backlash fixes etc.


pjwizr_1999@...
 

a little thought brings us all here but its fun to talk about it
anyway:

1: there is different wear and thus "clearance" at every point on the
leadscrew, even from new, but just how much is it?? this varies based
on the wear and corrosion, original surface quality, etc. YMMV, look
at the screw and see whether there is any hope for using a device
that presumes a fairly constant degree of clearance, you can use a
wire gauging technique to compare the end threads and middle threads,
to see if the thread grooves in the middle have appreciable wear (
measure the width - not the depth of the grooves - the real wear is
on the sides) - ALL single nuts will have this lash characteristic on
a non-perfect screw fit, even single ball-type nuts that dont use a
recirculating ball track will only follow their groove with the major
load on one side while moving. all that radial preload does is drive
the ball or the nut thread into the groove harder that the motive
force is pushing it out. to maintain a constant accuracy with
minimal lash you have to have a tapered groove and a follower that
bears on TWO opposing sides at once. no use of moglice, or any other
fixed nut design will change this. notice that the follower device
does not have to bear on two sides of the same groove!! see next para

2. if you can use the concept with two nuts as shown and explained in
the SHOPTASK EL DORADO page http://www.shoptask.com/feature3.htm
using double loaded nuts, you have a chance at getting the lash down
very low, but you have to live with the other space and load
restrictions. this design bears the preload on two opposing sides and
as long as the preload friction is less than the power needed to move
the carriage riding on the nut - now you have the nut tracking the
center of the space between the two groove surfaces and are getting a
fairly constant track with very low lash, and the wear is not
primarily on one or the other side of the screw thread surface. you
can use this principle to defeat some of the lash in the bronze nut
using a homemade delrin nut added under the slide ( does NOT replace
the bronze nut) and a spring around a small machine screw in the same
space ( or just a spring attached to the bronze nut) but it has
limited load capacity - a big cut will override the spring force.

3. most SB with some wear have a problem with the space between the
leadscrew handle end and the handle nut - this is where the leadscrew
on the compound and crosslide get lash quickly as this is where the
screw is fixed in place at the end, try putting a brass or delrin
shim here and preload just a teensy bit with handle nut tension to
get the big stuff out, then go after the little pieces. its easy to
saw a thin split in the bronze nut and put a small clamp on it to
close down the threads a little (radial preload!!)to see if you can
get some of the manufactured or worn clearance out without replacing
the leadscrew if you really find too much lash in the nut. easy way
to make a tap for delrin ( or even bronze if you are only using it
once or twice) is starting with a piece of the screw material -
fairly cheap to buy a little piece - just cut a taper on one end, few
slots, relief for the cutting edge, and grind a square on the other
end - making sure you cut or polish it to just a few thou undersize
so the delrin nut will have some inherent tension holding it onto the
(worn) threads. if the nut is too loose you can polish the tap down
and try again. polishing the space between the teeth is where you
will get the best result, not very much from cutting down the
diameter. even mild steel will harden just a little bit but you could
case-harden or get a screw length of drill rod or 0-1 if you get
really serious.

soapbox sagging and windbag empty now, keep the chips outta your
drawers and always tie your apron in the back!


gorvil
 

Hi All,

This backlash thread (pun intended) has been very interesing to me.
There are lots of good ideas and great suggestions. I am in the
middle of fixing my cross slide to reduce backlash, so this is very
timely.

Small Parts Inc. sells lead screws and anti-baclash nuts that appear
from the catalog photo to be of the spring loaded double nut type
mentioned earlier in this thread. I am not suggesting that anyone
should go out and buy from them, but the photo makes it easy to see
how the system works. They have an online catalog at
www.smallparts.com.

Micro-Mark sells low temperature alloys of various types for
casting. They sell tools and supplies for the model railroaders (I
mean HO not 1/7 scale live steamers) so you can get small amounts
(11.4 oz.) for under $15.00.

I made a new nut for my cross slide last night. This was the third
try but the learning process is part of the reason for doing this in
the first place. I made a simple threading tool to cut the threads.
I turned down a 3" x 1/2" round bar to under a 1/4" on 2/3 of it's
length. I drilled and tapped a 6-32 set screw in the small end of
the bar and about 1/2" from the end I drilled a 3/32" hole through
the diameter of the bar. A piece of 3/32" drill rod ground slightly
under sized to the thread on my cross slide made the cutting tool.
It helps to grind a flat on the side of the drill rod with a slight
bevel against the set screw to help hold it in place. The 1/2" end
of the bar mounts up in my boring bar holder. You can replace the
cutter as often as needed to finish the job or regrind it to change
the profile of the cut. I think a tool of this sort is called a
broaching bar when you use it to make keyways or rifleing.

I found that my new nut fits very tightly on the end of the screw
where there is less wear and fairly loosely in the center part of the
screw where most of the action has been. I have some Delrin rod
stock, so I may try the spring loaded anti-backlash double nut system.

