\version "2.17.0" 

% With staff-size 15 the beam of the grace notes collides with 
% the tie of the first voice. With the default staff-size, no 
% collision occurs:
#(set-global-staff-size 15)

\new Staff {
%   \time 2/4
  \partcombine 
    \relative c'' { f2~ f }
    \relative c'' { b2 \grace { c16[ b] } a2 }
}