Indexing Jig

This pen shows a two line message of parallel text.

Copyright © 2004 by Davis Multimedia, Int'l. All Rights Reserved.
As Printed in January 2004, Volume 29, No. 7 of The Engravers Journal.


  The finished stars and stripes pen we will create in this article.  

     Laser engraving specialists are sometimes faced with the often daunting job of engraving on a curved or cylindrical surface. One example of this is engraving a two line message on a pen where, for example, it’s necessary to engrave one line of text parallel to the pen’s axis and then rotate the pen to bring the next portion of the pen’s circumference into position to receive the second line.
    As most laser owners are aware, one way to laser engrave cylindrical surfaces is to use a cylindrical attachment, a device which is available with most lasers, which rotates the item as you engrave. Cylindrical attachments are great, especially if you want to engrave a line of text or a design reading around the circumference rather than parallel with the center line. However some of the cylindrical attachments are better suited for holding wine bottles and other large items than they are for pens and the like.They are also an expensive extra that is rarely needed in the average shop.
    In any event, I have found that a number of lasering jobs involving limited-coverage engraving on cylindrical surfaces can be done with excellent accuracy and repeatability using an inexpensive home-made indexing fixture I created myself. The principle used in my fixture is that you hold the cylindrical item “between centers” so that it can rotate on its axis. You set up a line of text or other graphical element (not extending too far out from the top of the curve) and then engrave the item as if it were a flat engraving job. To engrave a second line of text, you rotate the item on its axis to provide the desired spacing between lines and engrave that set up, again treating it as a “flat” layout.
    What triggered my invention was my experience as a woodworker for over 20 years. I immediately thought of an indexing pin on a wood lathe as a good starting point for my fixture. This utilizes a disc with holes equally spaced around it and a pin that creates a stop when it’s inserted in the hole.

Figure 1: Samples of indexing wheels with different numbers of stops for different projects. Figure 2: A knife was mounted to the fixture and slips into the notches on the indexing wheels.

     Because my fixture would be fabricated from acrylic and vector cut on the laser, I was able to easily make several interchangeable indexing wheels to accommodate virtually any number of steps around a cylinder. For example, Figure 1 shows three separate indexing wheels which contain 10, 14 and 8 indexing notches, respectively. Using a wheel containing a greater number of notches provides a greater number of rotary indexing steps having a smaller angle and reduced line-to-line spacing between set ups.
    The indexing wheels can then be mounted to the fixture, allowing the pen or other item to be rotated between notches (Fig. 2). Instead of using a pin, I mounted a “knife” which slips into the notches I cut around the indexing wheels.
    To more easily understand how to set up the computer files to work with this fixture, we’ll use the U.S. Flag pen shown in the opening photo as an example. This set up is much more complicated than a couple lines of text but it really demonstrates the operation of the fixture. Let’s start with the lower barrel of the pen (stripes).
    The first thing we do is measure the diameter of the cylinder. I use dial calipers for this. Note: The more accurate this dimension is, the more uniform the widths of the alternating red and white stripes will be.
    We know that the U.S. flag has 7 red stripes so let’s divide the cylinder into 14 equally spaced parts. We’ll use the 14 step indexing wheel for this since 7 can be divided equally into 14.
    The diameter of this particular pen barrel when finished is .613". Draw a circle of that diameter and then divide it into 14 equal segments. Then measure the width of the segment. The length of the pen barrel is 2.375". Figure 3 shows how we transfer these dimensions into a framework suitable for drawing wavy stripes.

Figure 3: Transferring the dimensions to a grid helps us to draw the wavy stripes. Figure 4: The grid show the 5-star and 4-star segments used to create all 50 stars.

    For clarity, I drew only two stripes in the grid. Normally we would only need to draw one because it’s a repeat pattern. If the pattern around the pen were totally different or non-uniform, we’d have to use the whole grid.
    The next step is to set up the laser. Position the indexing jig in the engraver with the wooden barrel mounted to it, and align the first segment of the grid exactly on top of the barrel in the laser engraver. In CorelDRAW create a box that represents the engravable area on the pen barrel (.25" high x length of segment). With the red dot pointer activated on the laser engraver, vector trace the outline of the box (with the engraver lid open of course). Adjust the location of the engraving box until it traces accurately onto the pen barrel. Now position the grid with all segments so that your first segment is centered within the engraving box. Once we’re aligned, set up your layers. This is essential with complex patterns but not really necessary if the pattern doesn’t change from one segment to the next.
    In our Corel layout, the first stripe in the pattern will be assigned to a layer we will call “PRINT.” All of the layers in the file will have the print function disabled except for this one. Also set up a layer called “STRIPES.” This layer will accommodate the stripes or segments of the pattern that aren’t being printed (engraved). When we get to the upper barrel of the pen (stars), the reason for this will become clearer.
    Before you begin cutting, make sure that the indexing knife on the jig is engaged in the 14 step indexing wheel. With the pen barrel mounted to the indexing jig and accurately aligned inside the engraver, you can now engrave your pattern.
    The “PRINT” layer should be the only active (printable) layer and the 1st segment of your pattern should be the only design element on that layer. I find that toggling the “visible” icon in the layer’s palette off and on is a quick way to see what you have on any given layer.