I'll post the results when I'm done.

Glen Reeser


Paul R. Hvidston <p.hvidston@...>
 

I'm sure someone will jump in if I'm wrong, but my understanding is that you
cannot use a spring-loaded anti-backlash nut unless the spring-force exceeds
the maximum working force on the screw. To use a spring, it would need to be
stiff enough to cause significant friction on the screw. The spring loading
would be rigid in one direction, and springy in the other.

Cheers!

Paul R. Hvidston, N6MGN
ACKSYS Engineering
Upland, CA

----- Original Message -----
From: <Gorvil@aol.com>
To: <southbendlathe@yahoogroups.com>
Sent: Thursday, October 11, 2001 11:28 AM
Subject: [southbendlathe] Re: backlash fixes etc.


Hi All,

This backlash thread (pun intended) has been very interesing to me.
There are lots of good ideas and great suggestions. I am in the
middle of fixing my cross slide to reduce backlash, so this is very
timely.

Small Parts Inc. sells lead screws and anti-baclash nuts that appear
from the catalog photo to be of the spring loaded double nut type
mentioned earlier in this thread. I am not suggesting that anyone
should go out and buy from them, but the photo makes it easy to see
how the system works.


Christopher C Stratton
 

Glen wrote about his machined nut:

I found that my new nut fits very tightly on the end of the screw
where there is less wear and fairly loosely in the center part of the
screw where most of the action has been. I have some Delrin rod
stock, so I may try the spring loaded anti-backlash double nut system.
Nice job on machining ID acme threads... not something I'm sure I'm ready
to tackle.

Anyway, I just spoke to Bruce at Moglice. 100 grams of the stuff
(about 3 cubic inches) goes for about $35 bucks. He recommended using
tight stretched teflon pipe thread tape (!) as a release agent. The
idea is that it generates more clearence especially at the major and
minor diameters.

I asked about using new rolled screw material. Apparently the problem
is that rolled screws have nice tooth sides, but iffy crests and
roots. We talked about reaming out the moglice nut to increase the
minor diameter tolerance, and I was thinking maybe one could grind
down the outside of the screw slightly once the nut had been cast.
Have to be carefully of generating burrs though. Or maybe chase the
thread root somehow upsetting the crest of a thread on a piece of
annealed screw stock while dressing the sides a bit narrower in that
region to be sure not to cut them.

I think I'm going to try the spring idea first, although $35 for
moglice + $25 for good rolled alloy screw stock + $.50 for teflon tape
sounds reasonable, too. If I do this I think I will go to 1/2" screw
stock rather than the present 3/8". And maybe sneak a thrust bearing
in there, too.

On a related topic, I think my bronze nut is slightly loose in the top
slide. Have to figure out how I'm going to tighten that up and/or make
the shell for a moglice one stay tight.

Chris


Christopher C Stratton
 

I'm sure someone will jump in if I'm wrong, but my understanding is that you
cannot use a spring-loaded anti-backlash nut unless the spring-force exceeds
the maximum working force on the screw. To use a spring, it would need to be
stiff enough to cause significant friction on the screw. The spring loading
would be rigid in one direction, and springy in the other.
I think the spring loaded nuts are made of low friction materials, so
it's okay.

For the add-on external springs, I guess one would have to decide
between setting up for OD work or for ID work. For OD work, the screw
pushes in and the spring pulls out with enough force to overcome
dovetail friction/stickiness. That should be less than cutting force.

For ID work the screw would pull out and the spring push in.

At least that's how it works in the machine shop in my head ;-)

Chris


Yasmiin Davis <yasmiin@...>
 

This may be a dumb idea because I have never tried it but how about taking the old nut and slitting the bottom of it.  Then put a cap screw through the flat part so you could adjust the diameter of the inside of the nut.  Then spread clover compound on the parts with less wear and lap them -  turn the lead screw inside the now adjustable nut.  You could check your progress with bluing periodically till the lead screw has a uniform thread down its whole length.  Then make a new nut with moglice.  Yes the lead screw would have a non standard major diameter but who cares.  Using the existing nut as a lap should maintain the length wise accuracy of the lead screw.
 
Yasmiin

-----Original Message-----
From: stratton@... [mailto:stratton@...]On Behalf Of ccs@...
Sent: Thursday, October 11, 2001 12:32 PM
To: southbendlathe@...
Subject: Re: [southbendlathe] Re: backlash fixes etc.


Glen wrote about his machined nut:

> I found that my new nut fits very tightly on the end of the screw
> where there is less wear and fairly loosely in the center part of the
> screw where most of the action has been.  I have some Delrin rod
> stock, so I may try the spring loaded anti-backlash double nut system.

Nice job on machining ID acme threads... not something I'm sure I'm ready
to tackle.

Anyway, I just spoke to Bruce at Moglice.  100 grams of the stuff
(about 3 cubic inches) goes for about $35 bucks.  He recommended using
tight stretched teflon pipe thread tape (!) as a release agent.  The
idea is that it generates more clearence especially at the major and
minor diameters.

