Mass Finishing Process Considerations

The mechanical finishing department in virtually every metal-working plant is used as a general rectification shop. This is inevitable because metal forming machinery does not produce consistent burrs or truly consistent surface and edge finish conditions. Equipment in the mechanical finishing department should be sufficiently versatile to meet the changing quality of parts.

Cleanliness. Most mass finishing processes tolerate some oil, scale, and dirt on components. In fact, mass finishing is frequently used as a highly effective means of cleaning components, having both mechanical and chemical action; however, some operations, particularly super finishing on very high-precision parts, demand cleanliness or at least consistency of any surface contamination. The finishing department should be notified of any changes in prior operations, such as change of machining lubricants or heat treatment methods.

Ancillary Equipment. Most mass finishing processes are wet operations. After parts are unloaded from the mass finishing equipment, parts are rinsed, corrosion protection is applied, and the parts are dried. A decision must be made whether these processes are separate from the mass finishing machine or if combination compounds could incorporate these steps in the finishing machine.

There is a requirement to separate parts from media by screening or magnetic separation. Media reclassification is also frequently incorporated into a screening separator.

Automation. Opportunities to automate mass finishing should be investigated. Mass finishing operation requires measured quantities of parts and media to be placed into the equipment; controlled addition of compound solution with change of solution at some point during the process cycle; unloading of the equipment; separation of parts from media; washing, drying, and conveying parts; and classification, conveying, and storage of media. Such a sequence may represent a substantial labor involvement and many opportunities for errors that are difficult to control. The cost of automation is repaid by improved work flow, improved quality, and lower labor costs.

Process Instructions and Control. Process instructions should cover all variables, which include:

• Machine cycle, including process times, barrel speeds, or vibratory frequency and amplitude, or G force and speed, depending on type of equipment

• Load levels, total load, and proportion of parts to media

• Compound and water flow rates

• Media and compound variables

• Pretreatment and post-treatment variables

Masking and Fixturing. Holes in components with rubber or plastic are plugged to keep media from being jammed into those holes during processing. Corners and edges that must be left sharp can also be masked. Some components are processed on fixtures in the mass finishing equipment. This is essential for spindle finishing. In some mass finishing equipment, large components are better processed in individual compartments within the machine. Occasionally, threads are masked that might get damaged or abraded too much during processing.

Causes of Difficulties Commonly Encountered in Mass Finishing. The following is a list of difficulties encountered during mass finishing processes:

Excessive impingement on part surface

• Insufficient amount of media in chamber

• Parts in work load too large for the machine

• Excessive equipment speed or amplitude

• Low solution level or flow rate

• Insufficient or wrong compounds

• Undesirable handling methods

• Media too large for part size

Rollover of edges, corners, and burrs

• Abrasive action too slow

• Media particles too large for part size

• Excessive equipment speed or amplitude

• Excessive work load for volume of media

Incorrect water level

• Processing time too long

Persistent lodging of media in holes, slots, or recessed areas

• Wrong shape of media

• Excessive wear or depreciation of media

• Media fracturing during operation

• Failure to classify media as required

Poor surface finish

• Insufficient cutdown in process

• Inadequate flushing and cleaning of equipment

• Incorrect compound

• In-process corrosion

• Water flow not correct

• Media too aggressive or too hard

• Part-on-part impingement

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