JDC Creations

Building a Robot from Scratch – Part 3, Beginning Construction

Posted on August 8th, 2011 by jdc15 under Building a Robot | 1 Comment »

After finalizing my basic designs, I began construction.  After coming across many design flaws and issues with factors I never had thought of initially, I put together a mechanical frame of my robot.  Currently everything is in place except for a few sensors.  Nothing is plugged in or connected yet but I now have a plan.

Step 1 – Get More Supplies!

I think I’m beginning to say this a lot.  Every time I seem to do something I seem to have forgotten a key component as to what I needed to do.  As stated before I obtained some plastic and metal from local suppliers.  This was 1/8th inch thick Lexan plastic, otherwise known as engineering plastic or polycarbonate, and approximately 1mm thick aluminum scraps.  I’m not sure the exact alloy and thickness, but it was good enough for my purposes.  The one key component I was missing was glue!  I went to my local hardware store, Canadian Tire, and obtained some 5-minute epoxy to hold everything together.  As you will soon see, this was a big mistake!  As I soon learned, 5-minute epoxy is a lot weaker than other glues out there, and is not a good choice for bonding joints together.  Use screws instead.  Period.

Step 2 – Cut Out the Pieces

After obtaining glue, the first thing I had to do was cut out the pieces.  As Lexan is machinable, I was able to cut it with a combination of a table saw and a skill saw.  Since the aluminum was so thin, I was able to easily cut it with snips.  Later I learned that aluminum has a higher strength-to-weight ratio than Lexan.  Since aluminum is cheaper (per square inch), stronger, easier to work with, and lighter than an equivalent structural component made of Lexan, if I were to design this robot again, I would make it entirely out of aluminum.  The only downside to aluminum is perhaps difficulty in folding it and getting it perfectly flat.  Other than aesthetics, it outperformed Lexan in every way.  Back on topic, here are pictures of the cut out pieces:

The hands were bent into shape with pliers and the holes were drilled with, of course, a drill.  A 0.26″ bit was used as that’s how wide the little stubs will be.  The rods were cut painfully by hand with a hacksaw.  It took 3 hours.

Step 3 – Glue Stuff Together

There really isn’t much to say about this.  After cutting out the pieces I (attemted to) glue them together.  Here are some progress shots:

Now I’ll tell you about all of the problems I had.  In almost all cases, the glue failed miserably.  First off, when mixing epoxy, I didn’t mix it thoroughly enough when applying it the first time.  As a result, I had to spend hours painfully sanding it off.  As it was half cured, it wasn’t completely dry, so it didn’t peel off, and as it wasn’t completely uncured either, it didn’t wipe off.  Thus I had to sand down and scratch up my beautifully cut Lexan.

This was extremely frustrating as I not only had to spend 6 hours re-sanding and re-gluing everything, but I also had to tarnish my hard work.  The moral of this story is to glue it right the first time.  Another problem I ran into was the joins on the servos and arms.  They broke off several times.  To rectify this I sanded the surface down before re-gluing and upgraded to  a stronger glue.  I ended up with JB-Weld.  When tested on metal it proved far superior to epoxy.

I actually bent the aluminum into all sorts of contorted shapes before it tore off.  Unfortunately, the glue proved not much stronger than the epoxy on plastic.  However, it eventually worked out after several tries and I managed to assemble the robot.  Thus I’ve summarized all the things I learned about epoxy glue in this list:

  1. Don’t use it if at all possible.  Just kidding.  But seriously don’t use it on joints.  The bond is NEVER stronger than the underlying material.  Don’t listen to what other people say as it simply isn’t true.  Use screws instead. In my case I had already designed it with glue in mind.  Thus there’s no turning back for me, but for you, try implementing screws into your preliminary design.  Epoxy only works well when there is a large surface area over which to put it.
  2. Mix it for one minute before use.  Don’t skimp out on this part.  Actually time yourself mixing it.  If you don’t mix it properly, you will dearly regret scraping off half cured glue for 6 hours.
  3. Sand and clean the surface before use.  If you don’t sand it, the glue will just peel off.  This is especially true of plastics.  One thing to note is that touching the bond surface with your fingers leaves oils that will cause the glue to peel off.  Don’t touch the surface after cleaning it.
  4. Avoid 5-minute epoxy.  It’s less than half the strength of 30 minute or longer epoxy.  Also keep in mind the type of glue that you’re working with.  Plastic welder works better on plastics that epoxy.  JB-Weld works better on metal than regular epoxy.  Do some research first.  Google “Glue Wars” for an objective comparison between types of glue.  Not all epoxies are the same.

And that pretty much sums it up.

Step 4 – Final Construction Bits and Completion

Again there’s not much to say here.  I simply glued the rest of it together after many failed attempts.  I ended up using strong aluminum tape on the legs as glue alone wouldn’t hold.  I also glued on the circuitry.  Nothing’s soldered together yet and sensors are missing but the basic construction is done.  Next time I’ll write up details on the electronics.  Here are some assembly photos:

I didn’t mention this earlier, but I paid special attention to the servo positioning before putting it together.  I had to make sure that the servos would be able to move in the desired angle range before permanently gluing it.  For example, the foreleg must be able to rotate like an actual human foreleg.  The 180 degree span goes from a straight leg to one bent completely backward.  I had to align the servo to match this.  To help with this process, I programmed an Arduino and wired it up on a breadboard to have to placed to plug in a servo.  One spot moved it to the 0 degree position and the other moved it to the 180 degree position.  You can see the circuit in the third photo.  And finally, the finished result:

It stands!  Thankfully my calculations were correct and it was able to stand by itself without falling over.  Thank you for reading another part of my building a robot series!  If you have any questions please leave a comment below.


One Response to “Building a Robot from Scratch – Part 3, Beginning Construction”

  1. Lakshmikant says:

    Really interesting dude! keep it up… we have a lot to learn from you…

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