Broaching & Sawing Processes Instructor - Sam Chiappone

Broaching Broaching is the process of removing metal with a tool which has “teeth” arranged in a row. Each tooth is successively higher than the previous tooth and removes more material. In broaching, one stroke or cycle of the machine produces a finished part. Broaching is used to produce both internal and external features. Production rates are high and tolerances of +/- .0005” are possible.

Broaching

Broaching

Broaching

Broaching Chip Formation Chip formation involves three basic requirements: The cutting tool must be harder than the part material There must be interference between the tool and the part as designated by the feed rate and cut per tooth There must be a relative motion or cutting velocity between the tool and workpiece with sufficient force to overcome the resistance of the part material. http://www.generalbroach.com

Broaching Tool Feed Direction Tool Gullet Depth of cut per tooth Workpiece

Broaching Chip Formation As long as these three conditions exist, the portion of the material being machined that interferes with the free passage of the tool will be displaced to create a chip. Many combinations exist that may fulfill such requirements. Variations in tool material and tool geometry, feed and depth of cut, cutting velocity, and part material have an effect not only upon the formation of the chip, but also upon cutting force, cutting horsepower, cutting temperatures, tool wear and tool life, dimensional stability, and the quality of the newly created surface. http://www.generalbroach.com

Broaching The Mechanics of Chip Formation Empirical metal-cutting studies reveal several important characteristics of the chips formed during the broaching process: The cutting process generates heat The thickness of the chip is usually greater than the thickness of the layer from which it came The hardness of the chip is usually much greater than the hardness of the parent material, and The other three relative values are all affected by changes in cutting conditions and in properties of the material to be machined http://www.generalbroach.com

Broaching The Mechanics of Chip Formation These observations also indicate that the process of chip formation is one of deformation or plastic flow of the material, with the degree of deformation dictating the type of chip that will be produced. http://www.generalbroach.com

Broaching Plastic Deformation Originally, it was thought that chips formed in metal cutting were created in much the same way that wood chips are formed when split by an axe. This may be partially true for brittle materials such as cast iron, but it does not hold true for the majority of metals. The process by which chips are formed with metal-cutting tools is called plastic deformation, and was first described by Rosenhain at the Stratsfordshire Iron and Steel Institute in 1906. http://www.generalbroach.com

Broaching Plastic Deformation What actually happens in this shearing process is that the metal immediately ahead of the cutting edge of the tool is severely compressed resulting in temperatures high enough to allow plastic flow. When the resisting stresses in a material exceed their elastic limit, a permanent relative motion occurs and further deformation is withstood. This strengthening is called work or strain hardening, and is characteristic of all steels, but demonstrated most dramatically in stainless steels. http://www.generalbroach.com

Broaching How and Where Heat is Generated The force or energy that is put into the tool creates movement in a group of metal atoms in the workpiece. This group is a finite number of atoms which are forced to change their positions in relationship to each other. As the atoms in the metal ahead of the tool are disturbed, the friction involved in their sliding over one another is thought to be responsible for 60% or more of the total heat generated. This internal friction, and the heat it generates, can be compared to the friction and heat caused by bending a paper clip back and forth until it breaks. http://www.generalbroach.com

Broaching How and Where Heat is Generated As the tool continues to push through the work piece, a chip eventually slides up the cutting face of the tool. This sliding creates an external friction which again releases heat. This external friction accounts for about 30% of the total heat generated. The third area of heat generation is on the land or flank of the tool. This area accounts for about 10% of the heat generated. This is assuming that the tools are sharp and made correctly as far as clearance angles and face angles are concerned. As the tool wears, the above percentages will vary, especially when there is excess wear on the land, or if the clearance angle is insufficient for the material or the part configuration. This contact zone will actually increase as the part continues to close in after the cut resulting in extremely high pressures on the land area of the tool.

Advantages & Disadvantages Rough to finish in one pass Production rates are high Cutting time is quick Rapid load and unload of parts External and internal features Any form that can be produced on a broaching tool can be produced Production tolerances are excellent Surface finishes are equal to milling Operator skill is low

Advantages & Disadvantages Tooling cost can be high In some cases--not suited for low production rates Parts to be broached must be strong enough to withstand the forces of the process Surface to be broached must be accessible

Methods of Operation Pull broaching - broach is pulled through or across stationary work Push broaching - broach is pushed through or across work Surface broaching - either the work or the broach moves across the other Continuous broaching - the work is moved continuously against stationary broaches. The path of the movement may be straight or circular.

Machines Vertical single and double slide - Table moves part into position for broaching, part is broached and the table retracts for unloading. Vertical push broaching - Used for internal features such as holes, rounds, or slots. Vertical pull down Tool is suspended above work Lowered into pull mechanism in the base of the machine Advantages Part positioning is easy Large parts are handled efficiently

Machines Vertical single and double slide

Machines Horizontal Versatile machine capable of producing internal and external features key-ways gear teeth riffling High cutting speeds in the range of 10-40ft/min with return speeds of 110 ft/min MRR of ¼ in per stroke is possible

Methods of Operation

Machines Horizontal

Machines Rotary Parts are mounted to a rotating table and are moved to different stations for different operations Primarily used on small parts Typical operations include: Slotting Holes Key-ways

Broaching Tools Design & Construction Considerations Material to be broached Size and shape of cut Quality of surface finish Part tolerance Productions rates Type of machine Workholding method Strength of the workpiece

Sawing Depending on part tolerance, sawing can be a vital first operation or the total process.

Machines Classifications Reciprocating saws Horizontal hacksaw and vertical sawing machines Light to heavy duty Simple and most economical to operate Manual to fully automatic feed mechanisms Uses blades similar to hacksaw blades

Machines Classifications Circular saws Sometimes called cold sawing machines Saw blades are large and rotate at low Rpms Cutting is similar to a milling operation due to geometry of saw blade

Machines Classifications Band saws Irregular shapes Very versatile Profile cuts Internal cuts external configurations Blades are continuous HSS - Carbide tipped Diamond impregnated Filing

Machines Classifications Band saws 2 1 3

Saw Blades Circular Straight Band

Saw Blades Terminology Set of the blade - Directional offset, left-right, from the blades centerline. Sometimes referred to as the “kref.” The kref provides clearance for the blade as it cuts through the work. Straight - one tooth left one tooth right. Typically used for brass, copper, and plastic. Raker - Three tooth sequence, left, right, straight. Typically used for steel and cast iron. Wavy - Alternate arrangement of several teeth to the right and left. Used to cut tubes and light sheet metal.

Saw Blades

Saw Blades Tooth forms Variable positive Variable Standard Skip Hook