CUTTING FLUIDS FOR BROACHING
The three functional roles of cutting fluids are to provide lubrication for the cutting tool, to reduce heat at the interface of the tool and the work piece, and to flush away metal chips and fines from the cutting zone. In addition, to these roles cutting fluids can improve the surface finish of the machined part.
In broaching, temperature reduction is an important aspect of increasing tool life. Studies have indicated that the reduction of heat at the tool interface by as little as 50 degrees can increase tool life by as much as 50% in addition to increased tool life through temperature reduction, the sheer angle of the chip is increased thereby reducing the power requirements and the possibility of part degradation.
While the application of cutting fluids to the point of cut is critical in broaching operations, it is not always an easy thing to accomplish. An internal broach tool, for example, may receive an adequate supply of cutting fluid upon entering the work piece however; upon entering the work the fluid is retarded. This has been noted in some horizontal broaching operations where surface finish, and cutting tool life are good at the starting half of a horizontal internal shape but poor at the final half. During horizontal internal broaching, the flow of cutting fluid into the interior of a work piece is restricted by the cutting teeth. Fluid trapped between the tooth spaces flows by gravity to the lower half of the tool; the upper teeth may be cutting dry in a very short time. The problem can be resolved by submerging the work piece in cutting fluid during the entire broaching operation. When broaching long internal shapes such as rifle barrels, high pressure streams of cutting fluid can be forced through the bore and around the broach. This helps to accomplish two functional roles. First it insures the cutting fluid is getting to the cutting zone and that the fluid pressure will flush the chips out of the work piece. High temperature alloys and exotic metals can cause special problems because cutting forces are higher and more heat is generated. One approach is to reduce the cutting speed of the broach thereby allowing heat transfer by conduction and using a high water based synthetic which reduces heat. Many synthetic cutting fluids provide for exceptional lubricating properties while also delivering the advantage of faster cooling than conventional straight oils. In that synthetics have the viscosity of water it is imperative that the broaching tool be fully flooded with the cutting fluid at the point of the cut. The usual approach to selecting a cutting fluid for a particular broaching operation would be to consult with cutting fluid supplier, or involves trying several fluids to determine which give the best performance. The concern with trying several fluids would be that between each trial that the coolant system be completely flushed out so that there is no residue from the other test. The best performance is a combination of finished part quality, tool life and cutting fluid compatibility with the broach and disposal requirements. Regardless of the type of product you ultimately select, you must deliver the cutting fluid to the point of cut in order for the fluid to perform its function.
The following list of work piece materials and the cutting fluids used in broaching them is a suggested possibility upon which the performance of the tool and the material will finally determine what to use.
Material Product type Concentration ratio
Low carbon steel Soluble oil 10%
Free-machining steel Semi-synthetic 5-10%
High carbon steel Semi-synthetic 5-10%
Alloy steel Soluble oil or straight oil 10%
Tool steel Heavy duty soluble oil 5-10%
Cast iron Synthetic 5-10%
Stainless 300 Heavy duty soluble oil 5-10%
Stainless 400 Heavy duty soluble oil 5-10%
High temperature allow Heavy duty soluble oil 5-10%
Titanium Heavy duty soluble oil 5-10%
Low silicon aluminum Heavy duty soluble oil 5-10%
High silicon aluminum Heavy duty semi-synthetic 5-10%