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Re: broaching & sawing
Re: broaching & sawing -- NDR	Post Reply	Top of thread	Forum

Posted by: WhiteTiger ® 02/04/2006, 01:13:01 Author Profile eMail author Edit

Sawing, aside from simple cut off operations, is primarily used for prototype, one of a kind, and very short run production. Cold sawing and bandsawing are the major profiling/contouring methods. The advantages are that for unique or very small lot parts there is a minimum of machine setup and operating time and relatively low power consumption since only the material of the kerf need be removed. Intricate contours can be accurately sawed from plate, sheet and block material without complex fixturing or tooling, provided the machinist is skilled and the layout is done well. The down side is that tolerances must be loose on the high side and for precision fits and dimensions secondary operations are mandatory. Production sawing is almost unheard of today aside from cut-to-length operations on bulk stock. Almost anything that would have been a bandsaw operation decades back is now either punched or auto-cut on cnc guided torch, plasma or waterjet machines. The exceptions are for one of a kind work or extremely short runs in small specialty shops. Broaching is far more involved as far as tooling and machine requirements are concerned. Even simple keyway broaching requires a dedicated machine to achieve any significant production volume. Most products which would have been broached in the early 1900's are now hobbed, swaged, drawn, extruded or produced on advanced rotary cutter machines or machining centers. Broaching still has applications, particularly in producing specific, precise internal contours. An example would be the broaching of open end and box wrenches. Due to the nature of the process, high precision tolerances are difficult to achieve with broaching in most cases. Where general tolerances run in the hundredth range rather than the thousandth, broaching can be very cost effective since the cutters have a relatively long life owing to the distribution of cutting stresses over many more cutting teeth than is common with rotary cutting tools. Depth of cut per tooth is normally very minimal as well, since the chips have to be able to tranport out of the cut in the spaces between teeth. It all adds up to longer cutting tool life which decreases total process cost per part. One of the seeming paradoxes of broaching is the fact that where most removal and forming processes have difficulty with precision internal work, proper broaching can often produce equal or higher tolerance and quality work than the competing processes and do it at less total cost. This is particularly true of producing axial internal profiles in spaces considered deep holes, such as splining the interior of milling machine spindles, or producing deep section planetary ring gears. The biggest downsides to broaching are expense in finances and shop floorspace for the dedicated machinery, and the need for a good toolroom to keep the broaches in shape and to produce any "specials" that may be needed, either by profile grinding standards or producing the broaches from blank stock. Broaching these days is mostly relegated to mid-level production volumes where a skilled workforce can adapt quickly to changing work demands and make flexible semi-automatic processes pay.

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