January 13, 2004
The balance between waterjet cutting production rate and part precision always has been difficult to achieve because of the jet's complex behavior. Because its shape at any point along the tool path is a result of multiple independent variables — including the speed and acceleration with which it is moving — the jet is particularly difficult to manage. The cutting jet bends back along the path, flops from side to side, produces a speed-dependent tapered kerf, and produces a wider kerf when moving slowly. Precise parts can be made in spite of all these factors, but at the expense of the production rate, by moving slowly along the entire path.
In an earlier article on waterjet cutting, I described software's role in managing the speed along the tool path as a function of the path shape, so that parts can be made more quickly while achieving the surface finish specifications. This article discusses software that automatically tilts the cutting head to make a square edge on the part while moving at higher speeds that normally result in a tapered kerf. Let's begin by discussing some of the facts about taper.
All of the independent jet cutting variables affect taper. Most of these factors are determined during cutting-rate setup. The only remaining factor of interest is the speed at which the jet is moved along the path. As the jet slows, the kerf moves from a taper widest at the top at high speed to a kerf widest at the bottom at extremely low speeds. Figure 1shows taper over a portion of the speed range.
Thin materials usually are cut at high speeds and thick materials are cut at low speeds. This leads to the surprising result that the taper is greatest in thin materials. The taper in a 1/8-in.-thick steel part may be as much as 0.007 in. and as little as 0.0005 in. in the same part made of 2-in.-thick steel. Of course, if you cut 1/8-in. steel at the same speed as 2-in. steel, the taper could be nearly eliminated at a great sacrifice in productivity. Some software packages handle this variable by permitting you to assign a minimum taper quality to certain portions of the path. Doing so can eliminate the need to perform secondary operations to remove a taper and is justified in these cases.
Taper Angle as a Function of Speed
The primary reason for controlling taper is part appearance. If your customer thinks the part looks bad because of the tapered edge, you can't sell it, and that is the end of the story.
Taper often causes clamping problems during secondary machining operations. Tapered parts are difficult to hold firmly in a chuck or vise. Often the first step of machining a jet-cut part is to make a skin cut to remove taper and allow solid clamping.
For some parts the cut surface must butt squarely against an adjacent part. A bolted joint is one example.
Sometimes the cut edge must run against another surface while carrying load evenly across the edge. Jet-cut gears, sprockets, and cams are examples of this part type.
Finally, a small amount of taper is desireable in some parts. Stamping dies and cutting tools require a small relief angle that can be formed with a tilting jet.
In all of these cases, taper control during the jet cutting process lowers cost by delivering a useful part without secondary processing.
Two ingredients are necessary for removing taper by tilting the cutting jet—a mechanism for tilting the jet and software that correctly anticipates the taper and drives the tilting head accordingly. The tilting mechanism is attached to the XY table normally used without tilt. It is important that the center of rotation for the tilt be close to the point where the jet enters the top of the workpiece. Otherwise, keeping the jet entry point on the path would require large motions of the XY axes when tilt occurs.
Simplified 2-D Version of Nozzle Tip Pivot Showing Two Positions
Figure 2shows a 2-D linkage that tilts on one axis and illustrates the principle involved. Note that the tool point moves only slightly as the tool is tilted. A similar mechanism with three linkage arms permits tilting in two directions, while keeping the tool point almost fixed. Note that the largest motion of the tip is in the vertical direction.
Figure 3shows the actual head used for the taper control. In this mechanism, one of the three linkage arms is driven in two directions by servomotors enclosed in the rounded housings. The remaining two linkage arms are enclosed in the small bellows.
Because of the slight vertical motion of the tip as the head tilts, a compensating motion must be made in the Z direction. The cutting table then becomes a full five-axis machine with X, Y, Z and two angular axes.
Tilting Head Attached to XY Table
1. The software uses a built-in cutting model to calculate the speed at every point along the tool path that is required to produce the desired part edge surface finish for the part material and thickness. This is the normal calculation for pure XY cutting that is done by the advanced controllers described in an earlier article.
2. An extension of the cutting model that predicts taper is used to calculate the amount of tilt required to make a square edge or an edge with the desired taper.
3. The actuator commands are calculated from the desired tilt angles. In this step, slight adjustments to the X,Y, and Z actuator commands can be made to compensate for the fact that the pivot point for the tilt is not exactly at the point at which the jet enters the material.
The computing time to accomplish steps 2 and 3 is about 20 times the time to complete step 1. At this point, the motion plan for the entire path is stored in the control memory and can be run multiple times to make multiple parts.
Gear Cut With Tilting Mechanism
Combining a jet tilting mechanism with taper-compensating software produces taper-free parts in fractions of the time it takes to make them simply by slowing down the process. Figure 4shows a spur gear made with the tilting head shown in Figure 3. The gear is resting on a surface plate next to an angle plate to show its squareness. It took 6.3 minutes to make this gear from -in. aluminum using tilt and 13.6 minutes at the minimum taper speed without tilt.
Finer abrasives produce a finer surface finish on the cut edge, while cutting at the same rates as coarse abrasives. However, finer abrasives also produce a kerf with more taper than coarse abrasives. A tilting head removes this extra taper and, in fact, all taper so that there is no longer a taper penalty to achieving better edge finish.