Design Evaluation of Multiroll Mills for Small-diameter wire rolling

Paper by K. Kuroda, T. Kuboki, Y. Imamura

Presentation by Adam SladeMonday, 17 September, 2007

Background

Wire rolling is being utilized rather than the more traditional wire drawing method of production. A round rod is used as a starting place for the ensuing rolling reductions.

Purpose of the Paper

Demonstrate differences in forces and advantages/disadvantages of wire rolling using different numbers of rollers

Show wire rolling to be a good alternative to wire drawing, specifically wire rolling with multiple rollers

Why?

“Conversion from drawing to rolling ensures a high reduction in area per pass, because the severe sliding frictional condition is almost eliminated.”

The Reality: most of the contact area of the rollers with the wire involves sliding friction, and no data is provided as a comparison between the two methods

Comparison to Previous Work

Many papers/studies had been previously been made examining the advantages and disadvantages of multiple rolls in wire manufacture

Authors’ angle: “…none of the previous studies has compared the deformation and loading characteristics of the three mills on an even basis, i.e. on the same roll and groove geometry design basis, and direct one-to-one comparisons have not been made of all three mills on a numerical and experimental basis.”

Authors’ Possible Motive

“Such study may enable further development of the multiroll cold wire rolling mill in the next decade.”

The authors created a four-roll micromill named the ‘super-micromill’

Objectives in Design

There was to be no tension in the wire between stands (as opposed to the drawing method)

Driving torque given from one source to all stands through a common drive (one motor)

Compact design

Limitations of Experimentation

Only to examine 2, 3 and 4 roll stands– Stand composed of more than five rolls is not

realistic because of complexity– Why a 5 roll stand is not examined…

The Setup

2 roll unit driven by two shafts

3 and 4 roll unit driven by single shaft

Roll force measured using load cell

Driving torque calculated by revolution rate and power consumption

Specifications for prototype mill

Authors offer no explanation for calculation of nominal roll diameter, or rolling speed

Obtaining Results

Contact Area with rollers

Comparing the Model to the Data

Conclusion: Why bother with any further actual experimentation?

Considerations for further models

Mother wire to come from smallest available hot-rolled rod on the market

– 5 or 6 mm

Minimum diameter wire taken as the “smallest wire in the world”– The authors report it to be 1.2 mm– Minimum diameter produced from first source investigated found to

be 0.14 mm for Cu, 0.25 mm for aluminum, 0.38 mm for carbon steel

– Reference obtained from the authors’ own previous paper

Roll diameter comes from “accepted market data”– Minimum ratio of roll diameter to wire diameter is 20– Maximum roll size based on the following statement:

“It is known that, the larger the roll shaft and the machine size, the larger is the bearing load but the higher the machine cost.”

Finite Element Analysis

CORMILL finite element code developed by the University of Tokyo

For three-dimensional “rigid-plastic” steady state analysis of rolling

Performed over one-half of the contact area with each roller

Results

As the reduction in area increases, ovality increases– This tendency

becomes greater as the ratio of roll diameter to wire diameter increases

Manipulation of ResultsOver-emphasis on

“ovality” favors the 4-roll configuration. Ovality the only consideration in precision.

“…the four-roll mill is most advantageous in ensuring precision when subjected to smaller-diameter wire rolling.”

Comparison of Results

“It has been said that, the smaller the number of rolls, the higher the reduction becomes, i.e. better performance can be obtained in the order two-roll>three-roll>four-roll.” but…

This paper compares the performance given equal dimensional tolerances, on unequal measuring techniques.

Loading Results

“… the two-roll mill requires greater torque than either the three- or four-roll mills.”

Total rolling force is equivalent for all rollers, indicating the same power requirement for each configuration

Torque inequalities

The three different configurations require different torque requirements. Not an even comparison.

Assumption of constant frictional work on each roller likely false, due to different contact areas/deformations.

Force vs. Torque

Rolling force is approximately equal

Power requirement based on force, not torque

Lower portion shows the inequalities in the setup of equipment

Calculations based on…

Three data points enough?

More Unfair Comparisons

Measuring rolling force per one roll

Not linear as shown… Isn’t it obvious that the rolling

force per roll should decrease this drastically for an increased number of total rolls?

Red lines indicate the 2- and 3-roll positions normalized (as if all rollers had four members)

– the three-roll actually has the lowest total rolling force

Conclusion – Disadvantages and Considerations

Prolonged manufacture time, due to greater restrictions on reduction ratio

More complex machinery – twice as many rollers = twice as much maintenance

Other effects on the final product not considered (additional work hardening, heat introduced into machinery due to greater deformations, etc.)

Conclusion – Advantages and Implications

The four-roll micomill would provide a good way to create high tolerance small diameter wire for a lower energy requirement over drawing, and a very slightly lower energy requirement over a two-roll mill

This improve the efficiency of the wire industry greatly, if high tolerances are more desirable than the increase in equipment and increase in processing time

References

Four of the sources are the author’s own work, all but four are from Japan (presumably written by coworkers), and those four are from 1983, 1983, 1982, and 1952