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Lead screw / Halfnut adjustment
- Thread starter Dman1114
- Start date
- Joined
- Mar 31, 2014
- Messages
- 607
There may be a way to shim it but I think if you move the carriage to the tailstock end as far as possible and put a straight edge on the lead screw you'll find it's sagging under it's own weight when not engaged, probably when it's engaged the split nut is acting somewhat like a steady rest and bringing it into it's true position.
- Joined
- Nov 16, 2012
- Messages
- 5,596
Remove the pumpkin... It's large head is almost certainly causing a gravitational force on the leadscrew, causing to flex. If that doesn't work, see if you can shim or adjust the half-nuts. Don't use the lathe that way, it will cause problems -especially when working at the extreme ends of the bed.
Ray
Ray
- Joined
- Nov 16, 2012
- Messages
- 5,596
... Also, get a long straight edge and see if the LS is bent. A machine crash can bend a LS.
I don't really think pumpkin head gravity is bending the LS. A shaft that diameter and length will not bend visibly in the middle when supported at both ends. Disengage the HN, turn the machine on and see if it wobbles. If so, it's bent. Best you can do is try to straighten it.
Could also could be extremely worn-out bearings at the supporting ends. In some cases, there's no bearing at the TS end and the LS is just stuffed in a hole. The hole could be way oversized. Drill it out and make a bushing.
Ray
I don't really think pumpkin head gravity is bending the LS. A shaft that diameter and length will not bend visibly in the middle when supported at both ends. Disengage the HN, turn the machine on and see if it wobbles. If so, it's bent. Best you can do is try to straighten it.
Could also could be extremely worn-out bearings at the supporting ends. In some cases, there's no bearing at the TS end and the LS is just stuffed in a hole. The hole could be way oversized. Drill it out and make a bushing.
Ray
Sorry not familiar with the term pumpkin ?!!!
It does not seam to be bent . I don't believe it is
Turning the screw by hand and and engaging watching it position doesn't affect
I removed the tail stock end support for the lead screw and it doesn't have any accessive wear.
Seems though I need to move the whole half nut mechanism down .30 or so
It flexes up about .030 or so. Does not seem to change from tail to headstock
I think I'll pull some more stuff off and see if I can get to the bottom of this.
It does not seam to be bent . I don't believe it is
Turning the screw by hand and and engaging watching it position doesn't affect
I removed the tail stock end support for the lead screw and it doesn't have any accessive wear.
Seems though I need to move the whole half nut mechanism down .30 or so
It flexes up about .030 or so. Does not seem to change from tail to headstock
I think I'll pull some more stuff off and see if I can get to the bottom of this.
- Joined
- Dec 17, 2012
- Messages
- 745
It's very old machinist jargon, derived from the ancient Greek word "pepon"(πέπων), to indicate the issue of the Cucurbita pepo I evidenced in color in the photo below.
:biggrin:
- Joined
- Nov 24, 2013
- Messages
- 1,791
ahahahahahahahaha
- Joined
- Nov 16, 2012
- Messages
- 5,596
Thanks for not being mad and taking it outside of humor...
Anyhow, you'll want to get that addressed. In many cases, a skewed or "influenced" LS will cause ghost lines to show-up in a fine finish cut. The lines will have the exact same spacing as the LS pitch. -Have seen this many times...
Ray
Ok so i Just pulled it out the apron...
Everything looks really good. nothing worn out. i measured the throw with the half nuts flipped over thinking they were backwards but they are the same.
Could not find any way to adjust. They are on a rotating cam. so no way to adjust the Throw..
so.... i put it back on tightened it up . and nothing changed.
well i got to thinking... i loosed the four apron bolts and put a gap between the carriage and the apron now the issue is reversed.
so i drew up the four bolts till the issue is now gone.
the half nuts engage and all is well no more Flex.
So the ??? now. Is there a Gasket I'm missing between the apron and the carriage ? What should i use to shim this or should i use a gasket ?? I'm guessing it needs to be around .020 - .030 .
This machine does not seem like it has ever turned a thread. Not really sure what its been doing since 1998?? i know the guy that i bought it from said he's never threaded on it ( i believe him cause he didn't even realize the forward/reverse switch needs to be off to start the lathe)
anyhow i think this may have been like this from day one.
So Does shimming between the top of the apron and the bottom of the carriage sound like a good fix??
Everything looks really good. nothing worn out. i measured the throw with the half nuts flipped over thinking they were backwards but they are the same.
Could not find any way to adjust. They are on a rotating cam. so no way to adjust the Throw..
so.... i put it back on tightened it up . and nothing changed.
well i got to thinking... i loosed the four apron bolts and put a gap between the carriage and the apron now the issue is reversed.
so i drew up the four bolts till the issue is now gone.
the half nuts engage and all is well no more Flex.
So the ??? now. Is there a Gasket I'm missing between the apron and the carriage ? What should i use to shim this or should i use a gasket ?? I'm guessing it needs to be around .020 - .030 .
This machine does not seem like it has ever turned a thread. Not really sure what its been doing since 1998?? i know the guy that i bought it from said he's never threaded on it ( i believe him cause he didn't even realize the forward/reverse switch needs to be off to start the lathe)
anyhow i think this may have been like this from day one.
So Does shimming between the top of the apron and the bottom of the carriage sound like a good fix??
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