Cutting Blade Design 3

[spelletier]

I am looking for resources to help with the design of a guillotine type blade or simply cutting blades in general. I have a basic idea of my dimensions but would like to know if there is a source of best practices when it comes to the design of cutting blades geometry and material selection.

Thanks everyone!

 

[btrueblood]

Really depends on what you are trying to cut (steel? leather? fabric? rubber? ice? wood? ...), and what the finished product has to look like (square cornered? shear lips/burrs ok? ...)

There are literally volumes of books written on cutting edges of tools and how best to prepare them.

 

[EdStainless]

Slice, chop, shear, or cut? Each requires a different style and system.
Lots of details are needed.

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P.E. Metallurgy, Plymouth Tube

 

[spelletier]

I will be cutting a packing seal made from braided Nylon and Kevlar. When it comes to finished product, as little fraying and deformation as possible would be ideal.

For a little more insight I am looking into automating a packing cutter similar to this

Any resources at all when it comes to tool cutting edge would be very much appreciated.

I apologise if I am not being clear enough I am still very much learning.

 

[EdStainless]

In order to get good clean cuts the material needs to be constrained from moving sideways in a guide.
The blades needs to be stiff and well guided so that it doesn't deform.
Sometimes the cutting blade is held into a thicker holder for support.
Couldn't you buy one and automate it? (air cylinders, safety switches, sensors, controls)

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P.E. Metallurgy, Plymouth Tube

 

[btrueblood]

Mmm....tricky. I'd see if Dupont (or the other guys, Teijin aramid that makes Twaron?) has something on their website about shearing kevlar fibers. I know it tends to wear out (abrade) high-carbon steel blades pretty quickly, and many people use carbide and other ceramics in place of steel. If this is mixed strands of the two fibers, you have a second problem that the nylon is fairly soft compared to the kevlar, and it's like you are cutting a bar of steel inside a stick of butter...good support for the strand is essential.

 

[MikeHalloran]

Such an automated machine would pull bulk material from a roll, cut it off to a precise length, and stop when it had cut a predetermined number of pieces.

You can buy such machines from these folks:

https://www.markenmfg.com/automatic-cutting-machines/#

Of course there are other manufacturers, but Marken provided a machine that worked out of the box, for less money than I had already spent on unsuccessful homebrews, for cutting bulk refrigeration hose, and they were enormously helpful both before and after the sale.



Mike Halloran
Pembroke Pines, FL, USA

 

[spelletier]

Couldn't you buy one and automate it? (air cylinders, safety switches, sensors, controls)

Ideally yes, We could just buy the cutter that's on the market, unfortunately all the cutters I have found to date are limited to upto 1" packing while we need upto 2"-2.5".

I'd see if Dupont (or the other guys, Teijin aramid that makes Twaron?) has something on their website about shearing kevlar fibers.

That's a good place to start thank you! I will check the manufacturers sites. From what you and Ed have advised it seems like the cutter will definitely need to be held on all sides to avoid problems with the cut due to the mixed fibers.

You can buy such machines from these folks:

https://www.markenmfg.com/automatic-cutting-machin...#

I will definitely keep this in mind, Unfortunately the company I am interning with already invested a decent sum into an attempted cutter and plc+drive system around it. The previous person who worked on this system never saw it to completion so they are still pretty invested in the homebrew option.

 

[btrueblood]

It's not the cutter but the material being cut that needs to be held/clamped/prevented from distorting (as much as possible). Cutters for kevlar fabrics, for example, have fine serrations that are intended to grip the individual strands as a mating serration in the shear moves to cut it (think pinking shears but finer teeth).

 

[EdStainless]

There are high quality kitchen knives and scissors that are micro-serrated. This may be what you need. These are usually single ground (back face of blade is flat). These blades excel at cutting things that want to slide out. You need to full restrain the packing, and have a stiff blade or the cuts will be very uneven.

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P.E. Metallurgy, Plymouth Tube

 

[Compositepro]

DuPont has a machining and cutting guide for Kevlar, but the bottom line is that is that the blade has to be very sharp. Scissors for fibers are often micro-serrated in order to keep the fiber from sliding as opposed to getting cut. For your application you do not want scissor type cutters. You want a thin, very sharp razor blade that squeezes the cord against a hard plastic or aluminum anvil. The blade must be thin to be able to cut through thick material. The anvil must be hard compared to the material you are cutting, but soft compared to the blade. The razor blade should be disposable because it is far more economical to replace the blade than to try sharpening it. Kevlar filaments are 12 microns in diameter (0.0005") and one uncut filament causes problems because the fiber is very strong and tough.

 

[MikeHalloran]

For those super-size packings, first thing I'd do is ask the packing manufacturers what they use to cut the stuff.

Failing that, I'd whip the stuff on both sides of the cut line, and try running a hot blade through it. I'm thinking something that looks like a traditional safety razor blade, but longer, with an electric current running through it, sufficient to get it to a few hundred degrees F. Keep a fire extinguisher nearby.


A little searching reveals that hot knives are the hot setup for cutting stuff like climbing ropes, which are very difficult to cut with a cold knife. There must be one big enough to cut a hawser, which is basically what you need.



Mike Halloran
Pembroke Pines, FL, USA

 

[3DDave]

I think Kevlar doesn't melt, at least not if exposed to air, and can withstand temps to 450C. I would be looking at a laser cutting system, but that won't slot into the current scheme. I expect the answer will lie with a circular knife blade, probably several feet in diameter, with some coolant to keep the nylon from melting and fusing to the blade.

 

[MikeHalloran]

Damn, that Kevlar is weird stuff.
I bought a spool of Kevlar dial cord for experiments with small mechanisms.
The official recommended crimp splices wouldn't hold,
even when assembled with the official recommended expensive crimping pliers,
even under just finger tension.
And the fibers splintered going over a pulley of the official recommended radius.
I'd do a burn test if I hadn't thrown it away in disgust.



Mike Halloran
Pembroke Pines, FL, USA

 

[Compositepro]

Kevlar (aramid) is used to make protective clothing for firemen.