Re: backlash fixes etc.


Christopher C Stratton
 

First let me mention one thing I found on the 9" lathe last night. I
was getting endplay in the crossfeed screw bearing no matter how tight
I turned the nut in the hand crank. Turns out the micrometer collar
was slightly too short and the 5/16" bored crank was bottoming out on
the shoulder to the 3/8" bore of the collar. A simply brass shim
washer (between collar and crank) fixed that.

i think that we are all eluding to the fact that SOME backlash is
acceptable as long as its CONSISTENT for the length of screw travel.
that way the accuracy is maintained.
It depends on your goal. For a manual machine, this is true - some
backlash is okay, while consistency is paramount. For a computerized
machine the opposite is actually the case - backlash is very bad (for
some types of cuts) but consistency is not paramount as it can be
compensated in software. Of course it's hard to make a tight screw
that isn't a consistent pitch.

we are assuming that the screw
is ok, and only the nut is worn out. not true. consider that your
lead screw is worn more near the headstock than at bracket end. i
think someone pointed out this example.
I am in favor of replacing the screw, as long as I can find new stock
that is as good as the original. This is partly as I'd rather
carefully pack up the original components in a safe place and run my
experiments (fit altering, polymer casting, etc) on easily replaced
generic screw stock.

can anyone definitely state what the thread is on the screw?
i would assume that it is standard acme. is it some weird pitch like
and odd number? (i admit, i havn't gone out to measure mine, and i
know my screw is worn so i wouldn't trust my screw as the standard)
On the 9" south bend the screw is 10 tpi. I thought it was 3/8"
diameter, but it may be 7/16". I think I might try to fit 1/2-10 as a
replacement. (The 13" is 5/8-8). We're assuming it is acme - one
reason for replacing both screw and nut.

i think scott pointed out converting to a ball screw is propably best
for accuracy. he might be right, and i don't want to seem like i am
second guessing his expertise. however, consider that ball screws
don;t have enough friction to resist turning of not held in place
with some sort shaft lock or stepper motor power. this could be a
issue when using the compound off of the apron drive. also, i don't
think that manufacturing a ball screw retrofit is as easy as it might
seem as i think that the ball screws and maybe the nuts are hardened.
i will eat my words later talking about machining 4130 futher on...
I have a 5/8-5 double nut left hand ballscrew (about $200 for the set)
partially fit to my 13" lathe. I had to mill out the channel in the
carriage a little bit, using a ball nose mill to keep everything nice
and round to avoid creating stress risers. If anyone else attempts
this, be smarter that I was and put round stock under the V ways to
set it on the mill table... (I tried to shim it level off the flat
surfaces) I'm making a new screw-in bearing sleeve so I just remove
the entire screw, sleeve, collar and handwheel assembly intact (can't
get it apart anyway) and replace it with the ballscrew based one set
up for a cog belt pulley instead of a handwheel.

with fixes like moglice and babbit wearing surfaces i sounds like its
almost too much trouble. not only in material cost, but the time and
detours you would need to take to do the job completely. and what do
you do if the fix doesn;t hold? you are only taking out play in the
nut. you are not fixing the wear in the screw. and you are using a
potentially worn screw as the mold for both and makeing that defect
the reference surface for the entire screw.
There is one thing I think that would make using moglice in
conjunction with a new screw easier than using a new nut. This is
machining the top of the nut to fit into the hole in the slide. For
the block the ballscrew fits into on the big lathe, I finally put the
old screw in a 5c block holder on the mill table and indicated the nut
boss. Then I switched to an appropriate collet holding a piece of bar
single pointed with the same threads 15/16-whatever threads as on the
outside of the ballnut and screwed the block that would become the nut
mount onto that. Programed the mill to circular mill the boss - one
might be able to do it on a manual machine with a boring head having a
cutter turned in rather than out.

Constrasting with moglice, I think one could machine a sleeve that fit
the apron with an oversize bore in about the right place for the
screw. Assemble it with some clay or something to dam around the
screw. Take the carriage off the lathe, set it upside down on the
workbench and inject moglice through the hole that I think I remember
is present in the middle of the carriage.

the spring loaded pair of nuts is a good idea. the spring force is an
issue.
Springs work best with a low friction nut material... moglice is
supposed to be lower friction than bronze.

from green bay: a 4130 or 1018 acme threaded rod 2G, lead
error of less than 0.001 /inch in "standard" sizes (0.25-16 up to
0.75-6/8/10/12) is anywhere from $3-$7 per foot. a round ampco or sae
bronze acme nut roughly 1" long is around $20 for the matching
sizes.
I'd get 2C class screw from Mcmaster, but otherwise agree. Bruce at
moglice said stainless screws are often better made than the carbon
steel ones... I would be the alloy steel ones are pretty good, too.
I've turned ballscrews with carbide - acme screws are deeper but
I would guess still do able.

Chris

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