Mechanical checks on
an axis:- When there is an axis related problem one
always wonders if it is a mechanical problem or
electrical problem. It is not a rocket science and
it is just logic. Here is a simple step by step
procedure to ascertain if it is mechanical or
electrical.
1)
Backlash Check-Determine
if the axis you are checking has a scale or an
encoder as a position feedback device.
a.
If it is an encoder set
an indicator on the axis. Move the machine using a
hand held pendant .001 inches at a time 10 times.
Move the axis in opposite direction by 10 clicks. If
the indicator does not come back to 0, the
difference is the backlash. One will also see the
first few clicks your numbers on the control moved
but the axis did not move. Backlash is the lost
movement of an axis when the direction of movement
is reversed.
b.
Scale is used to avoid
the backlash showing up on the part. If a scale is
used as position feedback device then the above test
will not work and the procedure below should be
used. Scale has two components. Scale and a reader
head. The scale or the reader head is placed to the
body of the machine and the other part is fixed to
the moving section of the machine. Get access to the
belt or a pulley or some moving rotary mechanism.
Move the machine using a hand held pendant .001
inches at a time 10 times. You will get an idea how
much degree (example 1 tooth of a belt) every time
the motor or the belt is moving for every .001
movement by the pendant. Now reverse the movement
direction by one click. If the axis moves almost the
same amount then there is no backlash. If the
movement is much larger then there is backlash. You
can eyeball this as close as .001 inch.
i.
The backlash can be
compensated by putting a value in the backlash
compensation parameter in every control.
ii.
If it is a scale this
compensation does not work on most of the controls.
iii.
Since lots of new
machines are more precise they come with scales. The
newer controls have a way of compensation to work in
spite of having a scale.
iv.
Backlash can come from
the ball screw, thrust bearing, or loose nut.
v.
Backlash could be
different at different places of the table depending
on the wear on the ball screw.
vi.
Usually the center part
of the table will have the most wear. If the
backlash cannot be adjusted with parameters or if it
changes every time check the tightness of the gib.
When you run the indicator on a straight line if it
does not stay straight check the gib. Higher end
machines do not have gibs, due to the nature of Ways
constructed.
vii.
For simplicity sake,
make sure the indicator is not .0001 inches. This
will create different set of issues.
viii.
How much is acceptable.
This depends on how well the axes are tuned. If it
is tuned like a BMW, above .004 of backlash would
create a problem. I have seen machines run with a
scale up to .01 backlash. There are a few parameters
you can adjust to get this going. But it has other
complications and I will leave it for another day’s
discussion
2) Lost motion Check
- Setup the gauge as mentioned above. Push and pull
the axis and see whether it moves and comes back to
position. It is quite normal to be able to push the
axis out of position. But it should spring back. The
amount it is not coming back is the lost motion.
This originates from the same sources as mentioned
above. The same way, try to move the axis sideways.
If there is looseness, try adjusting the gib. The
indicator can be set at the edge of the screw and
checked if it is the screw or the thrust bearing.
The lost motion should be less than .0002. If this
is more the axis performance will be poor.
3)
Repeatability checks - Setup the gauge as mentioned
above and zero the gauge out. Try moving the axis
one-inch away and back. Try moving it 2, 4, 6, 12
inches away and back. If the difference is same
every time, it is mechanical. If it is different
every time and the difference increases
proportionately, than the problem is electrical.
4) Hand check - With no brakes or motor try moving
the ball screw by hand to see if it is free and not
binding. Though this is rare, it is always a good
idea to check and make sure there is no binding.
There are screws which would have a nick at a place
or two. The nut should be snug on the screw. If it
is too loose that means the screw is worn out. All
ball screws are not made equal. I have seen some
reworked ball screws work lesser by quite a few
years. I have also seen some screws can run only at
half the speed. Always find the right place to send
it to for repair.
Author: Ven Swaminathan.
Email - Ven@cnconestop.com
For more information visit
http://cnconestop.com/CNC.htm
