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Oster Power Threading Handbook

Thread Cutting Fluids

Cutting fluids are extremely important to successful thread cutting in general, but are critical to generating good threads with chaser type equipment.


The lubricity of the cutting fluid allows the tool to move over the surface of the material with minimum friction and therefore minimizes the power required and the heat generated. Some of the energy put into cutting the thread is used to shear the chip from the material, some is lost to the friction of the chaser with the material being turned, and some is lost to the friction of the chip with the face of the die. Good cutting fluids reduce the loading of the die at all of these points. If the cutting fluid does not do its job, more energy is required and this energy is converted to high temperatures at the cutting edge. These high temperatures result in rapid wear of the die, welding of the chip to the cutting edge and torn of disfigured threads.

Lubricity, or the friction-reducing character of the cutting fluid, can be increased by a number of methods. The most important methods involve the use of liquid film and metal film lubricants. Liquid film lubricants are the components of the cutting fluid which do not react chemically with the surface of the work, but in themselves are slippery and reduce friction. These components of cutting oils are similar to those found in machine and motor oils. The metal film lubricants are formed by the oil when it combines chemically with the surface of the material being cut. Generally, this is done by adding elements of the oil which will rapidly form inter-metallic compounds on the surface being cut. These compounds are easily machined and have very low coefficients of friction. Typical additives for forming metal film lubricants for threading are sulfur and chlorine.

Oster Bestoil contains numerous specifically formulated organic compounds which contribute to both the liquid film and metal film lubricity. Bestoil is very high in active sulfur content and chlorine. It is specifically formulated for the extreme conditions found in chaser threading. Bestoil was developed by the Oster Manufacturing Company many years ago for use with our line of threading machines. Development of the oil has continued over the years to keep abreast of the newest technologies in lubrication engineering and to remain effective even in today's machines which are faster and used regularly for difficult-to-machine alloys.

Heat Capacity

Cutting fluids for threading must absorb heat at a rapid rate and conduct that heat away from the cutting surface as quickly as possible. Excessive heat at the cutting surface leads to rapid wear of the dies. As the temperature of the cutting edge increases, the hardness of the die material gets lower and wear is accelerated. Also, excessive temperatures lead to welding of minute particles of the material being cut to the surface of the die. This results in torn threads, more friction, more heat, and a general degradation of the threading system. Water would be an excellent coolant for threading because it absorbs heat very rapidly. Unfortunately, it has very low lubricity. Oster Bestoil has a very high specific heat, and therefore absorbs heat readily. It tends to keep the cutting surface cool and yet has all of the necessary lubricity to prevent excess wear.

Viscosity and Wetting Properties

Cutting oils used in chaser type threading need to have an ability to cling to the surface of the material being cut in order to promote formation of the metal film lubricants and to absorb as much heat as possible. Further, they must retain reasonable viscosity levels over a wide range of temperatures. They must continue to operate efficiently from freezing temperatures to near the boiling point. In service, the oil which is in intimate contact with the cutting surface may reach temperatures even several hundred degrees higher.


Good cutting fluids must remain effective throughout their life. Bestoil is formulated so that it contains no volatile components which evaporate from the surface and change the characteristic of the oil with time. It is also resistant to oxidation. The make-up of oil required to replace oil which is carried away with the finished pipe is sufficient to maintain the balance of materials in the oil reservoir so that the oil will remain effective for long periods of time. There is no need to dump the contents of the reservoir because the oil is "worn out". The only reason to change the oil is if it becomes contaminated with foreign materials such as other oils, solvents, grits, etc. Keep the oil free of particulate matter as much as possible by keeping the filter, chip tray and reservoir clean. Never mix oils of different types. The results may work well, but they may result in damage to the dies, or work, or both.