Looking for a knife-edge splitting formula

[CChunter]

I've searched high and low to try and find a formula that would fall under this situation, with no luck. I'm trying to find the force required to push a knife edge blade through a piece of polyethlene pipe, like the hand held plastic pipe cutters. I know how to calculate the force required to shear a section or punch a hole, but can't find anything for causing a split with a knife edge.

Thanks for any leads!

 

[MiketheEngineer]

How sharp is it??

Try it it by testing?? Probably cheapest solution.

 

[GregLocock]

The usual approach using a work equation will be foxed by the squishiness of the tube.

Test it.

Cheers

Greg Locock


New here? Try reading these, they might help FAQ731-376

http://eng-tips.com/market.cfm?

 

[tomwalz]

+1 for testing

Thomas J. Walz
Carbide Processors, Inc.

http://www.carbideprocessors.com

Good engineering starts with a Grainger Catalog.

 

[CChunter]

The blade will be ground from tool steel, the design is basically a hydrualic driven guillotine cutter. I have a ballpark idea of how much force would be needed, based on the largest length of blade contact x depth of cut x shear strength of HDPE. I may try to set up a test cut using a hydraulic press, I was just hoping there was already some method for this madness.

Thanks guys

 

[hydtools]

Seem like the cut would be a combination of shearing as the blade starts the cut and material failure in tension as the blade moves through the cut. Similar to cutting tree branches with a pruning shear.
Test.
Or a time to copy or at least take cues from existing cutters.

Ted

 

[IRstuff]

I'm unclear why you dismissed shearing and hole punching as not applicable to your specific problem. Seems to me that both situations involve the edge pushing its way through a material.

The difficulty would seem to be how one models how the material moves or doesn't move away from the edge; as with post-tensioned slabs, external forces pushing the split to closure would change the force required to extend the split. The force would be a combination of actual cleaving of the material, pushing it aside, and the friction of the edge moving along the material.

TTFN
faq731-376

 

[chicopee]

Would this be a longitudinal split in the pipe?

 

[racookpe1978]

The "theoretical" solution is nice, but it will fall (fail ?) as the assumed sharpness of the actual-real-world "edge" becomes dull.

So, imagine a 3 degree "wedge" with a perfectly sharp angle 0.001 thick. It might create a shear "load" based on the perfect theoretical force of chopping through the polyeth tube.

But a real world knife edge will vary from 5 thousandth to 25 thousandth on that face, and those variations will "move" randomly up and down the knife through use. So, do you use 0.002? 0.005? Some weighted average edge thickness?

Test a few cuts.

 

[Cockroach]

I think we're getting verification mixed up with validation. CCHunter wants to have some theoretical estimate of the cutting force, then verify the calculus model experimentally.

I would approach this issue similar to a jetway problem. Determine the force as the result of the minimum expected shear on the plastic pipe. Obviously blade length would be linearly dependent with force, similar to a key. But in this case you want the blade strength, I.e. Stress to be vastly sperior to the shear strength of the plastic pipe. Then add in some sort of efficiency factor.

I would estimate the validity of the model using something like paper, then sheets of plastic, etc to try and get experimental results for refinement of the model.

Make sense?

Regards,
Cockroach

 

[racookpe1978]

Hmmmmn.

A problem you've not addressed here in print is the "collapse" (deformation) of the (soft, plastic, hollow) polyeth. pipe as the blade hits and compresses the top, then bulges the sides of the pipe BEFORE it can get enough pressure to begin cutting though.

So the starting force needed will not be the force to begin cutting though/punching through polyetl plastic, but rather what force you need to start a blade through an essential flat upper surface of an unsupported pipe. (Unless you somehow dynamically capture the round pipe virtually all the way around.

Then, once you've started through the "flat" upper surface of the pipe, now you're going to go through two separate near-vertical walls, then the "flat" bottom.

I'd use a bandsaw type (moving blade) cutter, not a forced-through vertical slicer. You would probably not need teeth on the band-saw type blade.

 

[CChunter]

Thanks for input guys, the attached PDF is a blade style that produces decent results. I've seen and tested machines like these many years ago for production, and they did well. This machine will only be used for shearing/splitting pipe into smaller lengths so it can be re-ground and reused, customer stated they are not concerned with the finished cut, but this blade will provide a decent cut anyways. I was only hoping there was a formula or method is use that could determine the force required to perform this type of operation.

Thanks again for the input.