# Need help on a project - Rotating a box

I'd like to turn a box, dimensions 50" by 35", weighing ~50 lbs, 90 degrees CCW. I'd then like to turn it back to it's starting position. The box would be angled back at 10-20 degrees.

Where do I start? How do I find out what type of stepping motor I should use? I want a fluid motion but it also needs to be semi slow (like 3-5 seconds). The box needs to turn slow and under control. Turning fast would damage the goods inside the box.

Anywhere you can direct me where I can better understand how to move forward would be awesome. Thanks for your time!

A stepper can move as slowly as you want. The main thing you have to figure out is how much torque you need to move the platform.
Note that, if the load is perfectly balanced over the rotational axis of the platform, you actually need very little torque if you don’t need it to accelerate quickly – but then, you have to also decelerate it slowly.
As a first approximation, I would multiply the weight of the box, with the radius of the platform, and that’s the torque I’d be looking for. If the platform radius is 30 cm, and the box is 25 kg, then I’d look for 750 kg*cm of torque, for a reasonably well-controlled load. This is a gross back-of-napkin calculation – the mechanical engineers have better models based on the actual mass distribution of the system.

However, that’s 10x more than you’ll get from “big hobby size” steppers, like this: http://www.circuitspecialists.com/step-motor-nema-34-85bygh450c-03.html
Thus, you’re looking for at least a 10:1 reduction gear ratio in a separate gearbox. Because you don’t need fast movement, that’s quite alright – you’ll have a nice, well-controlled movement if you ramp up the speed to the stepper controller to match your load, and then ramp down using the same curve again at the end of the movement.

If all you need is a controlled acceleration and deceleration, rather than a “direct control” kind of system, then you can get away with a much smaller motor, but then you have to be very careful in calibrating your acceleration/deceleration curve to the actual load to avoid jerkiness or overload.

Thanks for the info, makes sense. So a stepper like this - http://www.automationdirect.com/adc/Shopping/Catalog/Motion_Control/Stepper_Systems/Motors_-z-_Cables/STP-MTR-23055 which does 166oz-in would give me ~10 lbs of torque per inch. When we talk radius of the platform are we talking dimensions of the box or the metal plate connected to the gear box?

So if I was using the dimensions of the box and it's radius was say 22.5 inches and it weighs 30 lbs. So I need 675 lbs*in of torque. So 675/10 == ~67 so I would need a gear reduction of 67:1 right? that would be a pretty big gear box right?

What about a different stronger stepping motor then, like 400oz*in with gear reduction at 20:1 or more?

I guess my real questions is if the diameter of the gear on the stepping motor is 1 inch, does that mean with a 67 gear reduction the gear on my rotational shaft has to be 67 inches in diameter?

Let me give you an example of the design I'm building. That way it's application is more straight forward.

Kahlid74: When we talk radius of the platform are we talking dimensions of the box or the metal plate connected to the gear box?

I am talking about the box, unless the plate is significantly heavier than the box.

I guess my real questions is if the diameter of the gear on the stepping motor is 1 inch, does that mean with a 67 gear reduction the gear on my rotational shaft has to be 67 inches in diameter?

You can use more than one gear! As long as the losses don't build up, and the play/backlash is not a problem. Two gear steps with 10:1 ratio is the same as one gear step with 100:1 ratio.

Also, my suggestions are very much rules of thumb. And the rule of thumb is there to give you enough that you can achieve a reasonably stable movement with a reasonably limited change in spin (linear velocity) by simply applying torque over time.

As long as you have force to overcome friction (which can be made almost arbitrarily small for a spinning platform), you can use a weak motor, on the condition that you accept the very slow acceleration and deceleration you'll be able to impart, and you drive the stepper based on some synchronous feedback mechanism so you "know" when to turn on/off the different windings. You can use a 1:1 gear ratio (direct coupling) if you can get a synchronous feedback mechanism, and it will drive the plate at whatever the maximum possible acceleration is given the torque available.

Another warning: I'm not a mechanical engineer! I have a background in simulation software that simulates physical/mechanical systems, and mostly indirect exposure to the actual practicalities.

Love the picture, by the way :-)