How to machine the back plate to minimize the lathe chuck’s runout?
1. Make sure you have .0001” graduation indicator on your hand. For example:
Shars 303-3113
2. Turn off the power, pull the belt (lathe belt) by hand, slowly rotate lathe
spindle 360 °. Check the runout of the lathe spindle at point A & B with a
dial indicator. In order to obtain the lathe chuck’s clamping accuracy, the
permissible runout on the lathe spindle should be within standard runout
(please see the following form). Note that you are not comparing the two
points, just confirming the runout of your spindle.
A
B
3. Machining the back plate
1) Connect the back plate on the lathe spindle, and make sure the size of flange
center hole is the same as the diameter of the spindle.
If the back plate is a D type, you need just clean all studs and install it into
the spindle. Make sure it is vertical to spindle.
Measure the chuck’s D1, for example measure 202-5433, D1=5.118”
Back plate and chuck must be interference fit to each other. The tolerance of
chuck’s recess diameter D1 is H7 and the tolerance of the back plate’s recess
diameter D1 is K6.
K6 tolerance:
Chuck Size D1 Size (mm)
3”
55±0.005
4”
72±0.005
5”
95±0.005
6”
130±0.007
8”
165±0.008
10”
206±0.008
12”
270±0.01
16”
340±0.02
20”
440±0.02
2) Measure the recess diameter D1 of the chuck
D1
3) For example, 202-6109
6109 back plate has a D1=5.16”and there is 1mm extra
space on the shoulder of the back plate D1. The customer needs to turn this
down to fit the chuck. If the tolerance of the back plate’s recess diameter D1 is
tighter, the back plate and chuck will contact tighter, so the chuck’s TIR will be
better.
Machine here
D1
4）If
If you install the back plate onto another lathe spindle to complete the
machining process and
nd then install the back plate on your lathe, the chuck will
hold the run out from the other lathe spindle. Under this condition, there is no
way to make sure the chuck’s runout is true. Therefore, this procedure is wrong.
When you install the back plate into the chuck, machine the back plate’s D1,
the standard of concentricity is the outer flange back plate not the threaded
middle hole on the back plate. You cannot guarantee chucks reach TIR after
back plate connects to chuck. Therefore, this procedure is wrong.
You must connect back plate on the lathe spindle it will be used on, making
sure it is tight on the lathe spindle. Make sure the center of chuck and center of
lathe spindle is concentric, then machine the shoulder diameter D1, so the
runout will be reached.
After machining the back plate, if the back plate can be easily put into the
chuck then that means the back plate and chuck are not interference fit to each
other. The chuck’s runout will then be out of tolerance and the back plate and
chuck will not be aligned concentric to each other. If the chuck holds a dowel
rod to test the runout, it should indicate that runout will be out of tolerance.
The following steps show how to make the back plate and chuck
interference fit.
1. Place the chuck on a flat surface with the back side facing up
2. Place the back plate into the recess of the lathe chuck
3. Using a rubber hummer, gently tap the back plate’s top and bottom, then
tap left side and right side; make sure the back plate fits parallel into the
recess
If the machined back plate’s D1 is too small, interference fit cannot be
reached. You have to get another back plate and machine it again.
4. After machining the back plate, see fig G0. Test the T.I.R. on points a and b.
Test point A, check if the back plate and spindle are concentric.
Test point B, check if the back plate is vertical to the spindle.
If the T.I.R A and B are smaller than permissible error, then go to the next
step.
For example: 202-5433,
5433, 6” chuck, TIR at point A must be smaller than
0.005mm=0.0001968”. The number 0.005mm shows on fig G0.
B
A
After finish machining D1, you also need to machine the outer diameter of back
plate A down 1mm, make sure back plate A and D1 are concentric. Note: You
must test the T.I.R. at the point A not point C, because the point A will connect
to chuck.
Do not use the following indicator that is photographed below, the
precision of that indicator is only .001 " and the tolerances of the chuck’s D1
and back plate’s D1 are small, so you have to use an indicator above 0.0001”
to test.
