As a Techshop member I have seen many awesome designs created on the Trotec Engraver. These projects include designs that are both cut and etched in various materials including plywood, lucite, and glass. After taking a class on the Trotec laser, it was my goal to make small keepsake boxes with clear plastic lids that screwed on to seal in the contents.
Step 1: Drawing Boxes in AutoCAD
I had seen the box designs that other Techshop members had made, most of which had been downloaded from two box designer websites. Unfortunately, neither of these sites give me designs that I needed.
This site provided the designs for pretty good boxes, but unfortunately did not include the T-slot option that I was looking for to hold the boxes together:
This site has the best interface for box-making I can image, but the output boxes have problems. Specifically, the corners of the boxes do not come together correctly, leaving a missing cube of material at the corner where the three sides meet. I wasted a day trying to get this site to give me anything I could use. I also didn’t like the way the T-slots were designed with the bolts extending completely through the nuts.
What I finally chose to do was to draw the boxes myself in AutoCAD for output to TechShop’s Trotec laser. TechShop has an agreement with Autodesk to provide all of their latest CAD and modeling software on all of TechShop’s computers, so I knew I would have the software tools I needed to work with.
I had some experience with AutoCAD’s three-dimensional modeling when I worked as a mechanical designer. This past experience made drawing the boxes easier. The simple rule of designing finger joint boxes is wherever one side fills a space at a corner, the adjoining side needs to be cut away. It is also essential to decide which of the three sides will fill the corners where the three faces come together. An additional design decision that I made was to cut the T-slots so that the bolts did not fully pass through the nuts and cut into the sides of the box.
I used a caliper to measure the dimensions of all of the parts, including the thickness of the plywood and the #4 bolt diameter. Interestingly, neither of these dimensions were as written in the engineering specifications. Specifically, the ¼” plywood that I used was anything but ¼” thick. In most cases ¼” thick plywood is actually about 0.223” thick. Knowing this dimension is critical for the box to fit together snugly and without the sides overlapping. Also, the diameter of a #4 threaded bolt is generally for a clearance hole for this bolt to fit through, not the actual dimension of the bolt itself. I wanted both the holes and the slots in my design to have interference fits, not clearance fits The diameter that I chose was 0.10”, although this could be tighter for the cuts in wood, and looser for the holes in the Lucite.
Here find a screen capture of my box drawn in AutoCAD using the above dimensions. I have chosen to create boxes that use the golden mean as the ratio of the height to width for both the ends and bottom of the box. Using the golden mean in these dimensions should give a pleasing final appearance.
Step 2: Determining the Kerf
The next step in this process was to determine the amount of material that the laser burned away when it cut out a part. In woodworking, the width of the material that is lost to sawdust is called the kerf. With experimentation, I found that the offset needed for TechShop’s Trotec laser is about 0.00374” (one half of the kerf) when cutting ¼” plywood. I also found that the kerf of the laser was less when cutting ¼” solid pine, or cutting Lucite. I can imagine that this kerf would be different on different brands of lasers than Trotec. Here find a copy of my box with all of the edges offset by this dimension. In AutoCAD I made a Polyline out of each side of the box and used the Offset command with a dimension of 0.00374”, then deleted the original lines.
Step 3: Selecting Lineweight and Color
Next I created a new Layer in AutoCAD that I named “Zero Weight.” For output to the Trotec, it is essential that this layer have its color set to,“255,0,0” Red. Trotec uses this pure red to signify a cut line. It is also essential that this line have a Lineweight of zero. If the line has any Lineweight at all, the Trotec will assume you want to etch the line and not cut the line, a raster instead of vector process. Here I set a Lineweight of 0.00 mm.
Step 4: Lineweight and Color Selected
Since the default function of AutoCAD is to set line characteristics of lines by layer, I then used the Change command to set the Properties of all of my box elements to my Zero Weight layer.
Step 5: Using AutoCAD's Plot Command
Now that the model was complete, I used AutoCAD’s Plot command. In this dialog box I needed to make three changes:
One change was to uncheck the “Fit to paper” box and set the plot scale to 1:1. The default setting will scale the sides of the box to whatever your media setting is, causing the box to not fit together. In early attempts, I made postage stamp size box ends that wouldn’t fit together.
Next I needed to set the Properties for the Trotec laser. Many of these settings are highly specific to this machine, but may still be useful to read.
Finally, I needed to change the default “Plot area” from “Display” to “Window” and select the area of my drawing to be printed. This step will be familiar to anyone who has printed from AutoCAD in the past. In this case I selected only one side of my box to print at a time. If you lay out the sides of the box close together, and you are using a large enough piece of material, you can cut more than one box side at a time.
Step 6: Selecting "Custome Properties"
For the Trotec laser you will be given a standard Windows print dialog. On this dialog select “Custom Properties.”
Step 7: Selecting the Trotec Laser Properties
This will take you to the Trotec laser Properties. In this dialog box under Material Settings select Wood, and then select Plywood.
For some reason I am not clear on, if you select Birch or any other kind of wood than Plywood you will not be able to set the red cut settings, only the black etch settings. From experience, the Trotec cuts real wood more cleanly and with a narrower kerf than it cuts plywood.
