Hard Milling Is Not That Hard

Hard milling of injection mold cores, cavities and inserts is one of the best ways to shorten delivery dates and save money. In fact, if you are not already hard milling, you will soon find yourself at a distinct competitive disadvantage. Injection mold making is a perfect fit for hard milling, and this has permanently changed the mold making process.

A friend of mine decided to try his hand at hard milling without making much of an investment in his spindle, machine rigidity or carbide inserts. You can guess what happened: he broke the inserts, the surface finish was horrible and the dimensions were not even close.

Mold making and milling
Using Iscar Carbide Cutters

If you are serious about hard milling, you should definitely invest in a machine that is designed for this purpose.

Benefits of hard milling

Reduce set upsReduce EDM workReduce hand fitting
Geometry is true to CADReduce polishingExpedite workflow
Reduce grindingShorten delivery datesGain new clients

Those are some pretty compelling reasons to use hard milling! After all, who doesn’t want to achieve any or all of the above benefits?

Hard milling machines are different

A CNC  machining center used for hard milling should have

  • A column and base that is heavy
  • Box ways with linear roller bearings
  • A spindle that is core cooled
  • Ball screws that are dual supported
  • Thermal stability and structural rigidity
Mold making and Lean Mfg.
Mold Making And Lean Mfg.
You don’t need CAD/CAM software that is engineered specifically for hard milling. But your quality will suffer as a consequence.

This is because the requirements for hard milling are different than conventional milling of soft steel. The typical software will cause the cutter to move in a jerky manner, which will shorten tool life dramatically and fail to achieve the desired accuracy and surface finish.
Other tool path requirements

  • The need to control how the cutter enters and exits the cut
  • The need to maintain a steady, constant chip load
  • Ensure that the shock conditions for each roughing and finishing pass are maintained

The idea is to produce a surface that is true to the model, dimensionally accurate, has a good surface finish and do it quickly! Sounds like a lot to ask, but it is done everyday by progressive mold making shops around the world. Having the right software is essential.

Hard milling and spindles
The cutting tool and the tool holder act as one unit. The spindle must be able to protect the integrity of this unit. Therefore, it must be designed for the high speeds that are necessary.

Direct drive spindles are called for in hard milling applications. Gear and belt driven spindles are not advised. The control of heat and vibration is also extremely important.
The importance of the spindle cannot be overlooked because it is the link between the machine tool and the cutter. If the spindle is inadequate, the entire process will suffer greatly.

Hard milling CNC tooling?
Shrink fit toolholders and an HSK interface are mandatory for hard milling. Sure, you can use other methods, but this combination has proven to be very effective. The shrink fit toolholders are the most accurate available today and they are very easy to use.

The HSK interface is simply the most accurate, secure and stiff type in use today. Failing to use the proper toolholders will shorten tool life dramatically because the chip load will fall on on flute and quickly cause premature wear.

Conclusion
The hard milling of plastic injection molds is becoming a standard method of mold making. There are so many benefits associated that shops who are slow to “get onboard” will soon be at a serious loss to compete.
With today’s sophisticated CNC milling machines, coated carbide end mills, ceramic inserts, advanced software and reasonable prices, hard milling is within the reach of almost any competitive mold shop.

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