Page 16 - MetalForming August 2019

Tooling by Design
By Peter Ulintz
Processing Considerations for Metal Spinning
This article, the second of a series on metal spinning processes and their tools, discusses key process parameters for successful spinning. Last month’s col- umn covered basic spinning technology and equipment.
Metal spinning imparts contin- uous local plastic deforma- tion through the feeding motion of the roller(s), providing inher- ent advantages over global deformation processes such as deep drawing. Con- versely, localized deformations make it challenging to control final part dimensions and shape.
Assessing spinnability—the ability of sheet metal to undergo deformation by spinning without wrinkling or frac- turing—requires the evaluation of the workpiece, tooling and process.
Workpiece Parameters
Key workpiece parameters include material type and grade, blank thick- ness, blank diameter, minimum wall thickness, corner radii and part draw- ing conventions.
Nearly all ductile metal alloys avail- able in sheet form, including aluminum, stainless steel, copper, brass, tin, silver and gold, are used in metal spinning.
With aluminum, there can be sig-
Peter Ulintz has worked in the metal stamping and tool and die industry since 1978. His back- ground includes tool and die making, tool engi- neering, process design, engineering manage- ment and advanced product development. As an educator and technical
presenter, Peter speaks at PMA national seminars, regional roundtables, international conferences, and college and university programs. He also pro- vides onsite training and consultations to the met- alforming industry.
Peter Ulintz
Technical Director, PMA pulintz@pma.org
nificant differences in mechanical behavior between different alloys and tempers. For example, AA1100-H14 can have a total elongation approaching 60 percent, making it the most formable of the aluminum alloys. Although AA3003-H14 has the same temper designation (H14), it is less formable, with approximately 30-per- cent total elongation.
Ferritic stainless steels such as Type 439 behave much like low-carbon steels, but require more force to cause yielding. Austenitic stainless steels such as Type 304 have total elongations exceeding 60 percent but require far greater force to continue yielding (deforming) as they rapidly strengthen due to cold working.
The ratio of blank diameter (Do) to mandrel diameter (d), known as the spinning ratio or spinning coefficient (m), is expressed as m=d/Do.
A larger value of m means a larger material deformation is possible, which implies better spinnability. Lower val- ues of m and/or thinner blanks make the spinning process more difficult.
Obtaining uniform wall thickness during spinning generally requires a high feed ratio—the roller-feed rate rel- ative to the spindle speed, expressed as mm/revolution or in./revolution— and the number and the direction of each roller pass. When the roller moves forward, the material thins slightly; when it moves backward, it displaces metal in the opposite direction to help maintain consistent wall thickness. Conversely, high feed ratios also can produce buckling, wrinkling, rough sur- face finish and dimensional variation.
Superior surface finish and improved dimensional stability, while attainable with relatively low feed ratios, comes at the expense of wall thinning and cycle time. When feed ratios are too low, exces- sive thinning can lead to fracturing.
Another impactful factor: corner radii. This can affect spinning accuracy and wall thickness along the length of the part. Product designs should avoid sharp corners when possible and a formed radius should not be less than two to three times the blank thickness. Radii as small as one material thickness may be possible with thick blanks.
The part drawing should specify an inside-diameter tolerance whenever possible. Depending on the process parameters selected to produce the spun part, including feed ratio and roller nose radius, the thickness of the side wall and the bottom of the spun part can vary as much as 25 percent. If the expected thinning range of the material is accounted for on the part drawing, then the outside diameter may be spec- ified instead.
Tooling Parameters
Key tooling parameters include roller diameter and nose radius, mandrel diameter and blank support. Other important factors in tooling selection include production volume, the size of the part and the type of material being deformed. Whether made by the spin- ning-machine manufacturer or inhouse by the spinner, rollers come in different diameters and radii, both of which can significantly influence material thinning and surface finish. The often-cited work of professor Masujiro Hayama ( Yoko- hama National University), shows that low mandrel speed, small roller diam- eter and low-viscosity lubricants pro- duce the best surface finish. Hayama proposed the following formula to determine roller diameter:
Dr = 0.1D + (120 ±60) mm
Where, Dr is the roller diameter in mm and D is the flat-blank diameter in mm.
Roller diameter should be less than the diameter of the spun component.
14 MetalForming/August 2019
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