Lubrication is a vital ingredient in the metalformer's quest for part quality, machine productivity, tooling longevity and, in the final analysis...profit.
David Houcque, Tribology Research Scientist
and Arthur E. Dampts, Director, IRMCO, Evanston, IL
Fig.1 -- Illustration of test specimens before and after test
execution.
As a researcher and developer of proprietary lubricants designated for deep drawing, forming, blanking and piercing a variety of materials, IRMCO felt the need to be proactive in the establishment of a universal standard for rating potential lubricant performance with a lab instrument.
Throughout IRMCO's 81 years in the industry it has been involved with many traditional devices and methods for evaluating lubricant performance. Many of these methods, for example, limited dome height (LDH), and drawbead simulator (DBS) provide some indication of potential lubricant performance though they can be time and cost prohibitive as a research tool ($50,000 to $100,000 for the device without any computer or mathematical calculation capability).
In addition, the resulting data aren't tied to any universal lubricant ranking system. With a significant portion of IRMCO's operating budget spent on research, quick turnaround of lab results and the need to rank lubricant performance are paramount. With more than 500 candidate formulations each year (10/wk on average), IRMCO needs to apply the most recent results to the next formulation.
In 1994, IRMCO had the opportunity to participate in a joint lubricant performance study conducted by John Deere Horicon Works and Ohio State University (as outlined in MetalForming, November 1994, article titled "A New Method for Ranking Sheet Metal Lubricants"). This experience introduced IRMCO to the prototype "Interlaken System" (as described in MetalForming, October 1994, article titled "An Affordable Sheet Formability Test System").
In December 1994, IRMCO ordered one of the first commercial Interlaken Systems and since has been evaluating and refining the system. This research also allowed us to reach two very important goals:
The Interlaken device allows for quick formability testing (10 lubricant tests/hr.). The Draw-Scale then ranks the test results to determine potential lubricant performance in a stamping operation.
IRMCO Draw-Scaletm(IDS)
In creating the Draw-Scale, a series of lubricants was selected to display a range of
lubricating values from light-duty to heavy-duty applications.
The scale values for each tested product (lubricant) were obtained by calculating its strain ratio relative to the value established with an unlubricated strip. Strain data was normalized, as indicated in Table 1, to create a draw scale that ranks the lubricants against reference values where an unlubricated specimen carries a rating of 100 and PTFE is rated 200, as determined experimentally.
Fig. 3 shows the ranking of lubricants for the cold-rolled steels that was generated from Table 1.
|
|||||||||||||||
Testing Procedure
A simplified schematic of the test system is shown in Fig. 2. The metal specimen, a
9.5 in. x 0.5 in. x 0.03 in. rectangular strip, is completely clamped around its edges and
the shape is developed entirely at the expense of specimen thickness. Prior to testing,
two parallel lines of 0.5 in. are scribed in the center region of the strip. Specimen
surfaces are carefully cleaned before lubricant is applied.
Fig. 2 -- Simplified representation of the testing device.
Each lubricant is rolled on the cleaned specimen using a coating rod (rod number 12) to obtain effective lubricant film thickness. Strips are weighed prior to testing.
The specimen is locked in the die to ensure a pure stretching operation. The punch stretches the specimen until it fails. To ensure reproducibility of the results, each lubricant is tested at least five times.
The Interlaken testing devices key components are a short stroke clamping actuator, and two dimensional tooling that allows the die to be opened and the specimen removed without a large clamping stroke. Three standard die sets are usually available: the friction die set, the formability die set for thin metals and the formability die set for thick metals and aluminum. The dies in the machine are interchangeable. The friction die set has been used for these tests. More detailed information concerning the machine can be found in references [1]-[5].
The degree and type of lubrication applied to the strip surface can strongly influence strain distribution, but the effects are difficult to predict in advance. Better lubrication tends to permit metal in contact with the punch to deform and slide over the punch, reducing strain localization. A rough (unlubricated) surface, on the other hand, may behave similar to poor lubrication, retarding metal being drawn in from the flange, and forcing higher strains in the punch stretching region.
Moreover, better lubrication creates a more uniform strain distribution and greater punch depth. Our technique compares the variation in length for a given lubricant to that of an unlubricated specimen. A high strain value indicates low friction. Lower friction signifies a more suitable lubricant.
