Looking for a knife-edge splitting formula
Looking for a knife-edge splitting formula
(OP)
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!
Thanks for any leads!





RE: Looking for a knife-edge splitting formula
Try it it by testing?? Probably cheapest solution.
RE: Looking for a knife-edge splitting formula
Test it.
Cheers
Greg Locock
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RE: Looking for a knife-edge splitting formula
Thomas J. Walz
Carbide Processors, Inc.
www.carbideprocessors.com
Good engineering starts with a Grainger Catalog.
RE: Looking for a knife-edge splitting formula
Thanks guys
RE: Looking for a knife-edge splitting formula
Test.
Or a time to copy or at least take cues from existing cutters.
Ted
RE: Looking for a knife-edge splitting formula
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

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RE: Looking for a knife-edge splitting formula
RE: Looking for a knife-edge splitting formula
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.
RE: Looking for a knife-edge splitting formula
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
RE: Looking for a knife-edge splitting formula
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.
RE: Looking for a knife-edge splitting formula
Thanks again for the input.