Figure 5: Entering the height into the “Nudge Offset” box switches the 5-star pattern position to the 4-star pattern position.

    When you’re sure that the top stripe is the only element on the “PRINT” layer, you can engrave into the pen barrel. Once the item is engraved to the desired depth, slide the indexing knife out of the wheel and advance the barrel and wheel by two notches in preparation for the second cut. Re-insert the knife and engrave the second stripe. Repeat this seven times to complete the cylinder.
    One characteristic of the indexing jig as opposed to rotary attachments is that when you engrave around a cylinder using the indexing jig, you will always end up exactly where you started. With the rotary attachment, if the diameter that you enter is slightly off when you make your way around the cylinder, your alignment will also be slightly off.
    When engraving the stars on the upper barrel of the U.S. flag pen, we will be using two different pattern segments instead of just one as we did with the stripes (Fig. 4).
    To arrive at a total of 50 stars, I used a grid with 11 segments and alternated between five stars and four stars per segment. The first and last segments each have five stars, which isn’t really a problem because, when assembling the pen, I positioned the clip in between the two rows of five stars.
    With the upper barrel of the pen mounted on the indexing jig and the 11 step indexing wheel installed, we’re ready to set up our layers. Let’s create a new layer called “STARS.” Make sure the “printable” icon is turned off. We also want to move the stripe that was on the “PRINT” layer to the “STRIPE” layer. When you start engraving the stars, you’ll want to do all of the segments with five stars in them first. As before with the stripe segments, align the first star segment with the upper pen barrel which is mounted on the indexing jig in the laser engraver. Again use the red dot pointer to make sure the stars are perfectly centered on the barrel. Put the row of five stars on the “PRINT” layer. They should be the only design element on that layer and that should be the only active layer.




Figure 6: Engraving box: Grid in starting position for engraving 5-star segments. Engraving box: Grid nudged into 2nd position for engraving the 4-star segments.

    Now you are ready to engrave the pen barrel. Once the engraving is at the desired depth, slide the indexing knife out of the wheel and advance the barrel and wheel by two notches. Re-insert the knife and engrave the next row of five stars. When you’ve completed the 6th row of stars switch the layout to the row of 4 stars. To do this, enter the height of the segment (in this case .188") into the “nudge offset box” in CorelDRAW (Fig. 5). This is an excellent way to toggle up and down (using the arrow keys on your keyboard) through the segments of your pattern and still be perfectly aligned with the pen barrel on the indexing jig.
    Select the grid including all segments and stars. Use the “up” arrow to nudge the second segment of the pattern (four stars) into the engraving box (Fig. 6).
    When switching from the five star segment to the four star segment, the five stars which were on the “PRINT” layer must be moved to the “STAR” layer and the four stars in the second segment must be moved to the “PRINT” layer.
    The indexing wheel has to be advanced two more notches after the last five star segment is engraved. This sets you up to start engraving the first four star segment. Continue advancing the indexing wheel by two notches after each four star segment is engraved until you’ve completed all five segments and are back at the beginning. That completes the indexing jig’s involvement in this project.
    As you can see, a complex pattern with completely different shapes throughout the entire design would have you moving segments on and off the print layer after each segment was engraved. However, if you follow that approach, your chances for success are all but guaranteed.

Figure 7: When using the indexing jig your inlayed pieces match and you get a nice tight fit. When using the rotary attachment the fit is not identical and not as clean. Figure 8: The pen kit costs are kept reasonable because the stars are cut from a flat piece of veneer.

    This technique is especially wellsuited for doing inlays, i.e. when you laser cut a cavity and then vector cut an inlay to fit precisely within it. One important factor to note regarding the indexing jig versus the rotary attachment for this type of inlay work is that unless all of your inlayed pieces are vector cut from a cylinder with the same diameter as the engraved cylinder, you will not get tight, clean-fitting pieces (Fig. 7).
    The walls that are engraved on a cylinder using the indexing jig are straight sided or parallel to each other, whereas the walls created using a rotary attachment are all tapered toward the center of the cylinder. For the U.S. flag pen, being able to vector cut several hundred stars from a flat piece of veneer in a couple of minutes is extremely convenient as opposed to turning wooden cylinders to cut the stars. This also makes it possible for me to sell these pens in kit form (Fig. 8) and keep the costs reasonable.
    Even if your lasering needs are not as complicated as this stars and stripes inlaying job, you can take full advantage of using an indexing fixture like mine. It’s an excellent technique when you need to mass produce multiple jobs involving simple additions of two or three lines of copy. Try it. You’ll like it.