Page 32 - MetalForming-Nov-2018-issue

FABRICATION
Controlling Taper in Abrasive-Waterjet
Machining
Despite technology and software advancements, parts may exhibit taper. Here’s how to minimize or eliminate the issue.
In abrasive waterjet machining, taper refers to a difference in kerf width between the top and bottom of the cut. For the most part, with today’s technology, abrasive waterjet users should not have to consider taper. Depending on the brand or style of waterjet-cutting machine, taper-com- pensation software and hardware accessories exist to help mitigate the issue. However, despite all the advance- ments, operators may still experience taper in the finished product. Let’s examine how utilizing the tools avail- able, as well as understanding effective abrasive-waterjet operations, can elim- inate the issue.
Taper Variations Identified
V-Shaped Taper—This represents the most common type of taper, with a greater amount of kerf at the top of the material than at the bottom (Fig. 1). V-shaped taper occurs when some jet- stream cutting energy disperses as the stream cuts deeper into the material. The energy of the jet stream may not have totally pierced the material, caus- ing buildup. Slightly more material is
Fig. 1—V-shaped taper.
removed at the top of a cut, where the jet stream enters the material, than at the bottom, where the jet stream exits. V-shaped taper usually is associated with rapid cutting. In general, the greater the nozzle speed, the more pro- nounced the taper.
Reverse Taper—Slow cutting speeds can produce reverse taper, where the jet stream removes more material at the bottom of the cut than at the top.
Fig. 2—Reverse taper.
The natural flair of an abrasive-waterjet stream is conical, and as the stream travels farther from the nozzle, the flair will become more dispersed. Reverse taper also can occur when cutting soft materials, as material rigidity plays less of a factor in maintaining the focus of the jet stream’s energy. Be cautious when reducing cut speed to combat V- shaped taper, as slowing cut speeds excessively can cause reverse taper, add machine wear and result in increased abrasive usage.
Barrel Taper—Here, kerf width is greatest in the middle of the cut. This type of taper can occur when cutting thick materials, as the jet stream must linger for an extended period to com- plete a full pierce.
Fig. 3—Barrel taper.
Barrel taper also occurs in laminated materials where the top and bottom surfaces are harder than material at the core. Once the jet stream punches through the top layer, the stream’s ener- gy disperses through the soft core before piercing the bottom layer. This action can result in delamination.
Strategies for Taper Control
Many strategies can be employed to control taper, including manual interventions such as tuning the machine, slowing down cut speed, using a low standoff (the distance between the end of the cutting nozzle and the material) or choosing a differ- ent nozzle. Other options include using software to select quality designations
30 MetalForming/November 2018
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