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How many Nm required to cut red Plastic Tubing

boylesg

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I am trying to make an autocutter to cut the red tubing in the attached photo CAM.jpg

I have a NEMA 17 stepper motor which can provide 0.4Nm

And this needs to turn the cam also in the photo and push a stanley knife blade down with enough force to slice through the red tubing.

However it seems that 0.4nM is no where near enough.

But I don't have any context, through previous projects, to give much idea of the ball park amount of torque I would need.

Looking on ebay at various NEMA stepper motors I found this table attachment NEMA.jpg

Can anyone give me at least a rough idea of the minimum torque (from NEMA.jpg) I am likely to need to cut through the plastic tubing?
 

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betwixt

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Have you considered using a gear mechanism to convert motor torque to the linear motion of the blade? It would make more sense to make the motor pass through several steps to produce the required depth of cut, a high gear ratio would make it easy.
Alternatively, can the cam be replaced by a rotating screw shaft as a simple way to convert rotary to linear motion?

Brian.
 

FvM

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The knife force will be quantified in N rather than Nm (torque). Nm comes into play when you design the mechanism to drive the knife. Without a design with respective gear ratio and efficiency as well as a cutting speed specification, you don't know the required motor torque and power.
 

boylesg

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Have you considered using a gear mechanism to convert motor torque to the linear motion of the blade? It would make more sense to make the motor pass through several steps to produce the required depth of cut, a high gear ratio would make it easy.
Alternatively, can the cam be replaced by a rotating screw shaft as a simple way to convert rotary to linear motion?

Brian.
I did, but it appears that the gear mechanisms on ebay are way more expensive than just getting a NEMA with more torque.

Apart from the fact I would not have to modify my build if I used a NEMA with more torque.

- - - Updated - - -

I don't know whether these give your all a better idea.
The current NEMA is not capable of pushing the block down against the springs.
It looks a sthough I have one of these NEMAs: https://www.ebay.com.au/itm/Nema-17...696892&hash=item288b0a36dd:g:rvgAAOSwIZJd~Fuf

It says it is capable of 44Ncm.

I assume that is 44N of torque at a radius of 1cm.

So would that be 22N at 2cm etc?

Give me a ball park figure of what sort of torque might be capable of pushing down the block against my springs.

88Ncm? 176Ncm? 880Ncm? I have no context with this problem.

 

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KlausST

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Hi,

If I understand correctly, then the excentric red part presses the knife to the plastic tube to cut it off.
This needs a lot of force.

I´d do some test with a rope, a bucket and water.
Fix the rope in a way that it when you pull on it it presses the knife to the plastic tube
at the other end of the rope fix a bucket. The bucek should be freely hanging down on the rope. Maybe you need some metal ring or similar to redirect the rope.

Then gently fill water into the bucket until the plastic tube gets cut off.

Measure the weigh of bucket with water, then calculate force in Newton.

Klaus
 

boylesg

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Hi,

If I understand correctly, then the excentric red part presses the knife to the plastic tube to cut it off.
This needs a lot of force.

I´d do some test with a rope, a bucket and water.
Fix the rope in a way that it when you pull on it it presses the knife to the plastic tube
at the other end of the rope fix a bucket. The bucek should be freely hanging down on the rope. Maybe you need some metal ring or similar to redirect the rope.

Then gently fill water into the bucket until the plastic tube gets cut off.

Measure the weigh of bucket with water, then calculate force in Newton.

Klaus
Good thinking. That just did not occur to me at all.

Even more simply, if I can figure out how to balance an ice cream container on the wood block, then I could just keep adding water until the blade cuts through through the plastic tube.
 

betwixt

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You have to push the blade AND compress the springs AND overcome the friction of the cam using that method so even more force is needed. A simple fix might be to replace one of the metal supports with a plain rod, attach the motor to a screw thread on the other side and use the motor in forward or reverse to cut and retract the blade. A nut on the screw thread can move the blade carrier. With say a 4mm screw thread, to traverse the cutting distance would probably take five or six complete revolutions of the motor so you would have a huge increase in blade pressure without changing the motor at all.

The drawback is you don't get to eat the ice cream.

Brian.
 

boylesg

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So 4.75kg seems to be sufficient.

F=ma = 4.75 * 9.8 = 46.55N

F = T / (L * sin Angle)

Length of the cam from the centre of rotation is about 4.7cm or 0.047m
Angle of engagement is about 45 degrees.

46.55 = T / (0.047 * sin 45)
46.55 * 0.031819 = T
T = 1.48Nm

Does that seem about right?

This Nema should be sufficient if my calculations are correct.

https://www.ebay.com.au/itm/Stepper...hash=item4675a345ad:m:mQRSPwlho--zmS1nP_QF3ZQ
 
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KlausST

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Hi,

I didn`t verify your calculations.

Mind:
* You need to add some friction torque.
* the given torque is the "holding torque", but you need to consider "torque during rotation"

Klaus
 

BradtheRad

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Let gravity assist in slicing through the tube. Have the motor lift a weight up several inches. Then drop the weight onto the knife, which cuts through the tube.

Or the miniature saw & cutoff disks (made for the Dremel Moto-tool).

small saw and grinding disks.jpeg

The mandrel can be chocked into an ordinary drill. (No need to get an expensive Dremel.) I joined a small motor to the mandrel by a tube of the right size.
 

dick_freebird

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A windlass at the motor shaft and a pulley at the blade arm
can be assembled from hardware- or hobby-store small
brass parts. Stout bit of braided fishing line and there's
your linear motion. Maybe a tension spring for return.
Easy peasy and no fancy tolerances to get your mechanical
advantage.
 

KlausST

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Hi,

Another approch is to additionally make the tube to rotate.
This reduces force, but maybe increases time.

Klaus
 

Kajunbee

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You might reduce the amount of force needed by going with a v-shaped blade. If your handy with a grinder and blade sharpener you could fashion one from a thin piece of metal like a putty knife blade. A takeaway might be that the blade will have to travel further to complete the cut.
 

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