Multicolor Plot Tips

Tips on making multi-color plots with AxiDraw

You can make multi-color plots with the AxiDraw by using multiple pens, using each to draw certain parts of your file, and changing pens for each color. Here are some tips about working with this process.

Selecting part of your file to plot

When you plot a document from the Plot tab of AxiDraw Control, by default the entire document will be plotted.

If you separate your Inkscape document into different layers, you can use the Layers tab to plot one layer, or a set of layers, with names that begin with a given number. There are additional controls that you can use, documented in AxiDraw Layer Control, to assign particular behaviors to layers as well.

You might, for example, have one layer named "1 Red" and another layer named "2 Blue". If you use the Layers tab, and enter "1" in the box there (in the box labeled "plot only layers beginning with"), then only layer "1 Red" will be plotted when you click Apply. If you enter "2" in the box and click apply, then only layer "2 Blue" will be plotted.

With AxiDraw Control version 1.7.0 and newer, one additional mode of selecting parts of your document to plot is available. You can also switch certain layers to be visible or not visible, by using the "eye" icon in Inkscape's Layers panel (Layer > Layers... from the menu). Only visible layers will be plotted. (If you are using an older version of the AxiDraw software, you can download a newer snapshot from the AxiDraw Releases page at github.)

Making a multicolor plot

To make a two-color plot, one might use the following procedure:

• Insert the first pen, and set the machine up as you normally would (check pen-up/pen-down heights, make sure that corner of paper is below tip of pen, etc.).
• Plot the first part of the file, for example with the Layers tab.
• After the first part finishes, remove the first pen and insert the second pen. Take care to position the pen at a similar height to the that of the first pen.
• Plot the second part of the file, for example with the Layers tab.

This procedure can, of course, be extended to as many different colors as you like.

Troubleshooting pen alignment

If you are getting poor alignment between different colors, there are a few different possible causes, some of which are quite obvious. We'll try to list the most common ones.

Pen choice matters

First and foremost, the type of pen that you use will largely dictate how well you can align two of them.

Aside: Classic vintage pen plotters used identical, short, stout pens, typically with stiff, wide tips. This helped in a few ways: The short, stout pens flex less upon both insertion and use. The small size leads to lower moving mass, which cuts down on resonant "wiggles" in the paths that you draw. The fact that they are identical in barrel size means that you can swap out two of them without changing where the center point is located. Wider tips (particularly on the felt-tip plotter pens) hide minor misalignment issues.

The AxiDraw lets you use many different types of pen, which trades off the easy precision that vintage pen plotters have for a great flexibility in terms of what writing instrument you use.

Some suggestions:

• Use the same family of pen. If you have pens from different families, they will generally have different barrel diameters, which will lead to an offset in where the pen is centered.

• Use pens with a stiffer barrel. Thinner or more flexible pens will bend more when you tighten the nylon thumbscrew to hold them in place. This flexing will lead to an offset in where the pen tip is located.

• Use pens with a wider tip. Using felt-tip pens and other wide-nib writing implements can hide minor imperfections in alignment.

• Use pens with a stiffer tip. Some felt-tip markers have flexible tips (even ultra-fine point Sharpie pens have some flex). This can bend back and forth, reducing precision of your plot.

Even if you do everything perfectly, you may still be limited, particularly with fine tip pens, on how precisely the pen tips are centered on the barrel.

Vertical versus diagonal pen alignment

When making multi-pen plots, we strongly recommend that you use only the vertical (rather than diagonal) position for your pen.

The reason for this is that when the pen is mounted in the diagonal position, any (even minor) difference between the height of the two pens translates into a horizontal misalignment as well.

Don't bump the AxiDraw or Paper

It may be obvious, but take care not to bump either the AxiDraw or the paper while switching pens. No amount of care with the pen can compensate for having the paper in the wrong position.

Keep the XY motors energized

Precise positioning with the AxiDraw requires that the XY motors are continuously left on (energized) for the entire duration of your plotting session, start to finish. Any time that the motors are disabled (de-energized) and turned back on again, you cannot be certain that the motor positions will be consistent with those in the previous session.

If you need to cancel a portion of the plot, be sure to use the physical pause button, rather than (for example) disconnecting power or USB. To return the AxiDraw to the home position, use the Resume tab in AxiDraw Control, rather than pushing it by hand.

When doing initial setup, it's perfectly fine to disable the motors (for example when initially homing the machine) but be aware that position consistency is not maintained between subsequent sessions.

Use a consistent method and height for inserting your pens

The most important thing is to use a consistent method for inserting your pens every time.

A good method to ensure that each pen is inserted to the same height as the previous is to use a spacer to set the height. A typical spacer might be a small, flat piece of metal or plastic, about 3 mm (1/8 inch) thick or a little bit thicker. For example, a thin bottle cap or a few coins stacked together. When inserting your pens, set the pen holder to the pen-up position and then rest the tip of the pen on the spacer above your paper when tightening the thumbscrew. This will give a consistent height for each of the pens that you use.

When tightening the thumbscrew for your pens, try to use consistent, light pressure: Only enough pressure to ensure that the pen will not work itself loose. Excessive pressure can cause bending of the pen, which can affect the position of the pen tip.

You may also find it helpful to orient the pens every time before inserting them into the holder. While you won't want to do this on collectable pens, a good method is to use a permanent marker and make a mark on each pen, indicating which way is "forward" towards the pen-holding thumbscrew.

Testing pens and alignment

A great test for alignment is to draw a simple rectangle (perhaps 2 inches or 5 cm on a side) on the computer and then to plot that rectangle on a piece of scratch paper. You can do this well before you begin plotting your "real" documents, in order to understand the performance and behaviors of your particular pens. Check that, when drawing the same rectangle with two different pens, you can get good overlap between the two. This is usually straightforward when using wider-tip pens, such as felt-tip markers, and may take a little more work for consistency (see the previous section) if you are using finer-point pens.

Working harder at testing pens and alignment

Aside: The methods described below are for achieving high precision alignment when using fine-tip pens. This is well above and beyond the level of effort needed in most AxiDraw use cases.

Suppose that you want to improve the precision of alignment. First, try plotting the same rectangle two times without changing anything in between. If your AxiDraw is working correctly, you should see almost no variation between the two, except perhaps that the rectangle looks darker after two passes.

Next, try taking the pen out and putting it back in the holder between the two plots. If you plot a rectangle, remove the pen and then put the pen back in the same place, then you should again see no difference between how it looks after the two passes. If you do see a difference, the issue is most likely that the pen tip is not centered in the barrel. Try making a mark (as noted in the previous section) and consistently inserting it twice with the same orientation.

If your pen-change technique is good, (with consistent technique, position, and alignment), you should be able to get the plot of two rectangles to overlap just as well as if you had not taken the pen out between the two plots. If your technique is good but the plots still not very consistent, it may be the case that you have a flexible (or semi-flexible) tip on your pen that is getting bent into different positions as you plot. This can sometimes happen with certain types of fine felt-tip markers.

If your pen only shows good consistently between plots at a certain angles of orientation, it may be useful to see if the issue is the centering of the tip within the barrel. Try plotting the rectangle four times, once each with the orientation mark facing North/East/South/West. If the pen tip is perfectly centered within the barrel, there will be no difference between them. Frequently, with fine enough pens, you'll see some variation. You may, with certain types of pens, be able to bend the tip very gently towards center, to improve its precision.

Once you've learned to characterize a single pen, try matching several pens. You may find that it's helpful to make a mark on each for proper orientation.