Because they affect both machining precision and the service life of the metalworking fluid, slideway lubricants are integral to the productivity of modern machine shops. Here are some tips from Mobil Industrial Lubricants on selecting the correct slideway lubricants for your application.

Posted: December 30, 2011


Because they affect both machining precision and the service life of the metalworking fluid, slideway lubricants are integral to the productivity of modern machine shops. Here are some tips on selecting the correct slideway lubricants for your application.

The selection of slideway lubricants is integral to the productivity of modern machine shops. Slideway lubricants can have a direct impact on the productivity of machine tools by affecting both machining precision and the service life of the metalworking fluid. The optimum slideway lubricant must not only provide superior friction control to ensure high machining precision, but also provide excellent separability from aqueous coolants that are commonly used in metalworking operations.

Slideway lubrication is typically a total-loss, open system. As such, slideway lubricants can contaminate the coolant circulation system and eventually form “tramp oil” if aqueous coolants are being used. In fact,“tramp oil” is one of the primary contaminants found in such product and can adversely affect its working life.

The ability of the slideway lubricant to separate quickly and completely from water based coolants is an important characteristic. If oil separation is not complete, the negative effects on aqueous coolant performance can result in higher operating costs and unscheduled machine downtime. High quantities of tramp oil in aqueous coolants can:

  • Change the coolant concentration, making monitoring difficult
  • Affect lubricity leading to tool wear and poor surface finish
  • Increase the risk of bacterial growth and undesirable odors
  • Reduce coolant pH levels, potentially causing corrosion
  • Promote excessive coolant foaming

Conversely, contamination of the slideway lubricant with aqueous coolants can compromise the performance of the slideway operation. Emulsification through poor separation characteristics may result in:

  • Reduced lubricity leading to increased friction
  • Increased slip-stick, and reduced machining precision
  • Potentially higher energy consumption
  • Wear of slideway contact surfaces or coating materials
  • Corrosion of components and machines

To summarize, good slideway lubricant/coolant separability properties allow the machine tool to operate with optimal precision, and will maximize the life and performance of the aqueous metal working coolant. There are a number of standardized separability tests recognized in the machine tool industry and they are described below.

The Toyoda test simulates a slideway lubricant contaminating a coolant reservoir. 90 ml of coolant (5 percent concentration) is placed in a cylinder with 10 ml of slideway lubricant. Agitated vertically for 15 seconds, the cylinder is then allowed to rest for 16 hours. The volume, in ml, of three phases is then measured: separated oil (top), ‘‘cream” or emulsified oil and water (middle), and separated coolant (bottom).

In the example above, the test result is recorded as 90/0/10 (90 ml coolant, 0 ml cream, 10 ml slideway lubricant) which indicates “complete separation” between oil and coolant. A result of 98/2/0 (98 ml coolant, 2 ml cream, 0 ml slideway lubricant) indicates that a stable emulsion has formed and the coolant and slideway lubricant do not separate readily.

This test simulates the situation where aqueous coolant contaminates the slideway lubricant. Slideway lubricant is tested against 11 standard coolants at a ratio of 80/20, 8 ml of slideway lubricant and 2 ml of coolant. The mixture is agitated at 1500 rpm for one minute and then visually inspected after 1 hour, 1 day and 7 days. A 1 to 6 rating is given according to the following key:
1 = Complete separation
2 = Partial separation
3 = Oil + intermediate
4 = Oil + intermediate + emulsion
5 = Intermediate + emulsion
6 = All intermediate

Even if a slideway lubricant with good separability characteristics is in use, cross contamination of coolants with various machine tool oils requires attention. Oil contamination in the coolant comes not only from slideway lubricants, even though this is the most common source, but also from other lubricants such as hydraulic oil, gear oil and grease.

If not periodically removed, tramp oil can accumulate and cover the coolant surface, preventing contact with oxygen. This situation provides a suitable environment for growth of anaerobic bacteria in the coolant resulting in reduced service life and foul smells. Moreover, tramp oil can become emulsified into the coolant through system agitation compromising the performance of the coolant. High volumes of tramp oil can also interfere with coolant monitoring.

Routine monitoring of the coolant concentration is a very important process to maximize coolant life. The simplest method is to use a refractometer. Normally the line will be sharp but if there is a significant amount of emulsified slideway lubricant in the coolant, the view in the refractometer will be obscured, indicating high levels of tramp oil.

Many modern machine tools are equipped with automatic oil skimmers that run constantly to remove tramp oil. Belt skimmers can also be purchased separately. In larger systems, filters and centrifuges are commonly used to remove tramp oil and other contaminants. Alternatively, removal of tramp oil can be achieved manually using for example an industrial vacuum cleaner.

Offsite laboratory analysis is another option to quantify the presence of tramp oil. Coolant concentration can be measured by titration and then compared to fresh coolant to determine the emulsified tramp oil level.

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