5754
« on: September 14, 2009, 04:30:31 PM »
Z AXIS ADJUSTMENTS – BACKLASH REMOVAL –BEARING PRELOAD
The Z axis components were adjusted by feel. The axis was turned using a makeshift handle attached to the screw. Now we get into testing for refining of the axis components. This may get a little long, but a lot of how to and description is included which may be of value to someone.
The Z axis calculated steps per unit and a basic velocity of 40 was used as rough motor tuning units in Mach. I just needed something so I could jog around using the controller. What I wanted to see is if the carriage is trying to lift or bind in any way. The Z axis ballscrew is not a precision ground screw such that the overall straightness meets a tolerance, but, that tolerance is not like a ground screw. The screw is anchored at the spindle end and the bearing at the tail end allows axial float ( just used for radial loading ) Slowly moving the carriage a slight adjustment of the ballscrew nut to saddle had to made. This was repeated until no movement ( of the carriage off the ways was happening – initially it was around .002” ).
Then the velocity was increased until it skipped at 120 IPM. The velocity was reduced and set at 80 IPM and no skipping happened. ( Just for info the X will do 200 IPM and is set at 100 and it only took 11 oz in to just move the axis ( 31 in oz with a new / different stepper installed ). The difference is the internal pole attract / detent. So we have an initial velocity that works, but, it is governed by how well things are aligned and associated torque required.
I will elaborate more in the refinement comments.
BACKLASH CHECK and REMOVAL
The steps per unit were confirmed. Now backlash was checked by moving one direction and then the other at different distances. The backlash was 0.012”…..
The bearing at the spindle end provides axial anchoring so any play between the outer rings must be removed. The ball nut is adjustable so that is another backlash point. Belt, motor quality, pulleys can play into this but the main two were noted first.
The bearings in the anchor end were re-shimmed. The bearings sit in a housing and the outer rings of he bearings are fixed by compressing them together via the mounting plate and the bearing housing. The inner ring of the bearings receives a preload by tightening a bolt into the screw.
Another back lash check was done, didn’t touch the rough ball nut adjustment, backlash now reduced to 0.008” and I know that the outer bearing rings are making contact. I then tighten the screw which adds the preload on the inner bearing rings. Another backlash check is made and it improves, but the steppers are skipping which indicates the bearing preload is too much. So I reduce it, the steppers don’t skip, and backlash is now at .004”.
Now the ball screw nut is progressively tightened until back lash comes down to around 0.001” and the motors are not skipping. Remember the carriage movement check done in the beginning. When all this stuff is tight there is not much give and alignment is very important. Not bad for a rough testing.
You will never have “0” backlash ( the X axis has 0.0003”) . Remember that lubrication needs to be between the moving components. So absolute “0” backlash is rather difficult to achieve if not impossible.
You can however do your best at each component of the axis train to eliminate it.
There is another part to these adjustments, since as you increase the loading on the bearings and ball screw nut, the torque required to move everything also can increase. So you need to play some to find a good combination.
TURNING TORQUE
Torque will be measured using the gauge shown in one of my other posts.
The torque to rotate the axis was measured and it took 80 in oz ( that’s a lot ). The belt was removed and the motor static detent torque was measured at 20 in oz. Can’t do anything about the motor so your left with 60 in oz. to try to reduce. A stated before the X axis has 21 in oz and one less bearing when measured.
The belt is replaced so I can jog the axis.
The ball nut preload was reduced / played with such that the backlash didn’t change. Then I further played with the anchor assembly preload again not causing an increase in backlash. That done the belt was removed and the torque measured. It’s now 35 in oz to just start to move the carriage. So the torque was refined rather easily and reduced almost in half. ( 60 down to 35 in oz). I would guess that at most the best I can reduce it is around 10 in oz.
The ball screw is not perfectly straight so when it turns you are putting a radial load into the bearings. I believe I can reduce required torque by adjusting the bearing mounted at the tail end of the lathe. That requires shimming the mounting plate and just takes some time fooling with it.
But for now, I know that I can almost eliminate the backlash (0.001” current on the Z ). Everything needs to be removed, but I just needed to confirm that all is within what I would like to accomplish before continuing on.
You say what’s the big thing about the torque readings?
Well if you can minimize the required torque for the axis without undue strain on the components keeping backlash within a desired tolerance, then you have a refined machine which will improve velocity and acceleration.
I will note that if you go back to the speeds, feeds, and torque that I was after, they have already been exceeded.
LIFE IS GOOD!
RICH
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