Fixtures/Fluids/Sharpening

FIXTURING
Fixturing is important. The forces on any type of broach fixture will probably exceed those encountered in any other machining process, simply because so many more cutting teeth are in contact with the work at one time in broaching. Fixtures are also important because of the tremendous cost savings they can produce by reducing work handling time and labor.

Nevertheless, the principal function of a fixture is to locate and hold a workpiece rigidly during the cutting stroke of a broach tool. Other functions - such as guiding the tool, speeding loading and unloading, or coordinating the broaching machine with other machines - are all secondary.

One trend today in fixture design is the automation of fixture action to assist in integrating broaching machines into transfer lines and other automatic machine systems. A second trend is toward universal fixtures that can hold similar, but not necessarily identical, workpieces.

But fixture design is basically the job of the machine-tool builder; the user need only provide the necessary dimensional, machining, and production data for the job.

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 workpiece, 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 workpiece 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 workpiece 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 workpiece 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 workpiece. 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 provide4 exceptional lubricating properties while also delivering the advantage of faster cooling than conventional straight oils. In that synthetics have the viscosity of water (32 viscosity) 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 involves trying several fluids t determine which give the best performance. 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 workpiece materials and the cutting fluids used in broaching them should be considered as a starting point rather than a recommendation.

BROACH SHARPENING
This section deals only with the sharpening of high-speed-steel broaches. Not only are they the most common broach types, but the other principal cutting edge, carbides, now are used in throwaway forms.

The original grinding is the responsibility of the broach producer. Resharpenings are the responsibility of the user, but the broach tool may be returned to the producer for resharpening in the producer's plant.

With proper care. and use most broaches may be sharpened numerous times However, the high-speed tool-steels used in making broaches include some of the most difficult-to-grind steels known. For this reason it is not unusual for a secead-choice tool steel (in terms of tool life) to be chosen over a slightly better steel if the first choice is extremely difficult to grind. This decision frequently arises with abrasion-resistant PM-4 orT-15 high speed tool steel. The choice of a less-abrasion-resistant grade that costs less to resharpen often will more than offset any cost savings gained from the longer tool life of the PM-4 or T-15 tool.

Internal Broaches
Internal broaches are sharpened by grinding them only on the face. Metal removal on the top of the teeth changes the dimensions of the broached surface. Grinding on the tool face requires a small grinding wheel inclined at an angle greater than the face angle because of the geometry involved. Internal broaches may be retapered to remove abrasion in some cases.

Surface Broaches
Surface broaches normally are resharpened on the face, but may be reground on the top of the teeth if excessive wear lands exist. When this is done the original dimensions may be re-establfshed by shimming the broach on its holder. Care also must be taken to regrind the gullet space to the original tooth depth so that adequate chip space is maintained. Chipbreaker notches must be reground.

Grinding wheels suitable for broach resharpening are mostly of the vitrified aluminum oxide type, usually with grain sizes between 46 and 100.