Overview

So say I want an arc between the x and z axis in the same way the following creates an arc between the x and y axis (MVCE):

\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}
\tikzset{
x={(0:1cm)},y={(50:0.7cm)},z={(90:1cm)}
}
\draw[->] (0,0,0) --++ (1,0,0) node[right]{$x$};
\draw[->] (0,0,0) --++ (0,1,0) node[right]{$y$};
\draw[->] (0,0,0) --++ (0,0,1) node[right]{$z$};
\draw (0,0,0) ++(0:.3) arc (0:90:.3);
\end{tikzpicture}
\end{document} One solution to this exact problem is to do scope and tell arc that the y-axis is the z-axis.

\begin{scope}
\tikzset{y=(90:1cm)}
\draw (0,0,0) ++(0:.3) arc (0:90:.3);
\end{scope}

The problem

However, I need this more generic since I don't necessarily know how the y-axis is defined, so I'm looking for a way to do, essentially this:

\begin{scope}
\switchYZAxes
\draw (0,0,0) ++(0:.3) arc (0:90:.3);
\end{scope}

or

\begin{scope}
\tikzset{y=\theZAxis}
\draw (0,0,0) ++(0:.3) arc (0:90:.3);
\end{scope}

The latter is problematic because it seems like I can't send polar coordinates to the \tikszet via a macro (which is the background for this question).

Any ideas? Thanks :)

With \usetikzlibrary{3d} you can use:

\begin{scope}[canvas is xy plane at z=0, blue, thick]
\draw (0:0.3) arc (0:90:0.3);
\end{scope}

\begin{scope}[canvas is yz plane at x=0, red, thick]
\draw (0:0.3) arc (0:90:0.3);
\end{scope}

which yields: Code:

\documentclass[border=1mm]{standalone}
\usepackage{tikz}
\usetikzlibrary{3d}

\begin{document}
\begin{tikzpicture}
\tikzset{
x={(0:1cm)},y={(50:0.7cm)},z={(90:1cm)}
}
\draw[->] (0,0,0) --++ (1,0,0) node[right]{$x$};
\draw[->] (0,0,0) --++ (0,1,0) node[right]{$y$};
\draw[->] (0,0,0) --++ (0,0,1) node[right]{$z$};

%\draw (0,0,0) ++(0:.3) arc (0:90:.3);
\begin{scope}[canvas is xy plane at z=0, blue, thick]
\draw (0:0.3) arc (0:90:0.3);
\end{scope}
\begin{scope}[canvas is yz plane at x=0, red, thick]
\draw (0:0.3) arc (0:90:0.3);
\end{scope}

\end{tikzpicture}
\end{document}
\documentclass[border=5mm]{standalone}
\usepackage{tikz}
\begin{document}

\begin{tikzpicture}
\tikzset{
x={(0:1cm)},y={(50:0.7cm)},z={(90:1cm)}
}

\draw[->] (0,0,0) --++ (1,0,0) node[right]{$x$};
\draw[->] (0,0,0) --++ (0,1,0) node[right]{$y$};
\draw[->] (0,0,0) --++ (0,0,1) node[right]{$z$};
\draw (0,0,0) ++(0:.3) arc (0:90:.3);

\coordinate (A) at (\rad, 0, 0);
\coordinate (B) at (0, 0, \rad);
\draw[red] (A) edge[in = 0, out = 90] (B);
\end{tikzpicture}
\end{document} I'd recommend tikz-3dplot for that. You only need to draw the arcs in rotated planes. (The first scope is not necessary here, but I put it in case you want to extend the code.)

\documentclass[tikz,border=3.14pt]{standalone}
\usepackage{tikz-3dplot}
\begin{document}
\tdplotsetmaincoords{70}{20}
\begin{tikzpicture}[tdplot_main_coords,font=\sffamily]
\draw[->] (0,0,0) --++ (1,0,0) node[right]{$x$};
\draw[->] (0,0,0) --++ (0,1,0) node[right]{$y$};
\draw[->] (0,0,0) --++ (0,0,1) node[right]{$z$};
\tdplotsetrotatedcoords{0}{0}{0}
\begin{scope}[tdplot_rotated_coords]
\draw[red] (0.3,0,0) arc (0:90:0.3);
\end{scope}
\tdplotsetrotatedcoords{0}{90}{90}
\begin{scope}[tdplot_rotated_coords]
\draw[blue] (0.3,0,0) arc (0:90:0.3);
\end{scope}
\end{tikzpicture}
\end{document} 