Reddit reviews High Torque Nema 23 CNC Stepper Motor 114mm 425oz.in/3Nm CNC Mill Lathe Router

Already a lot of great answers by clever people here! I can add a bit on motors and electricals, but I also want to say that you're probably underestimating how big a 3' arm is. Imagine that on your desk- it takes up half a table! Sizing the motors for static torque alone doesn't work well, as the inertia at the end effector increases with length^2 which is proportional to dynamic torque, speed, and vibration. Larger limb sections are also heavier and more complicated to make, which makes them even more heavy. Sizing down a little bit will make the arm dramatically more stable and performant.

> Belts or Gears for the actuators?

For 3 lb @ 35" you're looking at a minimum torque of 12.2 N-m at the shoulder. That will require reduction. Belts are far cheaper than gears, especially if you have a 3d printer- plastic pullys work great, although they need to be well glued to metal shafts (NB that a shaft key will greatly reduce strength and durability). Red loctite is great for that. A single belt reduction can do 5x, although you can do 10x+ with idlers. Mcmaster is a good place for belts, but amazon has a small selection that can be cheaper.

Note that belts can be very rigid: highly tensioned, fiber reinforced belts at moderate torque (otherwise the teeth start pulling out) are actually stiffer than most gears, which have a grease film and a gap between teeth that has a slight initial give/backlash. The reason you switch from belts to gears is because you need to tension the belts more tightly for higher torque. Once the tension becomes hard on the bearings and gearbox frame, you switch to gears. Basically you want to avoid gears if at all possible; they're expensive, hard to find, and hard to mount without metal backplates and the ability to cut bearing mounts. SDP/SI is a good place to get gears.

> Once I know how much torque I need, how do I know which type of motor is best for me? Stepper, Servo, Brushless?

Depends how much you want to spend. Hobby servos won't work for a 35" arm, even the $350 dynamixels. You also don't want to be designing your own brushless drivers, and the range of robotics controllers for bldc is limited. You are basically stuck between NEMA 23 and odrive.

NEMA 23 is the cheap choice- you can get very big NEMA 23s on amazon, hook them up to a single-stage 5x reduction, and have gobs of torque and good control. You can even get NEMA 34 for affordable prices. The drivers are stupidly cheap- for <$70 all-in you can have an arduino-controlled joint with 15 N-m of torque and top out solidly over 500 rpm. Add a couple heat sinks and you can increase that a lot- 500+ watts no problem, or 7 watts per dollar.

Downsides are you don't get any regen (not so important on an arm), low/no backdriveability (although this can be nice since the robot usually holds position when it turns off), very loud operation, low efficiency, and pretty low acceleration. Brushless motors require higher reduction and closed loop control, but are quiet, efficient, and can be used to build very responsive + high regen robots. Driving them is the weak link: the 56 V odrive dual driver cost a whopping $150. However for $70-80 per motor you get 40-90 amps continuous for 2 to 5 kilowatts, WITH regen and accuracy to >512 steps. That can be over 20 watts per dollar for the motor, reduction, sensors and driver. The limiting factor is even finding motors that can handle that power.

If your budget is <$500, go for steppers. If it's >$800, I'd go for brushless. You'll get an immense amount of speed and power, both of which are very good for an arm with a 3' reach. Note that 3' is a very large arm- the weight of the arm itself will be very limiting if you don't used fairly sophisticated techniques. 8"-12" sections are a hassle to 3d print. Rotational inertia increases with reach^2 so you'll need quadratically more power for the same acceleration (and to fight wobble). A 26" arm will require only half the power.

> Do I start my design from the end effector or do I start at the base?

I'd start at the end effector- that will set your payload weight and the torque required at the next joint, and so on back to the shoulder. Doing it the other way requires a lot more iteration.

The one thing I always say on posts like this is to learn how to use bearings. Bearings are the #1 cause of wobble in poorly designed arms, and the easiest way to tell if the designer had any clue what they were doing. Use 608 bearings for everything you can. They're incredibly cheap and precise because they're used in skateboards- 20 to 50 cents each. They're deep groove bearings, which are excellent for machinery, and can take 300 lbs radial and 150 lbs axial static load and 2-3x that for dynamic load. They're easily a 50x better value than any other types of bearings. If you want other bearings (maybe very large thin section) go to onlinebearingstore, despite having a 2000s era website/name they're really great. Unrelated, theoringstore is also really great.

The most important thing to know about bearings is that they always, always need a preload. The bearing will not meet specs if it does not have some axial force. It will have a very noticeable play and will wear out quickly. This is why you always use bearings in pairs- not because they can't take it, but because you can't preload a single bearing. You need two bearings to be pressed together. I like disc springs for this, but shims and even just bolts also work well for providing the axial force. You can usually just set your preload by feel (so make it possible to bolt down one bearing closer to the other), but if you want to do the math it's good to aim for an axial force of 50% of the maximum radial force you expect. That can come from static load, or torque from twisting the bearing.

https://www.amazon.com/Torque-Stepper-Motor-425oz-Router/dp/B00PNEPW4C

I'm looking for the stepper motor which can make one revolution in the shortest amount of time possible. In other words, something which can make a catapulting motion. Should I be looking for high torque motors?


I found this one but I'm not sure if it has the right specs for what I'm looking for... Could anybody advise? Thanks!

These are what I have:
https://www.amazon.com/Torque-Stepper-Motor-425oz-Router/dp/B00PNEPW4C/ref=pd_cp_60_4?_encoding=UTF8&pd_rd_i=B00PNEPW4C&pd_rd_r=35Y9S4KD82HKA5H9YX3A&pd_rd_w=0KHvJ&pd_rd_wg=Iq2NL&psc=1&refRID=35Y9S4KD82HKA5H9YX3A

This is what you may be talking about:
https://www.amazon.com/Stepper-Motor-Bipolar-269oz-Router/dp/B00PNEPI0A/ref=pd_sim_469_4?_encoding=UTF8&pd_rd_i=B00PNEPI0A&pd_rd_r=61V9SWJJ3XHTTDF9QDE8&pd_rd_w=1Fgkr&pd_rd_wg=kmipE&psc=1&refRID=61V9SWJJ3XHTTDF9QDE8

Longer Length = More torque = Wider shaft needed