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- Jan 22, 2011
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That change of direction could point to a thrust bearing. Any end play in the leadscrew? Or it could be in the apron, as Ray mentioned.
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Surface Finish Issues with CNC Lathe
- Thread starter JLeather
- Start date
Ray,
There's nothing inside the apron except the ballnut. The apron on this lathe is only a sheetmetal cover for the ballnut. There is no manual mode. The only way I've been able to manually move the apron around is through a knob on the tailstock end of the leadscrew. There's no handwheel, and the gear rack on the lathe is no longer attached to anything. There's only the leadscrew.
Tony,
You're right that it's something related to leadscrew thrust. My money is still on the headstock-end bearings, but I can't figure out how to take them apart and don't have the time to really dig deep until after Christmas.
There's nothing inside the apron except the ballnut. The apron on this lathe is only a sheetmetal cover for the ballnut. There is no manual mode. The only way I've been able to manually move the apron around is through a knob on the tailstock end of the leadscrew. There's no handwheel, and the gear rack on the lathe is no longer attached to anything. There's only the leadscrew.
Tony,
You're right that it's something related to leadscrew thrust. My money is still on the headstock-end bearings, but I can't figure out how to take them apart and don't have the time to really dig deep until after Christmas.
Actually I think I just recently solved the issue. The lathe had no front gib. Back when it was a manual lathe there was 40 pounds of apron and assorted gears hanging from the front edge of the carriage, so a front gib wasn't really necessary. Now that it's a CNC there's no weight on the front of the carriage, and with little effort you can lift the front of the carriage off the ways. I found that the carriage easily racked when sliding along, causing the front edge to slightly ride up on the v-way which may well explain the issues I've been having with especially light cuts.
I decided to add a front gib to the lathe. I machined it out of 3/4" 7075 aluminum and attached a strip of 932 bearing bronze to the front edge for the bearing surface. The carriage does seem to have a good bit of drag now, but on the plus side I can no longer lift the front edge at all.
Completed Gib Assembly:
Installed Gib:
Initial results are very promising. I took a 2.5" test cut on a piece of 3" OD solid 6061. This picture was a pass taking a .002" cut and at an exaggeratedly-slow .001"/rev feed rate just to see what would happen:
Took a nice long pass on a test bar. 1 1/4" 6061, a 16" long pass without a follow rest. No sign of the striations I have been fighting. I think I'm good.
I decided to add a front gib to the lathe. I machined it out of 3/4" 7075 aluminum and attached a strip of 932 bearing bronze to the front edge for the bearing surface. The carriage does seem to have a good bit of drag now, but on the plus side I can no longer lift the front edge at all.
Completed Gib Assembly:
Installed Gib:
Initial results are very promising. I took a 2.5" test cut on a piece of 3" OD solid 6061. This picture was a pass taking a .002" cut and at an exaggeratedly-slow .001"/rev feed rate just to see what would happen:
Took a nice long pass on a test bar. 1 1/4" 6061, a 16" long pass without a follow rest. No sign of the striations I have been fighting. I think I'm good.
