# Force required to crop/cut a mild steel bar

I have cropping/cutting wedges with a thickness of 4 mm cutting either side of a round mild steel bar (17.5 mm). Imagine two wedges being driven together with a round mild steel bar in the middle (similar to the design of bolt cutters) These wedges are operated using 2 hydraulic cylinders (6" diameter, 2 1/2 travel) (one for each cutting wedge) to force them together (against the mild steel). The cropping wedges are made from BS 970 304S12 steel.

How can I determine the force required to crop this round bar? What equations/formulas are relevant?

• In the US at least, the value you're looking for is known as tonnage. It is used to describe the force required for shearing, punching, bending, and stamping processes with metals. Commented Jun 22, 2015 at 14:37

This is a simple shear force problem. However there are a few considerations.

1. You don't need 2 hydraulic cylinders. One should be mobile, the other should be stationary. Below is a simple and quick figure to show how they should align.

You should check Sheet Metal Shears they will give you an idea.

1. Below is the simple equation for shear. T is shear stress, it is around 400MPa for 0.3%C steels. If you don't have shear stress values you can use ultimate tensile stress values and multiply by 0.6-0.7, for an approximation. Use cross section area of your bar for A.

2. 304 steels aren't a great idea for this kind of work. It can't be hardened by heat treatment, just by cold working. So you won't have long life from your tool. Consider a martensitic stainless steel such as 410, you can harden it and have much higher life. If you don't need corrosion resistance then consider a heat treatable steel. However if you must use 304, try hardening it by hammering, or if you have rolling. That will increase tool life.

• How accurate is a simple equation based on cross-sectional area, though, when cutting a round bar? I would expect that since the wedges are initially only on contact with a very small part of the bar, the cutting force would be reduced, as when shearing a wide sheet at an angle from one side to the other.
– Air
Commented Jun 22, 2015 at 16:20
• Hi @Air this simple equation should be accurate enough to cut it. Shear forces act on the whole area of bar, so it is larger. In fact it should be larger because I didn't take into account the effect of friction and lateral displacement, and work hardening of material during cutting, dulling of the blade and lateral movement. These factors will have very small effect. If a much more accurate simulation of the process is needed some finite element analysis is needed. But again that simple formula is enough to cut that bar. Commented Jun 23, 2015 at 8:29

Fc = zckzA*f

Where:

Fc = cutting force in kg.

Zc=number of contact teeths (this number is maximun when the saw reach the diameter of the bar.

kz=especific pressure of cutting in kg/mm2 (this value depend of the cutting material).

A=width of saw in mm.

f = feed per stroke in mm.