space more usable)

      7)      Table top, and must be stable with a heavy load.

Find lowest cost/ best performance material.  Who’s excited?!??!?!

The first thing is to determine how long and wide to make the drill table.  The drill is about 50” tall, and the table is about 20x25.  The drill head sticks out an additional 8 inches, so when looking from the top, the drill takes up about 34 inches, call it 3 ft, and I’d like some room from the wall, so let’s say 40” depth of the table, for easy of manufacturer (and because I have the room) 40 wide.   The drilling surface is about 4 inches thick, so the table needs to be 25 inches tall to have the drilling surfaces of the 2 tables at the same height.  So the table will be 40x40x25.  Now the drill has a hole pattern on the bottom of the drilling surface that is 17x23 and uses ½-13 threads.  So to use the drill pattern to secure the drill to the table we will need supports that mate to those holes.  This means an extra support in the center of the table.  Well look how much we have done , and we don’t even know what it will be made of!

Onto material!  I settled on square tubing because it has superior strength, doesn’t have any crevices where swarf can build up,  and it looks nice :(and it wasn't in the requirements so that means i can choose what i want).  Now that we know what shape we want to use, we look to see which we will use.  So we know the load, and the span, and the material, and the shape, and we can calculate the deflection based on thiswhere E is the modulus of elasticity and I is the moment of inertia.

All that changes is the moment of inertia depending on the thickness, and size of the tubing.   Now we can chart the deflection of different materials versus their cost per inch, and find the best candidate.

Luckily I made one charting several steel tubes, and a few extruded aluminum tubes.

  

So based on cost and deflection, it looks like the 3x3 11 .120 wall steel is a winner.  And that is in a nutshell what engineers do.  Sure I could have just made it out of ¼ wall 3x3 or fancy 80/20 aluminum, and if I was charging 250 an hour, I would have, but engineers figure out how to make something as quickly and cheaply as possible, and if you have no cash or are making a trillion of something, then having an engineer to find out the best for the least is a necessity.
I look at it as the designer makes the requirements (frames the box) and the engineer makes the part you need fit in the requirements box.  sure lines are blurred sometimes, today i was the designer and engineer,. but i have to wear a lot of hats.

Now to figure the cost of hardware…