Building a Robotic Arm — Part 2

Building the mechanical assembly

In the first part of this series I listed the parts required to build the mechanical assembly for a robotic arm. You might remember my delight is discovering that no instructions were included! Yes, I’m one of those silly people who assemble IKEA furniture and then look at the instructions to see if I did it correctly. (Surely having parts left over just means I have assembled it more efficiently, right…) Realising that most people won’t want to spend an hour staring at a pixelated image of the assembled arm on the amazon product page to determine the best order in which to assemble it I decided to create some instructions.

Assembled robotic arm

There’s a few ways you can put these arms together. I opted to copy how the arm was assembled according to the pictures on the amazon product page.

A quick note on fasteners

Before we begin there are 4 types of fasteners used in this robot arm kit. To avoid any confusion in this guide I will refer to them by a number as shown in the image below.

Nuts and bolts
  • Machine screw #1 is used to attach the metal servo horn to the servo and also to attach the servo horn to the metal parts of the robotic arm. These screws came with the metal servo horns and were not part of the robotic arm kit. Some equivalent screws were included with the arm kit but I decided not to use them as they were shorter.
  • Bolt and nut #2 are used to connect all the non-moving metal parts together.
  • Bolt and nut #3 are used with a bearing (not pictured) to connect the 4 main moving joints of the robotic arm.
  • Bolt and nut #4 are used to attach the body of the servos to the robotic arm.

#1, #2, and #3 all have the same thread which I believe is an M2 thread. #4 appears to have an M4 thread.

Right, with that out of the way let’s get going!

Assembling the claw

First attach the servo using four #2 machine screws. These are screwed upwards into the four metal threaded tubes on the claw assembly.

Attach the servo to the claw assembly

Use a servo tester to rotate the servo fully anti-clockwise. Place the metal servo horn onto the servo so that when attached to the claw arm the claw is fully shut.

Aligning the servo horn

Use three #1 machine screws to attach the servo horn to the servo and the claw arm. Use the servo tester to check that the claw opens and closes.

Secure the servo horn to the claw assembly

Use a servo tester to align another servo to the middle of its travel range. Use a #1 machine screw to attach a servo horn to the servo. The alignment of the horn should be selected so that the claw opens in the axis of the servo’s longest dimension as shown in the picture below. Use two #1 machine screws to attach the claw to the servo horn.

Fully assembled claw — Servo at mid-point in travel range

Assembling the main arm section

Use six #2 bolts and nuts to attach two “U” sections together to form an “H”.

Two “U” sections ready for assembly
Assembled “H” section

Attach a servo bracket to the “H” section using a bearing and a #3 bolt/nut.

The bearing should be pressed into the hole on the “H” section from the outside.

Bearing installed on “H” section
Completed joint assembly

Next take an unused “U” section, a right-angle bracket, and four #2 nuts/bolts.

“U” section and right-angle bracket ready for assembly

Attach the two pieces together as shown in the image below.

Assembled “U” section and right-angle bracket

Prepare the two assembled metal sections for further assembly using four #2 nuts/bolts.

Attach another servo bracket to the assembly using a #3 nut/bolt and a bearing.

Attaching the second servo bracket

Attach a third servo bracket to the assembly using four #2 nuts/bolts.

Flip over the assembly. Use a servo tester to rotate a servo as far as it will travel in a clockwise direction. Attach the servo horn to the servo using a #1 machine screw.

Use four #4 nuts/bolts to secure the servo to the servo bracket.

Use four #1 machine screws to attach the servo horn to the “H” bracket.

The machining on the metal parts is not perfect. I could not get the 4th machine screw to thread properly.

Prepare another servo by rotating it fully clockwise and attaching the servo horn using a #1 machine screw.

Attach the servo to the metal bracket using four #4 nuts/bolts. Attach the servo horn to the “U” bracket using four #1 machine screws.

Flip the assembly again. Attach a servo bracket to the other side of the “H” assembly using a #3 bolt/nut and a bearing.

Nut position for attaching the servo bracket
Servo bracket attached to main assembly

That concludes the assembly of the main arm section.

Assembling the base

Assemble a “U” section and a servo bracket using a bearing and a #3 nut/bolt.

Attach the “U”/servo bracket assembly to a wide “U” section using four #2 bolts/nuts.

Use a servo tester to move a servo to its mid-point. Attach a servo horn to the servo using a #1 machine screw. Attach the servo to the servo bracket using four #4 bolts/nuts.

Attach two wide “U” sections using four #2 bolts/nuts to make a wide “H” section.

Wide “H” section

Join the two sub-assemblies using four #2 nuts/bolts.

Assembled base.

Final assembly

We now have 3 sub-assemblies complete: the main arm, the base and the claw. The new step is to put them all together.

Use four #3 bolts to attach the lower servo bracket on the main arm section to the “U” bracket and servo horn on the base. In this case the bolts are secured into the servo horn and not using nuts.

Prepare a servo by rotating it fully counter-clockwise using a servo tester. Attach the servo horn using a #1 machine screw. Use four #4 nuts/bolts to attach the servo to the lower servo bracket.

Use four #1 machine screws to attach the servo horn to the lower part of the main “H” section.

Due to poor manufacturing tolerances I was only able to get two machine screws aligned

Attach the claw to the main assembly using four #4 nuts/bolts.

Claw attached to the main assembly

The arm is now fully assembled. Use a servo tester to check that each of the servos moves as expected.

The completed robotic arm

During testing I noticed that the lowest servo joint was struggling to handle the force of lifting the unloaded arm. One possible solution to this is to run the servos at a higher voltage. The Tower Pro servos I used should operate fine at 6V; this will slightly increase the servo torque. Another option is to replace this servo, and possible the one above it, with more powerful and better quality models. This is likely the route I will take.

The next step is to design and manufacture the electronic circuit to control the robotic arm. Click below to read the other posts in this series:

Part 1 — Introduction
Part 2 — Building the Mechanical Assembly (You are here)
Part 3 — Designing the Electronics System (Pending)
Part 4 — Developing the Microcontroller Software (Pending)
Part 5 — Developing the PC control software (Pending)

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