07-09-2012, 09:39 AM
Broaching & Sawing Processes
Broaching_Sawing.pptx_neww.pptx (Size: 1.38 MB / Downloads: 74)
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.
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.
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
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.
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
Advantages
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
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