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Help with an improperly working DRO

tstr

Plastic
Joined
Nov 18, 2023
Hi, I'm a starting hobbyist who got my 14x40 manual lathe straight from China. It does what it is supposed to and I'm still learning but one thing that is annoying me is the DRO's X-axis. The problem is that it doesn't stay at 0 when I turn on the spindle. Example: I've done a pass let's say 1mm depth of cut and back the cross slide back, move to the end of the part, move the cross slide back to 1mm and turn off the spindle to measure the part. When turning the spindle back on the X-axis on DRO jumps a few mm even though the cross slide doesn't move, so I have to always re-zero the DRO when doing another pass. Now it's not a huge deal, but it's annoying and if fixable I would like to fix it. Problem is, that being a beginner I don't really know where to start. Any ideas?
 
By "a few mm" do you mean 1mm or 0.01mm? The latter would not be terribly unusual. The mechanical shock of the motor starting up on a typical import 14x40 can easily shift the cross slide by the amount of slack in the screw/nut, even if the screw doesn't rotate and your eye doesn't notice the cross slide moving.
At the high school we typically see a 0.0002 to 0.002" jump (usually toward the bottom of that range) in diameter reading on the DRO, and I point this out to the students as something to be aware of when setting up a finish pass.
A full 1mm (0.039") would be quite excessive. This might indicate a loose mounting for the DRO scale or the reader head. A 0.1mm jump would be twice what we see. A 0.01mm jump would be right in line with our experience.
BTW, you should not have to rezero the DRO. You may have to retract the cross slide a turn and then advance back to your mark after turning on the motor again, to take up the slack/backlash, but you should be able to set the DRO X diameter reading to match your measurement before turning the motor on because 1) the cross slide has not shifted yet, and 2) you had taken out the slack when making the test cut so when you retract/advance back to the mark your cross slide should be in the same place.
 
By "a few mm" do you mean 1mm or 0.01mm? The latter would not be terribly unusual. The mechanical shock of the motor starting up on a typical import 14x40 can easily shift the cross slide by the amount of slack in the screw/nut, even if the screw doesn't rotate and your eye doesn't notice the cross slide moving.
I had to check again as I haven't used the lathe for a few weeks due to other projects, but on lower spindle speeds the movement seems to be in the ballpark of 0,002-0,01mm so that is probably the slack in the screw.
But with higher speeds it is bigger; -0,35mm @1150rpm and -1,37mm @1850rpm. Also at 1850rpm I only needed to move the cross slide to -1,35mm for contact with the part.
So this would make me think that there is something loose in the x-axis scale or reader, but what and where?
 
No noticeable vibration, no idea of electrical noise, cables have been routed from behind the lathe. No variable speed, just gears.
 
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When you bring the cross slide back to zero, try backing off the handle just a smidge so there's no preload on the cross slide. That might do it. Other than that, just don't reset the cross slide until the spindle is running. That's how I always do it. Almost every machine will give a little shock/bounce when starting, it's not really avoidable without a VFD or soft start. Even then they might move a bit just due to vibration.
 
Ok, will try this. I have plans to add a VFD at some point, but will try this for now.
 








 
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