Problem with lead screw/half nut/s
#1
When I engage the horizontal(longitudinal) feed via the lever on the saddle the lead screw moves(bends)towards the bed by approx 2mm.
This is(I think) causing "waves" in the finish on turned items, how can I sort this please people?

TIA.

Roy.
Light travels faster than sound!
This is why some people appear bright until they open their mouth!
Preserve nature...…….Pickle a squirrel! 
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#2
The "waves" in your finish are most likely caused by chatter (lack of rigidity) in your set up - not your lead screw. A loose gib, too much tool stick-out, DULL tools, tool center height, even how solid (or not) the base your lathe is sitting on is all it takes to set up a vibration in the machine that will give you all kinds of finishes - except the one you want. Feeds and speeds are a big part of that too. Remember as your work diameter changes (lathe) so does the feed rate.





Snug up all of your gibs, start with a good SHARP tool - on center - and keep everything short (up close and personal) and see if you note any improvement in your finish.   Smiley-gen163

The lead screw on my lathe moves just slightly as well when engaging the half-nuts. But there are no "adjustments" available on my lathe to change the alignment of the half-nuts. I don't believe it really matters because I don't believe there is a manual lathe out there that doesn't have some small variation in the lead screw contact.
Willie
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#3
What kind of lathe? Does this deflection occur everywhere along the leadscrew, or just near one end?

Tom
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#4
(08-09-2019, 02:13 PM)TomG Wrote: What kind of lathe? Does this deflection occur everywhere along the leadscrew, or just near one end?

Tom

Chinese 8 x 16...….MX210-V if that's any help, deflection occurs everywhere along the lead screw.
Finish is better at the chuck than at the tailstock though.

Cheers.

Roy.
Light travels faster than sound!
This is why some people appear bright until they open their mouth!
Preserve nature...…….Pickle a squirrel! 
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#5
For headstock...……...read tailstock!

How do you edit a post? Bash aahhh……...I can edit this, but not my last post! Yikes

Cheers.

Roy.
Light travels faster than sound!
This is why some people appear bright until they open their mouth!
Preserve nature...…….Pickle a squirrel! 
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#6
(08-09-2019, 06:19 PM)Swarf_Newbie Wrote: I can edit this, but not my last post!

There's a time limit on editing your posts. I'm too lazy to check right now but I will later if you want to know.

Ed
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#7
The leadscrew deflection could mean that the half-nuts aren't lined up properly with the screw. I don't know how you would check/fix that, but it may be worth looking into.

Sometimes you can lessen chatter by slowing the spindle speed. Easier on a lathe with variable motor speed - you can do it on the fly. If the workpiece is long and thin, chatter is almost a foregone conclusion, without a follower rest.

All the advice in the previous posts are right on the money - sharp tools, on centre, proper speeds and feeds.
Mike

If you can't get one, make one.

Hawkeye, proud to be a member of MetalworkingFun Forum since Jan 2013.
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#8
I've yet to figure it out yet, haven't really tried, but I get the same thing on my Logan. Doesn't matter how light a cut, but if I don't use the auto feed it's good. I did some turning further from the headstock tho and didn't get it. Gibs are set right.
Logan 200, Clarke 7x12, Index 40H Mill, Boyer-Shultz 612 Surface Grinder, HF 4x6 Bandsaw, ...
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#9
Bent lead screw, worn/loose/tight v-belt, loose/tight change gears, worn/loose spindle bearings.... yada, yada, yada.
All depends on how far down the rabbit hole you want to go I suppose.

Changing the hard lumpy Chinese v-belt on my lathe to a Fenner linked belt made a big difference in the finishes I got after changing it. I still have to be vigilant with sharp tools and short stick-outs though.
Willie
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#10
As others have already said setup is the most important and most influential part of obtaining good finishes.  That said a 8X16 Chinese lathe is not a quality machine and will not give the service a good American, European, Japanese, or South Korean machine tool will impart.  Some may consider this China bashing, but its based on experience.  Even the "industrial grade" Jet machine tools are inferior.

Unless you have the skill set to do a complete tear down and hand scrape your lathe so it is assembled properly you'll never get good results.  Even then you'll have a machine tool of inferior engineering and a inferior bed casting.  Start saving so you can purchase a quality machine.  There's nothing wrong with having small equipment, the steam modelers rely on small machines, but the good ones understand they will only face extreme frustration if they use Chinese machine tools.
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