Manufacturing interventional cardiac devices, such as stents, has traditionally employed two-axis systems consisting of a rotary “spindle” axis mounted on top of a direct-drive linear stage. (See Figure 1) This combination of axes allows the workpiece to be moved very quickly underneath a fixed laser beam, and permits very precise and intricate profiles to be cut into the tubular workpiece material.

Fig. 1 - An example of an integrated two-axis linear/rotary system for stent manufacturing.
Typically these systems are programmed as if the two axes are both, in fact, linear in nature. In other words, the cylindrical pattern is “unrolled” to take the shape of a more typical flat workpiece. While this technique might ease the motion control programming burden, often the resulting pattern cycletime suffers due to the many directional changes the rotary axis (in particular) must undergo during laser processing. Alternatively, “free-running” the rotary axis in a constant direction and rasterscanning the required pattern using high-dynamic galvanometers is a better and far more time-efficient means of writing the same pattern to the cylindrical workpiece.

Figure 2 shows a typical “unrolled” stent pattern. Because of the many tight circular features in the pattern, both axes must accomplish many directional changes while executing the motion profile. As workpieces become smaller in diameter, the acceleration values implied by these directional changes become very high. At certain accelerations the stages can no longer effectively track the desired profile and cycle times must be increased (as acceleration values are decreased) to compensate. However, if the image shown in Figure 2 were sampled to create pixels whose sizes are an integer multiple of the laser spot size, then the entire pattern could easily be written using a raster-style from the resulting bitmap array.

Fig. 2 - A typical stent pattern “unrolled.”
Clever motion control programming, along with mark-on-the-fly features native to some modern advanced motions controllers, can facilitate writing the pattern onto a continuously rotating workpiece using a high-speed galvanometer. One axis of the scanner performs the linear scanning while the second axis compensates for the rotation of the workpiece during a single raster line, that is, making sure that the line is not diagonal on the workpiece. Of course, care must be taken to choose a rotational speed and scanning velocity that will not cause the laser spot to “walk off” the workpiece and become defocussed. The steps required to create such a project follow.

Create a BitMap from a Drawing

Fig. 3 - Bitmap dialog within CADFusion.
First, save the source DXF file to a bitmap format of the highest possible resolution. Ensure that the pixel size is scaled or chosen to be an integer multiple of the laser spot size being used to process the part. Other sampling strategies might include simple snipping/screen captures (for lower resolution trials) or third-party programs to convert vector files to bitmap images.

Create Bitmap Project

Next, the image must be post-processed (or sampled) to create an array of pixels that are used to dictate when the laser will be on as the scanner rasters over the top of the workpiece. One product that could be used for this task is CADFusion, a post-processor that converts drawings or images into motion controller programs.

he desired bitmap image should be selected and the software settings should be chosen. CADFusion allows easy selection of the drive hardware which simplifies Tracking Source (from where the servo encoder signals come for each of the two axes) as well as Marking Settings. For this example, the “Threshold, Bitmap’s Average Pixel Value” choice from the Conversion drop-down list in the post-processor is selected to mandate that pixels will be defined by this metric, as opposed to dithering, for example. At this point specific choices may also be made to define laser firing outputs and hardware definitions (See Figure 3).

After the Bitmap settings have been made, the image should be positioned on the canvas to represent its position within the motion control system during laser micromachining (i.e., at the same offset as the physical tooling underneath the galvo scanner).

Medical Design Briefs Magazine

This article first appeared in the July, 2015 issue of Medical Design Briefs Magazine.

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