inconsistent grading problem
 

hi i am quite novice to openfoam
i keep getting this error that i can not figure out how to correct


--> FOAM FATAL ERROR: (openfoam-2012)
Point merge failure between face 2 of block 8 and face 1 of block 9
Points: (0.500181 -0.500181 -0.5) (0.50903 -0.50903 -0.5)
This may be due to inconsistent grading.

From void Foam::blockMesh::calcTopologicalMerge()
in file blockMesh/blockMeshMergeTopological.C at line 453.

FOAM exitig

this is my mesh file
/*--------------------------------*- C++ -*----------------------------------*\
| ========= | |
| \\ / F ield | foam-extend: Open Source CFD |
| \\ / O peration | Version: 4.0 |
| \\ / A nd | Web: http://www.foam-extend.org |
| \\/ M anipulation | |
\*---------------------------------------------------------------------------*/
FoamFile
{
version 2.0;
format ascii;
class dictionary;
location "constant/polyMesh";
object blockMeshDict;
}
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //

scale 1;


vertices
(
(-5 -5 -0.5)
(-1 -5 -0.5)
(-1 -1 -0.5)
(-5 -1 -0.5)
(-5 -5 0.5)
(-1 -5 0.5)
(-1 -1 0.5)
(-5 -1 0.5)
(-1 1 -0.5)
(-5 1 -0.5)
(-1 1 0.5)
(-5 1 0.5)
(-1 5 -0.5)
(-5 5 -0.5)
(-1 5 0.5)
(-5 5 0.5)
(1 5 -0.5)
(10 5 -0.5)
(10 1 -0.5)
(1 1 -0.5)
(1 5 0.5)
(10 5 0.5)
(10 1 0.5)
(1 1 0.5)
(10 -1 -0.5)
(10 -5 -0.5)
(1 -5 -0.5)
(1 -1 -0.5)
(10 -1 0.5)
(10 -5 0.5)
(1 -5 0.5)
(1 -1 0.5)
(0.5 -0.5 -0.5)
(0.5 0.5 -0.5)
(-0.5 0.5 -0.5)
(-0.5 -0.5 -0.5)
(0.5 -0.5 0.5)
(0.5 0.5 0.5)
(-0.5 0.5 0.5)
(-0.5 -0.5 0.5)


);

blocks
(

hex (0 1 2 3 4 5 6 7) (220 220 1) simpleGrading (1 1 1)
hex (3 2 8 9 7 6 10 11) (220 220 1) simpleGrading (1 1 1)
hex (9 8 12 13 11 10 14 15) (220 220 1) simpleGrading (1 1 1)
hex (8 19 16 12 10 23 20 14) (220 220 1) simpleGrading (1 1 1)
hex (1 26 27 2 5 30 31 6) (220 220 1) simpleGrading (1 1 1)
hex (19 18 17 16 23 22 21 20) (220 220 1) simpleGrading (1 1 1)
hex (27 24 18 19 31 28 22 23) (220 220 1) simpleGrading (1 1 1)
hex (26 25 24 27 30 29 28 31) (220 220 1) simpleGrading (1 1 1)
hex (32 27 19 33 36 31 23 37) (220 220 1) simpleGrading (50 1 1)
hex (2 27 32 35 6 31 36 39) (220 220 1) simpleGrading (1 50 1)
hex (2 35 34 8 6 39 38 10) (220 220 1) simpleGrading (50 1 1)
hex (34 33 19 8 38 37 23 10) (220 220 1) simpleGrading (1 50 1)

);

edges
(
arc 35 34 (-0.7071067812 0 -0.5)
arc 39 38 (-0.7071067812 0 0.5)
arc 2 8 (-1.4142135624 0 -0.5)
arc 6 10 (-1.4142135624 0 0.5)
arc 34 33 (0 0.7071067812 -0.5)
arc 38 37 (0 0.7071067812 0.5)
arc 8 19 (0 1.4142135624 -0.5)
arc 10 23 (0 1.4142135624 0.5)
arc 33 32 (0.7071067812 0 -0.5)
arc 37 36 (0.7071067812 0 0.5)
arc 19 27 (1.4142135624 0 -0.5)
arc 23 31 (1.4142135624 0 0.5)
arc 32 35 (0 -0.7071067812 -0.5)
arc 36 39 (0 -0.7071067812 0.5)
arc 27 2 (0 -1.4142135624 -0.5)
arc 31 6 (0 -1.4142135624 0.5)
);

boundary
(
inlet
{
type patch;
faces
(
(13 15 11 9)
(9 11 7 3)
(3 7 4 0)
);
}
outlet
{
type patch;
faces
(
(21 17 18 22)
(22 18 24 28)
(28 24 25 29)
);
}
obstacle
{
type wall;
faces
(
(38 34 35 39)
(37 33 34 38)
(36 32 33 37)
(39 35 32 36)
);
}
tb
{
type symmetry;
faces
(
(15 13 12 14)
(14 12 16 20)
(20 16 17 21)
(0 4 5 1)
(1 5 30 26)
(26 30 29 25)
);
}
frontAndBack
{
type empty;
faces
(
(7 6 5 4)
(0 1 2 3)
(11 10 6 7)
(3 2 8 9)
(15 14 10 11)
(9 8 12 13)
(14 20 23 10)
(8 19 16 12)
(6 31 30 5)
(1 26 27 2)
(20 21 22 23)
(19 18 17 16)
(23 22 28 31)
(27 24 18 19)
(31 28 29 30)
(26 25 24 27)
(37 23 31 36)
(32 27 19 33)
(39 36 31 6)
(2 27 32 35)
(10 38 39 6)
(2 35 34 8)
(10 23 37 38)
(34 33 19 8)
);
}
);

mergePatchPairs
(
);
// * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * //


i really appreciate your help