MoldMaking Technology

AUG 2018

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Machining 14 MoldMaking Technology —— AUGUST 2018 the cutting area, reducing the need for re-cutting and improving tool life and surface finish. Basically, placing the core and cavity on a vertical fixture in an HMC provides better access for the spindle to reach both the core and the cavity. Also, gravity aids the natural flow of chips and coolant from the core and the cavity. By contrast, if the core and cavity were flat on the table of a VMC, both would retain chips and coolant. The flat orientation makes it difficult to remove chips and coolant from the cutting area, so the spindle and cutter continuously encounter a collection of chips and coolant. This results in "re-cutting" of the chips, "rubbing" them against the surface to be machined, which causes additional tool wear and poor surface finish on the cavity. The key to coolant longevity is maintaining good coolant performance as long as possible without needing to clean or recharge the coolant system. HMCs offer better coolant man- agement than VMCs. The flat orienta- tion of a VMC delays or prevents the coolant that "pools" in the cavity and core from returning to the machine coolant filtration system. This affects the machine's ability to handle chips effi- ciently. Typically, HMCs have more cool- ant volume and higher sophistication in coolant filtration. This lengthens the coolant's serviceable life and minimizes the labor and expendable investment. Another HMC advantage, permit- ted by its chip-management design, is the typical use of an index table (or full contouring fourth axis) under the work or pallet. This table facilitates index- ing or rotating the workpiece relative to the spindle, which provides angular access to the workpieces for shorter, stiffer tools that can produce better sur- face finishes. This also provides access to multiple sides of the workpiece. Minimizing workpiece handling, reducing lead time, cutting costs and improving quality by consolidating multi-face features into a single setup. Most VMC designs do not include an indexing (or rotary) table under the workpiece, which prohibits positioning the workpiece relative to the spindle. A VMC requires the part to be handled or re-fixed for each operation, which means that the machine and spindle are idle. So, not only does the HMC approach pro- vide more parts to the spindle in a single setup, which amortizes non-cutting time over more parts, it also requires less han- dling and fewer setups, which increases spindle utilization time on the HMC. Also, when comparing an HMC to a similar VMC, the HMC provides signifi- cantly more usable workspace at a ratio of more than two to one when consider- ing the total working volume. The HMC,

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