# What is the relationship between the type of material(ex: steel, iron etc.), thrust and torque?

If I am drilling through bars of different material(ex: iron, steel etc.) of the same thickness and under the same conditions of drill operations(feed rate and rpm), how will the torque and thrust force measurement change for different materials? For example, let us assume that the materials in question are iron, steel and aluminium.

• It's not recommended to drill these different materials at the same feed rate and RPM! What's the reason behind this question? – Jonathan R Swift Nov 7 '20 at 8:26
• It is just out of curiosity. I wanted to know how such a thing would affect thrust and torque – sumit808 Nov 7 '20 at 8:30
• each material behaves differently, so each needs their own rpm etc otherwise it breaks things or worse. – Solar Mike Nov 7 '20 at 8:32

Usually what happens when you are trying to determine parameters about drilling, lathing or milling a material you need to consider the following parameters:

• material that is being removed
• material of the bit that does the cutting
• the relative velocity between the material and the "bit".
• determine if a liquid medium is used to remove heat.

There are two main considerations:

• heat developing due to removal of the material.
• quick removal of the material from the cutting area.

The concentration of heat is a parameter that affects the cutting bit and can lead to its very fast degradation of the properties. That is the reason that high speed is -in many cases- detrimental to the "life" of a drill bit. High speed is usually correlated (or driven) from high torque or high power input. Way to mitigate the problems caused by heat are:

• reducing rotational speed and feed rate
• introducing a lubricant for removing the excess heat as quickly (water should not be used because it might corroded the material).

The second issue (which is more common to drilling), is that you need to pick a geometry that for the type of material (e.g. brittle/ductile, metal/plastic) removes the excess material as quickly as possible. Keeping the material near the cutting region, is wasteful, it dulls the cutting bit, and can also lead to jams.

As a bottom line:

Different combinations of materials, cutting bits and cutting bits geometries have different optimal feed rates and rpms for drilling (but also for milling and lathing).

What you are interested boils down to the "specific cutting force" ($$N/mm^2$$) parameter - which varies by material. Not just steel or aluminum - but what alloy (e.g. addition of lead acts as a lubricant and makes it easier to machine), and hardness treatment it has. In approximate terms: Aluminum 800 MPa, Iron 1500 MPa, Steel 2500-3000 MPa. So very roughly 3x as much electricity or torque is required to cut steel vs aluminum (if same speed, feed, and geometry).

You may also see it expressed as "Specific Energy" ($$J/mm^3$$) = how much energy is required to remove a cubic millimeter. According to Kalpajkian, the difference in specific energy can be more than 20x for some steel to aluminum alloys.

Good references for more detail:

For the same RPM and feed ,the relative torque answer is; 1 . cast iron ( grey) = vey low, 2 aluminum = low ,3 steel medium to high depending on hardness.