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RoboArm Information Page
written by Deve Krehbiel

Introducing The RoboArm System!

I can tell you, this has been a lot of fun! It is plagued with all kinds of challenges from Fusion 360 design to Movement for 5 stepper motors, to the associated electronics and wiring, it has kept me busy for months! Now that most of the quirks have been addressed, I will document where we are at this moment. I started off with re-creating something along the lines of a MeArm. The problem was, it wasn't big enough nor was it strong enough to really be useful. Robotic Arms are cool and all, but let's make it useful. To do that, we need larger parts, more versatility in movement, and much stronger stepper motors. It goes without saying we need to eliminate mini servo's for this project.

The Platform

I needed a standalone platform that is useful for other projects. Something that rotated very smoothly, is speed controlled, and can handle a lot of weight. This is the foundation for the RoboArm but I can also use it for taking pictures of small items from any angle. I am sure with a little imagination a person could think of other things to use it for, so it was a great place to start.

After MUCH trial and error, I concluded the way to make this happen is with a large 3D printed Bearing Case and 6.35mm ball bearings. This bearing is pressed into the large gear and glued into place. I then used two roller skate bearings to bolt the assembly down to the platform so it had the liberty of movement, but the turntable cannot just fall off. The Nema 17 High Torque motor is more than enough to turn the turntable with the heavy RoboArm and whatever it is picking up pushing down on it.

The Nema 17 needs to be adjustable to move into the main gear, and of course there is creating an effective gear in the first place. Fusion 360 does a fine job of creating gears that work nicely, but you have to decide the size of each gear. The Nema 17 gear has to be small enough to be able to tighten the motor to the platform. The skate bearing is so that we can put a plastic bolt through the entire job to hold down the turntable.

A matching skate bearing is embedded into the turntable so the bolt can spin the inner bearing without binding it. This holds the assembly together and allows for full movement. Lots of thought goes into this stuff! Sometimes more than I am capable of. The bearing/large gear is held into place with 3mm screws and embedded nuts. Everything has to be flat so there is nothing protruding from the turntable.

Once the turntable is properly designed and ready for use, we need to determine the best Stepper Motor to use. I started off with Nema 11 very small steppers in high hopes of them being able to drive the turntable. Not even close! So I tried a normal Nema 17 42 series motor like used on 3D printers. Close but no cigar! So the one that worked best is a Nema 17 higher torque, higher amperage motor.

The turntable and platform system was challenging but it turned out quite well. I also made a half case for an Arduino Mega 2560 that bolts under the platform, but after much more thought, there simply is not enough room under the platform for all of the electronics required for this project. I had no idea where this was all going. The first stepper motor controller was an Uno with a Shield with 1 amp stepper controllers. This proved to not be even close to what was really needed. As you will soon see, this project turned out to be quite involved!

The Stepper Motors

Having no prior knowledge of stepper motors, I proceeded to learn the hard way. What I have learned in this journey is a few important things. As an electronics tech by trade, it made no sense how they rate the voltage for these steppers. It turns out, steppers are not rated for voltage. What matters is the amperage. The voltage rating is irrelevant. So a .8 amp rated Nema 11 stepper will run fine using a 36 volt power supply as long as the TB650 3 amp max stepper controllers are set to .8 amps. Go figure. Additionally, the holding torque that is advertised is only providing you matched the motor with the correct amperage controller. Turns out the biggest stepper motor you can find with the highest holding torque will be totally useless using a 1 amp controller. So, before blaming your stepper motor supplier for not enough holding torque, be sure the controller is set for the proper amperage. THEN kick yourself for not getting a strong enough motor! I have several times. Pictured is the 5 steppers we will be using for this project. Each one has a specific purpose and is right-sized for that purpose.

So I am sure you would appreciate some specifications, so here you go from small to large:

  • The Nema 11 has a footprint of 28mm square and a bolt pattern of 23mm square. The part number I used is 28HD1411-02. It requires a .8 amp controller and has a holding torque of 1.6Ncm/2.3 oz. All of these are 1.8 degree, bipolar 4 wire motors.
  • The smaller Nema 17 has a footprint of 42mm square and a bolt pattern of 31mm square. The part number is 42SHD0217-24B. It requires a 2 amp controller and has a holding torque of 45Ncm/63.7 oz.
  • The larger Nema 17 has a footprint of 42mm square and a bolt pattern of 31mm square. The part number is 17HS19-2004SI. It requires a 2 amp controller and has a holding torque of 59Ncm/83.6 oz. These all have a 5mm D shaft.
  • The two Nema 23 motors have a footprint of 56mm square and a bolt pattern of 47mm square. The part number is 57BYGH627. It requires a 3 amp controller and has a holding torque of 19Kgcm/270 oz. This one has a 6.35mm round shaft.

The little Nema 11 will be used for the grabbing mechanism at the end of the Crane. It will easily handle the torque and replace the Mini Servo this project started with. The smaller of the Nema 17's will be used to move the grabber mechanism up and down. The larger Nema 17 will move the turntable. The two Nema 23's are huge and heavy, but will handle anything we throw at it when it comes to moving the Crane in both Axis'.

The 3D Printed Parts

Because the turntable is elevated, we cannot use a whimpy arm system. We also cannot make the upper Crane arm using a 220x220 3D Printer Bed without splitting the Arm in two. No problem, but the upper arm itself measures 410mm (16.14 inches). It needs to be reinforced substantially and it needs to have wiring holes at certain intervals to accommodate the four wires of the Nema 11. I also think it would be cool to put some LED's on the upper arm. Thinking ahead is crucial!

The First Member, or the cranes base arm is made very solid and has a hollow passage inside of it for the wires to fit into. It is approx. 235mm (9.25 inches) long but can be printed diagonally on a 220x220mm print bed. All 3D parts are made from PLA plastic and the arms are solid or 100% infill. This takes much more time to print, but we need a solid solution we can be proud of.

The Base of our project is a two-piece affair. One of them handles mounting of these massive motors. The center part handles the tower needed to start the First Member and hold the necessary gears. Part of the challenge of these base parts is getting the geometry correct and getting the two gears down on each other properly. This design enables us to put all of the motors low except for the little Nema 11 on the end. I feel this system is right sized in its approx. 36 inch horizontal reach and its approx. 18 inch vertical reach.