Well, let’s clarify the phrase “from scratch” to begin with. I prefer building stuff from the ground-up, if possible. Sometimes, that leads me to the Not-Invented-Here syndrome, but within limits, I believe building things from scratch has the most self-pedagogic value. This applies to software as well, but that is a rant for another time. In the meantime, let’s come to grips with building a robotic arm. “Come to grips”, get it? Sigh. In another life, I used to work on and build robots. A lot of that stuff led to COMRADE and IRIS.
Anyway, for this exercise, we will build a 3-DOF robotic arm from the ground up. That will mean that we’ll be sawing, drilling, soldering, and, perhaps most importantly, scavenging for materials. My aim is to introduce things from their basics. We’ll try to play most of it by the ear, and not really worry about precise calculations, apart from some very rough back-of-the-envelope calculations. The old maxim of hardware still stands: “Measure twice, cut once.”
Let’s flesh out some basic parameters for this robot, starting with the degrees of freedom. These will be:
- Rotation at the base about a vertical axis.
- Rotation of the “upper” arm about a horizontal axis.
- Rotation of the “forearm” about a horizontal axis.
The last operation will simply be the operation of the gripper, but we will worry about that later. The images below show some in-progress construction shots of the kind of thing we’ll be building.
Most of this post will be dedicated to the tools and materials you will need to get started with the mechanical construction of the arm. Some of them are optional and I note those cases.
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Always use the following items:
- Protective glasses: You’ll be generating a lot of “dust” when constructing the arm. Definitely get a pair of these.
- Gloves: Tools do not care whether they are drilling/cutting/sawing metal or flesh. A deformed workpiece can be replaced: a missing finger cannot. Take care.
- Mask: Not always needed, but useful when sawing. It can be as simple as a wet piece of cloth wrapped around your nose and mouth.
In addition, make sure that your electrical outlets have proper earthing. This will only become more important when we get to testing our electronic circuits.
- Hammer: The most common use of the hammer will be in conjunction with a drill punch to mark the center of holes you’ll drill in aluminium/plastic workpieces. Besides this, very handy for bending plate strips for mounting brackets.
- Punch: You’ll need this precisely mark the position of a hole you want to drill. The depression caused also prevents the drill bit from “walking” when you initially begin drilling.
- Drill/Driver: For drilling holes, obviously. I also use it to tighten screws by substituting a screwdriver bit for the usual drill bit. Keep a manual drill as backup for those occasions when there is a blackout before you can drill through that last millimetre.
- Drill bits: You’ll never need very thick drill bits, and HSS (High Speed Steel) bits will serve for drilling through plastic/aluminium. But you’ll need one or two carbide/cobalt bits for drilling through shafts.
- Hole saws: You’ll need these to build housings for ball bearings.
- Vice: You’ll need one bench vice, and probably one C-clamp. The bench vice is a necessity.
- Screwdrivers: The basic slotted and Philips-type screwdrivers will do fine. Depending upon what screws you’re using, a precision screwdriver may be a better option.
- Hacksaw: You’ll need this for sawing through plastic/aluminium. Also, make sure to get blades designed for sawing metal. Investing in a hacksaw with a more comfortable rubber grip will definitely make it easier.
- Pliers: For tightening nuts, and bending thinner pieces of work.
- Lubricant for drill bits: Helps extend the life of your drill bits. WD-40 works fine for this purpose.
- Ruler and markers: For general measurement and marking.
Optional Tools (Mechanical)
- Jigsaw: Handy for when you need to cut irregular shapes. Because it is electrically powered, all you need to do is guide it. Note though, that you need to clamp down your workpiece securely if you want to use this, with two C-clamps, for example.
- Reciprocating Saw: For when your arms are to lazy to use a hacksaw. Also electrically powered, you should not really need it.
- Aluminium sheets: You’ll need these mostly to build struts to build rigidity into your arm.
- HDPE sheets/rods/bars: A form of high-density plastic commonly used in making cutting boards in kitchens.
- Screws and nuts: For fastening things to other things.
- Gears and Chains: The primary form of power transmission across joints.
- Printer shafts: For mounting gears and other rotational elements.
- DC motors with gearboxes: The actuators.
- Ball bearings: For reducing friction.
The last few items in the list above are things which fall into the “scavenged” category. In Bangalore, there is a flea market on Sunday which sells all sorts of broken machinery, including printers, bike chains, and scavenged motors. You have a high chance of finding materials there. Similarly, aluminium sheets/HDPE bars are more or less easily available on S.P. Road in Bangalore.
There will be more things we’ll need later to build in feedback control, like potentiometers, or maybe even Hall Effect sensors, but this is all you need to get started. Any other things we’ll need in the later stages of construction, I’ll point those out when we come to them.
A Note on the Work Area
Make sure it is well-lit, and well-ventilated. When sawing, you’ll generate a lot of dust. Wear a mask, protective glasses, and your work gloves. It does not have to be anywhere special. I converted my kitchen into my workshop when I was building this.
Basically, my kitchen. At some point, I do intend to construct a proper workshop table, but for now, this should suffice. And no, I don’t cook in there.
Pictured here are gears, ball bearings, and printer shafts)
Pictured here are HDPE rods and aluminium sheets.
In the next post, we’ll play around a bit with building some components that we can repeatedly use in our design, like the ball bearing housings.