Modern Machine Shop

DEC 2018

Modern Machine Shop is focused on all aspects of metalworking technology - Providing the new product technologies; process solutions; supplier listings; business management; networking; and event information that companies need to be competitive.

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CNC TECH TALK MMS DECEMBER 2018 40 Processes and Programming AUTHOR | Mike Lynch Founder and President, CNC Concepts Inc. DEBURR ANY CROSSHOLE example, the headstock may be perpendicular to the bed. As it warms, only the height of the cutting tool's edge changes. This minimizes the amount of machined diameter variation from part to part as the machine warms up. When purchasing any new CNC machine, you should understand how the builder deals with thermal variation. More importantly, you must confirm that machined-surface variations caused by thermal growth during warm-up will not exceed tolerances. Otherwise, you could be in for a productivity-wasting surprise when you discover that your new machine must run for a warm-up period before it can be used in production. Some of the most severe repeatability issues have nothing to do with machine design. Instead, they are inf luenced by the machine's application. Variations of any kind—during a production run or from one time a job is run to the next—can impact repeatability. Things that change from cycle to cycle will cause the need for a time-con- suming adjustment. If the variation is great enough, it could result in scrap. Examples of variations during a production run include: • Tool wear. As cutting edges wear, machined surfaces will vary. External surfaces grow while internal surfaces shrink. • Dull tool replacement. When dull cutting tools are replaced, extreme caution is required to ensure that cutting edge(s) do not vary from their prede- termined position(s). From one time a job is run to the next include: • Workholding setup. Many factors affect work- piece stability (placement/alignment of the work- holding device, clamp location and force applied, and program zero assignment, for instance). • Cutting tool assembly, measurement and offset entry. Component and assembly variations result in rigidity variations that can lead to machining issues. • Machine condition. Variations caused by mis- haps and the neglect of preventive maintenance can result in sizing problems with jobs that have run successfully in the past.

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