I asked about using new rolled screw material.  Apparently the problem
is that rolled screws have nice tooth sides, but iffy crests and
roots.  We talked about reaming out the moglice nut to increase the
minor diameter tolerance, and I was thinking maybe one could grind
down the outside of the screw slightly once the nut had been cast.
Have to be carefully of generating burrs though.  Or maybe chase the
thread root somehow upsetting the crest of a thread on a piece of
annealed screw stock while dressing the sides a bit narrower in that
region to be sure not to cut them.

I think I'm going to try the spring idea first, although $35 for
moglice + $25 for good rolled alloy screw stock + $.50 for teflon tape
sounds reasonable, too.  If I do this I think I will go to 1/2" screw
stock rather than the present 3/8".  And maybe sneak a thrust bearing
in there, too.

On a related topic, I think my bronze nut is slightly loose in the top
slide.  Have to figure out how I'm going to tighten that up and/or make
the shell for a moglice one stay tight.

Chris


To unsubscribe from this group, send an email to:
southbendlathe-unsubscribe@...



Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.


dpantazis@...
 

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. 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.

we are starting to sound like the old iron group debating the value
and karma consequences based the dilemas of choosing paint colors,
the proper plastic knob colors, or the 'proper' brass rivets to
reattach nameplates when restoring machines. woodworkers always seem
to get their panties in a twitch over these things. granted overall
machine accuracy is a bit more important than the vanity of paint
color match, but i digress...

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)

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...

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.

the spring loaded pair of nuts is a good idea. the spring force is an
issue.
as an alternate, consider the way bridgeports are set up to take out
backlash in the table screws. as memory serves me, the nut is long
like a barrel and is split almost 1/2 to 3/4 through the diameter in
the middle of the barrel. a screw in a boss on one half spans across
the split and bears against a boss on the other half. by extending
the screw, putting pressure on the boss on the other side you are
effectively making the once parallel slit into a slight trapezoid.
this in effect causes the barrel to 'deflect'. ascii graphics...

|| goes to /&#92;

this causes the threads to be a bit mis-aligned, but the effect is
that you have cocked the two nuts appart a hair and they bear on
opposite sides of the threads. the amount of movement is VERY small.
backlash is reduced....

granted i AM and engineer and my nature is to try and 'fix it better
than it was made.' at the risk of sounding like a marketing
campaign....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.

so for around the same $35 in materials you have the raw materials
for what i think is a better chance of fixing the problem.

to fix the screw: turn down the ends for the bearings. turn, shoulder
and thread the shaft for the drive. maybe silver solder a bushing to
increase the shaft diameter. slot a keyway for the drive (which can
be done w/o a mill. manufacturing a new screw is pretty straight
forward.

the nut could be more tricky. you could turn it down to the 'right'
size. chucking it 'flat' in a four jaw you can turn it and machine
off the flat. drill and tap an old style nut. this should fit the
channel.

if you wanted to make a split barrel like i was decribing, you cold
turn into the oversize nut to a barrel. maybe silver solder a couple
of other sleeves or bosses on. maybe turn a couple of flanges onto
the ends of the oversize nut and and grind away 3/4 of their
circumferance to leave yourself the bosses. you would then need to
bore out a few of the threads in the middle of the barrel. split the
barrel in the middle and drill and tap the mounting screws and
install the take up screw. i think it sounds easier than it might be
since it would be machining really small parts, but not impossible.
this would have to be made to fit into the existing channel.

i would think that it would be as durable as the original and
adjustable.

dennis


Larry Crebs <larrysbp2000@...>
 

What is delrin?
I have
some Delrin rod
stock, so I may try the spring loaded anti-backlash
double nut system.

I'll post the results when I'm done.

Glen Reeser




------------------------ Yahoo! Groups Sponsor

To unsubscribe from this group, send an email to:
southbendlathe-unsubscribe@egroups.com



Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/


__________________________________________________
Do You Yahoo!?
Make a great connection at Yahoo! Personals.
http://personals.yahoo.com


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


gorvil
 

Delrin is a DuPont trade name for a plastic that is strong, wear
resistant, machineable, and very slippery. Check it out at this URL.

Glen Reeser




http://www.dupont.com/enggpolymers/english/products/delrin.html

--- In southbendlathe@y..., Larry Crebs <larrysbp2000@y...> wrote:
What is delrin?
I have
some Delrin rod
stock, so I may try the spring loaded anti-backlash
double nut system.

I'll post the results when I'm done.

Glen Reeser




------------------------ Yahoo! Groups Sponsor

To unsubscribe from this group, send an email to:
southbendlathe-unsubscribe@egroups.com



Your use of Yahoo! Groups is subject to
http://docs.yahoo.com/info/terms/


__________________________________________________
Do You Yahoo!?
Make a great connection at Yahoo! Personals.
http://personals.yahoo.co