How to subtract surface from pdegplot?

Hi, sorry for bad english. I want to subtract surface created from x,y,z coordinates from stl model imported by pdegplot. I imported stl model and extacted X,Y,Z values of this model, then by using griddata command i made dimention of X and x same and subtracted and get result like in image. In red section what i want, but result also create subtraction result in blue sections. So is the any way to get only result only in red section? The basic code is presented

model = createpde;
importGeometry(model,'BELT PULLEY.stl');
U= generateMesh(model)
pl2=pdeplot3D(model)
X=pl2.XData;
Y=pl2.YData;
Z=pl2.ZData;
 Xgrid = griddata(y,z,x,Y,Z,);
 
 hh=plot3(X-Xgrid,Y,Z)