Biomedical Engineering Reference
In-Depth Information
Fig. 8.29 Ball location for the repeatability test
20 independent measurements. For the
X
and
Z
axes we repeat the same procedure.
Finally, the repeatability for each axis is defined as the difference between the
maximum and the minimum obtained for each axis. The results of the test are as
follows:
X
max
= 8,378;
X
min
= 8,371;
Y
max
= 7,093;
Y
min
= 7,083;
Z
max
= 4,597; and
Z
min
= 4,591.
The repeatability in each axis is:
R
x
¼
N
xmax
N
xmin
¼
8
;
378
8
;
371
¼
7 steps
ð
Þ
(8.107)
R
y
¼
N
ymax
N
ymin
¼
7
;
093
7
;
083
¼
10 steps
ð
Þ
(8.108)
R
z
¼
N
zmax
N
zmin
¼
4
;
597
4
;
591
¼
6 steps
ð
Þ
(8.109)
To know the repeatability in micrometers, it is necessary to multiply the results
by 1.88 (displacement of each axis per step):
R
x
= 13.16
m
m;
R
y
= 18.8
m
m; and
R
z
= 11.28
m
m.
For the second test, a special computer program was written by Alberto Cabal-
lero. The ball fixed to the tool support is moved by 3,000 steps in the
Y
axis and
3,000 steps in the
Z
axis from the home position, and makes contact with another
ball in the
X
direction. The position on the
X
-axis is read and saved in a file. After
this, the machine returns to the home position. This process is repeated as many
times as necessary. The results of this test are as follows:
X
max
= 2,082;
X
min
=
2,067;
Y
= 3,000;
Z
= 3,000. The home position repeatability is:
R
=
N
max
-
N
min
=
2,072 - 2,067 = 15 (steps) or
R
= 1.88
m. In this case, the home
position repeatability is larger than the repeatability without the home position.
For this reason, we have used the home position procedure only at the start of the
workpiece treatment. The errors of the home position procedure were compensated
by intermediate measurements of the workpiece sizes.
15 = 28.2
m
