Machining Tool Steel
Tool steel doesn’t offer the end user much benefit in the plate or bar form. Tool steels must be fabricated into the desired shape before they are able to be used. Fabrication is achieved with various methods of machining.
The goal of machining tool steel is to produce a tool of the proper shape and the proper dimensions. The tool should also have a smooth surface and a balanced geometry, if possible. Avoid tearing of the surface, as a rough surface finish can lead to initiation of a crack. For more information see
Avoiding Tool Failure.
Tool steel is usually provided to the machinist in the annealed or soft condition. Steel mills provide the tool steel in the annealed condition so that the machinist can readily machine the tool steel into a useable tool. The typical annealed hardness for each grade can be found on the specific
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Each grade of tool steel will also have specific machinability ratings. The machinability ratings are listed as a percentage based on the machinability of W-1 tool steel. W-1 tool steel has the lowest alloy content and therefore, it is the most machinable. W-1 is assigned a machinability rating of 100% and the other grades are listed as some percentage lower than W-1. As the alloy content of the tool steel increases, the machinability of the grades decreases. The carbide type and distribution of the carbides will also affect machinibility.
Machinabilty ratings for various tool steel classes are listed below.
|Tool Steel Class, Grade
|W (W-1, W-2)||100%||S (S-5, S-7)||60-70
||O (O-1, O-6)||45-60
||A (A-2, A-6)||45-60
||D (D-2, D-3)||30-40
||H (H-11, H-13)||60-70
Machining of tool steel into a useable tool is achieved by the use of sturdy machine tools such as lathes and mills, both vertical and horizontal. Boring machines and gear cutters are other types of machine tools used to fabricate tool steels. A well-built and sturdy machine will operate with less vibration.
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Typical machining operations include turning, boring, drilling, milling and tapping. These operations may be performed on many various machine tools, singularly or simultaneously, with high tech CNC machines capable of machining on 5 axis.
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Rough machining and finish machining may be achieved with cutting tools made from high-speed steel, carbide or ceramics. Each type of cutting tool will have various advantages and disadvantages. Cutting tool choice will be determined by factors such as cutting speeds and feed rates, the amount of heat generated, use of coolant and type of machining operation being performed.
High speed steel tool bits
are very useful for interrupted cuts because they offer toughness levels that are not provided by carbide or ceramic cutting tools. Carbide and ceramic cutting tools offer longer life by providing a high degree of wear resistance and heat resistance.
Rigid tool holders, sturdy work holders and low friction bearings combine to produce less vibration in the machine, which will allow the machinist to hard machine the tool steels. Hard-machining is the machining of hardened tool steel up to RC 65 and may provide a smoother finish while saving operator time.
Cutting tools are available in many sizes and shapes with various coatings to extend the life of the cutting tool. Consult with a cutting tool manufacturer for the proper use of cutting tools for your application.
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