Weekly Challenge Group

It's over, I can't understand it at all, and the translation is not very good

Which language do you need?

Bending Fixture with a Motor

The idea is to take on the challenge with the available information, does not mean it is the appropriate method to bend a sheet. This approach would still require a stepper controller or a clutch with a stop. Due to the light motor, a sheet of wax would be used.

I like the idea with the slotted axle.

If you look at it, there are not many ways to do it based on the information provided. I guess if you put your mind to it, a resolve happens.

The information was intentionally kept loose so as not to limit creativity. 

My thoughts here went to a lever solution with belt drive. But I find your approach very interssant.

If you use a stepper motor, then the simplest design is a crank mechanism. It is not clear why it is necessary to use a stepper motor, pneumatics are simpler and cheaper.

Thats the challenge to not use pneumatics. Think about maybe not having a pressured air source in this case. Then you need stepper motors.

Stepper motors would be a poor choice for this operation. Steppers have relatively low torque combined with a lot of supplemental electronics to complicate the design. I would have gone with a gearmotor with a plain AC or DC motor.

Stepper motors have become popular because they are easy to integrate into microcontroller systems. That's great if you already have or need a microcontroller. But otherwise they are often just an overcomplication.

Can’t bend more than 180 anyway. But for different applications it’s quite easy to use a 4-bar linkage for movement near or slightly greater than 180. Or use rack and pinion for multi-turn. There are many options. Look at the bucket mechanism on an excavator.

Typically, bending over about 120 degrees wouldn’t be done in a single operation. It would be done in two discreet steps. Not because of actuator limitations but because of limitations in the punch and die making the bends.

[Edited to correct for the errors pointed out by StrongDinosaur]

I did some preliminary calculations for a real world design. If we use annealed 6061 aluminum for the material, it has a tensile strength of about 124 MPa (N/mm^2). If we air bend this part using a die opening of 20mm and a nose radius of 3 mm this will require a minimum of about 223 kg force. (Note air bending is different from the example sketch which shows a sharp bend and no radius. However, that would be quite impractical and would require an order of magnitude greater tonnage.)

A safety factor of 1.5 for excess tonnage would be reasonable for a total force of 334 kg

I did a simple layout and calculate a reasonable ram stroke of about 20mm which would require a crank arm radius of 10 mm. Which equates to a required torque of 334 kg*cm

A NEMA 17 stepper is rated at 4.3 kg*cm torque and a maximum 2344 RPM.

If we use a 3 stage gearbox. The first 2 stages being 5:1 ratio and the final stage being 6:1 for a total reduction of 150:1. Efficiency for each stage is assumed at 80% for a total efficiency of 0.8^3 or 51%. 4.3 kg*cm * 150 * 51% = 330 kg*cm torque. Close to what is required. ( I didn't select specific gears but the final drive will undoubtedly be hefty in order to sustain that torque. Total efficiency would likely be much better but I chose worst case.)

Speed is 2344RPM / 150 = 15.6 RPM or 1 revolution every 3.8 seconds.

In conclusion: Given a rather soft material of annealed aluminum and the specific NEMA 17 stepper. The project is certainly possible with air bends and a reasonable nose radius.

One could shorten the ram stroke and work with the number of stages to increase the efficiency but I estimate being able to cut the cycle time in half at best. If methods other than air bending are used then forces rise exponentially. I didn't bother carrying the design any further.

Bob, I don’t know how you got 2.2 tons in your calculations, please share it.

Aluminum is a rather plastic material. There are many calculators for professional benders on the Internet and they show the result 320kg.

Just try to imagine 320 kg (1/4 of average car weight) standing on this thin 3mm aluminum strip, it will be bended immediately. CAE says 420 Mpa in the center of strip with 320kg load with 24mm die, сalculations are simplified and aren’t taking into account the friction of the workpiece against the edges of the dies, but anyway it is more than the tensile strength, the material has long been in the plastic zone. Of course, in reality, you will need to attach a bit greater force to achieve the shape of the die, but i don't understand why your result is 2.2 tonnes.

Many types of aluminum sheet bend very easily. But duralumin alloy and special hardened sheets with increased rigidity are very difficult to bend, because they can simply burst at the bend. If we bend duralumin we can decrease stresses by pre-milling a groove in the place where it is planned to make a bend. 

But overall, Bob is right. The design of mechanisms must begin with calculations. We cannot know the necessary forces without knowing the material. You can bend, for example, tin so you will need no reducer at all.

Good catch. I essentially used the same industry standard formula you did. I used a die opening (V) of 20mm and not 24mm. I'm used to using much smaller die openings for a self contained die set versus standard press brake tooling. I also included the punch width of 4.76mm which is the width of 105 degrees wrapped around a 3mm punch radius. This 4.76 mm is subtracted from the 20mm die opening (V) for an effective opening of 15.25mm. While your calculator had an input for the radius, I don't see it in the equation. I also used softer aluminum than you did.

My mistake was I failed to convert Newtons to KG. I am off by almost a factor of 10. I grabbed a handy tensile strength and failed to account for the units in a standard equation that i already had set up.

Redoing the calculations it can be done with a 150:1 gearbox and 3.8 seconds per cycle. Quite a difference.

The solution is a lot more reasonable. If I had the time I would finish this out. However I'm going skiing for the next week. :)

By the way: tin has about 10% the tensile strength of soft aluminum. While it's much easier to bend, you won't get by without a speed reduction. It will still require about 16:1 reduction.

(To save confusion I edited my original post to correct the error)