Chuck ColonairClick image to enlargeby Charles Colonair

Hard milling materials up to 66 HRC requires the right techniques

 

Today's moldmakers are seeking reduced lead times while producing highly accurate moulds. And when mould and die applications demand hard milling of materials up to 66 HRC, using the right end mills, cutting parameters and machining methods is critical for success.

Hard milling is challenging due to several factors, including the requirement of machining hardened steels with defined cutting edges. Milling these materials requires high speed machining centres set-up for hard milling, along with CAD/ CAM software which has been programmed accordingly.

Using the latest solid carbide end mills along with proper toolholding, workholding, feeds, speeds and cooling process, moldmakers can reliably and economically produce complex moulds. Complicated contours can be accurately machined, and time-consuming grinding or polishing rework may be omitted entirely or at least partially.

1. Choose end mills which meet the following requirements:

  • Solid carbide substrate with a high hot hardness and toughness.
  • Coating and cutting geometry that is matched to the material hardness.
  • The concentricity of the tool should not exceed 5 microns. Meeting this requirement will extend tool life and promote excellent surface finish quality.

2. Determining optimal cutting parameters and methods will yield the best results for hard milling of moulds:

Select a speed and feed rate recommended for the type of workpiece and material hardness as well as the engagement conditions of the tool. Consult your end mills partner for technical data recommendations for specific applications. When hard milling, dry machining is preferable to avoid thermal cracks in the cutting tool. For example, when using multi-flute end mills, cold air provides an effective cooling of the tool's cutting edges, resulting in efficient chip material removal.

Cooled air reduces temperatures in the cutting area, which in turn permits higher cutting speeds and up to 30 per cent longer tool life. This type of cooling enables modern TiALN coatings to achieve their full potential, avoiding damage to the cutting edge from thermal shock. In addition, the cold air nozzle helps to remove the tiny chips produced in copy milling, including from deep recesses or cavities in the mould.

3. Proper clamping of the tool and workpiece are also important for successful hard milling:

  • High rigidity and good vibration damping of the machine tool and the work holding devices.
  • High concentricity of toolholder for high surface finish quality and consistent tool life.

Hard milling with solid carbide end mills is an established and reliable way to manufacture moulds with hardness of up to 66 HRC. Secondary and grinding operations can be omitted for increased productivity and enormous cost savings.

Hard Milling Application Example: Forging Die

This forging die application example was done with one hard milling operation during the material hardened stage using Emuge Hard-Cut End Mills, eliminating multiple steps such as soft machining, pre-finishing, finishing and polishing.

  • Machine: Hermle C40
  • Control: Heidenhain
  • Material: CPM 3V (PM-steel), 61 HRC
  • Coolant: Cold Air

chartClick image to enlargeResult: forging die completely finished on a five axis machining centre with excellent form and position tolerances. Time savings: soft machining, EDM and polishing was omitted.

Charles Colonair is the product manager-milling, for Emuge Corp., West Bolyston, MA.

 

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