Re: [CAD_CAM_EDM_DRO] Bent Nook ballscrews ?????

On Tue, 2003-08-12 at 08:16, Bill Kichman wrote:

> Can I jump in here?

Sure.

> Something from my statics and dynamics coursework from
> way back tells me there is somethign missing in this discussion. Maybe I am
> wrong but here goes.

OK.

> I am not seeing how the backlash preload is directly related to the cutting
> forces involved

With a spring preloaded system, one must preload the backlash ballnut to
an axial force greater than the largest anticipated cutting force in
order to keep the spacing constant. If we anticipate a cutting load of
500 pounds, we'd have to preload a spring system with 500 pounds of
force or the ballnut will move under cutting forces.

> except as follows. IF the preload arrangement is assumed to
> be a rigid arrangement, like Kim's and others' designs, with a pair of bolts
> between the 2 making it for all intents and purposes a rigid body, then the
> cutting force, at any given time, is seen mostly, if not entirely, at the
> (1) of the (2) assembled nuts opposite the side of the pushing force. The
> other nut is just staying in place against the pushed side of the leadscrew
> thread, eliminating backlash. Then during direction changes, or when
> chatter can be expected due to slop for example cutting a circle at certain
> points around its travel, the load is alternately placed to either nut back
> and forth rapidly for a short duration until the load is taken up by the
> opposite nut. The bolts and other fasteners transfer all the load with no
> problem. Now my point is, how is the preload tension in any way shape or
> form related to the applied force at the nut?

It isn't in a solidly mounted system.

About a month ago I wrote an email to this group asking if the proposed
servo motors (Camtronics) and drive coupling (direct) and ballscrew
(5TPI) were going to generate enough force for our CNC machines. People
wrote back telling me they wouldn't be and that the motors were going to
overheat, etc. In practice we've got tons of power. Yesterday we were
taking 0.020 inch deep cuts at 20 IPM with a carbide cutter on 1.5 inch
steel shafts. Not soft steel either. We've got no motor overheating,
the feed is extremely smooth at any speed, etc. I THINK THIS IS BECAUSE
WE"VE GOT A GOOD BALLSCREW BACKLASH SETUP.

I corresponded with one gentleman running the same motors saying he had
no power whatsoever. We are driving these things at 150 IPM, limited
only by the computer generated step speed.

I think that some people have problems with motor sizing/power because
they've got a spring preloaded ballnut backlash setup putting 500 pounds
of axial load on the ballscrew. WE've got our ballscrew assembly out of
the lathe right now and the ballnut turns very nicely, WHEN CLEAN !

> I would counter that the
> tension is only related to that amount which eliminates the slop in the
> ball-to-screw arrangement. The added applied forces will not increase or
> decrease the backlash IF THE BACKLASH ASSEMBLY IS RIGID.

Agreed, Bill.

> Now of course, if there is a hole in these comments, I welcome correction.
>
> R. William Kichman, P.E.
> Kichman Engineering Associates
> 103 Old Furnace Road
> Cornwall, PA 17016-0643
> tel/fax 717/270-0714
> ----- Original Message -----
> From: "Kim Lux" <lux@...>
> To: <CAD_CAM_EDM_DRO@yahoogroups.com>
> Sent: Tuesday, August 12, 2003 9:44 AM
> Subject: Re: [CAD_CAM_EDM_DRO] Bent Nook ballscrews ?????
>
>
> >
> > Comments below.
> >
> > On Mon, 2003-08-11 at 22:24, Jon Elson wrote:
> > > Kim Lux wrote:
> > >
> > > >A substantial amount of force would have to be greater than the pushing
> > > >capacity of the motor so that there was never a force that would
> > > >displace the nuts against each other. If the spring is weaker than the
> > > >pushing force of the motor, then it will displace the backlash under a
> > > >heavy cut.
> > > >
> > > >As soon as the nuts are adjusted for the maximum force the motor is
> > > >capable of moving, there isn't any force left to move the cutter.
> > > >
> > > >
> > > No, you're confusing drag with linear thrust.
> >
> > No, I'm pretty sure I'm not. Lets say we have an axis that needs, on
> > occasion, 500 lbs of cutting force. In order to ensure that we never
> > had backlash, we'd have to put a preload and the backlash nut of 500
> > lbs, to keep the backlash zero in the presence of our load. That then
> > puts the load on the ballscrew at 1000 lbs, ie 500lbs for the cutting
> > load and 500 lbs for the backlash.
> >
> > > If the system is designed to
> > > apply 1000 Lbs of linear thrust, you want somewhat over 1000 Lbs
> > > of preload force between the two ballnuts.
> >
> > Yes. Thus the total load on that ballscrew is 2000 lbs. It takes twice
> > the motor to turn the preloaded system.
> >
> > > On a good ground screw,
> > > 1000 Lbs of preload will cause a very mild drag, maybe 10 Oz-In
> > > when new, and decreasing as the screw and nuts wear in a little.
> >
> > Where do you get your information from ? A 3/4" 5 TPI ballscrew has a
> > force to torque ratio of about 31 lbs force per in pound of torque.
> > 1000lbs/31 inlbs/lb = 32ish inlbs of torque or 516 oz in of torque, just
> > to turn the backlash preload on the nuts.
> >
> > > You don't have to adjust the nuts until they bind up. That could be
> > > 5000 - 10,000 Lbs of preload, and will quickly destroy the balls.
> >
> > HUH ?
> >
> >
> > >
> > > Jon
> > >
> > >
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> >
> > OFF Topic POSTS: General Machining
> > If you wish to post on unlimited OT subjects goto:
> aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if
> you have trouble.
> > http://www.metalworking.com/news_servers.html
> >
> > http://groups.yahoo.com/group/jobshophomeshop I consider this to be a
> sister site to the CCED group, as many of the same members are there, for OT
> subjects, that are not allowed on the CCED list.
> >
> > NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM.
> DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> > bill
> > List Mom
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>
> OFF Topic POSTS: General Machining
> If you wish to post on unlimited OT subjects goto: aol://5863:126/rec.crafts.metalworking or go thru Google.com to reach it if you have trouble.
> http://www.metalworking.com/news_servers.html
>
> http://groups.yahoo.com/group/jobshophomeshop I consider this to be a sister site to the CCED group, as many of the same members are there, for OT subjects, that are not allowed on the CCED list.
>
> NOTICE: ALL POSTINGS TO THIS GROUP BECOME PUBLIC DOMAIN BY POSTING THEM. DON'T POST IF YOU CAN NOT ACCEPT THIS.....NO EXCEPTIONS........
> bill
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--
Kim Lux <lux@...>