Page 18 - MetalForming-May-2018-issue

Tooling by Design
By Peter Ulintz
Forming Speeds: Why Ask Why?
Historically, experienced tool- and-die makers spent 4 yr. or more training apprentices on the hows of their craft without explor- ing the whys, relying, instead, on accepted practices and handbook ref- erences. While handbooks are great, sometimes rules must be questioned.
Take forming speeds for example. Several handbooks list the recom- mended forming speed for low-carbon steel at 55 to 80 ft./min. Many stamping companies use this information to establish the number of parts produced per min., a potentially big mistake.
From a metal-stamping business viewpoint, deforming sheetmetal pro- duces revenue. Deforming more quick- ly generates profit. From a tooling view- point, deforming sheetmetal produces heat. Deforming more quickly gener- ates numerous problems, all of which contribute to the maximum speed at which a particular part can be formed.
Factors to Consider
Let’s first consider the single-action mechanical press found in most press shops. All presses have a maximum working capacity, usually specified in terms of tonnage ratings. When a mechanical press is rated with a max-
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
imum working capacity of 600 tons, this means the machine carries out 600 tons of work with the slide approx- imately 0.25 in. above bottom dead center (BDC). From high-school physics, we recall work defined as a force working through a distance, expressed as:
W = (f)x(d)
In addition, tool steels expand when their temperature rises. This expansion decreases punch-to-die clearances and can result in excessive stretching, lead- ing to tears or breaks in the stampings.
Addressing Tears or Breaks
If tearing (or necks) occur in pro- duction, slowing the press is a common practice, and with good reason. The slower stamping process produces less heat, whereby the tooling temperature may drop considerably and, as a result, the stamping no longer tears. Initially, it might appear that reducing the press speed allows the material more time to flow. However, the improved forming likely results from the die cooling enough to provide adequate clearance for the sheetmetal to flow.
Sometimes, a small increase in tool temperature can cause stampings to tear or break, a common problem in dies that have been cut to nominal material thickness with no allowance for material thickening in drawn cor- ners. The machined punch-to-die clearance, based on initial material thickness, may be insufficient in these areas because the material thickens when drawn into the corners.
It is not unusual to experience galling in areas of insufficient clear- ance after just a few hits during die tryout. Applying more lubricant, or coating the die steels with a high-tech surface treatment, is a common, though not proper, solution during die tryout. Unfortunately, when the die temperature rises during production, the problem often returns as thermal expansion reduces die clearances.
The cheaper and more reliable solution for addressing galling—pro- gramming and machining the proper offset clearances in draw corners— also happens to be the proper die- engineering practice. MF
capacity
( W )
of a
The working
press—a function of the machine’s structural design, gearing, motor size, flywheel energy and other variables— is fixed by nature of the manufacturer’s design. If the working distance (d) increases, when deep drawing a cylin- drical shell 2 in. above BDC, the force (f) available to carry out the work decreases.
Increased forming speed further complicates the situation because forming steel more quickly requires greater stresses. If the selected press does not have a reserve work capacity to accommodate the increased force requirements, the motor may overheat as it tries to maintain flywheel speed or, in extreme cases, the press may stall. Factor in the increased forces needed to pierce and trim at higher speeds and many problems can arise in progressive and transfer tooling, including excessive tool defections, damaging tipping moments, and increased snapthrough or negative ton- nage forces that can shock and break tools—and sometimes presses.
Tooling temperature, one of the most important factors, may limit deep-drawing and forming speeds. A high temperature at the tool-to-sheet interface can cause lubricants to break down. This leads to microscopic weld- ing of the sheetmetal to the tools, called galling. Historically, galling had been a criterion for establishing some of the forming-speed limits still found in handbooks.
16 MetalForming/May 2018
www.metalformingmagazine.com