Then click the (unlabeled) laser settings button located under the words Material Settings. This will take you to a dialog box that is even more specific to the Trotec.
Step 8: Selecting the Trotec Material Settings
In the Trotec material settings box, you are given options to assign specific cut properties to the colors in your drawing. I have seen quite elaborate images engraved into many materials using the Trotec laser, but in this case we will only be cutting wood and will not need these additional settings. Since the Trotec uses the line colors of your drawing to specify cut and etch characteristics, it is essential that your lines are drawn in pure RGB red.
Under Cut, the default power setting is 100.00, and the default frequency of the laser is 2000 PPI/Hz. The one change that is needed is to slow down the cut speed to 0.70. I found that it is not always assured that the laser will cut through the plywood if the cut speed is faster than this.
The other change is to set the material thickness to 0.25 inch. Since the focus of the laser is determined by the lens that is inserted, I am not sure this setting makes any difference at all. When you are done, click “OK.” This will take you back to the previous dialog box where you will commit your changes by clicking the large “JC” (Job Control) button at the bottom of the previous dialog box.
Step 9: Saving Changes
When you exit these layers of dialog boxes you will be offered this small dialog box asking if you want save your changes to a temporary configuration file. The answer to this question is “OK.” If you do this work a lot, it may save some time to commit your changes to a PC3 file that you keep with your project instead of creating a temporary file each time.
Step 10: Previewing Selection in AutoCAD
If in the AutoCAD print dialog box you chose to preview the selection you had made, your box end might have looked like this.
Step 11: Trotec JobControl Panel (image 1)
I have chosen to break the following screen into two images. If I had made one image, the resolution of the screen capture would be too low to see what was being displayed.
Once you have returned from the five layers deep of dialog boxes above, you will be brought to the Trotec JobControl panel. In the image below this one, you will see your print job located in the cue below the label Jobname. Click and drag your job into the cut area. To see a preview of your image, click the picture of the eye located on the toolbar beneath the Window menu.
In this image you will see blue crosshair in the cut field. This represents the physical location of the laser. Typically you will want to reposition the laser to the upper left of the cut field and have your job snap to this point. Starting the laser in a different location in the cut field may allow you to save material and reduce setup times.
Depending on the complexity of the box side you have designed, you may want to right click on your part and from the drop down menu select “Optimize Cut Sequence.” For this simple part, which was already created as a single Polyline, there is no optimization possible, but for more complex parts optimization could save cutting time by reducing the path of the laser. In the worst case, a single side of a box which was not optimized could be seen by the laser as only a series of lines, which would then be cut in the order in which they were drawn in AutoCAD. This would mean that the laser would do a large amount of repositioning for the small amount of cutting it was doing.
Note on this screen the setting that we made earlier for a cut speed of 0.70.
Finally, click the “Update” button. This will show the approximate production time for this job, in this case about 23 seconds. In time one can learn to estimate how long a job should take to cut. If this time is too long or too short it can be an indication that one of the settings made earlier was incorrect.
One mistake I made while learning this process was to set my Linewidth at greater than 0.00mm in AutoCAD. As discussed earlier, this resulted in the Trotec etching a line instead of cutting it. In this dialog, when my production time for this small part was over a minute and a half, I knew I had an incorrect setting somewhere.
Step 12: Trotec JobControl Panel (image 2)
Finally, near the lower right of this screen click the blue “Play” button. If you have just turned on the laser, this button will be a USB symbol. Click this USB symbol to connect your computer with the Trotec.
Step 13: Cutting With the Trotec Engraver
I will assume that you already know how to operate your Trotec Engraver, and so I will not go through the specifics of making sure the lens is clear, or how to calibrating the laser above the material you are cutting.
If all has gone well, the laser will start. It may appear that the laser is cutting such a thin line that it is only drawing on the surface of the material and not cutting all the way through, but when you lift up the material after the cut is made, the parts should drop out. The actual Trotec laser kerf in ¼” thick plywood is only about 0.00748” inch.
Step 14: First Piece of the Box Is Cut
This is the final box end. I should have shown a caliper in this picture to demonstrate that it is dimensionally accurate and cut exactly 1:1 scale as drawn. If it had not been exactly the correct dimensions the box would not have assembled correctly.
Step 15: The Finished Box!
Finally, here is the assembled keepsake box. For use, items would be placed in the box before the Lucite lid is screwed in place.
In this design I have chosen to use #4 nuts and bolts because the flat-to-flat dimension of a #4 nut is exactly ¼”, which is the thickness of the plywood. Using #4 nuts allows them to lay flat both the inside and outside of the box. I have also chosen to design the box around my bolt lengths so that I can thread into each nut, but not extend through the other side of the nut.
For other uses in future revisions of this design, I will experiment with boxes of different dimensions. To improve the appearance of the box, I will also use brass instead of steel hardware. I will also experiment with different wood finishes. Finally, I plan to make the boxes out of solid wood instead of plywood, although it is likely that I will still be limited to about a ¼” in thickness of material.
I hope you have enjoyed this Instructable. I made it at TechShop!