Let's talk about HEAT! What do metalcutting chips and toilet paper have
During my last blog we discussed what two things it takes to make a chip "HEAT and PRESSURE". Know let's talk a little more about the heat and how we regulate it. Every material; steel, stainless, cast iron, non-ferrous, superalloys, and hardened steels have what is called a machinability rating. The proper definition of machinability rating is; the relative difficulty of a machining operation with regard to tool life, surface roughness, and power consumption. I like to say machinability rating; is the amount of heat and pressure required at the shear zone to plastically deform the material and make a chip. Materials are not all created the same therefore; each specific material requires a different amount of heat and pressure at the shear zone to plastically deform and shear away the material in the form of a chip. As mentioned previously you do not want to be too hot as the extra heat will plastically deform the cutting tool. If too cold, the lack of heat will make the material more difficult to deform and the added pressure could fracture the tool. So how do we regulate the heat at the shear zone?
To regulate the amount of heat you must regulate the amount of material that passes the cutting edge of the tool. Think of a roll of toilet paper rotating as you pull paper off the roll. The roll of toilet paper is your part. The paper you pull off the roll is the chip. The faster the material is passed by the cutting edge of the tool the higher the temperature is at the shear zone. The slower the material passes the cutting edge the lower the temperature is at the shear zone. We regulate the amount of friction and heat generated at the shear zone by adjusting the velocity of the surface of the part in lathe operations and the velocity of the circumference of the cutting tool in machining center operations. We call this Surface Speed; which is defined as the rate of the surface of the workpiece that passes the cutting tool. Or based on our toilet paper analogy; it's the amount of toilet paper that is pulled off the surface of the roll.
This is measured as Surface Feet per Minute
in inch and Surface Meters per Minute in metric. As the roll of toilet paper or the diameter of your part gets smaller the circumference gets smaller therefore the rpm must increase in order to have the same amount of material being removed per minute of time. In order for the amount of Surface Feet per Minute is converted into RPM using the following formula; RPM = (3.82*SFM) ÷ DIA. Every manufacturer of cutting tools produces Surface Speed charts to aid in the selection of the proper speed or RPM selection. By selecting the proper Surface Feet per Minute for the material being machined, you can calculate the proper RPM to run to create the proper amount of heat at the shear zone to properly plasticize the workpiece material. Stay tune for my next blog when we will talks about "PRESSURE".