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Lego Gripper

Jim Wright (wrightja@gte.net )

Introduction

I was surfing the web looking for new robots I had not seen before and I came across a Web site for a company called K-Team. They sell a robot platform called Khepera that had a really nifty gripper arm on it. The page on the gripper arm is: http://www.k-team.com/robots/khepera/gripper.html. I found the price list for the gripper arm and it sells for 2,500 CHF, which is about $1,500. I though to myself that I could probably build one of these grippers for much less than that, so I did.

Building your own gripper

Parts List:

(1) Brass bar, Small Parts Number U-ZRTB-2/4-12
(1) #4 threaded rod, Small Parts Number TRX-440-12
(2) 16 stud Lego beam
(2) 2 stud Lego beam
(1) 3 stud Lego axle
(1) 4 stud Lego axle
(1) 8 stud Lego axle
(2) 8 tooth Lego gear
(1) 24 tooth Lego clutch gear (or standard 24 tooth gear)
(2) 16 tooth Lego crown gear
(1) Lego gray peg
(4) Lego 1/2 gray peg
(2) Lego bushing
(1) Lego 1/2 bushing (not pictured)
(1) 1 by 10 Lego plate
(1) 2 by 10 Lego plate
(2) 1 by 3 Lego plate
(1) Lego axle extender
(1) Heat shrink tubing (1/8 I think, I can't remember and it is not marked) (this is also not pictured)
(1) Micro servo modified for continuous rotation (not pictured)
(1) Micro servo (not pictured)
(1) Hunk of component PC board

 

Cut the #4 threaded rod into two parts, one measuring 3 and 3/16 inches and another measuring 3 and 9/16 inches.

Cut the brass bar into two parts both 2 and 3/4 inches, you can change this measurement depending on how long you want the grippers to be. Lance Keizer suggested to me a long time ago how to drill the holes. Open up your favorite CAD program and draw a 1/4 inch bar, down the centerline of the bar 1/4 inch from the back draw a circle for the first drill hole. Lego holes are 5/16 inch on the center, so from the center of the first hole draw another circle 5/16 inch (on center) away from it. Print this drawing out using true dimensions and paste (or tape) the pattern to the part and you have a very nice guide on where to drill the holes.

Drill a 1/8 inch hole for the first hole on one of the bars and as the second hole on the second bar. Drill and tap a #4 hole as the second hole on the first bar and the first hole of the second bar. It really does not matter which hole is the #4 tapped and which is the 1/8 inch hole just as long at they are reversed on the second bar.

Take the Heat Shrink and cut two pieces of it 11/16 inch and shrink them onto the end of both pieces of #4 threaded rod. Take the four Lego 1/2 pegs and cut to short end of off them so that they are flush to the Lego beam. Insert two of the modified 1/2 pegs into the first two holes of a 16 stud beam, and push the two #4 threaded rods, heat shrink end through the two pegs. They should fit very snugly but should still spin withut much force.
Take the two brass bars and thread them on to the rods. I slid a 8 tooth gear on the end of one of the rods and used that to spin the rod round to get the bar onto it, then slid that gear off and on to the other rod to thread the other bar on to that rod. Don't worry about the position of each bar at this point we will be lining them up latter on, for right now we want the bars on the rods somewhere in the middle of the two rods.
Cut a 3/8 inch section of heat shrink tube and a 11/16 inch piece as well. Shrink them onto the other end of the threaded rods. 3/8 inch section on the front rod and the longer piece on the second rod. Take the last two 1/2 peg and put them into the first two holes of the other 16 stud beam, and push this onto the rods, you will find this easier said than done. Use the 2 by 10 plate as a guide to get the whole assembly 9 Lego studs long. Slipping one of the 8 tooth gears on the end of each rod move the bar as close to the beam as possible. You might find that some of the heat shrink tubing is in the way and you can trim that off. Put both of the 8 tooth gears on the rods at this point.
Put the 1 by 10 plate on the top of the assembly at the third stud from the top. Put the 8 stud long axle in the fourth hole of the beam and put the clutch gear onto it. Put a bushing on the other side of the slide the axle extender on the other side of the axle. Put the 3 stud axle into the fourth hole on the other side and slide a 16 tooth crown gear on to it.

Using the 4 stud axle, slide the 1/2 bushing on to it then the other 16 tooth crown gear. Slide this into the 2 stud beam and put the other bushing on the end of this. Attached this seven studs down on the beam. Adjust the two crown gears so they mesh.

Attach the 2 stud beam to the 16 stud beam using the gray peg 5 holes up from the bottom.

OR

Make your assembly look like the picture to the left.

Take one of the servo control horns and cut off all of the arms, leaving a sort of round control horn. Hot glue this horn to the bushing on the 4 stud axle. Slip this on to the servo that is modified for continuous rotation, I did not leave the horn attachment screw in, I just slipped the horn on the servo. Hot glue the servo to a hunk of PC component board and hot glue the component board to the 16 stud beam. The PC component board is about the right spacing for the servo to connect to the lego and the control horn with the bushing hot glued to it to line up with the 4 stud axle..

Screw a control horn of the unmodified servo to the other 16 stud beam and connect the unmodified servo to it. I then hot glued the whole thing to another hunk of PC component board but you can attach it to anything you want at this point..

I'm still not to happy with the speed of the grippers going in and out, I'm looking for a motor that has enough torque and speed to drive the grippers directly. This is a photo of the original design where I pushed the crown gear to the second rod. If I find a motor I like, I will write up and addendum to this encoder article about this. If any one else finds a better motor for this gripper e-mail me.