Controlling Taper in Abrasive-Waterjet Machining

November 1, 2018
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abrasive waterjet machining

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-compensation software and hardware accessories exist to help mitigate the issue. However, despite all the advancements, operators may still experience taper in the finished product. Let’s examine how utilizing the tools available, as well as understanding effective abrasive-waterjet operations, can eliminate the issue. 

Taper Variations Identified

V shaped taper
Fig. 1—V-shaped taper.

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, causing buildup. Slightly more material is 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 pronounced the taper. 

Reverse taper
Fig. 2—Reverse 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. 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 complete a full pierce. 

Barrel taper
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 energy 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 different nozzle. Other options include using software to select quality designations in specific work areas of the material. Here, the machine automatically adjusts cut speed to minimize taper.

Operators can minimize taper issues by accurately setting up material in the machining software, and ensuring that the material is correctly listed, its machinability is true and that the thickness of the piece is precise. All of this information allows the machine’s software to calculate proper cut speed to produce a specified cut quality. 

For example, Intelli-Max Layout software from Omax provides a single button that utilizes all job and materials data, and allows the operator to assign the most taper-free edge possible. The Quality of Minimum Taper button in the software’s Available Qualities toolbar will decrease or increase cut speed as necessary to provide the minimum amount of taper.

Zero Taper is Possible

Some machines can remove V-shaped and reverse taper simply by tilting the cutting head to compensate. For example, Omax Tilt-A-Jet technology allows for automated taper compensation, using a predictive model to calculate taper in the kerf at each portion of a cutting path. The software commands the nozzle head to tilt in vertical alignment with the part edge, resulting in taper on the scrap side of the material being cut. The jet continuously adjusts at thousands of points per inch along the cutting path, and the software controls cutting speed at each point while optimizing acceleration and deceleration. This allows for more precise cutting and significant increases in cutting speed. The result: a square edge.

highest quality abrasive
Fig. 4—To better combat unwanted taper, use the highest-quality abrasive available. And, experiment if necessary. If experiencing taper with an 80-mesh abrasive, try a 120- or 150-mesh. 

Another product, A-Jet, also from Omax, assists in the elimination of taper. The A-Jet bevel cutting head allows for automated, articulated taper compensation and enables creation of complex 3D shapes.

Minimize Taper Without Accessories

Even without the use of automated articulated cutting heads, operators still can mitigate taper. Today’s waterjet software often offers built-in taper-compensation options as well as improved nozzle controls. In addition, operators can take steps to reduce or eliminate taper.

Adjusting cut qualities represents the easiest and least-intrusive method for adjusting taper without using accessories. Slow down the cutting head by raising the cut quality in areas that must exhibit zero taper. Keep in mind that slowing the cutting head will lead to slower cutting times and increased abrasive usage.

Since thinner materials tend to exhibit the greatest amount of taper, try stacking. Taper usually is most pronounced in material thicknesses of less than 0.125 in. If available, use a stack-height calculator in the machine’s control software to determine the optimum number of sheets to stack in order to produce the most parts in the least amount of time.

Note that standoff matters. The jet stream will flair once it exits the nozzle. To mitigate this, maintain a recommended distance between the nozzle and the material being cut. Usually, standoff measures between 0.03 and 0.08 in. depending on the diameter of the nozzle. The closer the nozzle to the material, the less the jet stream will spread, and the less taper it will produce.

Use the highest-quality abrasive available. If experiencing taper with an 80-mesh abrasive, experiment with a 120- or 150-mesh. Lower-quality abrasives tend to have less-consistent particle sizes. In the end, this comes down to purchasing abrasive from a reputable distributor.

Another tip: Guarantee that the Z axis is perpendicular to the material in both the X- and Y-axis directions. Use a pitch indicator to ensure a cutting-head angle of 90 deg. relative to the cutting table. In addition, be sure to properly seat the machine’s mixing tube, otherwise the machine will not cut with an accurate Z axis. For machines equipped with an Omax A-Jet or Tilt-A-Jet, operators can use the Calibrate Perpendicular feature available in the Intelli-Max Make Setup menu. 

Taper may result from uneven wear in the mixing tube. This is not a function of the motion-control equipment, but rather a consequence of how the abrasive wears inside the mixing tube. Rotating the mixing tube 90 deg. for every 8 to 10 hr. of cutting time allows the mixing tube to wear more evenly and last longer, while preventing taper.

Keep Learning

Lastly, to keep unwanted taper at bay, stay abreast of current techniques by following manufacturing and trade blogs, social media and magazines. Become comfortable with your abrasive-waterjet machine, and learn how it behaves and how to work its idiosyncrasies. And, measure parts periodically to ensure accuracy. Knowing how a particular machine will react to certain materials under certain conditions is the best method for predicting taper outcomes. MF

Information for this article was supplied by Omax Corp., Kent, WA.

Industry-Related Terms: Abrasive, Core, Edge, Hardware, Layer, Model, Piercing, Point, Scrap, Thickness
View Glossary of Metalforming Terms

 

See also: Omax Corporation

Technologies: Cutting

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