Test Results
On most bulk forming processes, the loads required for the forming operation are
often the primary concern. However, in stretch forming the prediction of strain
distributions and limit strains, which define the onset of local necking, are important.
Localized strain variations can be revealed simply by measuring the relative length at the
center region of the strip.
Since failure often occurred midway between the punch contact and the die contact regions, the first limit is reached when a localized neck becomes visible and the ultimate limit is given by subsequent fracture. By measuring the scribe or grids before and after the experiment, the percentage increase in length of the specimen is determined:
| Relative change of length = [(L1-L0)/L0] x 100 where L1 and L0 are the final and initial scribe lengths. |
The advantage of dealing with strain distribution at the center area of the strip is that the strain value is more constant than that in the edge areas. However, the main purpose of the test procedure is to rank lubricants not to calculate an accurate value of the strain distribution. As long as the same material/lubricant combinations are tested -- under the same conditions -- differences in effective strain distribution are always significant. By using the IDS, the results are consistent and informative. Among the different lubricants that we tested, the best product was a dry film lubricant. In addition, this dry film lubricant revealed good correlation with the field in sheet metal forming operations.
Fig. 3 -- IDS chart representation.
Conclusion
The experimental reproducibility of Draw-Scaletm(IDS) values is consistent, and serves to avoid the sensitivity
encountered when using the friction co-efficient as the measure of lubricant performance.
Furthermore, friction tests alone cannot completely characterize the complex performance
behavior of lubricants. However, each provide valuable indicators for the development and
selection of the most suitable lubricant for various applications.
Through the development of the IDS and the use of the Interlaken System, now the stamping industry can quickly and easily gain measurable lab data to determine potential lubricant performance at an extremely low cost. With the need for stamping production time to be used more wisely than ever before, this screening technique provides stampers with a great opportunity to reduce the number of lubricants justifying production evaluations. To share the knowledge and experience gained developing lubricant performance research techniques, IRMCO will be conducting educational workshops for metalstamping companies on how IDS and the Interlaken System can be incorporated into their lubricant selection process. MF
The IRMCO Draw-Scaletmand the mathematical formulations incorporated are the intellectual property of International Refining & Mfg. Co. (IRMCO). Any reproduction or use is prohibited without permission.
[1] -- R. M. Harycki et al., "A New Method for Ranking Sheet Metal Lubricants," MetalForming, Nov. 1994, pp. 39-47.
[2] -- F. I. Saunders, M. P. Miles, J. L. Siles and R. H. Wagoner, "A Better Sheet Formability Test," MetalForming, Nov. 1993, pp. 31-34.
[3] S. Sadagopan, T. Brovold, and R. W. Wagoner, "Affordable Sheet Formability Test System," MetalForming, Oct. 1994, pp. 63-69.
[4] W. Wang, "Analysis of Punch Friction Under Realistic Sheet Forming Conditions," PhD dissertation, The Ohio State University, 1994.
[5] InterlakenTechnology Corporation, "Operations Manual -- Interlaken Metal Formability System," March 1995.
[6] A. K. Ghosh and S. S. Hecker, "Failure in Thin Sheets Stretched Over Rigid Punches," Metallurgical Transactions, vol. 6A, 1975, pp. 1065-1074.
[7] W. R. D. Wilson and J. J. Wang, "Hydrodynamic Lubrication in Simple Stretch Forming Processes," Trans ASME, vol 106, 1994, pp. 70-77.
[8] N. M. Wang and Budiansky, "Analysis of Sheet Metal Stamping by a Finite-Element Method," J. Appl. Mech., vol. 45, 1978, pp. 73-82.
[9] J. A. Schey, Tribology in Metalworking, ASM, 1983.
[10] K. Lange, ed., Handbook of Metal Forming, McGraw-Hill, 1985.
[11] J. A. Newnham, "Sheet Metal Working Lubrication," Metal Deformation Processes, J. A. Schey, ed., Marcel Dekker, 1970.
[12] D. Houcque and A. E. Dampts, "Evaluation of lubricant performance by using a combination of experimental test and interpolation technique," IRMCO technical report, 1996.
Precision Metalforming
Association, 6363 Oak Tree Blvd., Independence, Ohio 44131
Phone: 216-901-8800 ---- Fax: 216-901-9190 ---- Send e-mail