# Ramadhan Fathurizki

## Forum Replies Created

Yes, maybe a video will help me and others to understand more about the area calculation because I still confused about how exactly we determine the worst case of possible cutting area (the maximum size of plastic that we could possibly put in there). I still think that this has to follow the shredder geometry of some sort, since we can’t just determine the worst case by ourselves (the scrap/cutted plastic size will follow the shredder geometry), otherwise our design will be redundant.

Halo berkahabadu, saya saat ini berada di daerah Pasuruan, Jawa Timur. Kira2 ada masukan atau pendapat lain tidak ya terkait postingan saya di atas?

Terimakasih.:)

As for why the blade thickness is important, that is because the plastic (or scrap) to be cut must have a dimension more than the blade thickness. If less, then the plastic (or scrap) will be “pushed away” instead of getting cut because It will receive no resistance force (as explained from the thread Calculations Required for Shredder that the plastic will need to be exerted 2 (type of) forces, which are equal in magnitude, perpendicular to the plane of cutting, and opposite to each other. This is true since if you want to deform (or in this case shred) a material, you will need to put It in static equilibrium condition.

As the blade rotates, even though the plastic (or scrap) has a dimension bigger than the blade thickness, It will always cut into the dimension of the blade thickness, which is why I thought the cutting area is supposed to be the blade thickness x the thickness of the material.

Hi, thefranky

I will try to answer your question.

1. The whole purpose of this calculation is to define the required horsepower, so you could determine which motor you gonna use. In turn, you won’t make a machine with insufficient power (the machine is not gonna work) or overpowered machine (the machine is redundant and cost higher than It should be).

2. Why should we calculate horsepower? Refer to answer no. 1. How will you know the RPM? Well, basically you can’t just determine the RPM if you’re using a direct transmission (without gearbox). With the gearbox, on the other hand, you can determine the RPM output (the one that will move your shredder), while the RPM input is determined by calculation using formula (Power input x efficiency = power output). You will obtain the value of required Power input, which value you can use to choose your motor, and using the motor specification, determine the gearbox ratio you should use (or use your gearbox ratio to choose your motor later. The choice is yours.)

3. For the answer to this question, please refer to answer no. 2.

4. The constant comes from the conversion of the power unit. This depends on what your starting units are. For this, I suggest you should google the constant for the conversion of your unit.

5. Why 80%, well It’s the rule of thumb for the yield strength that needs to be surpassed for a shear mode of failure (which is the mode used in cutting mechanism). Although from the reference book I read, It’s supposed to be 50%. I hope someone could clarify me on this one.

6. For the answer to this question, I’m still wondering tho’, but I think It depends on the blade thickness and material thickness (see my previous reply). I guess I will try to conduct some further research on It.

7. For a material to be cut (or shred), It must be in static equilibrium, so It will deform. That is why It needs 2 forces (or resultant of forces, see my previous reply), perpendicular to the plane of cut, and opposite to each other, which are active force (the cutting force), and reaction force (the resistance force) that acts as a support that holds the material/plastic/scrap in place. That is why in my previous reply I stated that when the scrap is less than the blade thickness (plus the gap, if that exist), the scrap won’t be able to be cut. That is why the cutting area will depend on the blade thickness (plus gap) as It becomes the defining parameters for cutting capability. I still wondering if this is right tho’, hope someone will have a better explanation for this.

Hey, I wanna ask. You said that the cutting area is found by the cross-section area of the material of cut that the shredder blade contacts. Is this actually means the product of blade thickness and thickness of the material/height of material?

Or, is It means that the cross-section is the area of cut that I want to make, regardless of the knife geometry?

In my opinion, the plastic to be cut must have a dimension more than the blade thickness for the plastic to receive support from the counter-blades (if I referring to the design by Dave Hakkens).

Attached below is the schematic of how cutting actually works in my opinion. Please give me your thoughts.

Thanks:)