How to center a typewriter (ttfamily) '+'

Question

I would like to draw a circular TikZ node with + in its center, but font=\ttfamily moves it up. Why? How to solve it?

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\begin{document}
\begin{tikzpicture}[
    or/.style = {circle,    
        minimum size=4mm,  
        very thick,  
        draw,  
        font=\ttfamily, 
    },
]
\node[or,font=\normalfont] (a) {+};
\draw[very thin,red] (a.west)--(a.east);

\node[or] (b) at (1,0) {+};
\draw[very thin,red] (b.west)--(b.east);

\end{tikzpicture}
\end{document}

Answer 1

By design, the + in Computer Modern has its horizontal bar slightly above what we would consider the visual center; this "fools" TiKZ's algorithm for centering an object on the node coordinates.

\documentclass[border=1]{standalone}

\newcommand{\ttplus}{%
  \raisebox{-.15ex}[\dimexpr\height-.15ex\relax][0pt]{\ttfamily+}}

\usepackage{tikz}
\begin{document}
\hbox{\fboxsep=0pt \fboxrule=.2pt % just to have no sep and lighter rules
\fbox{+}\,a\,\fbox{\ttfamily+}\,\fbox{\ttplus}}
\hbox{\begin{tikzpicture}[
    or/.style = {circle,
        minimum size=4mm,
        very thick,
        draw,
    },
]
\node[or,font=\normalfont] (a) {+};
\draw[very thin,red] (a.west)--(a.east);

\node[or] (b) at (1,0) {\ttplus};
\draw[very thin,red] (b.west)--(b.east);

\end{tikzpicture}}
\end{document}

Answer 2

Just to add to egreg's fine answer. Why it moves up? TikZ will center the enclosing box, as is obvious from the image below:

TeX aligns the letters symbols at their baseline in text mode and at a centerline, if they are in maths mode. Kerning is a partial solution as egreg, showed. Having the + in math is a better solution. It is not just a font issue, but also the way TikZ aligns which is closer to mathmode. Better even design a small cross using TikZ and have it placed at the exact  center.

\documentclass[border=3mm]{standalone}
\usepackage{tikz}
\def\boxes{\fbox{\fbox{y}\fbox{x}\fbox{+}\fbox{f}}}
\begin{document}
\begin{tikzpicture}[
    or/.style = {circle,    
        minimum size=6mm,  
        very thick,  
        draw,  
        font=\ttfamily, 
    },
]
\node[or,font=\normalfont] (a) {yx\textsuperscript{2}+};
\draw[very thin,red] (a.west)--(a.east);

\node[or] (b) at (1.3,0) {$x^2_3+$};
\draw[very thin,red] (b.west)--(b.east);
\fboxsep0pt\fboxrule0.1pt
\node[or] (c) at (2.5,0) {\fbox{\fbox{y}\fbox{x}\fbox{+}}};
\draw[very thin,red] (c.west)--(c.east);

\end{tikzpicture}

\texttt{\boxes} 
\end{document}