C
A
5. With the back plate and chuck installed onto the spindle, follow the next step
to check if there is the gap between the back plate’s D1 and the chuck’s D1
and if the back plate’s D1 and the chuck’s D1 are interference fit. If you feel
the back plate shaking, that means there is gap between the back plate and
chuck meaning the back plate’s D1 is machined too small. If you don’t feel
the back plate shaking, there is no gap between the back plate and chuck,
chuck
and they properly match each other.
1) Test the runout at point A and B
2) For a 6” chuck, if the T.I.R. B ≤0.0015”and T.I.R. A≤0.0002”, that means
the back plate and chuck are interference fit.
A
B
For example:
After installing 202-5433 6” chuck onto 6” back plate, the 6 " back plate’s TIR
should be smaller than a = 0.005mm = 0.0001968".
The following 3 photos show that the 6” back plate has runout of 0.0006".
0.0006”>0.0002”, so the back plate’s D1 is too small, you have to get a new one to
machine again.
Also the following indicator is wrong, precision only .001 ", and the tolerances of
chuck D1 and back plate D1 are small, you have to use indicator 0.0001” to test.
Examples: SHARS No: 303-3113 0.2" X 0.0001
The following 3 photos show the 6” chuck body is running out only 0.0008".
1) If 6” back plate’s TIR is smaller than .0002" and 6 "chuck’s TIR is smaller
than .0015", that means back plate and chuck are concentric. D1 of back plate is
machined correctly. Back plate and chuck are interference fit. The difference of
TIR between back plate and chuck is smaller, the back plate and chuck’s
concentricity is closer.
2) If the 6 " back plate’s TIR is 0.0006", is greater than 0.0002" , then when chuck
holding rod to test TIR, the 6” Chuck’s TIR can’t reach 0.003 ".
3) If the 6 " back plate’s TIR is .0002", the concentricity between chuck, back plate,
and spindle is 0.0002”. 6 "chuck ‘s TIR is .002" , the concentricity between chuck
and spindle is 0.002”. That means the difference of the concentricity of back plate
and chuck’s are too large, that means D1 of back plate is machined too small, there
is gap between back plate D1 and chuck D1, they are not interference fit. At last,
when chuck hold test rod to test TIR, the TIR will fail to reach the standard.
6. See figG1, test T.I.R. at A and B point.
Test point A, check if the chuck and spindle are concentric.
Test point B, check if the chuck is vertical to spindle.
If the result reaches the permissible error, go to next step.
7. See Fig G2. Install a rod or pin gage, test T.I.R. at the points A.
1) The distance from the jaw, L, is listed on Fig G2.
For example, if the chuck’s diameter is 6”=152.4mm<160mm, the test distance
L from the jaw should be 50mm = 1.9685”. If the test result is less than
0.08mm=0.00315”, the chuck reaches T.I.R.
2) For test rod diameter information, please see the following form, unit is mm.
Chuck
specification
80 mm
100
125
160
200
250
315
400
500
630
Unit is inch
rod diameter
(small)
8mm
12
12
12
12
20
32
32
45
45
rod diameter
(medium)
12mm
15
15
20
20
32
45
45
75
75
rod
diameter(large)
15mm
20
20
32
32
45
75
75
115
115
Chuck
specification
3”
4
5
6
8
10
12
16
20
24
rod diameter
(small)
0.315”
0.472
0.472
0.472
0.472
0.787
1.2598
1.2598
1.7716
1.7716
rod diameter
(medium)
0.472”
0.591
0.591
0.787
0.787
1.2598
1.7716
1.7716
2.9527
2.9527
rod
diameter(large)
0.591”
0.787
0.787
1.2598
1.2598
1.7716
2.9527
2.9527
4.5275
4.5275
In Summary, if you want to reach the chuck’s TIR, you must follow the above
steps to test TIR at 9 points.
1) Check spindle’s runout at point A and B on spindle.
2) Check if the back plate is concentric and vertical to spindle at point A and B
on back plate. What is the TIR at point A (concentric)? What is the TIR at
point B(vertical)?
3) Check if the back plate and chuck are interference fit at point A(back plate)
and B (chuck).
4) Check if the chuck is concentric and vertical to spindle at point A and B
5) Install rod to test chuck’s runout at point A